INTERMEDIATE MICROECONOMICS – 1
CONTENTS
PARICULARS
Syllabus
Hal Varian
2. Budget Constraint
Workbook Questions
3. Preferences
Workbook Questions
4. Utility
Workbook Questions
5. Choice
Workbook Questions
6. Demand
Workbook Questions
7. Revealed Preference
Workbook Questions
8. Slutsky Equation
Workbook Questions
9. Buying and Selling
Workbook Questions
10. Intertemporal Choice
Workbook Questions
12. Uncertainty
Workbook Questions
Answers
B. Douglas Bernheim and M. Whinston
11. Choices Involving Risk
C. Snyder and W. Nicholson
9. Production Function
10. Cost Function
11. Profit Maximization
Answers and Solutions
PAGE NO.
002-003
004-379
005-017
018-032
033-053
054-071
072-090
091-107
108-129
130-147
148-170
171-187
188-205
206-223
224-247
248-262
263-284
285-306
307-327
328-342
343-356
357-372
373-379
380-417
381-417
418-516
419-446
447-481
482-511
512-516
DEPARTMENT OF ECONOMICS
DELHI SCHOOL OF ECONOMICS
UNIVERSITY OF DELHI
Minutes of Meeting
Subject:
Course:
Date of Meeting:
Venue:
Chair:
B.A. (Hons) Economics – Third Semester (CBCS)
05 - Intermediate Microeconomics - I
Wednesday, 4th May, 2016
Department of Economics, Delhi School of Economics,
University of Delhi, Delhi – 110 007
Dr. Anirban Kar
Attended by:
1
2
3
4
5
6
7
8
9
10
11
12
13
Anil S. Kakrody
Rajiv Jha
Shashibala Garg
Leema Paliwal
Pintu Parui
Savitri Sidona
Vandana
Shilpa Chaudhary
Ravinder Jha
Naveen Thomas
Shikha Singh
Shalini Saksena
Surajit Deb
HRC
SRCC
LSR
St. Stephen
Ramjas
ARSD
Dyal Singh
JDM
MH
Jesus & Marry College
DRC
DCAC
Arya Bhatt
The course Committee decided to maintain the same syllabus as last year.
Course Description
The course is designed to provide a sound training in microeconomic theory. Since students are
already familiar with the quantitative techniques in the previous semesters, mathematical tools are
used to facilitate understanding of the basic concepts. This course looks at the behaviour of the
consumer and the producer and also covers the behaviour of a competitive firm.
Course Outline
1. Consumer Theory
Preference; utility; budget constraint; choice; demand; Slutsky equation; buying and selling;
choice under risk and intertemporal choice; revealed preference.
(a) Hal Varian (2010): Chapters 2-10, Chapter 12.1-12.4.
(b) B. Douglas Bernheim and M. Whinston (2009): Chapter 11.
2. Production, Costs and Perfect Competition
Technology, isoquants, production with one and more variable inputs, returns to scale, short run
and long run costs, cost curves in the short and long run; review of perfect competition.
(a) C. Snyder and W. Nicholson (2010): Chapters 9-11.
Readings
1. Hal Varian (2010): Intermediate Microeconomics: A Modern Approach, 8th edition, Affiliated
East West Press (India). The workbook by Varian and Bergstrom could be used for problems.
2. B. Douglas Bernheim and M. Whinston (2009): Microeconomics, Tata McGraw Hill (India).
3. C. Snyder and W. Nicholson (2010): Fundamentals of Microeconomics, Cengage Learning
(India).
Intermediate
Microeconomics
A Modern Approach
Eighth Edition
Hal R. Varian
University of California at Berkeley
W. W. Norton & Company • New York • London
CHAPTER
2
BUDGET
CONSTRAINT
The economic theory of the consumer is very simple: economists assume
that consumers choose the best bundle of goods they can afford. To give
content to this theory, we have to describe more precisely what we mean by
“best” and what we mean by “can afford.” In this chapter we will examine
how to describe what a consumer can afford; the next chapter will focus on
the concept of how the consumer determines what is best. We will then be
able to undertake a detailed study of the implications of this simple model
of consumer behavior.
2.1 The Budget Constraint
We begin by examining the concept of the budget constraint. Suppose
that there is some set of goods from which the consumer can choose. In
real life there are many goods to consume, but for our purposes it is convenient to consider only the case of two goods, since we can then depict the
consumer’s choice behavior graphically.
We will indicate the consumer’s consumption bundle by (x1 , x2 ). This
is simply a list of two numbers that tells us how much the consumer is choosing to consume of good 1, x1 , and how much the consumer is choosing to
TWO GOODS ARE OFTEN ENOUGH
21
consume of good 2, x2 . Sometimes it is convenient to denote the consumer’s
bundle by a single symbol like X, where X is simply an abbreviation for
the list of two numbers (x1 , x2 ).
We suppose that we can observe the prices of the two goods, (p1 , p2 ),
and the amount of money the consumer has to spend, m. Then the budget
constraint of the consumer can be written as
p1 x1 + p2 x2 ≤ m.
(2.1)
Here p1 x1 is the amount of money the consumer is spending on good 1,
and p2 x2 is the amount of money the consumer is spending on good 2.
The budget constraint of the consumer requires that the amount of money
spent on the two goods be no more than the total amount the consumer has
to spend. The consumer’s affordable consumption bundles are those that
don’t cost any more than m. We call this set of affordable consumption
bundles at prices (p1 , p2 ) and income m the budget set of the consumer.
2.2 Two Goods Are Often Enough
The two-good assumption is more general than you might think at first,
since we can often interpret one of the goods as representing everything
else the consumer might want to consume.
For example, if we are interested in studying a consumer’s demand for
milk, we might let x1 measure his or her consumption of milk in quarts per
month. We can then let x2 stand for everything else the consumer might
want to consume.
When we adopt this interpretation, it is convenient to think of good 2
as being the dollars that the consumer can use to spend on other goods.
Under this interpretation the price of good 2 will automatically be 1, since
the price of one dollar is one dollar. Thus the budget constraint will take
the form
(2.2)
p1 x1 + x2 ≤ m.
This expression simply says that the amount of money spent on good 1,
p1 x1 , plus the amount of money spent on all other goods, x2 , must be no
more than the total amount of money the consumer has to spend, m.
We say that good 2 represents a composite good that stands for everything else that the consumer might want to consume other than good
1. Such a composite good is invariably measured in dollars to be spent
on goods other than good 1. As far as the algebraic form of the budget
constraint is concerned, equation (2.2) is just a special case of the formula
given in equation (2.1), with p2 = 1, so everything that we have to say
about the budget constraint in general will hold under the composite-good
interpretation.
22 BUDGET CONSTRAINT (Ch. 2)
2.3 Properties of the Budget Set
The budget line is the set of bundles that cost exactly m:
p1 x1 + p2 x2 = m.
(2.3)
These are the bundles of goods that just exhaust the consumer’s income.
The budget set is depicted in Figure 2.1. The heavy line is the budget
line—the bundles that cost exactly m—and the bundles below this line are
those that cost strictly less than m.
x2
Vertical
intercept
= m/p 2
Budget line;
slope = – p1 /p2
Budget set
Horizontal intercept = m/p1
Figure
2.1
x1
The budget set. The budget set consists of all bundles that
are affordable at the given prices and income.
We can rearrange the budget line in equation (2.3) to give us the formula
x2 =
m p1
− x1 .
p2
p2
(2.4)
This is the formula for a straight line with a vertical intercept of m/p2
and a slope of −p1 /p2 . The formula tells us how many units of good 2 the
consumer needs to consume in order to just satisfy the budget constraint
if she is consuming x1 units of good 1.
PROPERTIES OF THE BUDGET SET
23
Here is an easy way to draw a budget line given prices (p1 , p2 ) and income
m. Just ask yourself how much of good 2 the consumer could buy if she
spent all of her money on good 2. The answer is, of course, m/p2 . Then
ask how much of good 1 the consumer could buy if she spent all of her
money on good 1. The answer is m/p1 . Thus the horizontal and vertical
intercepts measure how much the consumer could get if she spent all of her
money on goods 1 and 2, respectively. In order to depict the budget line
just plot these two points on the appropriate axes of the graph and connect
them with a straight line.
The slope of the budget line has a nice economic interpretation. It measures the rate at which the market is willing to “substitute” good 1 for
good 2. Suppose for example that the consumer is going to increase her
consumption of good 1 by Δx1 .1 How much will her consumption of good
2 have to change in order to satisfy her budget constraint? Let us use Δx2
to indicate her change in the consumption of good 2.
Now note that if she satisfies her budget constraint before and after
making the change she must satisfy
p1 x1 + p2 x2 = m
and
p1 (x1 + Δx1 ) + p2 (x2 + Δx2 ) = m.
Subtracting the first equation from the second gives
p1 Δx1 + p2 Δx2 = 0.
This says that the total value of the change in her consumption must be
zero. Solving for Δx2 /Δx1 , the rate at which good 2 can be substituted
for good 1 while still satisfying the budget constraint, gives
p1
Δx2
=− .
Δx1
p2
This is just the slope of the budget line. The negative sign is there since
Δx1 and Δx2 must always have opposite signs. If you consume more of
good 1, you have to consume less of good 2 and vice versa if you continue
to satisfy the budget constraint.
Economists sometimes say that the slope of the budget line measures
the opportunity cost of consuming good 1. In order to consume more of
good 1 you have to give up some consumption of good 2. Giving up the
opportunity to consume good 2 is the true economic cost of more good 1
consumption; and that cost is measured by the slope of the budget line.
1
The Greek letter Δ, delta, is pronounced “del-ta.” The notation Δx1 denotes the
change in good 1. For more on changes and rates of changes, see the Mathematical
Appendix.
24 BUDGET CONSTRAINT (Ch. 2)
2.4 How the Budget Line Changes
When prices and incomes change, the set of goods that a consumer can
afford changes as well. How do these changes affect the budget set?
Let us first consider changes in income. It is easy to see from equation
(2.4) that an increase in income will increase the vertical intercept and not
affect the slope of the line. Thus an increase in income will result in a parallel shift outward of the budget line as in Figure 2.2. Similarly, a decrease
in income will cause a parallel shift inward.
x2
m’/p2
Budget lines
m/p2
Slope = –p1/p 2
m/p1
Figure
2.2
m’/p1
x1
Increasing income. An increase in income causes a parallel
shift outward of the budget line.
What about changes in prices? Let us first consider increasing price
1 while holding price 2 and income fixed. According to equation (2.4),
increasing p1 will not change the vertical intercept, but it will make the
budget line steeper since p1 /p2 will become larger.
Another way to see how the budget line changes is to use the trick described earlier for drawing the budget line. If you are spending all of
your money on good 2, then increasing the price of good 1 doesn’t change
the maximum amount of good 2 you could buy—thus the vertical intercept of the budget line doesn’t change. But if you are spending all of
your money on good 1, and good 1 becomes more expensive, then your
HOW THE BUDGET LINE CHANGES
25
consumption of good 1 must decrease. Thus the horizontal intercept of
the budget line must shift inward, resulting in the tilt depicted in Figure 2.3.
x2
m/p2
Budget lines
Slope = –p1 /p2
Slope = –p'1 /p2
m/p'1
m/p1
x1
Increasing price. If good 1 becomes more expensive, the
budget line becomes steeper.
What happens to the budget line when we change the prices of good 1
and good 2 at the same time? Suppose for example that we double the
prices of both goods 1 and 2. In this case both the horizontal and vertical
intercepts shift inward by a factor of one-half, and therefore the budget
line shifts inward by one-half as well. Multiplying both prices by two is
just like dividing income by 2.
We can also see this algebraically. Suppose our original budget line is
p1 x1 + p2 x2 = m.
Now suppose that both prices become t times as large. Multiplying both
prices by t yields
tp1 x1 + tp2 x2 = m.
But this equation is the same as
p1 x1 + p2 x2 =
m
.
t
Thus multiplying both prices by a constant amount t is just like dividing
income by the same constant t. It follows that if we multiply both prices
Figure
2.3
26 BUDGET CONSTRAINT (Ch. 2)
by t and we multiply income by t, then the budget line won’t change at
all.
We can also consider price and income changes together. What happens
if both prices go up and income goes down? Think about what happens to
the horizontal and vertical intercepts. If m decreases and p1 and p2 both
increase, then the intercepts m/p1 and m/p2 must both decrease. This
means that the budget line will shift inward. What about the slope of
the budget line? If price 2 increases more than price 1, so that −p1 /p2
decreases (in absolute value), then the budget line will be flatter; if price 2
increases less than price 1, the budget line will be steeper.
2.5 The Numeraire
The budget line is defined by two prices and one income, but one of these
variables is redundant. We could peg one of the prices, or the income, to
some fixed value, and adjust the other variables so as to describe exactly
the same budget set. Thus the budget line
p1 x1 + p2 x2 = m
is exactly the same budget line as
m
p1
x1 + x2 =
p2
p2
or
p2
p1
x1 + x2 = 1,
m
m
since the first budget line results from dividing everything by p2 , and the
second budget line results from dividing everything by m. In the first case,
we have pegged p2 = 1, and in the second case, we have pegged m = 1.
Pegging the price of one of the goods or income to 1 and adjusting the
other price and income appropriately doesn’t change the budget set at all.
When we set one of the prices to 1, as we did above, we often refer to that
price as the numeraire price. The numeraire price is the price relative to
which we are measuring the other price and income. It will occasionally be
convenient to think of one of the goods as being a numeraire good, since
there will then be one less price to worry about.
2.6 Taxes, Subsidies, and Rationing
Economic policy often uses tools that affect a consumer’s budget constraint,
such as taxes. For example, if the government imposes a quantity tax, this
means that the consumer has to pay a certain amount to the government
TAXES, SUBSIDIES, AND RATIONING
27
for each unit of the good he purchases. In the U.S., for example, we pay
about 15 cents a gallon as a federal gasoline tax.
How does a quantity tax affect the budget line of a consumer? From
the viewpoint of the consumer the tax is just like a higher price. Thus a
quantity tax of t dollars per unit of good 1 simply changes the price of good
1 from p1 to p1 + t. As we’ve seen above, this implies that the budget line
must get steeper.
Another kind of tax is a value tax. As the name implies this is a tax
on the value—the price—of a good, rather than the quantity purchased of
a good. A value tax is usually expressed in percentage terms. Most states
in the U.S. have sales taxes. If the sales tax is 6 percent, then a good that
is priced at $1 will actually sell for $1.06. (Value taxes are also known as
ad valorem taxes.)
If good 1 has a price of p1 but is subject to a sales tax at rate τ , then
the actual price facing the consumer is (1 + τ )p1 .2 The consumer has to
pay p1 to the supplier and τ p1 to the government for each unit of the good
so the total cost of the good to the consumer is (1 + τ )p1 .
A subsidy is the opposite of a tax. In the case of a quantity subsidy,
the government gives an amount to the consumer that depends on the
amount of the good purchased. If, for example, the consumption of milk
were subsidized, the government would pay some amount of money to each
consumer of milk depending on the amount that consumer purchased. If
the subsidy is s dollars per unit of consumption of good 1, then from the
viewpoint of the consumer, the price of good 1 would be p1 − s. This would
therefore make the budget line flatter.
Similarly an ad valorem subsidy is a subsidy based on the price of the
good being subsidized. If the government gives you back $1 for every $2
you donate to charity, then your donations to charity are being subsidized
at a rate of 50 percent. In general, if the price of good 1 is p1 and good 1 is
subject to an ad valorem subsidy at rate σ, then the actual price of good 1
facing the consumer is (1 − σ)p1 .3
You can see that taxes and subsidies affect prices in exactly the same
way except for the algebraic sign: a tax increases the price to the consumer,
and a subsidy decreases it.
Another kind of tax or subsidy that the government might use is a lumpsum tax or subsidy. In the case of a tax, this means that the government
takes away some fixed amount of money, regardless of the individual’s behavior. Thus a lump-sum tax means that the budget line of a consumer
will shift inward because his money income has been reduced. Similarly, a
lump-sum subsidy means that the budget line will shift outward. Quantity
taxes and value taxes tilt the budget line one way or the other depending
2
The Greek letter τ , tau, rhymes with “wow.”
3
The Greek letter σ is pronounced “sig-ma.”
28 BUDGET CONSTRAINT (Ch. 2)
on which good is being taxed, but a lump-sum tax shifts the budget line
inward.
Governments also sometimes impose rationing constraints. This means
that the level of consumption of some good is fixed to be no larger than
some amount. For example, during World War II the U.S. government
rationed certain foods like butter and meat.
Suppose, for example, that good 1 were rationed so that no more than
x1 could be consumed by a given consumer. Then the budget set of the
consumer would look like that depicted in Figure 2.4: it would be the old
budget set with a piece lopped off. The lopped-off piece consists of all the
consumption bundles that are affordable but have x1 > x1 .
x2
Budget line
Budget
set
x1
Figure
2.4
x1
Budget set with rationing. If good 1 is rationed, the section
of the budget set beyond the rationed quantity will be lopped
off.
Sometimes taxes, subsidies, and rationing are combined. For example,
we could consider a situation where a consumer could consume good 1
at a price of p1 up to some level x1 , and then had to pay a tax t on all
consumption in excess of x1 . The budget set for this consumer is depicted
in Figure 2.5. Here the budget line has a slope of −p1 /p2 to the left of x1 ,
and a slope of −(p1 + t)/p2 to the right of x1 .
TAXES, SUBSIDIES, AND RATIONING
29
x2
Budget line
Slope = – p1/p 2
Budget set
Slope = – (p1 + t )/p 2
x1
x1
Taxing consumption greater than x1 . In this budget set
the consumer must pay a tax only on the consumption of good
1 that is in excess of x1 , so the budget line becomes steeper to
the right of x1 .
EXAMPLE: The Food Stamp Program
Since the Food Stamp Act of 1964 the U.S. federal government has provided
a subsidy on food for poor people. The details of this program have been
adjusted several times. Here we will describe the economic effects of one
of these adjustments.
Before 1979, households who met certain eligibility requirements were
allowed to purchase food stamps, which could then be used to purchase food
at retail outlets. In January 1975, for example, a family of four could receive
a maximum monthly allotment of $153 in food coupons by participating in
the program.
The price of these coupons to the household depended on the household
income. A family of four with an adjusted monthly income of $300 paid
$83 for the full monthly allotment of food stamps. If a family of four had
a monthly income of $100, the cost for the full monthly allotment would
have been $25.4
The pre-1979 Food Stamp program was an ad valorem subsidy on food.
The rate at which food was subsidized depended on the household income.
4
These figures are taken from Kenneth Clarkson, Food Stamps and Nutrition, American Enterprise Institute, 1975.
Figure
2.5
30 BUDGET CONSTRAINT (Ch. 2)
The family of four that was charged $83 for their allotment paid $1 to
receive $1.84 worth of food (1.84 equals 153 divided by 83). Similarly, the
household that paid $25 was paying $1 to receive $6.12 worth of food (6.12
equals 153 divided by 25).
The way that the Food Stamp program affected the budget set of a
household is depicted in Figure 2.6A. Here we have measured the amount
of money spent on food on the horizontal axis and expenditures on all other
goods on the vertical axis. Since we are measuring each good in terms of
the money spent on it, the “price” of each good is automatically 1, and the
budget line will therefore have a slope of −1.
If the household is allowed to buy $153 of food stamps for $25, then this
represents roughly an 84 percent (= 1 − 25/153) subsidy of food purchases,
so the budget line will have a slope of roughly −.16 (= 25/153) until the
household has spent $153 on food. Each dollar that the household spends
on food up to $153 would reduce its consumption of other goods by about
16 cents. After the household spends $153 on food, the budget line facing
it would again have a slope of −1.
OTHER
GOODS
OTHER
GOODS
Budget line
with food
stamps
A
Figure
2.6
Budget
line
without
food
stamps
Budget
line
without
food
stamps
FOOD
$153
Budget line
with food
stamps
FOOD
$200
B
Food stamps. How the budget line is affected by the Food
Stamp program. Part A shows the pre-1979 program and part
B the post-1979 program.
These effects lead to the kind of “kink” depicted in Figure 2.6. Households with higher incomes had to pay more for their allotment of food
stamps. Thus the slope of the budget line would become steeper as household income increased.
In 1979 the Food Stamp program was modified. Instead of requiring that
SUMMARY
31
households purchase food stamps, they are now simply given to qualified
households. Figure 2.6B shows how this affects the budget set.
Suppose that a household now receives a grant of $200 of food stamps a
month. Then this means that the household can consume $200 more food
per month, regardless of how much it is spending on other goods, which
implies that the budget line will shift to the right by $200. The slope
will not change: $1 less spent on food would mean $1 more to spend on
other things. But since the household cannot legally sell food stamps, the
maximum amount that it can spend on other goods does not change. The
Food Stamp program is effectively a lump-sum subsidy, except for the fact
that the food stamps can’t be sold.
2.7 Budget Line Changes
In the next chapter we will analyze how the consumer chooses an optimal
consumption bundle from his or her budget set. But we can already state
some observations here that follow from what we have learned about the
movements of the budget line.
First, we can observe that since the budget set doesn’t change when we
multiply all prices and income by a positive number, the optimal choice of
the consumer from the budget set can’t change either. Without even analyzing the choice process itself, we have derived an important conclusion:
a perfectly balanced inflation—one in which all prices and all incomes rise
at the same rate—doesn’t change anybody’s budget set, and thus cannot
change anybody’s optimal choice.
Second, we can make some statements about how well-off the consumer
can be at different prices and incomes. Suppose that the consumer’s income
increases and all prices remain the same. We know that this represents a
parallel shift outward of the budget line. Thus every bundle the consumer
was consuming at the lower income is also a possible choice at the higher
income. But then the consumer must be at least as well-off at the higher
income as at the lower income—since he or she has the same choices available as before plus some more. Similarly, if one price declines and all others
stay the same, the consumer must be at least as well-off. This simple observation will be of considerable use later on.
Summary
1. The budget set consists of all bundles of goods that the consumer can
afford at given prices and income. We will typically assume that there are
only two goods, but this assumption is more general than it seems.
2. The budget line is written as p1 x1 + p2 x2 = m. It has a slope of −p1 /p2 ,
a vertical intercept of m/p2 , and a horizontal intercept of m/p1 .
32 BUDGET CONSTRAINT (Ch. 2)
3. Increasing income shifts the budget line outward. Increasing the price
of good 1 makes the budget line steeper. Increasing the price of good 2
makes the budget line flatter.
4. Taxes, subsidies, and rationing change the slope and position of the
budget line by changing the prices paid by the consumer.
REVIEW QUESTIONS
1. Originally the consumer faces the budget line p1 x1 + p2 x2 = m. Then
the price of good 1 doubles, the price of good 2 becomes 8 times larger,
and income becomes 4 times larger. Write down an equation for the new
budget line in terms of the original prices and income.
2. What happens to the budget line if the price of good 2 increases, but
the price of good 1 and income remain constant?
3. If the price of good 1 doubles and the price of good 2 triples, does the
budget line become flatter or steeper?
4. What is the definition of a numeraire good?
5. Suppose that the government puts a tax of 15 cents a gallon on gasoline
and then later decides to put a subsidy on gasoline at a rate of 7 cents a
gallon. What net tax is this combination equivalent to?
6. Suppose that a budget equation is given by p1 x1 + p2 x2 = m. The
government decides to impose a lump-sum tax of u, a quantity tax on
good 1 of t, and a quantity subsidy on good 2 of s. What is the formula
for the new budget line?
7. If the income of the consumer increases and one of the prices decreases
at the same time, will the consumer necessarily be at least as well-off?
These workouts are designed to build your skills in describing economic
situations with graphs and algebra. Budget sets are a good place to start,
because both the algebra and the graphing are very easy. Where there
are just two goods, a consumer who consumes x1 units of good 1 and x2
units of good 2 is said to consume the consumption bundle, (x1 , x2 ). Any
consumption bundle can be represented by a point on a two-dimensional
graph with quantities of good 1 on the horizontal axis and quantities of
good 2 on the vertical axis. If the prices are p1 for good 1 and p2 for good
2, and if the consumer has income m, then she can afford any consumption
bundle, (x1 , x2 ), such that p1 x1 + p2 x2 ≤ m. On a graph, the budget line
is just the line segment with equation p1 x1 + p2 x2 = m and with x1 and
x2 both nonnegative. The budget line is the boundary of the budget set.
All of the points that the consumer can afford lie on one side of the line
and all of the points that the consumer cannot afford lie on the other.
If you know prices and income, you can construct a consumer’s budget line by finding two commodity bundles that she can “just afford” and
drawing the straight line that runs through both points.
Myrtle has 50 dollars to spend. She consumes only apples and bananas.
Apples cost 2 dollars each and bananas cost 1 dollar each. You are to
graph her budget line, where apples are measured on the horizontal axis
and bananas on the vertical axis. Notice that if she spends all of her
income on apples, she can afford 25 apples and no bananas. Therefore
her budget line goes through the point (25, 0) on the horizontal axis. If
she spends all of her income on bananas, she can afford 50 bananas and
no apples. Therfore her budget line also passes throught the point (0, 50)
on the vertical axis. Mark these two points on your graph. Then draw a
straight line between them. This is Myrtle’s budget line.
What if you are not told prices or income, but you know two commodity bundles that the consumer can just afford? Then, if there are just
two commodities, you know that a unique line can be drawn through two
points, so you have enough information to draw the budget line.
Laurel consumes only ale and bread. If she spends all of her income, she
can just afford 20 bottles of ale and 5 loaves of bread. Another commodity
bundle that she can afford if she spends her entire income is 10 bottles
of ale and 10 loaves of bread. If the price of ale is 1 dollar per bottle,
how much money does she have to spend? You could solve this problem
graphically. Measure ale on the horizontal axis and bread on the vertical
axis. Plot the two points, (20, 5) and (10, 10), that you know to be on the
budget line. Draw the straight line between these points and extend the
line to the horizontal axis. This point denotes the amount of ale Laurel
can afford if she spends all of her money on ale. Since ale costs 1 dollar a
bottle, her income in dollars is equal to the largest number of bottles she
can afford. Alternatively, you can reason as follows. Since the bundles
(20, 5) and (10, 10) cost the same, it must be that giving up 10 bottles of
ale makes her able to afford an extra 5 loaves of bread. So bread costs
twice as much as ale. The price of ale is 1 dollar, so the price of bread is
2 dollars. The bundle (20, 5) costs as much as her income. Therefore her
income must be 20 × 1 + 5 × 2 = 30.
When you have completed this workout, we hope that you will be
able to do the following:
• Write an equation for the budget line and draw the budget set on a
graph when you are given prices and income or when you are given
two points on the budget line.
• Graph the effects of changes in prices and income on budget sets.
• Understand the concept of numeraire and know what happens to the
budget set when income and all prices are multiplied by the same
positive amount.
• Know what the budget set looks like if one or more of the prices is
negative.
• See that the idea of a “budget set” can be applied to constrained
choices where there are other constraints on what you can have, in
addition to a constraint on money expenditure.
2.1 (0) You have an income of $40 to spend on two commodities. Commodity 1 costs $10 per unit, and commodity 2 costs $5 per unit.
(a) Write down your budget equation.
.
(b) If you spent all your income on commodity 1, how much could you
buy?
.
(c) If you spent all of your income on commodity 2, how much could you
buy?
Use blue ink to draw your budget line in the graph below.
x2
8
6
4
2
0
2
4
6
8
x1
(d) Suppose that the price of commodity 1 falls to $5 while everything
else stays the same. Write down your new budget equation.
On the graph above, use red ink to draw your new budget
line.
(e) Suppose that the amount you are allowed to spend falls to $30, while
the prices of both commodities remain at $5. Write down your budget
equation.
Use black ink to draw this budget line.
(f ) On your diagram, use blue ink to shade in the area representing commodity bundles that you can afford with the budget in Part (e) but could
not afford to buy with the budget in Part (a). Use black ink or pencil to
shade in the area representing commodity bundles that you could afford
with the budget in Part (a) but cannot afford with the budget in Part
(e).
2.2 (0) On the graph below, draw a budget line for each case.
(a) p1 = 1, p2 = 1, m = 15. (Use blue ink.)
(b) p1 = 1, p2 = 2, m = 20. (Use red ink.)
(c) p1 = 0, p2 = 1, m = 10. (Use black ink.)
(d) p1 = p2 , m = 15p1 . (Use pencil or black ink. Hint: How much of
good 1 could you afford if you spend your entire budget on good 1?)
x2
20
15
10
5
0
5
10
15
20
x1
2.3 (0) Your budget is such that if you spend your entire income, you
can afford either 4 units of good x and 6 units of good y or 12 units of x
and 2 units of y.
(a) Mark these two consumption bundles and draw the budget line in the
graph below.
y
16
12
8
4
0
4
8
12
16
x
(b) What is the ratio of the price of x to the price of y?
.
(c) If you spent all of your income on x, how much x could you buy?
.
(d) If you spent all of your income on y, how much y could you buy?
.
(e) Write a budget equation that gives you this budget line, where the
price of x is 1.
.
(f ) Write another budget equation that gives you the same budget line,
but where the price of x is 3.
.
2.4 (1) Murphy was consuming 100 units of X and 50 units of Y . The
price of X rose from 2 to 3. The price of Y remained at 4.
(a) How much would Murphy’s income have to rise so that he can still
exactly afford 100 units of X and 50 units of Y ?
.
2.5 (1) If Amy spent her entire allowance, she could afford 8 candy bars
and 8 comic books a week. She could also just afford 10 candy bars and
4 comic books a week. The price of a candy bar is 50 cents. Draw her
budget line in the box below. What is Amy’s weekly allowance?
Comic books
32
24
16
8
0
8
16
24
32
Candy bars
.
2.6 (0) In a small country near the Baltic Sea, there are only three
commodities: potatoes, meatballs, and jam. Prices have been remarkably stable for the last 50 years or so. Potatoes cost 2 crowns per sack,
meatballs cost 4 crowns per crock, and jam costs 6 crowns per jar.
(a) Write down a budget equation for a citizen named Gunnar who has
an income of 360 crowns per year. Let P stand for the number of sacks of
potatoes, M for the number of crocks of meatballs, and J for the number
of jars of jam consumed by Gunnar in a year.
.
(b) The citizens of this country are in general very clever people, but they
are not good at multiplying by 2. This made shopping for potatoes excruciatingly difficult for many citizens. Therefore it was decided to introduce
a new unit of currency, such that potatoes would be the numeraire. A
sack of potatoes costs one unit of the new currency while the same relative prices apply as in the past. In terms of the new currency, what is
the price of meatballs?
.
(c) In terms of the new currency, what is the price of jam?
.
(d) What would Gunnar’s income in the new currency have to be for him
to be exactly able to afford the same commodity bundles that he could
afford before the change?
(e) Write down Gunnar’s new budget equation.
.
Is
Gunnar’s budget set any different than it was before the change?
2.7 (0) Edmund Stench consumes two commodities, namely garbage and
punk rock video cassettes. He doesn’t actually eat the former but keeps
it in his backyard where it is eaten by billy goats and assorted vermin.
The reason that he accepts the garbage is that people pay him $2 per
sack for taking it. Edmund can accept as much garbage as he wishes at
that price. He has no other source of income. Video cassettes cost him
$6 each.
(a) If Edmund accepts zero sacks of garbage, how many video cassettes
can he buy?
.
(b) If he accepts 15 sacks of garbage, how many video cassettes can he
buy?
.
(c) Write down an equation for his budget line.
.
(d) Draw Edmund’s budget line and shade in his budget set.
Garbage
20
15
10
5
0
5
10
15
20
Video cassettes
2.8 (0) If you think Edmund is odd, consider his brother Emmett.
Emmett consumes speeches by politicians and university administrators.
He is paid $1 per hour for listening to politicians and $2 per hour for
listening to university administrators. (Emmett is in great demand to help
fill empty chairs at public lectures because of his distinguished appearance
and his ability to refrain from making rude noises.) Emmett consumes
one good for which he must pay. We have agreed not to disclose what
that good is, but we can tell you that it costs $15 per unit and we shall
call it Good X. In addition to what he is paid for consuming speeches,
Emmett receives a pension of $50 per week.
Administrator speeches
100
75
50
25
0
25
50
75
100
Politician speeches
(a) Write down a budget equation stating those combinations of the three
commodities, Good X, hours of speeches by politicians (P ), and hours of
speeches by university administrators (A) that Emmett could afford to
consume per week.
.
(b) On the graph above, draw a two-dimensional diagram showing the
locus of consumptions of the two kinds of speeches that would be possible
for Emmett if he consumed 10 units of Good X per week.
2.9 (0) Jonathan Livingstone Yuppie is a prosperous lawyer. He
has, in his own words, “outgrown those confining two-commodity limits.” Jonathan consumes three goods, unblended Scotch whiskey, designer tennis shoes, and meals in French gourmet restaurants. The price
of Jonathan’s brand of whiskey is $20 per bottle, the price of designer
tennis shoes is $80 per pair, and the price of gourmet restaurant meals
is $50 per meal. After he has paid his taxes and alimony, Jonathan has
$400 a week to spend.
(a) Write down a budget equation for Jonathan, where W stands for
the number of bottles of whiskey, T stands for the number of pairs of
tennis shoes, and M for the number of gourmet restaurant meals that he
consumes.
.
(b) Draw a three-dimensional diagram to show his budget set. Label the
intersections of the budget set with each axis.
(c) Suppose that he determines that he will buy one pair of designer tennis
shoes per week. What equation must be satisfied by the combinations of
restaurant meals and whiskey that he could afford?
.
2.10 (0) Martha is preparing for exams in economics and sociology. She
has time to read 40 pages of economics and 30 pages of sociology. In the
same amount of time she could also read 30 pages of economics and 60
pages of sociology.
(a) Assuming that the number of pages per hour that she can read of
either subject does not depend on how she allocates her time, how many
pages of sociology could she read if she decided to spend all of her time on
sociology and none on economics?
(Hint: You have two
points on her budget line, so you should be able to determine the entire
line.)
(b) How many pages of economics could she read if she decided to spend
all of her time reading economics?
.
2.11 (1) Harry Hype has $5,000 to spend on advertising a new kind of
dehydrated sushi. Market research shows that the people most likely to
buy this new product are recent recipients of M.B.A. degrees and lawyers
who own hot tubs. Harry is considering advertising in two publications,
a boring business magazine and a trendy consumer publication for people
who wish they lived in California.
Fact 1: Ads in the boring business magazine cost $500 each and ads in
the consumer magazine cost $250 each.
Fact 2: Each ad in the business magazine will be read by 1,000 recent
M.B.A.’s and 300 lawyers with hot tubs.
Fact 3: Each ad in the consumer publication will be read by 300 recent
M.B.A.’s and 250 lawyers who own hot tubs.
Fact 4: Nobody reads more than one ad, and nobody who reads one
magazine reads the other.
(a) If Harry spends his entire advertising budget on the business publication, his ad will be read by
lawyers with hot tubs.
recent M.B.A.’s and by
(b) If he spends his entire advertising budget on the consumer publication,
his ad will be read by
lawyers with hot tubs.
recent M.B.A.’s and by
(c) Suppose he spent half of his advertising budget on each publication.
His ad would be read by
lawyers with hot tubs.
recent M.B.A.’s and by
(d) Draw a “budget line” showing the combinations of number of readings
by recent M.B.A.’s and by lawyers with hot tubs that he can obtain if he
spends his entire advertising budget. Does this line extend all the way
to the axes?
Sketch, shade in, and label the budget set, which
includes all the combinations of MBA’s and lawyers he can reach if he
spends no more than his budget.
(e) Let M stand for the number of instances of an ad being read by an
M.B.A. and L stand for the number of instances of an ad being read by
a lawyer. This budget line is a line segment that lies on the line with
equation
With a fixed advertising budget, how many
readings by M.B.A.’s must he sacrifice to get an additional reading by a
lawyer with a hot tub?
.
M.B.A.’s × 1,000
16
12
8
4
0
4
8
12
16
Lawyers × 1,000
2.12 (0) On the planet Mungo, they have two kinds of money, blue
money and red money. Every commodity has two prices—a red-money
price and a blue-money price. Every Mungoan has two incomes—a red
income and a blue income.
In order to buy an object, a Mungoan has to pay that object’s redmoney price in red money and its blue-money price in blue money. (The
shops simply have two cash registers, and you have to pay at both registers
to buy an object.) It is forbidden to trade one kind of money for the other,
and this prohibition is strictly enforced by Mungo’s ruthless and efficient
monetary police.
• There are just two consumer goods on Mungo, ambrosia and bubble
gum. All Mungoans prefer more of each good to less.
• The blue prices are 1 bcu (bcu stands for blue currency unit) per
unit of ambrosia and 1 bcu per unit of bubble gum.
• The red prices are 2 rcus (red currency units) per unit of ambrosia
and 6 rcus per unit of bubble gum.
(a) On the graph below, draw the red budget (with red ink) and the
blue budget (with blue ink) for a Mungoan named Harold whose blue
income is 10 and whose red income is 30. Shade in the “budget set”
containing all of the commodity bundles that Harold can afford, given
its∗ two budget constraints. Remember, Harold has to have enough blue
money and enough red money to pay both the blue-money cost and the
red-money cost of a bundle of goods.
Bubble gum
20
15
10
5
0
5
10
15
20
Ambrosia
(b) Another Mungoan, Gladys, faces the same prices that Harold faces
and has the same red income as Harold, but Gladys has a blue income of
20. Explain how it is that Gladys will not spend its entire blue income
no matter what its tastes may be. (Hint: Draw Gladys’s budget lines.)
.
(c) A group of radical economic reformers on Mungo believe that the
currency rules are unfair. “Why should everyone have to pay two prices
for everything?” they ask. They propose the following scheme. Mungo
will continue to have two currencies, every good will have a blue price and
a red price, and every Mungoan will have a blue income and a red income.
But nobody has to pay both prices. Instead, everyone on Mungo must
declare itself to be either a Blue-Money Purchaser (a “Blue”) or a RedMoney Purchaser (a “Red”) before it buys anything at all. Blues must
make all of their purchases in blue money at the blue prices, spending
only their blue incomes. Reds must make all of their purchases in red
money, spending only their red incomes.
Suppose that Harold has the same income after this reform, and that
prices do not change. Before declaring which kind of purchaser it will be,
Harold contemplates the set of commodity bundles that it could afford
by making one declaration or the other. Let us call a commodity bundle
∗
We refer to all Mungoans by the gender-neutral pronoun, “it.” Although Mungo has two sexes, neither of them is remotely like either of
ours.
“attainable” if Harold can afford it by declaring itself to be a “Blue” and
buying the bundle with blue money or if Harold can afford the bundle
by declaring itself to be a “Red” and buying it with red money. On the
diagram below, shade in all of the attainable bundles.
Bubble gum
20
15
10
5
0
5
10
15
20
Ambrosia
2.13 (0) Are Mungoan budgets really so fanciful? Can you think of situations on earth where people must simultaneously satisfy more than one
budget constraint? Is money the only scarce resource that people use up
when consuming?
.
2.1 In Problem 2.1, if you have an income of $12 to spend, if commodity 1
costs $2 per unit, and if commodity 2 costs $6 per unit, then the equation
for your budget line can be written as
(a) x1 /2 + x2 /6 = 12.
(b) (x1 + x2 )/8 = 12.
(c) x1 + 3x2 = 6.
(d) 3x1 + 7x2 = 13.
(e) 8(x1 + x2 ) = 12.
2.2 In Problem 2.3, if you could exactly afford either 6 units of x and 14
units of y, or 10 units of x and 6 units of y, then if you spent all of your
income on y, how many units of y could you buy?
(a) 26
(b) 18
(c) 34
(d) 16
(e) None of the other options are correct.
2.3 In Problem 2.4, Murphy used to consume 100 units of x and 50 units
of y when the price of x was 2 and the price of y was 4. If the price of x
rose to 5 and the price of y rose to 8, how much would Murphy’s income
have to rise so that he could still afford his original bundle?
(a) 700.
(b) 500.
(c) 350.
(d) 1,050.
(e) None of the other options are correct.
2.4 In Problem 2.7, Edmund must pay $6 each for punk rock video cassettes. If Edmund is paid $48 per sack for accepting garbage and if his
relatives send him an allowance of $384, then his budget line is described
by the equation:
(a) 6V = 48G.
(b) 6V + 48G = 384.
(c) 6V − 48G = 384.
(d) 6V = 384 − G.
(e) None of the other options are correct.
2.5 In Problem 2.10, if in the same amount of time that it takes her
to read 40 pages of economics and 30 pages of sociology, Martha could
read 30 pages of economics and 50 pages of sociology, then which of these
equations describes combinations of pages of economics, E, and sociology,
S, that she could read in the time it takes to read 40 pages of economics
and 30 pages of sociology?
(a) E + S = 70.
(b) E/2 + S = 50.
(c) 2E + S = 110.
(d) E + S = 80.
(e) All of the above.
2.6 In Problem 2.11, ads in the boring business magazine are read by 300
lawyers and 1,000 MBAs. Ads in the consumer publication are read by
250 lawyers and 300 MBAs. If Harry had $3,000 to spend on advertising,
if the price of ads in the boring business magazine were $600, and if the
price of ads in the consumer magazine were $300, then the combinations
of recent MBAs and lawyers with hot tubs whom he could reach with his
advertising budget would be represented by the integer values along a line
segment that runs between the two points
(a) (2,500, 3,000) and (1,500, 5,000).
(b) (3,000, 3,500) and (1,500, 6,000).
(c) (0, 3,000) and (1,500, 0).
(d) (3,000, 0) and (0, 6,000).
(e) (2,000, 0) and (0, 5,000).
2.7 In the economy of Mungo, discussed in Problem 2.12, there is a third
creature called Ike. Ike has a red income of 40 and a blue income of
10. (Recall that blue prices are 1 bcu [blue currency unit] per unit of
ambrosia and 1 bcu per unit of bubble gum. Red prices are 2 rcus [red
currency units] per unit of ambrosia and 6 rcus per unit of bubble gum.
You have to pay twice for what you buy, once in red currency and once
in blue currency.) If Ike spends all of its blue income, but not all of its
red income, then it must be that it consumes
(a) at least 5 units of bubble gum.
(b) at least 5 units of ambrosia.
(c) exactly twice as much bubble gum as ambrosia.
(d) at least 15 units of bubble gum.
(e) equal amounts of ambrosia and bubble gum.
CHAPTER
3
PREFERENCES
We saw in Chapter 2 that the economic model of consumer behavior is very
simple: people choose the best things they can afford. The last chapter was
devoted to clarifying the meaning of “can afford,” and this chapter will be
devoted to clarifying the economic concept of “best things.”
We call the objects of consumer choice consumption bundles. This
is a complete list of the goods and services that are involved in the choice
problem that we are investigating. The word “complete” deserves emphasis: when you analyze a consumer’s choice problem, make sure that you
include all of the appropriate goods in the definition of the consumption
bundle.
If we are analyzing consumer choice at the broadest level, we would want
not only a complete list of the goods that a consumer might consume, but
also a description of when, where, and under what circumstances they
would become available. After all, people care about how much food they
will have tomorrow as well as how much food they have today. A raft in the
middle of the Atlantic Ocean is very different from a raft in the middle of
the Sahara Desert. And an umbrella when it is raining is quite a different
good from an umbrella on a sunny day. It is often useful to think of the
34 PREFERENCES (Ch. 3)
“same” good available in different locations or circumstances as a different
good, since the consumer may value the good differently in those situations.
However, when we limit our attention to a simple choice problem, the
relevant goods are usually pretty obvious. We’ll often adopt the idea described earlier of using just two goods and calling one of them “all other
goods” so that we can focus on the tradeoff between one good and everything else. In this way we can consider consumption choices involving
many goods and still use two-dimensional diagrams.
So let us take our consumption bundle to consist of two goods, and let
x1 denote the amount of one good and x2 the amount of the other. The
complete consumption bundle is therefore denoted by (x1 , x2 ). As noted
before, we will occasionally abbreviate this consumption bundle by X.
3.1 Consumer Preferences
We will suppose that given any two consumption bundles, (x1 , x2 ) and
(y1 , y2 ), the consumer can rank them as to their desirability. That is, the
consumer can determine that one of the consumption bundles is strictly
better than the other, or decide that she is indifferent between the two
bundles.
We will use the symbol to mean that one bundle is strictly preferred
to another, so that (x1 , x2 ) (y1 , y2 ) should be interpreted as saying that
the consumer strictly prefers (x1 , x2 ) to (y1 , y2 ), in the sense that she
definitely wants the x-bundle rather than the y-bundle. This preference
relation is meant to be an operational notion. If the consumer prefers
one bundle to another, it means that he or she would choose one over the
other, given the opportunity. Thus the idea of preference is based on the
consumer’s behavior. In order to tell whether one bundle is preferred to
another, we see how the consumer behaves in choice situations involving
the two bundles. If she always chooses (x1 , x2 ) when (y1 , y2 ) is available,
then it is natural to say that this consumer prefers (x1 , x2 ) to (y1 , y2 ).
If the consumer is indifferent between two bundles of goods, we use
the symbol ∼ and write (x1 , x2 ) ∼ (y1 , y2 ). Indifference means that the
consumer would be just as satisfied, according to her own preferences,
consuming the bundle (x1 , x2 ) as she would be consuming the other bundle,
(y1 , y2 ).
If the consumer prefers or is indifferent between the two bundles we say
that she weakly prefers (x1 , x2 ) to (y1 , y2 ) and write (x1 , x2 ) (y1 , y2 ).
These relations of strict preference, weak preference, and indifference
are not independent concepts; the relations are themselves related! For
example, if (x1 , x2 ) (y1 , y2 ) and (y1 , y2 ) (x1 , x2 ) we can conclude that
(x1 , x2 ) ∼ (y1 , y2 ). That is, if the consumer thinks that (x1 , x2 ) is at least
as good as (y1 , y2 ) and that (y1 , y2 ) is at least as good as (x1 , x2 ), then the
consumer must be indifferent between the two bundles of goods.
ASSUMPTIONS ABOUT PREFERENCES
35
Similarly, if (x1 , x2 ) (y1 , y2 ) but we know that it is not the case that
(x1 , x2 ) ∼ (y1 , y2 ), we can conclude that we must have (x1 , x2 ) (y1 , y2 ).
This just says that if the consumer thinks that (x1 , x2 ) is at least as good
as (y1 , y2 ), and she is not indifferent between the two bundles, then it must
be that she thinks that (x1 , x2 ) is strictly better than (y1 , y2 ).
3.2 Assumptions about Preferences
Economists usually make some assumptions about the “consistency” of
consumers’ preferences. For example, it seems unreasonable—not to say
contradictory—to have a situation where (x1 , x2 ) (y1 , y2 ) and, at the
same time, (y1 , y2 ) (x1 , x2 ). For this would mean that the consumer
strictly prefers the x-bundle to the y-bundle . . . and vice versa.
So we usually make some assumptions about how the preference relations
work. Some of the assumptions about preferences are so fundamental that
we can refer to them as “axioms” of consumer theory. Here are three such
axioms about consumer preference.
Complete. We assume that any two bundles can be compared. That is,
given any x-bundle and any y-bundle, we assume that (x1 , x2 ) (y1 , y2 ),
or (y1 , y2 ) (x1 , x2 ), or both, in which case the consumer is indifferent
between the two bundles.
Reflexive. We assume that any bundle is at least as good as itself:
(x1 , x2 ) (x1 , x2 ).
Transitive. If (x1 , x2 ) (y1 , y2 ) and (y1 , y2 ) (z1 , z2 ), then we assume
that (x1 , x2 ) (z1 , z2 ). In other words, if the consumer thinks that X is at
least as good as Y and that Y is at least as good as Z, then the consumer
thinks that X is at least as good as Z.
The first axiom, completeness, is hardly objectionable, at least for the
kinds of choices economists generally examine. To say that any two bundles
can be compared is simply to say that the consumer is able to make a choice
between any two given bundles. One might imagine extreme situations
involving life or death choices where ranking the alternatives might be
difficult, or even impossible, but these choices are, for the most part, outside
the domain of economic analysis.
The second axiom, reflexivity, is trivial. Any bundle is certainly at least
as good as an identical bundle. Parents of small children may occasionally
observe behavior that violates this assumption, but it seems plausible for
most adult behavior.
The third axiom, transitivity, is more problematic. It isn’t clear that
transitivity of preferences is necessarily a property that preferences would
have to have. The assumption that preferences are transitive doesn’t seem
36 PREFERENCES (Ch. 3)
compelling on grounds of pure logic alone. In fact it’s not. Transitivity is
a hypothesis about people’s choice behavior, not a statement of pure logic.
Whether it is a basic fact of logic or not isn’t the point: it is whether or not
it is a reasonably accurate description of how people behave that matters.
What would you think about a person who said that he preferred a
bundle X to Y , and preferred Y to Z, but then also said that he preferred
Z to X? This would certainly be taken as evidence of peculiar behavior.
More importantly, how would this consumer behave if faced with choices
among the three bundles X, Y , and Z? If we asked him to choose his most
preferred bundle, he would have quite a problem, for whatever bundle he
chose, there would always be one that was preferred to it. If we are to have
a theory where people are making “best” choices, preferences must satisfy
the transitivity axiom or something very much like it. If preferences were
not transitive there could well be a set of bundles for which there is no best
choice.
3.3 Indifference Curves
It turns out that the whole theory of consumer choice can be formulated
in terms of preferences that satisfy the three axioms described above, plus
a few more technical assumptions. However, we will find it convenient to
describe preferences graphically by using a construction known as indifference curves.
Consider Figure 3.1 where we have illustrated two axes representing a
consumer’s consumption of goods 1 and 2. Let us pick a certain consumption bundle (x1 , x2 ) and shade in all of the consumption bundles that are
weakly preferred to (x1 , x2 ). This is called the weakly preferred set. The
bundles on the boundary of this set—the bundles for which the consumer
is just indifferent to (x1 , x2 )—form the indifference curve.
We can draw an indifference curve through any consumption bundle we
want. The indifference curve through a consumption bundle consists of all
bundles of goods that leave the consumer indifferent to the given bundle.
One problem with using indifference curves to describe preferences is
that they only show you the bundles that the consumer perceives as being
indifferent to each other—they don’t show you which bundles are better
and which bundles are worse. It is sometimes useful to draw small arrows
on the indifference curves to indicate the direction of the preferred bundles.
We won’t do this in every case, but we will do it in a few of the examples
where confusion might arise.
If we make no further assumptions about preferences, indifference curves
can take very peculiar shapes indeed. But even at this level of generality,
we can state an important principle about indifference curves: indifference
curves representing distinct levels of preference cannot cross. That is, the
situation depicted in Figure 3.2 cannot occur.
EXAMPLES OF PREFERENCES
37
x2
Weakly preferred set:
bundles weakly
preferred to (x1, x2 )
x2
Indifference
curve:
bundles
indifferent
to (x1, x2 )
x1
x1
Weakly preferred set. The shaded area consists of all bundles that are at least as good as the bundle (x1 , x2 ).
In order to prove this, let us choose three bundles of goods, X, Y , and
Z, such that X lies only on one indifference curve, Y lies only on the other
indifference curve, and Z lies at the intersection of the indifference curves.
By assumption the indifference curves represent distinct levels of preference, so one of the bundles, say X, is strictly preferred to the other bundle,
Y . We know that X ∼ Z and Z ∼ Y , and the axiom of transitivity therefore implies that X ∼ Y . But this contradicts the assumption that X Y .
This contradiction establishes the result—indifference curves representing
distinct levels of preference cannot cross.
What other properties do indifference curves have? In the abstract, the
answer is: not many. Indifference curves are a way to describe preferences.
Nearly any “reasonable” preferences that you can think of can be depicted
by indifference curves. The trick is to learn what kinds of preferences give
rise to what shapes of indifference curves.
3.4 Examples of Preferences
Let us try to relate preferences to indifference curves through some examples. We’ll describe some preferences and then see what the indifference
curves that represent them look like.
Figure
3.1
38 PREFERENCES (Ch. 3)
x2
Alleged
indifference
curves
X
Z
Y
x1
Figure
3.2
Indifference curves cannot cross. If they did, X, Y , and
Z would all have to be indifferent to each other and thus could
not lie on distinct indifference curves.
There is a general procedure for constructing indifference curves given
a “verbal” description of the preferences. First plop your pencil down on
the graph at some consumption bundle (x1 , x2 ). Now think about giving a
little more of good 1, Δx1 , to the consumer, moving him to (x1 + Δx1 , x2 ).
Now ask yourself how would you have to change the consumption of x2
to make the consumer indifferent to the original consumption point? Call
this change Δx2 . Ask yourself the question “For a given change in good
1, how does good 2 have to change to make the consumer just indifferent
between (x1 + Δx1 , x2 + Δx2 ) and (x1 , x2 )?” Once you have determined
this movement at one consumption bundle you have drawn a piece of the
indifference curve. Now try it at another bundle, and so on, until you
develop a clear picture of the overall shape of the indifference curves.
Perfect Substitutes
Two goods are perfect substitutes if the consumer is willing to substitute
one good for the other at a constant rate. The simplest case of perfect
substitutes occurs when the consumer is willing to substitute the goods on
a one-to-one basis.
Suppose, for example, that we are considering a choice between red pencils and blue pencils, and the consumer involved likes pencils, but doesn’t
care about color at all. Pick a consumption bundle, say (10, 10). Then for
this consumer, any other consumption bundle that has 20 pencils in it is
EXAMPLES OF PREFERENCES
39
just as good as (10, 10). Mathematically speaking, any consumption bundle (x1 , x2 ) such that x1 + x2 = 20 will be on this consumer’s indifference
curve through (10, 10). Thus the indifference curves for this consumer are
all parallel straight lines with a slope of −1, as depicted in Figure 3.3.
Bundles with more total pencils are preferred to bundles with fewer total
pencils, so the direction of increasing preference is up and to the right, as
illustrated in Figure 3.3.
How does this work in terms of general procedure for drawing indifference
curves? If we are at (10, 10), and we increase the amount of the first good
by one unit to 11, how much do we have to change the second good to get
back to the original indifference curve? The answer is clearly that we have
to decrease the second good by 1 unit. Thus the indifference curve through
(10, 10) has a slope of −1. The same procedure can be carried out at any
bundle of goods with the same results—in this case all the indifference
curves have a constant slope of −1.
x2
Indifference curves
x1
Perfect substitutes. The consumer only cares about the total
number of pencils, not about their colors. Thus the indifference
curves are straight lines with a slope of −1.
The important fact about perfect substitutes is that the indifference
curves have a constant slope. Suppose, for example, that we graphed blue
pencils on the vertical axis and pairs of red pencils on the horizontal axis.
The indifference curves for these two goods would have a slope of −2, since
the consumer would be willing to give up two blue pencils to get one more
pair of red pencils.
Figure
3.3
40 PREFERENCES (Ch. 3)
In the textbook we’ll primarily consider the case where goods are perfect
substitutes on a one-for-one basis, and leave the treatment of the general
case for the workbook.
Perfect Complements
Perfect complements are goods that are always consumed together in
fixed proportions. In some sense the goods “complement” each other. A
nice example is that of right shoes and left shoes. The consumer likes shoes,
but always wears right and left shoes together. Having only one out of a
pair of shoes doesn’t do the consumer a bit of good.
Let us draw the indifference curves for perfect complements. Suppose
we pick the consumption bundle (10, 10). Now add 1 more right shoe, so
we have (11, 10). By assumption this leaves the consumer indifferent to
the original position: the extra shoe doesn’t do him any good. The same
thing happens if we add one more left shoe: the consumer is also indifferent
between (10, 11) and (10, 10).
Thus the indifference curves are L-shaped, with the vertex of the L occurring where the number of left shoes equals the number of right shoes as
in Figure 3.4.
LEFT SHOES
Indifference
curves
RIGHT SHOES
Figure
3.4
Perfect complements. The consumer always wants to consume the goods in fixed proportions to each other. Thus the
indifference curves are L-shaped.
EXAMPLES OF PREFERENCES
41
Increasing both the number of left shoes and the number of right shoes
at the same time will move the consumer to a more preferred position,
so the direction of increasing preference is again up and to the right, as
illustrated in the diagram.
The important thing about perfect complements is that the consumer
prefers to consume the goods in fixed proportions, not necessarily that
the proportion is one-to-one. If a consumer always uses two teaspoons of
sugar in her cup of tea, and doesn’t use sugar for anything else, then the
indifference curves will still be L-shaped. In this case the corners of the
L will occur at (2 teaspoons sugar, 1 cup tea), (4 teaspoons sugar, 2 cups
tea) and so on, rather than at (1 right shoe, 1 left shoe), (2 right shoes, 2
left shoes), and so on.
In the textbook we’ll primarily consider the case where the goods are
consumed in proportions of one-for-one and leave the treatment of the
general case for the workbook.
Bads
A bad is a commodity that the consumer doesn’t like. For example, suppose that the commodities in question are now pepperoni and anchovies—
and the consumer loves pepperoni but dislikes anchovies. But let us suppose
there is some possible tradeoff between pepperoni and anchovies. That is,
there would be some amount of pepperoni on a pizza that would compensate the consumer for having to consume a given amount of anchovies. How
could we represent these preferences using indifference curves?
Pick a bundle (x1 , x2 ) consisting of some pepperoni and some anchovies.
If we give the consumer more anchovies, what do we have to do with the
pepperoni to keep him on the same indifference curve? Clearly, we have
to give him some extra pepperoni to compensate him for having to put up
with the anchovies. Thus this consumer must have indifference curves that
slope up and to the right as depicted in Figure 3.5.
The direction of increasing preference is down and to the right—that
is, toward the direction of decreased anchovy consumption and increased
pepperoni consumption, just as the arrows in the diagram illustrate.
Neutrals
A good is a neutral good if the consumer doesn’t care about it one way
or the other. What if a consumer is just neutral about anchovies?1 In this
case his indifference curves will be vertical lines as depicted in Figure 3.6.
1
Is anybody neutral about anchovies?
42 PREFERENCES (Ch. 3)
ANCHOVIES
Indifference
curves
PEPPERONI
Figure
3.5
Bads. Here anchovies are a “bad,” and pepperoni is a “good”
for this consumer. Thus the indifference curves have a positive
slope.
ANCHOVIES
Indifference
curves
PEPPERONI
Figure
3.6
A neutral good. The consumer likes pepperoni but is neutral
about anchovies, so the indifference curves are vertical lines.
He only cares about the amount of pepperoni he has and doesn’t care at
all about how many anchovies he has. The more pepperoni the better, but
adding more anchovies doesn’t affect him one way or the other.
EXAMPLES OF PREFERENCES
43
Satiation
We sometimes want to consider a situation involving satiation, where
there is some overall best bundle for the consumer, and the “closer” he is
to that best bundle, the better off he is in terms of his own preferences.
For example, suppose that the consumer has some most preferred bundle
of goods (x1 , x2 ), and the farther away he is from that bundle, the worse
off he is. In this case we say that (x1 , x2 ) is a satiation point, or a bliss
point. The indifference curves for the consumer look like those depicted in
Figure 3.7. The best point is (x1 , x2 ) and points farther away from this
bliss point lie on “lower” indifference curves.
x2
Indifference
curves
x2
Satiation
point
x1
x1
Satiated preferences. The bundle (x1 , x2 ) is the satiation
point or bliss point, and the indifference curves surround this
point.
In this case the indifference curves have a negative slope when the consumer has “too little” or “too much” of both goods, and a positive slope
when he has “too much” of one of the goods. When he has too much of one
of the goods, it becomes a bad—reducing the consumption of the bad good
moves him closer to his “bliss point.” If he has too much of both goods,
they both are bads, so reducing the consumption of each moves him closer
to the bliss point.
Suppose, for example, that the two goods are chocolate cake and ice
cream. There might well be some optimal amount of chocolate cake and
Figure
3.7
44 PREFERENCES (Ch. 3)
ice cream that you would want to eat per week. Any less than that amount
would make you worse off, but any more than that amount would also make
you worse off.
If you think about it, most goods are like chocolate cake and ice cream
in this respect—you can have too much of nearly anything. But people
would generally not voluntarily choose to have too much of the goods they
consume. Why would you choose to have more than you want of something?
Thus the interesting region from the viewpoint of economic choice is where
you have less than you want of most goods. The choices that people actually
care about are choices of this sort, and these are the choices with which we
will be concerned.
Discrete Goods
Usually we think of measuring goods in units where fractional amounts
make sense—you might on average consume 12.43 gallons of milk a month
even though you buy it a quart at a time. But sometimes we want to
examine preferences over goods that naturally come in discrete units.
For example, consider a consumer’s demand for automobiles. We could
define the demand for automobiles in terms of the time spent using an
automobile, so that we would have a continuous variable, but for many
purposes it is the actual number of cars demanded that is of interest.
There is no difficulty in using preferences to describe choice behavior
for this kind of discrete good. Suppose that x2 is money to be spent on
other goods and x1 is a discrete good that is only available in integer
amounts. We have illustrated the appearance of indifference “curves” and
a weakly preferred set for this kind of good in Figure 3.8. In this case the
bundles indifferent to a given bundle will be a set of discrete points. The
set of bundles at least as good as a particular bundle will be a set of line
segments.
The choice of whether to emphasize the discrete nature of a good or not
will depend on our application. If the consumer chooses only one or two
units of the good during the time period of our analysis, recognizing the
discrete nature of the choice may be important. But if the consumer is
choosing 30 or 40 units of the good, then it will probably be convenient to
think of this as a continuous good.
3.5 Well-Behaved Preferences
We’ve now seen some examples of indifference curves. As we’ve seen, many
kinds of preferences, reasonable or unreasonable, can be described by these
simple diagrams. But if we want to describe preferences in general, it will
be convenient to focus on a few general shapes of indifference curves. In
WELL-BEHAVED PREFERENCES
GOOD
2
GOOD
2
x2
x2
1
2
3
A Indifference "curves"
GOOD
1
45
Bundles
weakly
preferred
to (1, x 2)
1
2
3
GOOD
1
B Weakly preferrred set
A discrete good. Here good 1 is only available in integer
amounts. In panel A the dashed lines connect together the
bundles that are indifferent, and in panel B the vertical lines
represent bundles that are at least as good as the indicated
bundle.
this section we will describe some more general assumptions that we will
typically make about preferences and the implications of these assumptions
for the shapes of the associated indifference curves. These assumptions
are not the only possible ones; in some situations you might want to use
different assumptions. But we will take them as the defining features for
well-behaved indifference curves.
First we will typically assume that more is better, that is, that we are
talking about goods, not bads. More precisely, if (x1 , x2 ) is a bundle of
goods and (y1 , y2 ) is a bundle of goods with at least as much of both goods
and more of one, then (y1 , y2 ) (x1 , x2 ). This assumption is sometimes
called monotonicity of preferences. As we suggested in our discussion of
satiation, more is better would probably only hold up to a point. Thus
the assumption of monotonicity is saying only that we are going to examine situations before that point is reached—before any satiation sets
in—while more still is better. Economics would not be a very interesting
subject in a world where everyone was satiated in their consumption of
every good.
What does monotonicity imply about the shape of indifference curves?
It implies that they have a negative slope. Consider Figure 3.9. If we start
at a bundle (x1 , x2 ) and move anywhere up and to the right, we must be
moving to a preferred position. If we move down and to the left we must be
moving to a worse position. So if we are moving to an indifferent position,
we must be moving either left and up or right and down: the indifference
curve must have a negative slope.
Figure
3.8
46 PREFERENCES (Ch. 3)
Second, we are going to assume that averages are preferred to extremes.
That is, if we take two bundles of goods (x1 , x2 ) and (y1 , y2 ) on the same
indifference curve and take a weighted average of the two bundles such as
1
1
1
1
x1 + y1 , x2 + y2 ,
2
2
2
2
then the average bundle will be at least as good as or strictly preferred
to each of the two extreme bundles. This weighted-average bundle has
the average amount of good 1 and the average amount of good 2 that is
present in the two bundles. It therefore lies halfway along the straight line
connecting the x–bundle and the y–bundle.
x2
Better
bundles
(x1, x2 )
Worse
bundles
x1
Figure
3.9
Monotonic preferences. More of both goods is a better
bundle for this consumer; less of both goods represents a worse
bundle.
Actually, we’re going to assume this for any weight t between 0 and 1,
not just 1/2. Thus we are assuming that if (x1 , x2 ) ∼ (y1 , y2 ), then
(tx1 + (1 − t)y1 , tx2 + (1 − t)y2 ) (x1 , x2 )
for any t such that 0 ≤ t ≤ 1. This weighted average of the two bundles
gives a weight of t to the x-bundle and a weight of 1 − t to the y-bundle.
Therefore, the distance from the x-bundle to the average bundle is just
a fraction t of the distance from the x-bundle to the y-bundle, along the
straight line connecting the two bundles.
WELL-BEHAVED PREFERENCES
47
What does this assumption about preferences mean geometrically? It
means that the set of bundles weakly preferred to (x1 , x2 ) is a convex set.
For suppose that (y1 , y2 ) and (x1 , x2 ) are indifferent bundles. Then, if averages are preferred to extremes, all of the weighted averages of (x1 , x2 ) and
(y1 , y2 ) are weakly preferred to (x1 , x2 ) and (y1 , y2 ). A convex set has the
property that if you take any two points in the set and draw the line segment connecting those two points, that line segment lies entirely in the set.
Figure 3.10A depicts an example of convex preferences, while Figures
3.10B and 3.10C show two examples of nonconvex preferences. Figure
3.10C presents preferences that are so nonconvex that we might want to
call them “concave preferences.”
x2
x2
(y1, y2)
x2
(y1, y2)
(y1, y2)
Averaged
bundle
(x1, x2)
Averaged
bundle
(x1, x2)
x1
A Convex
preferences
Averaged
bundle
(x1, x2)
x1
B Nonconvex
preferences
x1
C Concave
preferences
Various kinds of preferences. Panel A depicts convex preferences, panel B depicts nonconvex preferences, and panel C
depicts “concave” preferences.
Can you think of preferences that are not convex? One possibility might
be something like my preferences for ice cream and olives. I like ice cream
and I like olives . . . but I don’t like to have them together! In considering
my consumption in the next hour, I might be indifferent between consuming
8 ounces of ice cream and 2 ounces of olives, or 8 ounces of olives and 2
ounces of ice cream. But either one of these bundles would be better than
consuming 5 ounces of each! These are the kind of preferences depicted in
Figure 3.10C.
Why do we want to assume that well-behaved preferences are convex?
Because, for the most part, goods are consumed together. The kinds
of preferences depicted in Figures 3.10B and 3.10C imply that the con-
Figure
3.10
48 PREFERENCES (Ch. 3)
sumer would prefer to specialize, at least to some degree, and to consume
only one of the goods. However, the normal case is where the consumer
would want to trade some of one good for the other and end up consuming
some of each, rather than specializing in consuming only one of the two
goods.
In fact, if we look at my preferences for monthly consumption of ice
cream and olives, rather than at my immediate consumption, they would
tend to look much more like Figure 3.10A than Figure 3.10C. Each month
I would prefer having some ice cream and some olives—albeit at different
times—to specializing in consuming either one for the entire month.
Finally, one extension of the assumption of convexity is the assumption
of strict convexity. This means that the weighted average of two indifferent bundles is strictly preferred to the two extreme bundles. Convex
preferences may have flat spots, while strictly convex preferences must have
indifferences curves that are “rounded.” The preferences for two goods that
are perfect substitutes are convex, but not strictly convex.
3.6 The Marginal Rate of Substitution
We will often find it useful to refer to the slope of an indifference curve at
a particular point. This idea is so useful that it even has a name: the slope
of an indifference curve is known as the marginal rate of substitution
(MRS). The name comes from the fact that the MRS measures the rate
at which the consumer is just willing to substitute one good for the other.
Suppose that we take a little of good 1, Δx1 , away from the consumer.
Then we give him Δx2 , an amount that is just sufficient to put him back
on his indifference curve, so that he is just as well off after this substitution
of x2 for x1 as he was before. We think of the ratio Δx2 /Δx1 as being the
rate at which the consumer is willing to substitute good 2 for good 1.
Now think of Δx1 as being a very small change—a marginal change.
Then the rate Δx2 /Δx1 measures the marginal rate of substitution of good
2 for good 1. As Δx1 gets smaller, Δx2 /Δx1 approaches the slope of the
indifference curve, as can be seen in Figure 3.11.
When we write the ratio Δx2 /Δx1 , we will always think of both the
numerator and the denominator as being small numbers—as describing
marginal changes from the original consumption bundle. Thus the ratio
defining the MRS will always describe the slope of the indifference curve:
the rate at which the consumer is just willing to substitute a little more
consumption of good 2 for a little less consumption of good 1.
One slightly confusing thing about the MRS is that it is typically a
negative number. We’ve already seen that monotonic preferences imply
that indifference curves must have a negative slope. Since the MRS is the
numerical measure of the slope of an indifference curve, it will naturally be
a negative number.
THE MARGINAL RATE OF SUBSTITUTION
49
x2
Indifference
curve
Slope =
Δx2
Δx2
= marginal rate
Δx1 of substitution
Δx1
x1
The marginal rate of substitution (MRS). The marginal
rate of substitution measures the slope of the indifference curve.
The marginal rate of substitution measures an interesting aspect of the
consumer’s behavior. Suppose that the consumer has well-behaved preferences, that is, preferences that are monotonic and convex, and that he is
currently consuming some bundle (x1 , x2 ). We now will offer him a trade:
he can exchange good 1 for 2, or good 2 for 1, in any amount at a “rate of
exchange” of E.
That is, if the consumer gives up Δx1 units of good 1, he can get EΔx1
units of good 2 in exchange. Or, conversely, if he gives up Δx2 units of good
2, he can get Δx2 /E units of good 1. Geometrically, we are offering the
consumer an opportunity to move to any point along a line with slope −E
that passes through (x1 , x2 ), as depicted in Figure 3.12. Moving up and to
the left from (x1 , x2 ) involves exchanging good 1 for good 2, and moving
down and to the right involves exchanging good 2 for good 1. In either
movement, the exchange rate is E. Since exchange always involves giving
up one good in exchange for another, the exchange rate E corresponds to
a slope of −E.
We can now ask what would the rate of exchange have to be in order for
the consumer to want to stay put at (x1 , x2 )? To answer this question, we
simply note that any time the exchange line crosses the indifference curve,
there will be some points on that line that are preferred to (x1 , x2 )—that
lie above the indifference curve. Thus, if there is to be no movement from
Figure
3.11
50 PREFERENCES (Ch. 3)
(x1 , x2 ), the exchange line must be tangent to the indifference curve. That
is, the slope of the exchange line, −E, must be the slope of the indifference
curve at (x1 , x2 ). At any other rate of exchange, the exchange line would
cut the indifference curve and thus allow the consumer to move to a more
preferred point.
x2
Indifference
curves
Slope = – E
x2
x1
Figure
3.12
x1
Trading at an exchange rate. Here we are allowing the consumer to trade the goods at an exchange rate E, which implies
that the consumer can move along a line with slope −E.
Thus the slope of the indifference curve, the marginal rate of substitution,
measures the rate at which the consumer is just on the margin of trading
or not trading. At any rate of exchange other than the MRS, the consumer
would want to trade one good for the other. But if the rate of exchange
equals the MRS, the consumer wants to stay put.
3.7 Other Interpretations of the MRS
We have said that the MRS measures the rate at which the consumer is
just on the margin of being willing to substitute good 1 for good 2. We
could also say that the consumer is just on the margin of being willing to
“pay” some of good 1 in order to buy some more of good 2. So sometimes
BEHAVIOR OF THE MRS
51
you hear people say that the slope of the indifference curve measures the
marginal willingness to pay.
If good 2 represents the consumption of “all other goods,” and it is
measured in dollars that you can spend on other goods, then the marginalwillingness-to-pay interpretation is very natural. The marginal rate of substitution of good 2 for good 1 is how many dollars you would just be willing
to give up spending on other goods in order to consume a little bit more
of good 1. Thus the MRS measures the marginal willingness to give up
dollars in order to consume a small amount more of good 1. But giving up
those dollars is just like paying dollars in order to consume a little more of
good 1.
If you use the marginal-willingness-to-pay interpretation of the MRS, you
should be careful to emphasize both the “marginal” and the “willingness”
aspects. The MRS measures the amount of good 2 that one is willing to
pay for a marginal amount of extra consumption of good 1. How much
you actually have to pay for some given amount of extra consumption may
be different than the amount you are willing to pay. How much you have
to pay will depend on the price of the good in question. How much you
are willing to pay doesn’t depend on the price—it is determined by your
preferences.
Similarly, how much you may be willing to pay for a large change in
consumption may be different from how much you are willing to pay for
a marginal change. How much you actually end up buying of a good will
depend on your preferences for that good and the prices that you face. How
much you would be willing to pay for a small amount extra of the good is
a feature only of your preferences.
3.8 Behavior of the MRS
It is sometimes useful to describe the shapes of indifference curves by describing the behavior of the marginal rate of substitution. For example,
the “perfect substitutes” indifference curves are characterized by the fact
that the MRS is constant at −1. The “neutrals” case is characterized by
the fact that the MRS is everywhere infinite. The preferences for “perfect
complements” are characterized by the fact that the MRS is either zero or
infinity, and nothing in between.
We’ve already pointed out that the assumption of monotonicity implies
that indifference curves must have a negative slope, so the MRS always
involves reducing the consumption of one good in order to get more of
another for monotonic preferences.
The case of convex indifference curves exhibits yet another kind of behavior for the MRS. For strictly convex indifference curves, the MRS—the
slope of the indifference curve—decreases (in absolute value) as we increase
x1 . Thus the indifference curves exhibit a diminishing marginal rate of
52 PREFERENCES (Ch. 3)
substitution. This means that the amount of good 1 that the person is
willing to give up for an additional amount of good 2 increases the amount
of good 1 increases. Stated in this way, convexity of indifference curves
seems very natural: it says that the more you have of one good, the more
willing you are to give some of it up in exchange for the other good. (But
remember the ice cream and olives example—for some pairs of goods this
assumption might not hold!)
Summary
1. Economists assume that a consumer can rank various consumption possibilities. The way in which the consumer ranks the consumption bundles
describes the consumer’s preferences.
2. Indifference curves can be used to depict different kinds of preferences.
3. Well-behaved preferences are monotonic (meaning more is better) and
convex (meaning averages are preferred to extremes).
4. The marginal rate of substitution (MRS) measures the slope of the indifference curve. This can be interpreted as how much the consumer is
willing to give up of good 2 to acquire more of good 1.
REVIEW QUESTIONS
1. If we observe a consumer choosing (x1 , x2 ) when (y1 , y2 ) is available one
time, are we justified in concluding that (x1 , x2 ) (y1 , y2 )?
2. Consider a group of people A, B, C and the relation “at least as tall as,”
as in “A is at least as tall as B.” Is this relation transitive? Is it complete?
3. Take the same group of people and consider the relation “strictly taller
than.” Is this relation transitive? Is it reflexive? Is it complete?
4. A college football coach says that given any two linemen A and B, he
always prefers the one who is bigger and faster. Is this preference relation
transitive? Is it complete?
5. Can an indifference curve cross itself? For example, could Figure 3.2
depict a single indifference curve?
6. Could Figure 3.2 be a single indifference curve if preferences are monotonic?
REVIEW QUESTIONS
53
7. If both pepperoni and anchovies are bads, will the indifference curve
have a positive or a negative slope?
8. Explain why convex preferences means that “averages are preferred to
extremes.”
9. What is your marginal rate of substitution of $1 bills for $5 bills?
10. If good 1 is a “neutral,” what is its marginal rate of substitution for
good 2?
11. Think of some other goods for which your preferences might be concave.
In the previous section you learned how to use graphs to show the set of
commodity bundles that a consumer can afford. In this section, you learn
to put information about the consumer’s preferences on the same kind of
graph. Most of the problems ask you to draw indifference curves.
Sometimes we give you a formula for the indifference curve. Then
all you have to do is graph a known equation. But in some problems, we
give you only “qualitative” information about the consumer’s preferences
and ask you to sketch indifference curves that are consistent with this
information. This requires a little more thought. Don’t be surprised or
disappointed if you cannot immediately see the answer when you look
at a problem, and don’t expect that you will find the answers hiding
somewhere in your textbook. The best way we know to find answers is to
“think and doodle.” Draw some axes on scratch paper and label them,
then mark a point on your graph and ask yourself, “What other points on
the graph would the consumer find indifferent to this point?” If possible,
draw a curve connecting such points, making sure that the shape of the
line you draw reflects the features required by the problem. This gives
you one indifference curve. Now pick another point that is preferred to
the first one you drew and draw an indifference curve through it.
Jocasta loves to dance and hates housecleaning. She has strictly convex
preferences. She prefers dancing to any other activity and never gets tired
of dancing, but the more time she spends cleaning house, the less happy
she is. Let us try to draw an indifference curve that is consistent with her
preferences. There is not enough information here to tell us exactly where
her indifference curves go, but there is enough information to determine
some things about their shape. Take a piece of scratch paper and draw a
pair of axes. Label the horizontal axis “Hours per day of housecleaning.”
Label the vertical axis “Hours per day of dancing.” Mark a point a
little ways up the vertical axis and write a 4 next to it. At this point,
she spends 4 hours a day dancing and no time housecleaning. Other
points that would be indifferent to this point would have to be points
where she did more dancing and more housecleaning. The pain of the
extra housekeeping should just compensate for the pleasure of the extra
dancing. So an indifference curve for Jocasta must be upward sloping.
Because she loves dancing and hates housecleaning, it must be that she
prefers all the points above this indifference curve to all of the points on
or below it. If Jocasta has strictly convex preferences, then it must be
that if you draw a line between any two points on the same indifference
curve, all the points on the line (except the endpoints) are preferred to
the endpoints. For this to be the case, it must be that the indifference
curve slopes upward ever more steeply as you move to the right along it.
You should convince yourself of this by making some drawings on scratch
paper. Draw an upward-sloping curve passing through the point (0, 4)
and getting steeper as one moves to the right.
When you have completed this workout, we hope that you will be
able to do the following:
• Given the formula for an indifference curve, draw this curve, and find
its slope at any point on the curve.
• Determine whether a consumer prefers one bundle to another or is
indifferent between them, given specific indifference curves.
• Draw indifference curves for the special cases of perfect substitutes
and perfect complements.
• Draw indifference curves for someone who dislikes one or both commodities.
• Draw indifference curves for someone who likes goods up to a point
but who can get “too much” of one or more goods.
• Identify weakly preferred sets and determine whether these are convex sets and whether preferences are convex.
• Know what the marginal rate of substitution is and be able to determine whether an indifference curve exhibits “diminishing marginal
rate of substitution.”
• Determine whether a preference relation or any other relation between pairs of things is transitive, whether it is reflexive, and whether
it is complete.
3.1 (0) Charlie likes both apples and bananas. He consumes nothing else.
The consumption bundle where Charlie consumes xA bushels of apples
per year and xB bushels of bananas per year is written as (xA , xB ). Last
year, Charlie consumed 20 bushels of apples and 5 bushels of bananas. It
happens that the set of consumption bundles (xA , xB ) such that Charlie
is indifferent between (xA , xB ) and (20, 5) is the set of all bundles such
that xB = 100/xA . The set of bundles (xA , xB ) such that Charlie is just
indifferent between (xA , xB ) and the bundle (10, 15) is the set of bundles
such that xB = 150/xA .
(a) On the graph below, plot several points that lie on the indifference
curve that passes through the point (20, 5), and sketch this curve, using
blue ink. Do the same, using red ink, for the indifference curve passing
through the point (10, 15).
(b) Use pencil to shade in the set of commodity bundles that Charlie
weakly prefers to the bundle (10, 15). Use blue ink to shade in the set
of commodity bundles such that Charlie weakly prefers (20, 5) to these
bundles.
Bananas
40
30
20
10
0
10
20
30
40
Apples
For each of the following statements about Charlie’s preferences, write
“true” or “false.”
(c) (30, 5) ∼ (10, 15).
.
(d) (10, 15) Â (20, 5).
.
(e) (20, 5) º (10, 10).
.
(f ) (24, 4) º (11, 9.1).
.
(g) (11, 14) Â (2, 49).
.
(h) A set is convex if for any two points in the set, the line segment
between them is also in the set. Is the set of bundles that Charlie weakly
prefers to (20, 5) a convex set?
.
(i) Is the set of bundles that Charlie considers inferior to (20, 5) a convex
set?
.
(j) The slope of Charlie’s indifference curve through a point, (xA , xB ), is
known as his marginal
of
at that point.
(k) Remember that Charlie’s indifference curve through the point (10, 10)
has the equation xB = 100/xA . Those of you who know calculus will
remember that the slope of a curve is just its derivative, which in this
case is −100/x2A . (If you don’t know calculus, you will have to take our
word for this.) Find Charlie’s marginal rate of substitution at the point,
(10, 10).
.
(l) What is his marginal rate of substitution at the point (5, 20)?
.
(m) What is his marginal rate of substitution at the point (20, 5)?
.
(n) Do the indifference curves you have drawn for Charlie exhibit diminishing marginal rate of substitution?
.
3.2 (0) Ambrose consumes only nuts and berries. Fortunately, he likes
both goods. The consumption bundle where Ambrose consumes x1 units
of nuts per week and x2 units of berries per week is written as (x1 , x2 ).
The set of consumption bundles (x1 , x2 ) such that Ambrose is indifferent
between (x1 , x2 ) and (1, 16) is the set of bundles such that x1 ≥ 0, x2 ≥ 0,
√
and x2 = 20 − 4 x1 . The set of bundles (x1 , x2 ) such that (x1 , x2 ) ∼
√
(36, 0) is the set of bundles such that x1 ≥ 0, x2 ≥ 0 and x2 = 24 − 4 x1 .
(a) On the graph below, plot several points that lie on the indifference
curve that passes through the point (1, 16), and sketch this curve, using
blue ink. Do the same, using red ink, for the indifference curve passing
through the point (36, 0).
(b) Use pencil to shade in the set of commodity bundles that Ambrose
weakly prefers to the bundle (1, 16). Use red ink to shade in the set of
all commodity bundles (x1 , x2 ) such that Ambrose weakly prefers (36, 0)
to these bundles. Is the set of bundles that Ambrose prefers to (1, 16) a
convex set?
.
(c) What is the slope of Ambrose’s indifference curve at the point (9, 8)?
(Hint: Recall from calculus the way to calculate the slope of a curve. If
you don’t know calculus, you will have to draw your diagram carefully
and estimate the slope.)
.
(d) What is the slope of his indifference curve at the point (4, 12)?
.
Berries
40
30
20
10
0
10
20
30
40
Nuts
(e) What is the slope of his indifference curve at the point (9, 12)?
at the point (4, 16)?
.
(f ) Do the indifference curves you have drawn for Ambrose exhibit diminishing marginal rate of substitution?
.
(g) Does Ambrose have convex preferences?
.
3.3 (0) Shirley Sixpack is in the habit of drinking beer each evening
while watching “The Best of Bowlerama” on TV. She has a strong thumb
and a big refrigerator, so she doesn’t care about the size of the cans that
beer comes in, she only cares about how much beer she has.
(a) On the graph below, draw some of Shirley’s indifference curves between 16-ounce cans and 8-ounce cans of beer. Use blue ink to draw these
indifference curves.
8-ounce cans
8
6
4
2
0
2
4
6
8
16-ounce cans
(b) Lorraine Quiche likes to have a beer while she watches “Masterpiece
Theatre.” She only allows herself an 8-ounce glass of beer at any one
time. Since her cat doesn’t like beer and she hates stale beer, if there is
more than 8 ounces in the can she pours the excess into the sink. (She
has no moral scruples about wasting beer.) On the graph above, use red
ink to draw some of Lorraine’s indifference curves.
3.4 (0) Elmo finds himself at a Coke machine on a hot and dusty Sunday.
The Coke machine requires exact change—two quarters and a dime. No
other combination of coins will make anything come out of the machine.
No stores are open; no one is in sight. Elmo is so thirsty that the only
thing he cares about is how many soft drinks he will be able to buy with
the change in his pocket; the more he can buy, the better. While Elmo
searches his pockets, your task is to draw some indifference curves that
describe Elmo’s preferences about what he finds.
Dimes
8
6
4
2
0
2
4
6
8
Quarters
(a) If Elmo has 2 quarters and a dime in his pockets, he can buy 1 soft
drink. How many soft drinks can he buy if he has 4 quarters and 2 dimes?
.
(b) Use red ink to shade in the area on the graph consisting of all combinations of quarters and dimes that Elmo thinks are just indifferent to
having 2 quarters and 1 dime. (Imagine that it is possible for Elmo to
have fractions of quarters or of dimes, but, of course, they would be useless in the machine.) Now use blue ink to shade in the area consisting of
all combinations that Elmo thinks are just indifferent to having 4 quarters
and 2 dimes. Notice that Elmo has indifference “bands,” not indifference
curves.
(c) Does Elmo have convex preferences between dimes and quarters?
.
(d) Does Elmo always prefer more of both kinds of money to less?
(e) Does Elmo have a bliss point?
.
(f ) If Elmo had arrived at the Coke machine on a Saturday, the drugstore
across the street would have been open. This drugstore has a soda fountain that will sell you as much Coke as you want at a price of 4 cents an
ounce. The salesperson will take any combination of dimes and quarters
in payment. Suppose that Elmo plans to spend all of the money in his
pocket on Coke at the drugstore on Saturday. On the graph above, use
pencil or black ink to draw one or two of Elmo’s indifference curves between quarters and dimes in his pocket. (For simplicity, draw your graph
as if Elmo’s fractional quarters and fractional dimes are accepted at the
corresponding fraction of their value.) Describe these new indifference
curves in words.
.
3.5 (0) Randy Ratpack hates studying both economics and history. The
more time he spends studying either subject, the less happy he is. But
Randy has strictly convex preferences.
(a) Sketch an indifference curve for Randy where the two commodities
are hours per week spent studying economics and hours per week spent
studying history. Will the slope of an indifference curve be positive or
negative?
.
(b) Do Randy’s indifference curves get steeper or flatter as you move from
left to right along one of them?
.
Hours studying history
8
6
4
2
0
2
4
6
8
Hours studying economics
3.6 (0) Flossy Toothsome likes to spend some time studying and some
time dating. In fact her indifference curves between hours per week spent
studying and hours per week spent dating are concentric circles around
her favorite combination, which is 20 hours of studying and 15 hours of
dating per week. The closer she is to her favorite combination, the happier
she is.
(a) Suppose that Flossy is currently studying 25 hours a week and dating
3 hours a week. Would she prefer to be studying 30 hours a week and
dating 8 hours a week?
(Hint: Remember the formula for
the distance between two points in the plane?)
(b) On the axes below, draw a few of Flossy’s indifference curves and
use your diagram to illustrate which of the two time allocations discussed
above Flossy would prefer.
Hours dating
40
30
20
10
0
10
20
30
40
Hours studying
3.7 (0) Joan likes chocolate cake and ice cream, but after 10 slices of
cake, she gets tired of cake, and eating more cake makes her less happy.
Joan always prefers more ice cream to less. Joan’s parents require her to
eat everything put on her plate. In the axes below, use blue ink to draw a
set of indifference curves that depict her preferences between plates with
different amounts of cake and ice cream. Be sure to label the axes.
(a) Suppose that Joan’s preferences are as before, but that her parents
allow her to leave anything on her plate that she doesn’t want. On the
graph below, use red ink to draw some indifference curves depicting her
preferences between plates with different amounts of cake and ice cream.
3.8 (0) Professor Goodheart always gives two midterms in his communications class. He only uses the higher of the two scores that a student
gets on the midterms when he calculates the course grade.
(a) Nancy Lerner wants to maximize her grade in this course. Let x1 be
her score on the first midterm and x2 be her score on the second midterm.
Which combination of scores would Nancy prefer, x1 = 20 and x2 = 70
or x1 = 60 and x2 = 60?
.
(b) On the graph below, use red ink to draw an indifference curve showing
all of the combinations of scores that Nancy likes exactly as much as
x1 = 20 and x2 = 70. Also use red ink to draw an indifference curve
showing the combinations that Nancy likes exactly as much as x1 = 60
and x2 = 60.
(c) Does Nancy have convex preferences over these combinations?
.
Grade on second midterm
80
60
40
20
0
20
40
60
80
Grade on first midterm
(d) Nancy is also taking a course in economics from Professor Stern.
Professor Stern gives two midterms. Instead of discarding the lower grade,
Professor Stern discards the higher one. Let x1 be her score on the first
midterm and x2 be her score on the second midterm. Which combination
of scores would Nancy prefer, x1 = 20 and x2 = 70 or x1 = 60 and
x2 = 50?
.
(e) On the graph above, use blue ink to draw an indifference curve showing
all of the combinations of scores on her econ exams that Nancy likes
exactly as well as x1 = 20 and x2 = 70. Also use blue ink to draw an
indifference curve showing the combinations that Nancy likes exactly as
well as x1 = 60 and x2 = 50. Does Nancy have convex preferences over
these combinations?
.
3.9 (0) Mary Granola loves to consume two goods, grapefruits and
avocados.
(a) On the graph below, the slope of an indifference curve through any
point where she has more grapefruits than avocados is −2. This means
that when she has more grapefruits than avocados, she is willing to give
up
grapefruit(s) to get one avocado.
(b) On the same graph, the slope of an indifference curve at points where
she has fewer grapefruits than avocados is −1/2. This means that when
she has fewer grapefruits than avocados, she is just willing to give up
grapefruit(s) to get one avocado.
(c) On this graph, draw an indifference curve for Mary through bundle
(10A, 10G). Draw another indifference curve through (20A, 20G).
Grapefruits
40
30
20
10
0
10
20
(d) Does Mary have convex preferences?
30
40
Avocados
.
3.10 (2) Ralph Rigid likes to eat lunch at 12 noon. However, he also
likes to save money so he can buy other consumption goods by attending
the “early bird specials” and “late lunchers” promoted by his local diner.
Ralph has 15 dollars a day to spend on lunch and other stuff. Lunch at
noon costs $5. If he delays his lunch until t hours after noon, he is able
to buy his lunch for a price of $5 − t. Similarly if he eats his lunch t hours
before noon, he can buy it for a price of $5 − t. (This is true for fractions
of hours as well as integer numbers of hours.)
(a) If Ralph eats lunch at noon, how much money does he have per day
to spend on other stuff?
.
(b) How much money per day would he have left for other stuff if he ate
at 2 P.M.?
.
(c) On the graph below, use blue ink to draw the broken line that shows
combinations of meal time and money for other stuff that Ralph can just
afford. On this same graph, draw some indifference curves that would be
consistent with Ralph choosing to eat his lunch at 11 A.M.
Money
20
15
10
5
10
11
12
1
2
Time
3.11 (0) Henry Hanover is currently consuming 20 cheeseburgers and 20
Cherry Cokes a week. A typical indifference curve for Henry is depicted
below.
Cherry Cok
oke
40
30
20
10
0
10
20
30
40
Cheeseburgers
(a) If someone offered to trade Henry one extra cheeseburger for every
Coke he gave up, would Henry want to do this?
.
(b) What if it were the other way around: for every cheeseburger Henry
gave up, he would get an extra Coke. Would he accept this offer?
.
(c) At what rate of exchange would Henry be willing to stay put at his
current consumption level?
.
3.12 (1) Tommy Twit is happiest when he has 8 cookies and 4 glasses of
milk per day. Whenever he has more than his favorite amount of either
food, giving him still more makes him worse off. Whenever he has less
than his favorite amount of either food, giving him more makes him better
off. His mother makes him drink 7 glasses of milk and only allows him 2
cookies per day. One day when his mother was gone, Tommy’s sadistic
sister made him eat 13 cookies and only gave him 1 glass of milk, despite
the fact that Tommy complained bitterly about the last 5 cookies that she
made him eat and begged for more milk. Although Tommy complained
later to his mother, he had to admit that he liked the diet that his sister
forced on him better than what his mother demanded.
(a) Use black ink to draw some indifference curves for Tommy that are
consistent with this story.
Milk
12
10
8
6
4
2
0
2
4
6
8
10
12
14
16
Cookies
(b) Tommy’s mother believes that the optimal amount for him to consume
is 7 glasses of milk and 2 cookies. She measures deviations by absolute
values. If Tommy consumes some other bundle, say, (c, m), she measures
his departure from the optimal bundle by D = |7 − m| + |2 − c|. The
larger D is, the worse off she thinks Tommy is. Use blue ink in the graph
above to sketch a few of Mrs. Twit’s indifference curves for Tommy’s
consumption. (Hint: Before you try to draw Mrs. Twit’s indifference
curves, we suggest that you take a piece of scrap paper and draw a graph
of the locus of points (x1 , x2 ) such that |x1 | + |x2 | = 1.)
3.13 (0) Coach Steroid likes his players to be big, fast, and obedient. If
player A is better than player B in two of these three characteristics, then
Coach Steroid prefers A to B, but if B is better than A in two of these
three characteristics, then Steroid prefers B to A. Otherwise, Steroid is
indifferent between them. Wilbur Westinghouse weighs 340 pounds, runs
very slowly, and is fairly obedient. Harold Hotpoint weighs 240 pounds,
runs very fast, and is very disobedient. Jerry Jacuzzi weighs 150 pounds,
runs at average speed, and is extremely obedient.
(a) Does Steroid prefer Westinghouse to Hotpoint or vice versa?
.
(b) Does Steroid prefer Hotpoint to Jacuzzi or vice versa?
.
(c) Does Steroid prefer Westinghouse to Jacuzzi or vice versa?
.
(d) Does Coach Steroid have transitive preferences?
.
(e) After several losing seasons, Coach Steroid decides to change his way of
judging players. According to his new preferences, Steroid prefers player
A to player B if player A is better in all three of the characteristics that
Steroid values, and he prefers B to A if player B is better at all three
things. He is indifferent between A and B if they weigh the same, are
equally fast, and are equally obedient. In all other cases, Coach Steroid
simply says “A and B are not comparable.”
(f ) Are Coach Steroid’s new preferences complete?
.
(g) Are Coach Steroid’s new preferences transitive?
.
(h) Are Coach Steroid’s new preferences reflexive?
.
3.14 (0) The Bear family is trying to decide what to have for dinner. Baby Bear says that his ranking of the possibilities is (honey, grubs,
Goldilocks). Mama Bear ranks the choices (grubs, Goldilocks, honey),
while Papa Bear’s ranking is (Goldilocks, honey, grubs). They decide to
take each pair of alternatives and let a majority vote determine the family
rankings.
(a) Papa suggests that they first consider honey vs. grubs, and then the
winner of that contest vs. Goldilocks. Which alternative will be chosen?
.
(b) Mama suggests instead that they consider honey vs. Goldilocks and
then the winner vs. grubs. Which gets chosen?
.
(c) What order should Baby Bear suggest if he wants to get his favorite
food for dinner?
.
(d) Are the Bear family’s “collective preferences,” as determined by voting, transitive?
.
3.15 (0) Olson likes strong coffee, the stronger the better. But he can’t
distinguish small differences. Over the years, Mrs. Olson has discovered
that if she changes the amount of coffee by more than one teaspoon in
her six-cup pot, Olson can tell that she did it. But he cannot distinguish
differences smaller than one teaspoon per pot. Where A and B are two
different cups of coffee, let us write A Â B if Olson prefers cup A to
cup B. Let us write A º B if Olson either prefers A to B, or can’t tell
the difference between them. Let us write A ∼ B if Olson can’t tell the
difference between cups A and B. Suppose that Olson is offered cups A,
B, and C all brewed in the Olsons’ six-cup pot. Cup A was brewed using
14 teaspoons of coffee in the pot. Cup B was brewed using 14.75 teaspoons
of coffee in the pot and cup C was brewed using 15.5 teaspoons of coffee
in the pot. For each of the following expressions determine whether it is
true of false.
(a) A ∼ B.
.
(b) B ∼ A.
.
(c) B ∼ C.
.
(d) A ∼ C.
.
(e) C ∼ A.
.
(f ) A º B.
.
(g) B º A.
.
(h) B º C.
.
(i) A º C.
.
(j) C º A.
.
(k) A Â B.
.
(l) B Â A.
.
(m) B Â C.
.
(n) A Â C.
.
(o) C Â A.
.
(p) Is Olson’s “at-least-as-good-as” relation, º, transitive?
.
(q) Is Olson’s “can’t-tell-the-difference” relation, ∼, transitive?
.
(r) is Olson’s “better-than” relation, Â, transitive.
.
3.1 In Problem 3.1, Charlie’s indifference curves have the equation
xB = constant/xA , where larger constants correspond to better indifference curves. Charlie strictly prefers the bundle (7,15) to the bundle:
(a) (15,7).
(b) (8,14).
(c) (11,11).
(d) all three of these bundles.
(e) none of these bundles.
3.2 In Problem 3.2, Ambrose has indifference curves with the equation
1/2
x2 = constant − 4x1 , where larger constants correspond to higher indifference curves. If good 1 is drawn on the horizontal axis and good 2 on
the vertical axis, what is the slope of Ambrose’s indifference curve when
his consumption bundle is (1,6)?
(a) −1/6
(b) −6/1
(c) −2
(d) −7
(e) −1
3.3 In Problem 3.8, Nancy Lerner is taking a course from Professor Goodheart who will count only her best midterm grade and from Professor
Stern who will count only her worst midterm grade. In one of her classes,
Nancy has scores of 50 on her first midterm and 30 on her second midterm.
When the first midterm score is measured on the horizontal axis and her
second midterm score on the vertical, her indifference curve has a slope
of zero at the point (50,30). ¿From this information we can conclude
(a) this class could be Professor Goodheart’s but couldn’t be Professor
Stern’s.
(b) this class could be Professor Stern’s but couldn’t be Professor Goodheart’s.
(c) this class couldn’t be either Goodheart’s or Stern’s.
(d) this class could be either Goodheart’s or Stern’s.
3.4 In Problem 3.9, if we graph Mary Granola’s indifference curves with
avocados on the horizontal axis and grapefruits on the vertical axis, then
whenever she has more grapefruits than avocados, the slope of her indifference curve is −2. Whenever she has more avocados than grapefruits,
the slope is −1/2. Mary would be indifferent between a bundle with 24
avocados and 36 grapefruits and another bundle that has 34 avocados and
(a) 28 grapefruits.
(b) 32 grapefruits.
(c) 22 grapefruits.
(d) 25 grapefruits.
(e) 26.50 grapefruits.
3.5 In Problem 3.12, recall that Tommy Twit’s mother measures the
departure of any bundle from her favorite bundle for Tommy by the sum
of the absolute values of the differences. Her favorite bundle for Tommy
is (2,7)—that is, 2 cookies and 7 glasses of milk. Tommy’s mother’s
indifference curve that passes through the point (c, m) = (3, 6) also passes
through
(a) the point (4,5).
(b) the points (2,5), (4,7), and (3,8).
(c) the point (2,7).
(d) the points (3, 7), (2, 6), and (2, 8).
(e) None of the other options are correct.
3.6 In Problem 3.1, Charlie’s indifference curves have the equation
xB = constant/xA , where larger constants correspond to better indifference curves. Charlie strictly prefers the bundle (9,19) to the bundle:
(a) (19,9).
(b) (10,18).
(c) (15,17).
(d) More than one of these options are correct.
(e) None of the above are correct.
CHAPTER
4
UTILITY
In Victorian days, philosophers and economists talked blithely of “utility”
as an indicator of a person’s overall well-being. Utility was thought of as
a numeric measure of a person’s happiness. Given this idea, it was natural
to think of consumers making choices so as to maximize their utility, that
is, to make themselves as happy as possible.
The trouble is that these classical economists never really described how
we were to measure utility. How are we supposed to quantify the “amount”
of utility associated with different choices? Is one person’s utility the same
as another’s? What would it mean to say that an extra candy bar would
give me twice as much utility as an extra carrot? Does the concept of utility
have any independent meaning other than its being what people maximize?
Because of these conceptual problems, economists have abandoned the
old-fashioned view of utility as being a measure of happiness. Instead,
the theory of consumer behavior has been reformulated entirely in terms
of consumer preferences, and utility is seen only as a way to describe
preferences.
Economists gradually came to recognize that all that mattered about
utility as far as choice behavior was concerned was whether one bundle
had a higher utility than another—how much higher didn’t really matter.
UTILITY
55
Originally, preferences were defined in terms of utility: to say a bundle
(x1 , x2 ) was preferred to a bundle (y1 , y2 ) meant that the x-bundle had a
higher utility than the y-bundle. But now we tend to think of things the
other way around. The preferences of the consumer are the fundamental description useful for analyzing choice, and utility is simply a way of
describing preferences.
A utility function is a way of assigning a number to every possible
consumption bundle such that more-preferred bundles get assigned larger
numbers than less-preferred bundles. That is, a bundle (x1 , x2 ) is preferred
to a bundle (y1 , y2 ) if and only if the utility of (x1 , x2 ) is larger than the
utility of (y1 , y2 ): in symbols, (x1 , x2 ) (y1 , y2 ) if and only if u(x1 , x2 ) >
u(y1 , y2 ).
The only property of a utility assignment that is important is how it
orders the bundles of goods. The magnitude of the utility function is only
important insofar as it ranks the different consumption bundles; the size of
the utility difference between any two consumption bundles doesn’t matter.
Because of this emphasis on ordering bundles of goods, this kind of utility
is referred to as ordinal utility.
Consider for example Table 4.1, where we have illustrated several different ways of assigning utilities to three bundles of goods, all of which
order the bundles in the same way. In this example, the consumer prefers
A to B and B to C. All of the ways indicated are valid utility functions
that describe the same preferences because they all have the property that
A is assigned a higher number than B, which in turn is assigned a higher
number than C.
Different ways to assign utilities.
Bundle
A
B
C
U1
3
2
1
U2
17
10
.002
U3
−1
−2
−3
Since only the ranking of the bundles matters, there can be no unique
way to assign utilities to bundles of goods. If we can find one way to assign
utility numbers to bundles of goods, we can find an infinite number of
ways to do it. If u(x1 , x2 ) represents a way to assign utility numbers to
the bundles (x1 , x2 ), then multiplying u(x1 , x2 ) by 2 (or any other positive
number) is just as good a way to assign utilities.
Multiplication by 2 is an example of a monotonic transformation. A
Table
4.1
56 UTILITY (Ch. 4)
monotonic transformation is a way of transforming one set of numbers into
another set of numbers in a way that preserves the order of the numbers.
We typically represent a monotonic transformation by a function f (u)
that transforms each number u into some other number f (u), in a way
that preserves the order of the numbers in the sense that u1 > u2 implies
f (u1 ) > f (u2 ). A monotonic transformation and a monotonic function are
essentially the same thing.
Examples of monotonic transformations are multiplication by a positive
number (e.g., f (u) = 3u), adding any number (e.g., f (u) = u + 17), raising
u to an odd power (e.g., f (u) = u3 ), and so on.1
The rate of change of f (u) as u changes can be measured by looking at
the change in f between two values of u, divided by the change in u:
f (u2 ) − f (u1 )
Δf
=
.
Δu
u2 − u1
For a monotonic transformation, f (u2 ) − f (u1 ) always has the same sign as
u2 − u1 . Thus a monotonic function always has a positive rate of change.
This means that the graph of a monotonic function will always have a
positive slope, as depicted in Figure 4.1A.
v
v
v = f (u )
v = f (u )
u
u
A
B
A positive monotonic transformation. Panel A illustrates
a monotonic function—one that is always increasing. Panel B
illustrates a function that is not monotonic, since it sometimes
increases and sometimes decreases.
Figure
4.1
1
What we are calling a “monotonic transformation” is, strictly speaking, called a “positive monotonic transformation,” in order to distinguish it from a “negative monotonic
transformation,” which is one that reverses the order of the numbers. Monotonic
transformations are sometimes called “monotonous transformations,” which seems
unfair, since they can actually be quite interesting.
CARDINAL UTILITY
57
If f (u) is any monotonic transformation of a utility function that represents some particular preferences, then f (u(x1 , x2 )) is also a utility function
that represents those same preferences.
Why? The argument is given in the following three statements:
1. To say that u(x1 , x2 ) represents some particular preferences means that
u(x1 , x2 ) > u(y1 , y2 ) if and only if (x1 , x2 ) (y1 , y2 ).
2. But if f (u) is a monotonic transformation, then u(x1 , x2 ) > u(y1 , y2 ) if
and only if f (u(x1 , x2 )) > f (u(y1 , y2 )).
3. Therefore, f (u(x1 , x2 )) > f (u(y1 , y2 )) if and only if (x1 , x2 ) (y1 , y2 ),
so the function f (u) represents the preferences in the same way as the
original utility function u(x1 , x2 ).
We summarize this discussion by stating the following principle: a monotonic transformation of a utility function is a utility function that represents
the same preferences as the original utility function.
Geometrically, a utility function is a way to label indifference curves.
Since every bundle on an indifference curve must have the same utility, a
utility function is a way of assigning numbers to the different indifference
curves in a way that higher indifference curves get assigned larger numbers. Seen from this point of view a monotonic transformation is just a
relabeling of indifference curves. As long as indifference curves containing
more-preferred bundles get a larger label than indifference curves containing less-preferred bundles, the labeling will represent the same preferences.
4.1 Cardinal Utility
There are some theories of utility that attach a significance to the magnitude of utility. These are known as cardinal utility theories. In a theory
of cardinal utility, the size of the utility difference between two bundles of
goods is supposed to have some sort of significance.
We know how to tell whether a given person prefers one bundle of goods
to another: we simply offer him or her a choice between the two bundles
and see which one is chosen. Thus we know how to assign an ordinal utility
to the two bundles of goods: we just assign a higher utility to the chosen
bundle than to the rejected bundle. Any assignment that does this will be
a utility function. Thus we have an operational criterion for determining
whether one bundle has a higher utility than another bundle for some
individual.
But how do we tell if a person likes one bundle twice as much as another?
How could you even tell if you like one bundle twice as much as another?
One could propose various definitions for this kind of assignment: I like
one bundle twice as much as another if I am willing to pay twice as much
for it. Or, I like one bundle twice as much as another if I am willing to run
58 UTILITY (Ch. 4)
twice as far to get it, or to wait twice as long, or to gamble for it at twice
the odds.
There is nothing wrong with any of these definitions; each one would
give rise to a way of assigning utility levels in which the magnitude of the
numbers assigned had some operational significance. But there isn’t much
right about them either. Although each of them is a possible interpretation
of what it means to want one thing twice as much as another, none of them
appears to be an especially compelling interpretation of that statement.
Even if we did find a way of assigning utility magnitudes that seemed
to be especially compelling, what good would it do us in describing choice
behavior? To tell whether one bundle or another will be chosen, we only
have to know which is preferred—which has the larger utility. Knowing
how much larger doesn’t add anything to our description of choice. Since
cardinal utility isn’t needed to describe choice behavior and there is no
compelling way to assign cardinal utilities anyway, we will stick with a
purely ordinal utility framework.
4.2 Constructing a Utility Function
But are we assured that there is any way to assign ordinal utilities? Given
a preference ordering can we always find a utility function that will order
bundles of goods in the same way as those preferences? Is there a utility
function that describes any reasonable preference ordering?
Not all kinds of preferences can be represented by a utility function.
For example, suppose that someone had intransitive preferences so that
A B C A. Then a utility function for these preferences would have
to consist of numbers u(A), u(B), and u(C) such that u(A) > u(B) >
u(C) > u(A). But this is impossible.
However, if we rule out perverse cases like intransitive preferences, it
turns out that we will typically be able to find a utility function to represent
preferences. We will illustrate one construction here, and another one in
Chapter 14.
Suppose that we are given an indifference map as in Figure 4.2. We know
that a utility function is a way to label the indifference curves such that
higher indifference curves get larger numbers. How can we do this?
One easy way is to draw the diagonal line illustrated and label each
indifference curve with its distance from the origin measured along the
line.
How do we know that this is a utility function? It is not hard to see that
if preferences are monotonic then the line through the origin must intersect
every indifference curve exactly once. Thus every bundle is getting a label,
and those bundles on higher indifference curves are getting larger labels—
and that’s all it takes to be a utility function.
SOME EXAMPLES OF UTILITY FUNCTIONS
59
x2
Measures distance
from origin
4
3
2
1
Indifference
curves
0
x1
Constructing a utility function from indifference curves.
Draw a diagonal line and label each indifference curve with how
far it is from the origin measured along the line.
This gives us one way to find a labeling of indifference curves, at least as
long as preferences are monotonic. This won’t always be the most natural
way in any given case, but at least it shows that the idea of an ordinal utility
function is pretty general: nearly any kind of “reasonable” preferences can
be represented by a utility function.
4.3 Some Examples of Utility Functions
In Chapter 3 we described some examples of preferences and the indifference curves that represented them. We can also represent these preferences
by utility functions. If you are given a utility function, u(x1 , x2 ), it is relatively easy to draw the indifference curves: you just plot all the points
(x1 , x2 ) such that u(x1 , x2 ) equals a constant. In mathematics, the set of
all (x1 , x2 ) such that u(x1 , x2 ) equals a constant is called a level set. For
each different value of the constant, you get a different indifference curve.
EXAMPLE: Indifference Curves from Utility
Suppose that the utility function is given by: u(x1 , x2 ) = x1 x2 . What do
the indifference curves look like?
Figure
4.2
60 UTILITY (Ch. 4)
We know that a typical indifference curve is just the set of all x1 and x2
such that k = x1 x2 for some constant k. Solving for x2 as a function of x1 ,
we see that a typical indifference curve has the formula:
x2 =
k
.
x1
This curve is depicted in Figure 4.3 for k = 1, 2, 3 · · ·.
x2
Indifference
curves
k=3
k=2
k=1
x1
Figure
4.3
Indifference curves.
different values of k.
The indifference curves k = x1 x2 for
Let’s consider another example. Suppose that we were given a utility
function v(x1 , x2 ) = x21 x22 . What do its indifference curves look like? By
the standard rules of algebra we know that:
v(x1 , x2 ) = x21 x22 = (x1 x2 )2 = u(x1 , x2 )2 .
Thus the utility function v(x1 , x2 ) is just the square of the utility function u(x1 , x2 ). Since u(x1 , x2 ) cannot be negative, it follows that v(x1 , x2 )
is a monotonic transformation of the previous utility function, u(x1 , x2 ).
This means that the utility function v(x1 , x2 ) = x21 x22 has to have exactly
the same shaped indifference curves as those depicted in Figure 4.3. The
labeling of the indifference curves will be different—the labels that were
1, 2, 3, · · · will now be 1, 4, 9, · · ·—but the set of bundles that has v(x1 , x2 ) =
SOME EXAMPLES OF UTILITY FUNCTIONS
61
9 is exactly the same as the set of bundles that has u(x1 , x2 ) = 3. Thus
v(x1 , x2 ) describes exactly the same preferences as u(x1 , x2 ) since it orders
all of the bundles in the same way.
Going the other direction—finding a utility function that represents some
indifference curves—is somewhat more difficult. There are two ways to
proceed. The first way is mathematical. Given the indifference curves, we
want to find a function that is constant along each indifference curve and
that assigns higher values to higher indifference curves.
The second way is a bit more intuitive. Given a description of the preferences, we try to think about what the consumer is trying to maximize—
what combination of the goods describes the choice behavior of the consumer. This may seem a little vague at the moment, but it will be more
meaningful after we discuss a few examples.
Perfect Substitutes
Remember the red pencil and blue pencil example? All that mattered to
the consumer was the total number of pencils. Thus it is natural to measure
utility by the total number of pencils. Therefore we provisionally pick the
utility function u(x1 , x2 ) = x1 +x2 . Does this work? Just ask two things: is
this utility function constant along the indifference curves? Does it assign
a higher label to more-preferred bundles? The answer to both questions is
yes, so we have a utility function.
Of course, this isn’t the only utility function that we could use. We could
also use the square of the number of pencils. Thus the utility function
v(x1 , x2 ) = (x1 + x2 )2 = x21 + 2x1 x2 + x22 will also represent the perfectsubstitutes preferences, as would any other monotonic transformation of
u(x1 , x2 ).
What if the consumer is willing to substitute good 1 for good 2 at a rate
that is different from one-to-one? Suppose, for example, that the consumer
would require two units of good 2 to compensate him for giving up one unit
of good 1. This means that good 1 is twice as valuable to the consumer as
good 2. The utility function therefore takes the form u(x1 , x2 ) = 2x1 + x2 .
Note that this utility yields indifference curves with a slope of −2.
In general, preferences for perfect substitutes can be represented by a
utility function of the form
u(x1 , x2 ) = ax1 + bx2 .
Here a and b are some positive numbers that measure the “value” of goods
1 and 2 to the consumer. Note that the slope of a typical indifference curve
is given by −a/b.
62 UTILITY (Ch. 4)
Perfect Complements
This is the left shoe–right shoe case. In these preferences the consumer only
cares about the number of pairs of shoes he has, so it is natural to choose
the number of pairs of shoes as the utility function. The number of complete
pairs of shoes that you have is the minimum of the number of right shoes
you have, x1 , and the number of left shoes you have, x2 . Thus the utility
function for perfect complements takes the form u(x1 , x2 ) = min{x1 , x2 }.
To verify that this utility function actually works, pick a bundle of goods
such as (10, 10). If we add one more unit of good 1 we get (11, 10),
which should leave us on the same indifference curve. Does it? Yes, since
min{10, 10} = min{11, 10} = 10.
So u(x1 , x2 ) = min{x1 , x2 } is a possible utility function to describe perfect complements. As usual, any monotonic transformation would be suitable as well.
What about the case where the consumer wants to consume the goods
in some proportion other than one-to-one? For example, what about the
consumer who always uses 2 teaspoons of sugar with each cup of tea? If x1
is the number of cups of tea available and x2 is the number of teaspoons
of sugar available, then the number of correctly sweetened cups of tea will
be min{x1 , 12 x2 }.
This is a little tricky so we should stop to think about it. If the number
of cups of tea is greater than half the number of teaspoons of sugar, then
we know that we won’t be able to put 2 teaspoons of sugar in each cup.
In this case, we will only end up with 12 x2 correctly sweetened cups of tea.
(Substitute some numbers in for x1 and x2 to convince yourself.)
Of course, any monotonic transformation of this utility function will
describe the same preferences. For example, we might want to multiply by
2 to get rid of the fraction. This gives us the utility function u(x1 , x2 ) =
min{2x1 , x2 }.
In general, a utility function that describes perfect-complement preferences is given by
u(x1 , x2 ) = min{ax1 , bx2 },
where a and b are positive numbers that indicate the proportions in which
the goods are consumed.
Quasilinear Preferences
Here’s a shape of indifference curves that we haven’t seen before. Suppose
that a consumer has indifference curves that are vertical translates of one
another, as in Figure 4.4. This means that all of the indifference curves are
just vertically “shifted” versions of one indifference curve. It follows that
SOME EXAMPLES OF UTILITY FUNCTIONS
63
the equation for an indifference curve takes the form x2 = k − v(x1 ), where
k is a different constant for each indifference curve. This equation says that
the height of each indifference curve is some function of x1 , −v(x1 ), plus a
constant k. Higher values of k give higher indifference curves. (The minus
sign is only a convention; we’ll see why it is convenient below.)
x2
Indifference
curves
x1
Quasilinear preferences. Each indifference curve is a vertically shifted version of a single indifference curve.
The natural way to label indifference curves here is with k—roughly
speaking, the height of the indifference curve along the vertical axis. Solving for k and setting it equal to utility, we have
u(x1 , x2 ) = k = v(x1 ) + x2 .
In this case the utility function is linear in good 2, but (possibly) nonlinear in good 1; hence the name quasilinear utility, meaning “partly
linear” utility. Specific examples of quasilinear utility would be u(x1 , x2 ) =
√
x1 + x2 , or u(x1 , x2 ) = ln x1 + x2 . Quasilinear utility functions are not
particularly realistic, but they are very easy to work with, as we’ll see in
several examples later on in the book.
Cobb-Douglas Preferences
Another commonly used utility function is the Cobb-Douglas utility function
u(x1 , x2 ) = xc1 xd2 ,
Figure
4.4
64 UTILITY (Ch. 4)
where c and d are positive numbers that describe the preferences of the
consumer.2
The Cobb-Douglas utility function will be useful in several examples.
The preferences represented by the Cobb-Douglas utility function have the
general shape depicted in Figure 4.5. In Figure 4.5A, we have illustrated the
indifference curves for c = 1/2, d = 1/2. In Figure 4.5B, we have illustrated
the indifference curves for c = 1/5, d = 4/5. Note how different values of
the parameters c and d lead to different shapes of the indifference curves.
x2
x2
x1
A c = 1/2 d =1/2
x1
B c = 1/5 d =4/5
Cobb-Douglas indifference curves. Panel A shows the case
where c = 1/2, d = 1/2 and panel B shows the case where
c = 1/5, d = 4/5.
Figure
4.5
Cobb-Douglas indifference curves look just like the nice convex monotonic indifference curves that we referred to as “well-behaved indifference
curves” in Chapter 3. Cobb-Douglas preferences are the standard example of indifference curves that look well-behaved, and in fact the formula
describing them is about the simplest algebraic expression that generates
well-behaved preferences. We’ll find Cobb-Douglas preferences quite useful
to present algebraic examples of the economic ideas we’ll study later.
Of course a monotonic transformation of the Cobb-Douglas utility function will represent exactly the same preferences, and it is useful to see a
couple of examples of these transformations.
2
Paul Douglas was a twentieth-century economist at the University of Chicago who
later became a U.S. senator. Charles Cobb was a mathematician at Amherst College.
The Cobb-Douglas functional form was originally used to study production behavior.
MARGINAL UTILITY
65
First, if we take the natural log of utility, the product of the terms will
become a sum so that we have
v(x1 , x2 ) = ln(xc1 xd2 ) = c ln x1 + d ln x2 .
The indifference curves for this utility function will look just like the ones
for the first Cobb-Douglas function, since the logarithm is a monotonic
transformation. (For a brief review of natural logarithms, see the Mathematical Appendix at the end of the book.)
For the second example, suppose that we start with the Cobb-Douglas
form
v(x1 , x2 ) = xc1 xd2 .
Then raising utility to the 1/(c + d) power, we have
c
d
x1c+d x2c+d .
Now define a new number
c
.
c+d
We can now write our utility function as
a=
v(x1 , x2 ) = xa1 x1−a
.
2
This means that we can always take a monotonic transformation of the
Cobb-Douglas utility function that make the exponents sum to 1. This
will turn out to have a useful interpretation later on.
The Cobb-Douglas utility function can be expressed in a variety of ways;
you should learn to recognize them, as this family of preferences is very
useful for examples.
4.4 Marginal Utility
Consider a consumer who is consuming some bundle of goods, (x1 , x2 ).
How does this consumer’s utility change as we give him or her a little more
of good 1? This rate of change is called the marginal utility with respect
to good 1. We write it as M U1 and think of it as being a ratio,
M U1 =
ΔU
u(x1 + Δx1 , x2 ) − u(x1 , x2 )
=
,
Δx1
Δx1
that measures the rate of change in utility (ΔU ) associated with a small
change in the amount of good 1 (Δx1 ). Note that the amount of good 2 is
held fixed in this calculation.3
3
See the appendix to this chapter for a calculus treatment of marginal utility.
66 UTILITY (Ch. 4)
This definition implies that to calculate the change in utility associated
with a small change in consumption of good 1, we can just multiply the
change in consumption by the marginal utility of the good:
ΔU = M U1 Δx1 .
The marginal utility with respect to good 2 is defined in a similar manner:
M U2 =
ΔU
u(x1 , x2 + Δx2 ) − u(x1 , x2 )
=
.
Δx2
Δx2
Note that when we compute the marginal utility with respect to good 2 we
keep the amount of good 1 constant. We can calculate the change in utility
associated with a change in the consumption of good 2 by the formula
ΔU = M U2 Δx2 .
It is important to realize that the magnitude of marginal utility depends
on the magnitude of utility. Thus it depends on the particular way that we
choose to measure utility. If we multiplied utility by 2, then marginal utility
would also be multiplied by 2. We would still have a perfectly valid utility
function in that it would represent the same preferences, but it would just
be scaled differently.
This means that marginal utility itself has no behavioral content. How
can we calculate marginal utility from a consumer’s choice behavior? We
can’t. Choice behavior only reveals information about the way a consumer
ranks different bundles of goods. Marginal utility depends on the particular utility function that we use to reflect the preference ordering and its
magnitude has no particular significance. However, it turns out that marginal utility can be used to calculate something that does have behavioral
content, as we will see in the next section.
4.5 Marginal Utility and MRS
A utility function u(x1 , x2 ) can be used to measure the marginal rate of
substitution (MRS) defined in Chapter 3. Recall that the MRS measures
the slope of the indifference curve at a given bundle of goods; it can be
interpreted as the rate at which a consumer is just willing to substitute a
small amount of good 2 for good 1.
This interpretation gives us a simple way to calculate the MRS. Consider a change in the consumption of each good, (Δx1 , Δx2 ), that keeps
utility constant—that is, a change in consumption that moves us along the
indifference curve. Then we must have
M U1 Δx1 + M U2 Δx2 = ΔU = 0.
UTILITY FOR COMMUTING
67
Solving for the slope of the indifference curve we have
MRS =
M U1
Δx2
=−
.
Δx1
M U2
(4.1)
(Note that we have 2 over 1 on the left-hand side of the equation and 1
over 2 on the right-hand side. Don’t get confused!)
The algebraic sign of the MRS is negative: if you get more of good 1 you
have to get less of good 2 in order to keep the same level of utility. However,
it gets very tedious to keep track of that pesky minus sign, so economists
often refer to the MRS by its absolute value—that is, as a positive number.
We’ll follow this convention as long as no confusion will result.
Now here is the interesting thing about the MRS calculation: the MRS
can be measured by observing a person’s actual behavior—we find that
rate of exchange where he or she is just willing to stay put, as described in
Chapter 3.
The utility function, and therefore the marginal utility function, is not
uniquely determined. Any monotonic transformation of a utility function
leaves you with another equally valid utility function. Thus, if we multiply
utility by 2, for example, the marginal utility is multiplied by 2. Thus the
magnitude of the marginal utility function depends on the choice of utility
function, which is arbitrary. It doesn’t depend on behavior alone; instead
it depends on the utility function that we use to describe behavior.
But the ratio of marginal utilities gives us an observable magnitude—
namely the marginal rate of substitution. The ratio of marginal utilities
is independent of the particular transformation of the utility function you
choose to use. Look at what happens if you multiply utility by 2. The
MRS becomes
2M U1
.
MRS = −
2M U2
The 2s just cancel out, so the MRS remains the same.
The same sort of thing occurs when we take any monotonic transformation of a utility function. Taking a monotonic transformation is just relabeling the indifference curves, and the calculation for the MRS described
above is concerned with moving along a given indifference curve. Even
though the marginal utilities are changed by monotonic transformations,
the ratio of marginal utilities is independent of the particular way chosen
to represent the preferences.
4.6 Utility for Commuting
Utility functions are basically ways of describing choice behavior: if a bundle of goods X is chosen when a bundle of goods Y is available, then X
must have a higher utility than Y . By examining choices consumers make
we can estimate a utility function to describe their behavior.
68 UTILITY (Ch. 4)
This idea has been widely applied in the field of transportation economics
to study consumers’ commuting behavior. In most large cities commuters
have a choice between taking public transit or driving to work. Each of
these alternatives can be thought of as representing a bundle of different
characteristics: travel time, waiting time, out-of-pocket costs, comfort, convenience, and so on. We could let x1 be the amount of travel time involved
in each kind of transportation, x2 the amount of waiting time for each kind,
and so on.
If (x1 , x2 , . . . , xn ) represents the values of n different characteristics of
driving, say, and (y1 , y2 , . . . , yn ) represents the values of taking the bus, we
can consider a model where the consumer decides to drive or take the bus
depending on whether he prefers one bundle of characteristics to the other.
More specifically, let us suppose that the average consumer’s preferences
for characteristics can be represented by a utility function of the form
U (x1 , x2 , . . . , xn ) = β1 x1 + β2 x2 + · · · + βn xn ,
where the coefficients β1 , β2 , and so on are unknown parameters. Any
monotonic transformation of this utility function would describe the choice
behavior equally well, of course, but the linear form is especially easy to
work with from a statistical point of view.
Suppose now that we observe a number of similar consumers making
choices between driving and taking the bus based on the particular pattern
of commute times, costs, and so on that they face. There are statistical
techniques that can be used to find the values of the coefficients βi for i =
1, . . . , n that best fit the observed pattern of choices by a set of consumers.
These statistical techniques give a way to estimate the utility function for
different transportation modes.
One study reports a utility function that had the form4
U (T W, T T, C) = −0.147T W − 0.0411T T − 2.24C,
(4.2)
where
T W = total walking time to and from bus or car
T T = total time of trip in minutes
C = total cost of trip in dollars
The estimated utility function in the Domenich-McFadden book correctly
described the choice between auto and bus transport for 93 percent of the
households in their sample.
4
See Thomas Domenich and Daniel McFadden, Urban Travel Demand (North-Holland
Publishing Company, 1975). The estimation procedure in this book also incorporated
various demographic characteristics of the households in addition to the purely economic variables described here. Daniel McFadden was awarded the Nobel Prize in
economics in 2000 for his work in developing techniques to estimate models of this
sort.
SUMMARY
69
The coefficients on the variables in Equation (4.2) describe the weight
that an average household places on the various characteristics of their
commuting trips; that is, the marginal utility of each characteristic. The
ratio of one coefficient to another measures the marginal rate of substitution between one characteristic and another. For example, the ratio of the
marginal utility of walking time to the marginal utility of total time indicates that walking time is viewed as being roughly 3 times as onerous as
travel time by the average consumer. In other words, the consumer would
be willing to substitute 3 minutes of additional travel time to save 1 minute
of walking time.
Similarly, the ratio of cost to travel time indicates the average consumer’s
tradeoff between these two variables. In this study, the average commuter
valued a minute of commute time at 0.0411/2.24 = 0.0183 dollars per
minute, which is $1.10 per hour. For comparison, the hourly wage for the
average commuter in 1967, the year of the study, was about $2.85 an hour.
Such estimated utility functions can be very valuable for determining
whether or not it is worthwhile to make some change in the public transportation system. For example, in the above utility function one of the
significant factors explaining mode choice is the time involved in taking
the trip. The city transit authority can, at some cost, add more buses to
reduce this travel time. But will the number of extra riders warrant the
increased expense?
Given a utility function and a sample of consumers we can forecast which
consumers will drive and which consumers will choose to take the bus. This
will give us some idea as to whether the revenue will be sufficient to cover
the extra cost.
Furthermore, we can use the marginal rate of substitution to estimate
the value that each consumer places on the reduced travel time. We saw
above that in the Domenich-McFadden study the average commuter in
1967 valued commute time at a rate of $1.10 per hour. Thus the commuter
should be willing to pay about $0.37 to cut 20 minutes from his or her
trip. This number gives us a measure of the dollar benefit of providing
more timely bus service. This benefit must be compared to the cost of
providing more timely bus service in order to determine if such provision
is worthwhile. Having a quantitative measure of benefit will certainly be
helpful in making a rational decision about transport policy.
Summary
1. A utility function is simply a way to represent or summarize a preference ordering. The numerical magnitudes of utility levels have no intrinsic
meaning.
2. Thus, given any one utility function, any monotonic transformation of
it will represent the same preferences.
70 UTILITY (Ch. 4)
3. The marginal rate of substitution, MRS, can be calculated from the
utility function via the formula MRS = Δx2 /Δx1 = −M U1 /M U2 .
REVIEW QUESTIONS
1. The text said that raising a number to an odd power was a monotonic
transformation. What about raising a number to an even power? Is this a
monotonic transformation? (Hint: consider the case f (u) = u2 .)
2. Which of the following are monotonic transformations? (1) u = 2v − 13;
(2) u = −1/v 2 ; (3) u = 1/v 2 ; (4) u = ln v; (5) u = −e−v ; (6) u = v 2 ;
(7) u = v 2 for v > 0; (8) u = v 2 for v < 0.
3. We claimed in the text that if preferences were monotonic, then a diagonal line through the origin would intersect each indifference curve exactly
once. Can you prove this rigorously? (Hint: what would happen if it
intersected some indifference curve twice?)
4. What kind of preferences
are represented by a utility function of the
√
form u(x1 , x2 ) = x1 + x2 ? What about the utility function v(x1 , x2 ) =
13x1 + 13x2 ?
5. What kind of preferences are represented by a utility function of the form
√
√
u(x1 , x2 ) = x1 + x2 ? Is the utility function v(x1 , x2 ) = x21 + 2x1 x2 + x2
a monotonic transformation of u(x1 , x2 )?
√
6. Consider the utility function u(x1 , x2 ) = x1 x2 . What kind of preferences does it represent? Is the function v(x1 , x2 ) = x21 x2 a monotonic
transformation of u(x1 , x2 )? Is the function w(x1 , x2 ) = x21 x22 a monotonic
transformation of u(x1 , x2 )?
7. Can you explain why taking a monotonic transformation of a utility
function doesn’t change the marginal rate of substitution?
APPENDIX
First, let us clarify what is meant by “marginal utility.” As elsewhere in economics, “marginal” just means a derivative. So the marginal utility of good 1 is
just
M U1 = lim
Δx1 →0
u(x1 + Δx1 , x2 ) − u(x1 , x2 )
∂u(x1 , x2 )
=
.
Δx1
∂x1
Note that we have used the partial derivative here, since the marginal utility
of good 1 is computed holding good 2 fixed.
APPENDIX
71
Now we can rephrase the derivation of the MRS given in the text using calculus.
We’ll do it two ways: first by using differentials, and second by using implicit
functions.
For the first method, we consider making a change (dx1 , dx2 ) that keeps utility
constant. So we want
∂u(x1 , x2 )
∂u(x1 , x2 )
dx1 +
dx2 = 0.
du =
∂x1
∂x2
The first term measures the increase in utility from the small change dx1 , and
the second term measures the increase in utility from the small change dx2 . We
want to pick these changes so that the total change in utility, du, is zero. Solving
for dx2 /dx1 gives us
∂u(x1 , x2 )/∂x1
dx2
=−
,
dx1
∂u(x1 , x2 )/∂x2
which is just the calculus analog of equation (4.1) in the text.
As for the second method, we now think of the indifference curve as being
described by a function x2 (x1 ). That is, for each value of x1 , the function x2 (x1 )
tells us how much x2 we need to get on that specific indifference curve. Thus the
function x2 (x1 ) has to satisfy the identity
u(x1 , x2 (x1 )) ≡ k,
where k is the utility label of the indifference curve in question.
We can differentiate both sides of this identity with respect to x1 to get
∂u(x1 , x2 )
∂u(x1 , x2 ) ∂x2 (x1 )
+
= 0.
∂x1
∂x2
∂x1
Notice that x1 occurs in two places in this identity, so changing x1 will change
the function in two ways, and we have to take the derivative at each place that
x1 appears.
We then solve this equation for ∂x2 (x1 )/∂x1 to find
∂x2 (x1 )
∂u(x1 , x2 )/∂x1
=−
,
∂x1
∂u(x1 , x2 )/∂x2
just as we had before.
The implicit function method is a little more rigorous, but the differential
method is more direct, as long as you don’t do something silly.
Suppose that we take a monotonic transformation of a utility function, say,
v(x1 , x2 ) = f (u(x1 , x2 )). Let’s calculate the MRS for this utility function. Using
the chain rule
∂f /∂u ∂u/∂x1
∂v/∂x1
=−
MRS = −
∂v/∂x2
∂f /∂u ∂u/∂x2
∂u/∂x1
=−
∂u/∂x2
since the ∂f /∂u term cancels out from both the numerator and denominator.
This shows that the MRS is independent of the utility representation.
This gives a useful way to recognize preferences that are represented by different utility functions: given two utility functions, just compute the marginal
rates of substitution and see if they are the same. If they are, then the two
utility functions have the same indifference curves. If the direction of increasing
preference is the same for each utility function, then the underlying preferences
must be the same.
72 UTILITY (Ch. 4)
EXAMPLE: Cobb-Douglas Preferences
The MRS for Cobb-Douglas preferences is easy to calculate by using the formula
derived above.
If we choose the log representation where
u(x1 , x2 ) = c ln x1 + d ln x2 ,
then we have
∂u(x1 , x2 )/∂x1
∂u(x1 , x2 )/∂x2
c/x1
=−
d/x2
c x2
=−
.
d x1
MRS = −
Note that the MRS only depends on the ratio of the two parameters and the
quantity of the two goods in this case.
What if we choose the exponent representation where
u(x1 , x2 ) = xc1 xd2 ?
Then we have
MRS = −
∂u(x1 , x2 )/∂x1
∂u(x1 , x2 )/∂x2
xd2
cxc−1
1
c d−1
dx1 x2
cx2
=−
,
dx1
=−
which is the same as we had before. Of course you knew all along that a monotonic
transformation couldn’t change the marginal rate of substitution!
In the previous chapter, you learned about preferences and indifference
curves. Here we study another way of describing preferences, the utility
function. A utility function that represents a person’s preferences is a
function that assigns a utility number to each commodity bundle. The
numbers are assigned in such a way that commodity bundle (x, y) gets
a higher utility number than bundle (x0 , y 0 ) if and only if the consumer
prefers (x, y) to (x0 , y 0 ). If a consumer has the utility function U (x1 , x2 ),
then she will be indifferent between two bundles if they are assigned the
same utility.
If you know a consumer’s utility function, then you can find the
indifference curve passing through any commodity bundle. Recall from
the previous chapter that when good 1 is graphed on the horizontal axis
and good 2 on the vertical axis, the slope of the indifference curve passing
through a point (x1 , x2 ) is known as the marginal rate of substitution. An
important and convenient fact is that the slope of an indifference curve is
minus the ratio of the marginal utility of good 1 to the marginal utility of
good 2. For those of you who know even a tiny bit of calculus, calculating
marginal utilities is easy. To find the marginal utility of either good,
you just take the derivative of utility with respect to the amount of that
good, treating the amount of the other good as a constant. (If you don’t
know any calculus at all, you can calculate an approximation to marginal
utility by the method described in your textbook. Also, at the beginning
of this section of the workbook, we list the marginal utility functions for
commonly encountered utility functions. Even if you can’t compute these
yourself, you can refer to this list when later problems require you to use
marginal utilities.)
Arthur’s utility function is U (x1 , x2 ) = x1 x2 . Let us find the indifference
curve for Arthur that passes through the point (3, 4). First, calculate
U (3, 4) = 3 × 4 = 12. The indifference curve through this point consists
of all (x1 , x2 ) such that x1 x2 = 12. This last equation is equivalent to
x2 = 12/x1 . Therefore to draw Arthur’s indifference curve through (3, 4),
just draw the curve with equation x2 = 12/x1 . At the point (x1 , x2 ),
the marginal utility of good 1 is x2 and the marginal utility of good 2 is
x1 . Therefore Arthur’s marginal rate of substitution at the point (3, 4) is
−x2 /x1 = −4/3.
Arthur’s uncle, Basil, has the utility function U ∗ (x1 , x2 ) = 3x1 x2 − 10.
Notice that U ∗ (x1 , x2 ) = 3U (x1 , x2 ) − 10, where U (x1 , x2 ) is Arthur’s
utility function. Since U ∗ is a positive multiple of U minus a constant, it
must be that any change in consumption that increases U will also increase
U ∗ (and vice versa). Therefore we say that Basil’s utility function is a
monotonic increasing transformation of Arthur’s utility function. Let us
find Basil’s indifference curve through the point (3, 4). First we find that
U ∗ (3, 4) = 3 × 3 × 4 − 10 = 26. The indifference curve passing through
this point consists of all (x1 , x2 ) such that 3x1 x2 − 10 = 26. Simplify this
last expression by adding 10 to both sides of the equation and dividing
both sides by 3. You find x1 x2 = 12, or equivalently, x2 = 12/x1 . This
is exactly the same curve as Arthur’s indifference curve through (3, 4).
We could have known in advance that this would happen, because if two
consumers’ utility functions are monotonic increasing transformations of
each other, then these consumers must have the same preference relation
between any pair of commodity bundles.
When you have finished this workout, we hope that you will be able
to do the following:
• Draw an indifference curve through a specified commodity bundle
when you know the utility function.
• Calculate marginal utilities and marginal rates of substitution when
you know the utility function.
• Determine whether one utility function is just a “monotonic transformation” of another and know what that implies about preferences.
• Find utility functions that represent preferences when goods are perfect substitutes and when goods are perfect complements.
• Recognize utility functions for commonly studied preferences such as
perfect substitutes, perfect complements, and other kinked indifference curves, quasilinear utility, and Cobb-Douglas utility.
4.0 Warm Up Exercise. This is the first of several “warm up exercises” that you will find in Workouts. These are here to help you see
how to do calculations that are needed in later problems. The answers to
all warm up exercises are in your answer pages. If you find the warm up
exercises easy and boring, go ahead—skip them and get on to the main
problems. You can come back and look at them if you get stuck later.
This exercise asks you to calculate marginal utilities and marginal
rates of substitution for some common utility functions. These utility
functions will reappear in several chapters, so it is a good idea to get to
know them now. If you know calculus, you will find this to be a breeze.
Even if your calculus is shaky or nonexistent, you can handle the first three
utility functions just by using the definitions in the textbook. These three
are easy because the utility functions are linear. If you do not know any
calculus, fill in the rest of the answers from the back of the workbook and
keep a copy of this exercise for reference when you encounter these utility
functions in later problems.
u(x1 , x2 )
2x1 + 3x2
4x1 + 6x2
ax1 + bx2
√
2 x1 + x2
ln x1 + x2
v(x1 ) + x2
x1 x2
xa1 xb2
(x1 + 2)(x2 + 1)
(x1 + a)(x2 + b)
xa1 + xa2
M U1 (x1 , x2 )
M U2 (x1 , x2 )
M RS(x1 , x2 )
4.1 (0) Remember Charlie from Chapter 3? Charlie consumes apples and
bananas. We had a look at two of his indifference curves. In this problem
we give you enough information so you can find all of Charlie’s indifference
curves. We do this by telling you that Charlie’s utility function happens
to be U (xA , xB ) = xA xB .
(a) Charlie has 40 apples and 5 bananas. Charlie’s utility for the bundle
(40, 5) is U (40, 5) =
The indifference curve through (40, 5)
includes all commodity bundles (xA , xB ) such that xA xB =
So the indifference curve through (40, 5) has the equation xB =
On the graph below, draw the indifference curve showing all of
the bundles that Charlie likes exactly as well as the bundle (40, 5).
Bananas
40
30
20
10
0
10
20
30
40
Apples
(b) Donna offers to give Charlie 15 bananas if he will give her 25 apples.
Would Charlie have a bundle that he likes better than (40, 5) if he makes
this trade?
What is the largest number of apples that Donna
could demand from Charlie in return for 15 bananas if she expects him to
be willing to trade or at least indifferent about trading?
(Hint: If
Donna gives Charlie 15 bananas, he will have a total of 20 bananas. If he
has 20 bananas, how many apples does he need in order to be as well-off
as he would be without trade?)
4.2 (0) Ambrose, whom you met in the last chapter, continues to thrive
on nuts and berries. You saw two of his indifference curves. One indif√
ference curve had the equation x2 = 20 − 4 x1 , and another indifference
√
curve had the equation x2 = 24 − 4 x1 , where x1 is his consumption of
nuts and x2 is his consumption of berries. Now it can be told that Ambrose has quasilinear utility. In fact, his preferences can be represented
√
by the utility function U (x1 , x2 ) = 4 x1 + x2 .
(a) Ambrose originally consumed 9 units of nuts and 10 units of berries.
His consumption of nuts is reduced to 4 units, but he is given enough
berries so that he is just as well-off as he was before. After the change,
how many units of berries does Ambrose consume?
.
(b) On the graph below, indicate Ambrose’s original consumption and
sketch an indifference curve passing through this point. As you can verify,
Ambrose is indifferent between the bundle (9,10) and the bundle (25,2).
If you doubled the amount of each good in each bundle, you would have
bundles (18,20) and (50,4). Are these two bundles on the same indifference
curve?
(Hint: How do you check whether two bundles are
indifferent when you know the utility function?)
Berries
20
15
10
5
0
5
10
15
20
Nuts
(c) What is Ambrose’s marginal rate of substitution, M RS(x1 , x2 ), when
he is consuming the bundle (9, 10)? (Give a numerical answer.)
What is Ambrose’s marginal rate of substitution when he is consuming
the bundle (9, 20)?
.
(d) We can write a general expression for Ambrose’s marginal rate of
substitution when he is consuming commodity bundle (x1 , x2 ). This is
M RS(x1 , x2 ) =
Although we always write M RS(x1 , x2 )
as a function of the two variables, x1 and x2 , we see that Ambrose’s utility
function has the special property that his marginal rate of substitution
does not change when the variable
changes.
4.3 (0) Burt’s utility function is U (x1 , x2 ) = (x1 + 2)(x2 + 6), where x1
is the number of cookies and x2 is the number of glasses of milk that he
consumes.
(a) What is the slope of Burt’s indifference curve at the point where he is
consuming the bundle (4, 6)?
Use pencil or black ink to draw
a line with this slope through the point (4, 6). (Try to make this graph
fairly neat and precise, since details will matter.) The line you just drew
is the tangent line to the consumer’s indifference curve at the point (4, 6).
(b) The indifference curve through the point (4, 6) passes through the
,0), (7,
), and (2,
). Use blue ink to
points (
sketch in this indifference curve. Incidentally, the equation for Burt’s
indifference curve through the point (4, 6) is x2 =
.
Glasses of milk
16
12
8
4
0
4
8
12
16
Cookies
(c) Burt currently has the bundle (4, 6). Ernie offers to give Burt 9
glasses of milk if Burt will give Ernie 3 cookies. If Burt makes this trade,
he would have the bundle
wise decision?
on your graph.
Burt refuses to trade. Was this a
Mark the bundle (1, 15)
(d) Ernie says to Burt, “Burt, your marginal rate of substitution is −2.
That means that an extra cookie is worth only twice as much to you as
an extra glass of milk. I offered to give you 3 glasses of milk for every
cookie you give me. If I offer to give you more than your marginal rate
of substitution, then you should want to trade with me.” Burt replies,
“Ernie, you are right that my marginal rate of substitution is −2. That
means that I am willing to make small trades where I get more than 2
glasses of milk for every cookie I give you, but 9 glasses of milk for 3
cookies is too big a trade. My indifference curves are not straight lines,
you see.” Would Burt be willing to give up 1 cookie for 3 glasses of
milk?
Would Burt object to giving up
2 cookies for 6 glasses of milk?
.
(e) On your graph, use red ink to draw a line with slope −3 through the
point (4, 6). This line shows all of the bundles that Burt can achieve by
trading cookies for milk (or milk for cookies) at the rate of 1 cookie for
every 3 glasses of milk. Only a segment of this line represents trades that
make Burt better off than he was without trade. Label this line segment
on your graph AB.
4.4 (0) Phil Rupp’s utility function is U (x, y) = max{x, 2y}.
(a) On the graph below, use blue ink to draw and label the line whose
equation is x = 10. Also use blue ink to draw and label the line whose
equation is 2y = 10.
(b) If x = 10 and 2y < 10, then U (x, y) =
If x < 10 and 2y = 10,
.
then U (x, y) =
(c) Now use red ink to sketch in the indifference curve along which
U (x, y) = 10. Does Phil have convex preferences?
.
y
20
15
10
5
0
5
10
15
20
x
4.5 (0) As you may recall, Nancy Lerner is taking Professor Stern’s
economics course. She will take two examinations in the course, and her
score for the course is the minimum of the scores that she gets on the two
exams. Nancy wants to get the highest possible score for the course.
(a) Write a utility function that represents Nancy’s preferences over alternative combinations of test scores x1 and x2 on tests 1 and 2 respectively.
U (x1 , x2 ) =
.
4.6 (0) Remember Shirley Sixpack and Lorraine Quiche from the last
chapter? Shirley thinks a 16-ounce can of beer is just as good as two
8-ounce cans. Lorraine only drinks 8 ounces at a time and hates stale
beer, so she thinks a 16-ounce can is no better or worse than an 8-ounce
can.
(a) Write a utility function that represents Shirley’s preferences between
commodity bundles comprised of 8-ounce cans and 16-ounce cans of beer.
Let X stand for the number of 8-ounce cans and Y stand for the number
of 16-ounce cans.
.
(b) Now write a utility function that represents Lorraine’s preferences.
.
(c) Would the function utility U (X, Y ) = 100X +200Y represent Shirley’s
preferences?
Would the utility function U (x, y) = (5X +
10Y )2 represent her preferences?
Would the utility function
U (x, y) = X + 3Y represent her preferences?
.
(d) Give an example of two commodity bundles such that Shirley likes
the first bundle better than the second bundle, while Lorraine likes the
second bundle better than the first bundle.
.
4.7 (0) Harry Mazzola has the utility function u(x1 , x2 ) = min{x1 +
2x2 , 2x1 + x2 }, where x1 is his consumption of corn chips and x2 is his
consumption of french fries.
(a) On the graph below, use a pencil to draw the locus of points along
which x1 + 2x2 = 2x1 + x2 . Use blue ink to show the locus of points for
which x1 + 2x2 = 12, and also use blue ink to draw the locus of points
for which 2x1 + x2 = 12.
(b) On the graph you have drawn, shade in the region where both of the
following inequalities are satisfied: x1 + 2x2 ≥ 12 and 2x1 + x2 ≥ 12.
At the bundle (x1 , x2 ) = (8, 2), one sees that 2x1 + x2 =
x1 + 2x2 =
Therefore u(8, 2) =
and
.
(c) Use black ink to sketch in the indifference curve along which Harry’s
utility is 12. Use red ink to sketch in the indifference curve along which
Harry’s utility is 6. (Hint: Is there anything about Harry Mazzola that
reminds you of Mary Granola?)
French fries
8
6
4
2
0
2
4
6
8
Corn chips
(d) At the point where Harry is consuming 5 units of corn chips and 2
units of french fries, how many units of corn chips would he be willing to
trade for one unit of french fries?
.
4.8 (1) Vanna Boogie likes to have large parties. She also has a strong
preference for having exactly as many men as women at her parties. In
fact, Vanna’s preferences among parties can be represented by the utility
function U (x, y) = min{2x − y, 2y − x} where x is the number of women
and y is the number of men at the party. On the graph below, let us try
to draw the indifference curve along which Vanna’s utility is 10.
(a) Use pencil to draw the locus of points at which x = y. What point
on this gives Vanna a utility of 10?
Use blue ink to draw
the line along which 2y − x = 10. When min{2x − y, 2y − x} = 2y − x,
there are (more men than women, more women than men)?
Draw a squiggly red line over the part of the blue line for which
U (x, y) = min{2x − y, 2y − x} = 2y − x. This shows all the combinations
that Vanna thinks are just as good as (10, 10) but where there are (more
men than women, more women than men)?
Now draw a
blue line along which 2x − y = 10. Draw a squiggly red line over the part
of this new blue line for which min{2x − y, 2y − x} = 2x − y. Use pencil
to shade in the area on the graph that represents all combinations that
Vanna likes at least as well as (10, 10).
(b) Suppose that there are 9 men and 10 women at Vanna’s party. Would
Vanna think it was a better party or a worse party if 5 more men came
to her party?
.
(c) If Vanna has 16 women at her party and more men than women,
and if she thinks the party is exactly as good as having 10 men and 10
If Vanna has
women, how many men does she have at the party?
16 women at her party and more women than men, and if she thinks the
party is exactly as good as having 10 men and 10 women, how many men
does she have at her party?
.
(d) Vanna’s indifference curves are shaped like what letter of the alphabet?
.
y
20
15
10
5
0
5
10
15
20
x
4.9 (0) Suppose that the utility functions u(x, y) and v(x, y) are related
by v(x, y) = f (u(x, y)). In each case below, write “Yes” if the function
f is a positive monotonic transformation and “No” if it is not. (Hint for
calculus users: A differentiable function f (u) is an increasing function of
u if its derivative is positive.)
(a) f (u) = 3.141592u.
.
(b) f (u) = 5, 000 − 23u.
.
(c) f (u) = u − 100, 000.
.
(d) f (u) = log10 u.
.
(e) f (u) = −e−u .
.
(f ) f (u) = 1/u.
.
(g) f (u) = −1/u.
.
4.10 (0) Martha Modest has preferences represented by the utility function U (a, b) = ab/100, where a is the number of ounces of animal crackers
that she consumes and b is the number of ounces of beans that she consumes.
(a) On the graph below, sketch the locus of points that Martha finds
indifferent to having 8 ounces of animal crackers and 2 ounces of beans.
Also sketch the locus of points that she finds indifferent to having 6 ounces
of animal crackers and 4 ounces of beans.
Beans
8
6
4
2
0
2
4
6
8
Animal crackers
(b) Bertha Brassy has preferences represented by the utility function
V (a, b) = 1, 000a2 b2 , where a is the number of ounces of animal crackers that she consumes and b is the number of ounces of beans that she
consumes. On the graph below, sketch the locus of points that Bertha
finds indifferent to having 8 ounces of animal crackers and 2 ounces of
beans. Also sketch the locus of points that she finds indifferent to having
6 ounces of animal crackers and 4 ounces of beans.
Beans
8
6
4
2
0
2
(c) Are Martha’s preferences convex?
4
6
8
Animal crackers
Are Bertha’s?
.
(d) What can you say about the difference between the indifference curves
you drew for Bertha and those you drew for Martha?
.
(e) How could you tell this was going to happen without having to
draw the curves?
.
4.11 (0) Willy Wheeler’s preferences over bundles that contain nonnegative amounts of x1 and x2 are represented by the utility function
U (x1 , x2 ) = x21 + x22 .
(a) Draw a few of his indifference curves. What kind of geometric figure are they?
convex preferences?
Does Willy have
.
x2
8
6
4
2
0
2
4
6
8
x1
4.12 (0) Joe Bob has a utility function given by u(x1 , x2 ) = x21 +2x1 x2 +
x22 .
(a) Compute Joe Bob’s marginal rate of substitution: M RS(x1 , x2 ) =
.
(b) Joe Bob’s straight cousin, Al, has a utility function v(x1 , x2 ) = x2 +x1 .
Compute Al’s marginal rate of substitution. M RS(x1 , x2 ) =
.
(c) Do u(x1 , x2 ) and v(x1 , x2 ) represent the same preferences?
Can you show that Joe Bob’s utility function is a monotonic
transformation of Al’s? (Hint: Some have said that Joe Bob is square.)
.
4.13 (0) The idea of assigning numerical values to determine a preference
ordering over a set of objects is not limited in application to commodity
bundles. The Bill James Baseball Abstract argues that a baseball player’s
batting average is not an adequate measure of his offensive productivity.
Batting averages treat singles just the same as extra base hits. Furthermore they do not give credit for “walks,” although a walk is almost as
good as a single. James argues that a double in two at-bats is better than
a single, but not as good as two singles. To reflect these considerations,
James proposes the following index, which he calls “runs created.” Let A
be the number of hits plus the number of walks that a batter gets in a season. Let B be the number of total bases that the batter gets in the season.
(Thus, if a batter has S singles, W walks, D doubles, T triples, and H
home runs, then A = S +D +T +H +W and B = S +W +2D +3T +4H.)
Let N be the number of times the batter bats. Then his index of runs
created in the season is defined to be AB/N and will be called his RC.
(a) In 1987, George Bell batted 649 times. He had 39 walks, 105 singles,
32 doubles, 4 triples, and 47 home runs. In 1987, Wade Boggs batted 656
times. He had 105 walks, 130 singles, 40 doubles, 6 triples, and 24 home
runs. In 1987, Alan Trammell batted 657 times. He had 60 walks, 140
singles, 34 doubles, 3 triples, and 28 home runs. In 1987, Tony Gwynn
batted 671 times. He had 82 walks, 162 singles, 36 doubles, 13 triples, and
7 home runs. We can calculate A, the number of hits plus walks, B the
number of total bases, and RC, the runs created index for each of these
players. For Bell, A = 227, B = 408, RC = 143. For Boggs, A = 305,
B = 429, RC = 199. For Trammell, A = 265, B = 389, RC = 157. For
Gwynn, A =
,B=
, RC =
.
(b) If somebody has a preference ordering among these players, based only
on the runs-created index, which player(s) would she prefer to Trammell?
.
(c) The differences in the number of times at bat for these players are
small, and we will ignore them for simplicity of calculation. On the graph
below, plot the combinations of A and B achieved by each of the players.
Draw four “indifference curves,” one through each of the four points you
have plotted. These indifference curves should represent combinations of
A and B that lead to the same number of runs-created.
Number of total bases
480
400
320
240
160
80
0
60
120
180
240
300
360
Number of hits plus walks
4.14 (0) This problem concerns the runs-created index discussed in the
preceding problem. Consider a batter who bats 100 times and always
either makes an out, hits for a single, or hits a home run.
(a) Let x be the number of singles and y be the number of home runs
in 100 at-bats. Suppose that the utility function U (x, y) by which we
evaluate alternative combinations of singles and home runs is the runscreated index. Then the formula for the utility function is U (x, y) =
.
(b) Let’s try to find out about the shape of an indifference curve between
singles and home runs. Hitting 10 home runs and no singles would give
him the same runs-created index as hitting
singles and no home
runs. Mark the points (0, 10) and (x, 0), where U (x, 0) = U (0, 10).
(c) Where x is the number of singles you solved for in the previous part,
mark the point (x/2, 5) on your graph. Is U (x/2, 5) greater than or less
than or equal to U (0, 10)?
Is this consistent with the
batter having convex preferences between singles and home runs?
.
Home runs
20
15
10
5
0
5
10
15
20
Singles
4.1 In Problem 4.1, Charlie has the utility function U (xA , xB ) = xA xB .
His indifference curve passing through 10 apples and 30 bananas will also
pass through the point where he consumes 2 apples and
(a) 25 bananas.
(b) 50 bananas.
(c) 152 bananas.
(d) 158 bananas.
(e) 150 bananas.
4.2 In Problem 4.1, Charlie’s utility function is U (A, B) = AB, where
A and B are the numbers of apples and bananas, respectively, that he
consumes. When Charlie is consuming 20 apples and 100 bananas, then
if we put apples on the horizontal axis and bananas on the vertical axis,
the slope of his indifference curve at his current consumption is
(a) −20.
(b) −5.
(c) −10.
(d) −1/5.
(e) −1/10.
1/2
4.3 In Problem 4.2, Ambrose has the utility function U (x1 , x2 ) = 4x1 +
x2 . If Ambrose is initially consuming 81 units of nuts and 14 units of
berries, then what is the largest number of units of berries that he would
be willing to give up in return for an additional 40 units of nuts?
(a) 11
(b) 25
(c) 8
(d) 4
(e) 2
4.4 Joe Bob from Problem 4.12 has a cousin Jonas who consume goods 1
and 2. Jonas thinks that 2 units of good 1 is always a perfect substitute
for 3 units of good 2. Which of the following utility functions is the only
one that would not represent Jonas’s preferences?
(a) U (x1 , x2 ) = 3x1 + 2x2 + 1, 000.
(b) U (x1 , x2 ) = 9x21 + 12x1 x2 + 4x22 .
(c) U (x1 , x2 ) = min{3x1 , 2x2 }.
(d) U (x1 , x2 ) = 30x1 + 20x2 − 10, 000.
(e) More than one of the above does not represent Jonas’s preferences.
4.5 In Problem 4.7, Harry Mazzola has the utility function U (x1 , x2 ) =
min{x1 + 2x2 , 2x1 + x2 }. He has $40 to spend on corn chips and french
fries. If the price of corn chips is 5 dollars per unit and the price of french
fries is 5 dollars per unit, then Harry will
(a) definitely spend all of his income on corn chips.
(b) definitely spend all of his income on french fries.
(c) consume at least as many units of corn chips as of french fries, but
might consume both.
(d) consume at least as many units of french fries as of corn chips, but
might consume both.
(e) consume an equal number of units of french fries and corn chips.
4.6 Phil Rupp’s sister Ethel has the utility function U (x, y) = min{2x +
y, 3y}. Where x is measured on the horizontal axis and y on the vertical
axis, her indifference curves consist of
(a) a vertical line segment and a horizontal line segment that meet in a
kink along the line y = 2x.
(b) a vertical line segment and a horizontal line segment that meet in a
kink along the line x = 2y.
(c) a horizontal line segment and a negatively sloped line segment that
meet in a kink along the line x = y.
(d) a positively sloped line segment and a negatively sloped line segment
that meet along the line x = y.
(e) a horizontal line segment and a positively sloped line segment that
meet in a kink along the line x = 2y.
CHAPTER
5
CHOICE
In this chapter we will put together the budget set and the theory of preferences in order to examine the optimal choice of consumers. We said earlier
that the economic model of consumer choice is that people choose the best
bundle they can afford. We can now rephrase this in terms that sound more
professional by saying that “consumers choose the most preferred bundle
from their budget sets.”
5.1 Optimal Choice
A typical case is illustrated in Figure 5.1. Here we have drawn the budget
set and several of the consumer’s indifference curves on the same diagram.
We want to find the bundle in the budget set that is on the highest indifference curve. Since preferences are well-behaved, so that more is preferred
to less, we can restrict our attention to bundles of goods that lie on the
budget line and not worry about those beneath the budget line.
Now simply start at the right-hand corner of the budget line and move to
the left. As we move along the budget line we note that we are moving to
higher and higher indifference curves. We stop when we get to the highest
74 CHOICE (Ch. 5)
indifference curve that just touches the budget line. In the diagram, the
bundle of goods that is associated with the highest indifference curve that
just touches the budget line is labeled (x∗1 , x∗2 ).
The choice (x∗1 , x∗2 ) is an optimal choice for the consumer. The set
of bundles that she prefers to (x∗1 , x∗2 )—the set of bundles above her indifference curve—doesn’t intersect the bundles she can afford—the bundles
beneath her budget line. Thus the bundle (x∗1 , x∗2 ) is the best bundle that
the consumer can afford.
x2
Indifference
curves
Optimal
choice
x*2
x*1
Figure
5.1
x1
Optimal choice. The optimal consumption position is where
the indifference curve is tangent to the budget line.
Note an important feature of this optimal bundle: at this choice, the
indifference curve is tangent to the budget line. If you think about it a
moment you’ll see that this has to be the case: if the indifference curve
weren’t tangent, it would cross the budget line, and if it crossed the budget
line, there would be some nearby point on the budget line that lies above
the indifference curve—which means that we couldn’t have started at an
optimal bundle.
OPTIMAL CHOICE
75
Does this tangency condition really have to hold at an optimal choice?
Well, it doesn’t hold in all cases, but it does hold for most interesting cases.
What is always true is that at the optimal point the indifference curve can’t
cross the budget line. So when does “not crossing” imply tangent? Let’s
look at the exceptions first.
First, the indifference curve might not have a tangent line, as in Figure 5.2. Here the indifference curve has a kink at the optimal choice, and
a tangent just isn’t defined, since the mathematical definition of a tangent
requires that there be a unique tangent line at each point. This case doesn’t
have much economic significance—it is more of a nuisance than anything
else.
x2
Indifference
curves
x*2
Budget line
x*1
x1
Kinky tastes. Here is an optimal consumption bundle where
the indifference curve doesn’t have a tangent.
The second exception is more interesting. Suppose that the optimal
point occurs where the consumption of some good is zero as in Figure 5.3.
Then the slope of the indifference curve and the slope of the budget line
are different, but the indifference curve still doesn’t cross the budget line.
Figure
5.2
76 CHOICE (Ch. 5)
We say that Figure 5.3 represents a boundary optimum, while a case
like Figure 5.1 represents an interior optimum.
If we are willing to rule out “kinky tastes” we can forget about the
example given in Figure 5.2.1 And if we are willing to restrict ourselves only
to interior optima, we can rule out the other example. If we have an interior
optimum with smooth indifference curves, the slope of the indifference curve
and the slope of the budget line must be the same . . . because if they were
different the indifference curve would cross the budget line, and we couldn’t
be at the optimal point.
x2
Indifference
curves
Budget
line
x*1
x1
Boundary optimum. The optimal consumption involves consuming zero units of good 2. The indifference curve is not tangent to the budget line.
Figure
5.3
We’ve found a necessary condition that the optimal choice must satisfy.
If the optimal choice involves consuming some of both goods—so that it is
an interior optimum—then necessarily the indifference curve will be tangent
to the budget line. But is the tangency condition a sufficient condition for
a bundle to be optimal? If we find a bundle where the indifference curve
is tangent to the budget line, can we be sure we have an optimal choice?
Look at Figure 5.4. Here we have three bundles where the tangency
condition is satisfied, all of them interior, but only two of them are optimal.
1
Otherwise, this book might get an R rating.
OPTIMAL CHOICE
77
So in general, the tangency condition is only a necessary condition for
optimality, not a sufficient condition.
x2
Indifference
curves
Optimal
bundles
Nonoptimal
bundle
Budget line
x1
More than one tangency. Here there are three tangencies,
but only two optimal points, so the tangency condition is necessary but not sufficient.
However, there is one important case where it is sufficient: the case
of convex preferences. In the case of convex preferences, any point that
satisfies the tangency condition must be an optimal point. This is clear
geometrically: since convex indifference curves must curve away from the
budget line, they can’t bend back to touch it again.
Figure 5.4 also shows us that in general there may be more than one
optimal bundle that satisfies the tangency condition. However, again convexity implies a restriction. If the indifference curves are strictly convex—
they don’t have any flat spots—then there will be only one optimal choice
on each budget line. Although this can be shown mathematically, it is also
quite plausible from looking at the figure.
The condition that the MRS must equal the slope of the budget line at
an interior optimum is obvious graphically, but what does it mean economically? Recall that one of our interpretations of the MRS is that it is that
rate of exchange at which the consumer is just willing to stay put. Well,
the market is offering a rate of exchange to the consumer of −p1 /p2 —if
Figure
5.4
78 CHOICE (Ch. 5)
you give up one unit of good 1, you can buy p1 /p2 units of good 2. If the
consumer is at a consumption bundle where he or she is willing to stay put,
it must be one where the MRS is equal to this rate of exchange:
MRS = −
p1
.
p2
Another way to think about this is to imagine what would happen if the
MRS were different from the price ratio. Suppose, for example, that the
MRS is Δx2 /Δx1 = −1/2 and the price ratio is 1/1. Then this means the
consumer is just willing to give up 2 units of good 1 in order to get 1 unit of
good 2—but the market is willing to exchange them on a one-to-one basis.
Thus the consumer would certainly be willing to give up some of good 1 in
order to purchase a little more of good 2. Whenever the MRS is different
from the price ratio, the consumer cannot be at his or her optimal choice.
5.2 Consumer Demand
The optimal choice of goods 1 and 2 at some set of prices and income is
called the consumer’s demanded bundle. In general when prices and
income change, the consumer’s optimal choice will change. The demand
function is the function that relates the optimal choice—the quantities
demanded—to the different values of prices and incomes.
We will write the demand functions as depending on both prices and
income: x1 (p1 , p2 , m) and x2 (p1 , p2 , m). For each different set of prices and
income, there will be a different combination of goods that is the optimal
choice of the consumer. Different preferences will lead to different demand
functions; we’ll see some examples shortly. Our major goal in the next
few chapters is to study the behavior of these demand functions—how the
optimal choices change as prices and income change.
5.3 Some Examples
Let us apply the model of consumer choice we have developed to the examples of preferences described in Chapter 3. The basic procedure will be the
same for each example: plot the indifference curves and budget line and
find the point where the highest indifference curve touches the budget line.
Perfect Substitutes
The case of perfect substitutes is illustrated in Figure 5.5. We have three
possible cases. If p2 > p1 , then the slope of the budget line is flatter than
the slope of the indifference curves. In this case, the optimal bundle is
SOME EXAMPLES
79
where the consumer spends all of his or her money on good 1. If p1 > p2 ,
then the consumer purchases only good 2. Finally, if p1 = p2 , there is a
whole range of optimal choices—any amount of goods 1 and 2 that satisfies
the budget constraint is optimal in this case. Thus the demand function
for good 1 will be
⎧
⎨ m/p1
x1 = any number between 0 and m/p1
⎩
0
when p1 < p2 ;
when p1 = p2 ;
when p1 > p2 .
Are these results consistent with common sense? All they say is that
if two goods are perfect substitutes, then a consumer will purchase the
cheaper one. If both goods have the same price, then the consumer doesn’t
care which one he or she purchases.
x2
Indifference
curves
Slope = –1
Budget line
Optimal choice
x*1 = m/p1
x1
Optimal choice with perfect substitutes. If the goods are
perfect substitutes, the optimal choice will usually be on the
boundary.
Perfect Complements
The case of perfect complements is illustrated in Figure 5.6. Note that
the optimal choice must always lie on the diagonal, where the consumer is
purchasing equal amounts of both goods, no matter what the prices are.
Figure
5.5
80 CHOICE (Ch. 5)
In terms of our example, this says that people with two feet buy shoes in
pairs.2
Let us solve for the optimal choice algebraically. We know that this
consumer is purchasing the same amount of good 1 and good 2, no matter
what the prices. Let this amount be denoted by x. Then we have to satisfy
the budget constraint
p1 x + p2 x = m.
Solving for x gives us the optimal choices of goods 1 and 2:
x1 = x2 = x =
m
.
p1 + p 2
The demand function for the optimal choice here is quite intuitive. Since
the two goods are always consumed together, it is just as if the consumer
were spending all of her money on a single good that had a price of p1 + p2 .
x2
Indifference
curves
x*2
Optimal choice
Budget line
x*
1
x1
Optimal choice with perfect complements. If the goods
are perfect complements, the quantities demanded will always
lie on the diagonal since the optimal choice occurs where x1
equals x2 .
Figure
5.6
2
Don’t worry, we’ll get some more exciting results later on.
SOME EXAMPLES
x2
81
x2
Optimal choice
Budget line
Optimal choice
Budget line
1
2
3
x1
A Zero units demanded
1
2
3
x1
B 1 unit demanded
Discrete goods. In panel A the demand for good 1 is zero,
while in panel B one unit will be demanded.
Neutrals and Bads
In the case of a neutral good the consumer spends all of her money on the
good she likes and doesn’t purchase any of the neutral good. The same
thing happens if one commodity is a bad. Thus, if commodity 1 is a good
and commodity 2 is a bad, then the demand functions will be
m
p1
x2 = 0.
x1 =
Discrete Goods
Suppose that good 1 is a discrete good that is available only in integer
units, while good 2 is money to be spent on everything else. If the consumer chooses 1, 2, 3, · · · units of good 1, she will implicitly choose the
consumption bundles (1, m − p1 ), (2, m − 2p1 ), (3, m − 3p1 ), and so on. We
can simply compare the utility of each of these bundles to see which has
the highest utility.
Alternatively, we can use the indifference-curve analysis in Figure 5.7. As
usual, the optimal bundle is the one on the highest indifference “curve.” If
the price of good 1 is very high, then the consumer will choose zero units
of consumption; as the price decreases the consumer will find it optimal to
consume 1 unit of the good. Typically, as the price decreases further the
consumer will choose to consume more units of good 1.
Figure
5.7
82 CHOICE (Ch. 5)
Concave Preferences
Consider the situation illustrated in Figure 5.8. Is X the optimal choice?
No! The optimal choice for these preferences is always going to be a boundary choice, like bundle Z. Think of what nonconvex preferences mean. If
you have money to purchase ice cream and olives, and you don’t like to
consume them together, you’ll spend all of your money on one or the other.
x2
Indifference
curves
Nonoptimal
choice
X
Budget
line
Optimal
choice
Z
Figure
5.8
x1
Optimal choice with concave preferences. The optimal
choice is the boundary point, Z, not the interior tangency point,
X, because Z lies on a higher indifference curve.
Cobb-Douglas Preferences
Suppose that the utility function is of the Cobb-Douglas form, u(x1 , x2 ) =
xc1 xd2 . In the Appendix to this chapter we use calculus to derive the optimal
ESTIMATING UTILITY FUNCTIONS
83
choices for this utility function. They turn out to be
c m
c + d p1
d m
x2 =
.
c + d p2
x1 =
These demand functions are often useful in algebraic examples, so you
should probably memorize them.
The Cobb-Douglas preferences have a convenient property. Consider the
fraction of his income that a Cobb-Douglas consumer spends on good 1. If
he consumes x1 units of good 1, this costs him p1 x1 , so this represents a
fraction p1 x1 /m of total income. Substituting the demand function for x1
we have
p1 c m
c
p1 x1
=
.
=
m
m c + d p1
c+d
Similarly the fraction of his income that the consumer spends on good 2 is
d/(c + d).
Thus the Cobb-Douglas consumer always spends a fixed fraction of his
income on each good. The size of the fraction is determined by the exponent
in the Cobb-Douglas function.
This is why it is often convenient to choose a representation of the CobbDouglas utility function in which the exponents sum to 1. If u(x1 , x2 ) =
, then we can immediately interpret a as the fraction of income spent
xa1 x1−a
2
on good 1. For this reason we will usually write Cobb-Douglas preferences
in this form.
5.4 Estimating Utility Functions
We’ve now seen several different forms for preferences and utility functions
and have examined the kinds of demand behavior generated by these preferences. But in real life we usually have to work the other way around: we
observe demand behavior, but our problem is to determine what kind of
preferences generated the observed behavior.
For example, suppose that we observe a consumer’s choices at several
different prices and income levels. An example is depicted in Table 5.1.
This is a table of the demand for two goods at the different levels of prices
and incomes that prevailed in different years. We have also computed
the share of income spent on each good in each year using the formulas
s1 = p1 x1 /m and s2 = p2 x2 /m.
For these data, the expenditure shares are relatively constant. There are
small variations from observation to observation, but they probably aren’t
large enough to worry about. The average expenditure share for good 1 is
about 1/4, and the average income share for good 2 is about 3/4. It appears
84 CHOICE (Ch. 5)
Table
5.1
Some data describing consumption behavior.
Year
1
2
3
4
5
6
7
p1 p2
1 1
1 2
2 1
1 2
2 1
1 4
4 1
m
100
100
100
200
200
400
400
x1 x2
25 75
24 38
13 74
48 76
25 150
100 75
24 304
s1
.25
.24
.26
.24
.25
.25
.24
s2
.75
.76
.74
.76
.75
.75
.76
Utility
57.0
33.9
47.9
67.8
95.8
80.6
161.1
1
3
that a utility function of the form u(x1 , x2 ) = x14 x24 seems to fit these
data pretty well. That is, a utility function of this form would generate
choice behavior that is pretty close to the observed choice behavior. For
convenience we have calculated the utility associated with each observation
using this estimated Cobb-Douglas utility function.
As far as we can tell from the observed behavior it appears as though the
1
3
consumer is maximizing the function u(x1 , x2 ) = x14 x24 . It may well be that
further observations on the consumer’s behavior would lead us to reject this
hypothesis. But based on the data we have, the fit to the optimizing model
is pretty good.
This has very important implications, since we can now use this “fitted”
utility function to evaluate the impact of proposed policy changes. Suppose,
for example, that the government was contemplating imposing a system of
taxes that would result in this consumer facing prices (2, 3) and having an
income of 200. According to our estimates, the demanded bundle at these
prices would be
1 200
= 25
x1 =
4 2
3 200
= 50.
x2 =
4 3
The estimated utility of this bundle is
1
3
u(x1 , x2 ) = 25 4 50 4 ≈ 42.
This means that the new tax policy would make the consumer better off
than he was in year 2, but worse off than he was in year 3. Thus we can use
the observed choice behavior to value the implications of proposed policy
changes on this consumer.
Since this is such an important idea in economics, let us review the
logic one more time. Given some observations on choice behavior, we try
to determine what, if anything, is being maximized. Once we have an
estimate of what it is that is being maximized, we can use this both to
IMPLICATIONS OF THE MRS CONDITION
85
predict choice behavior in new situations and to evaluate proposed changes
in the economic environment.
Of course we have described a very simple situation. In reality, we normally don’t have detailed data on individual consumption choices. But we
often have data on groups of individuals—teenagers, middle-class households, elderly people, and so on. These groups may have different preferences for different goods that are reflected in their patterns of consumption
expenditure. We can estimate a utility function that describes their consumption patterns and then use this estimated utility function to forecast
demand and evaluate policy proposals.
In the simple example described above, it was apparent that income
shares were relatively constant so that the Cobb-Douglas utility function
would give us a pretty good fit. In other cases, a more complicated form
for the utility function would be appropriate. The calculations may then
become messier, and we may need to use a computer for the estimation,
but the essential idea of the procedure is the same.
5.5 Implications of the MRS Condition
In the last section we examined the important idea that observation of demand behavior tells us important things about the underlying preferences
of the consumers that generated that behavior. Given sufficient observations on consumer choices it will often be possible to estimate the utility
function that generated those choices.
But even observing one consumer choice at one set of prices will allow
us to make some kinds of useful inferences about how consumer utility will
change when consumption changes. Let us see how this works.
In well-organized markets, it is typical that everyone faces roughly the
same prices for goods. Take, for example, two goods like butter and milk.
If everyone faces the same prices for butter and milk, and everyone is
optimizing, and everyone is at an interior solution . . . then everyone must
have the same marginal rate of substitution for butter and milk.
This follows directly from the analysis given above. The market is offering everyone the same rate of exchange for butter and milk, and everyone
is adjusting their consumption of the goods until their own “internal” marginal valuation of the two goods equals the market’s “external” valuation
of the two goods.
Now the interesting thing about this statement is that it is independent
of income and tastes. People may value their total consumption of the two
goods very differently. Some people may be consuming a lot of butter and
a little milk, and some may be doing the reverse. Some wealthy people
may be consuming a lot of milk and a lot of butter while other people may
be consuming just a little of each good. But everyone who is consuming
the two goods must have the same marginal rate of substitution. Everyone
86 CHOICE (Ch. 5)
who is consuming the goods must agree on how much one is worth in terms
of the other: how much of one they would be willing to sacrifice to get some
more of the other.
The fact that price ratios measure marginal rates of substitution is very
important, for it means that we have a way to value possible changes in
consumption bundles. Suppose, for example, that the price of milk is $1
a quart and the price of butter is $2 a pound. Then the marginal rate of
substitution for all people who consume milk and butter must be 2: they
have to have 2 quarts of milk to compensate them for giving up 1 pound
of butter. Or conversely, they have to have 1 pound of butter to make
it worth their while to give up 2 quarts of milk. Hence everyone who is
consuming both goods will value a marginal change in consumption in the
same way.
Now suppose that an inventor discovers a new way of turning milk into
butter: for every 3 quarts of milk poured into this machine, you get out
1 pound of butter, and no other useful byproducts. Question: is there
a market for this device? Answer: the venture capitalists won’t beat a
path to his door, that’s for sure. For everyone is already operating at a
point where they are just willing to trade 2 quarts of milk for 1 pound
of butter; why would they be willing to substitute 3 quarts of milk for 1
pound of butter? The answer is they wouldn’t; this invention isn’t worth
anything.
But what would happen if he got it to run in reverse so he could dump
in a pound of butter get out 3 quarts of milk? Is there a market for this
device? Answer: yes! The market prices of milk and butter tell us that
people are just barely willing to trade one pound of butter for 2 quarts of
milk. So getting 3 quarts of milk for a pound of butter is a better deal than
is currently being offered in the marketplace. Sign me up for a thousand
shares! (And several pounds of butter.)
The market prices show that the first machine is unprofitable: it produces
$2 of butter by using $3 of milk. The fact that it is unprofitable is just
another way of saying that people value the inputs more than the outputs.
The second machine produces $3 worth of milk by using only $2 worth of
butter. This machine is profitable because people value the outputs more
than the inputs.
The point is that, since prices measure the rate at which people are just
willing to substitute one good for another, they can be used to value policy
proposals that involve making changes in consumption. The fact that prices
are not arbitrary numbers but reflect how people value things on the margin
is one of the most fundamental and important ideas in economics.
If we observe one choice at one set of prices we get the MRS at one
consumption point. If the prices change and we observe another choice we
get another MRS. As we observe more and more choices we learn more
and more about the shape of the underlying preferences that may have
generated the observed choice behavior.
CHOOSING TAXES
87
5.6 Choosing Taxes
Even the small bit of consumer theory we have discussed so far can be used
to derive interesting and important conclusions. Here is a nice example
describing a choice between two types of taxes. We saw that a quantity
tax is a tax on the amount consumed of a good, like a gasoline tax of
15 cents per gallon. An income tax is just a tax on income. If the
government wants to raise a certain amount of revenue, is it better to raise
it via a quantity tax or an income tax? Let’s apply what we’ve learned to
answer this question.
First we analyze the imposition of a quantity tax. Suppose that the
original budget constraint is
p1 x1 + p2 x2 = m.
What is the budget constraint if we tax the consumption of good 1 at a
rate of t? The answer is simple. From the viewpoint of the consumer it is
just as if the price of good 1 has increased by an amount t. Thus the new
budget constraint is
(5.1)
(p1 + t)x1 + p2 x2 = m.
Therefore a quantity tax on a good increases the price perceived by
the consumer. Figure 5.9 gives an example of how that price change might
affect demand. At this stage, we don’t know for certain whether this tax will
increase or decrease the consumption of good 1, although the presumption
is that it will decrease it. Whichever is the case, we do know that the
optimal choice, (x∗1 , x∗2 ), must satisfy the budget constraint
(p1 + t)x∗1 + p2 x∗2 = m.
(5.2)
The revenue raised by this tax is R∗ = tx∗1 .
Let’s now consider an income tax that raises the same amount of revenue.
The form of this budget constraint would be
p1 x1 + p2 x2 = m − R∗
or, substituting for R∗ ,
p1 x1 + p2 x2 = m − tx∗1 .
Where does this budget line go in Figure 5.9?
It is easy to see that it has the same slope as the original budget line,
−p1 /p2 , but the problem is to determine its location. As it turns out, the
budget line with the income tax must pass through the point (x∗1 , x∗2 ). The
way to check this is to plug (x∗1 , x∗2 ) into the income-tax budget constraint
and see if it is satisfied.
88 CHOICE (Ch. 5)
x2
Indifference
curves
Optimal choice
with income tax
Original
choice
x*2
1
x*1
Budget constraint
with quantity tax
slope = – (p + t )/p
1
Figure
5.9
Budget constraint
with income tax
slope = – p /p
Optimal
choice
with
quantity
tax
2
x1
2
Income tax versus a quantity tax. Here we consider a quantity tax that raises revenue R∗ and an income tax that raises
the same revenue. The consumer will be better off under the
income tax, since he can choose a point on a higher indifference
curve.
Is it true that
p1 x∗1 + p2 x∗2 = m − tx∗1 ?
Yes it is, since this is just a rearrangement of equation (5.2), which we
know to be true.
This establishes that (x∗1 , x∗2 ) lies on the income tax budget line: it is an
affordable choice for the consumer. But is it an optimal choice? It is easy
to see that the answer is no. At (x∗1 , x∗2 ) the MRS is −(p1 + t)/p2 . But the
income tax allows us to trade at a rate of exchange of −p1 /p2 . Thus the
budget line cuts the indifference curve at (x∗1 , x∗2 ), which implies that there
will be some point on the budget line that will be preferred to (x∗1 , x∗2 ).
Therefore the income tax is definitely superior to the quantity tax in
the sense that you can raise the same amount of revenue from a consumer
and still leave him or her better off under the income tax than under the
quantity tax.
This is a nice result, and worth remembering, but it is also worthwhile
REVIEW QUESTIONS
89
understanding its limitations. First, it only applies to one consumer. The
argument shows that for any given consumer there is an income tax that
will raise as much money from that consumer as a quantity tax and leave
him or her better off. But the amount of that income tax will typically differ
from person to person. So a uniform income tax for all consumers is not
necessarily better than a uniform quantity tax for all consumers. (Think
about a case where some consumer doesn’t consume any of good 1—this
person would certainly prefer the quantity tax to a uniform income tax.)
Second, we have assumed that when we impose the tax on income the
consumer’s income doesn’t change. We have assumed that the income tax
is basically a lump sum tax—one that just changes the amount of money
a consumer has to spend but doesn’t affect any choices he has to make.
This is an unlikely assumption. If income is earned by the consumer, we
might expect that taxing it will discourage earning income, so that after-tax
income might fall by even more than the amount taken by the tax.
Third, we have totally left out the supply response to the tax. We’ve
shown how demand responds to the tax change, but supply will respond
too, and a complete analysis would take those changes into account as well.
Summary
1. The optimal choice of the consumer is that bundle in the consumer’s
budget set that lies on the highest indifference curve.
2. Typically the optimal bundle will be characterized by the condition that
the slope of the indifference curve (the MRS) will equal the slope of the
budget line.
3. If we observe several consumption choices it may be possible to estimate
a utility function that would generate that sort of choice behavior. Such a
utility function can be used to predict future choices and to estimate the
utility to consumers of new economic policies.
4. If everyone faces the same prices for the two goods, then everyone will
have the same marginal rate of substitution, and will thus be willing to
trade off the two goods in the same way.
REVIEW QUESTIONS
1. If two goods are perfect substitutes, what is the demand function for
good 2?
90 CHOICE (Ch. 5)
2. Suppose that indifference curves are described by straight lines with a
slope of −b. Given arbitrary prices and money income p1 , p2 , and m, what
will the consumer’s optimal choices look like?
3. Suppose that a consumer always consumes 2 spoons of sugar with each
cup of coffee. If the price of sugar is p1 per spoonful and the price of coffee
is p2 per cup and the consumer has m dollars to spend on coffee and sugar,
how much will he or she want to purchase?
4. Suppose that you have highly nonconvex preferences for ice cream and
olives, like those given in the text, and that you face prices p1 , p2 and have
m dollars to spend. List the choices for the optimal consumption bundles.
5. If a consumer has a utility function u(x1 , x2 ) = x1 x42 , what fraction of
her income will she spend on good 2?
6. For what kind of preferences will the consumer be just as well-off facing
a quantity tax as an income tax?
APPENDIX
It is very useful to be able to solve the preference-maximization problem and get
algebraic examples of actual demand functions. We did this in the body of the
text for easy cases like perfect substitutes and perfect complements, and in this
Appendix we’ll see how to do it in more general cases.
First, we will generally want to represent the consumer’s preferences by a utility
function, u(x1 , x2 ). We’ve seen in Chapter 4 that this is not a very restrictive
assumption; most well-behaved preferences can be described by a utility function.
The first thing to observe is that we already know how to solve the optimalchoice problem. We just have to put together the facts that we learned in the
last three chapters. We know from this chapter that an optimal choice (x1 , x2 )
must satisfy the condition
MRS(x1 , x2 ) = −
p1
,
p2
(5.3)
and we saw in the Appendix to Chapter 4 that the MRS can be expressed as the
negative of the ratio of derivatives of the utility function. Making this substitution
and cancelling the minus signs, we have
∂u(x1 , x2 )/∂x1
p1
=
.
∂u(x1 , x2 )/∂x2
p2
(5.4)
From Chapter 2 we know that the optimal choice must also satisfy the budget
constraint
(5.5)
p1 x1 + p2 x2 = m.
This gives us two equations—the MRS condition and the budget constraint—
and two unknowns, x1 and x2 . All we have to do is to solve these two equations
APPENDIX
91
to find the optimal choices of x1 and x2 as a function of the prices and income.
There are a number of ways to solve two equations in two unknowns. One way
that always works, although it might not always be the simplest, is to solve the
budget constraint for one of the choices, and then substitute that into the MRS
condition.
Rewriting the budget constraint, we have
x2 =
m
p1
− x1
p2
p2
(5.6)
and substituting this into equation (5.4) we get
∂u(x1 , m/p2 − (p1 /p2 )x1 )/∂x1
p1
=
.
∂u(x1 , m/p2 − (p1 /p2 )x1 )/∂x2
p2
This rather formidable looking expression has only one unknown variable, x1 ,
and it can typically be solved for x1 in terms of (p1 , p2 , m). Then the budget
constraint yields the solution for x2 as a function of prices and income.
We can also derive the solution to the utility maximization problem in a more
systematic way, using calculus conditions for maximization. To do this, we first
pose the utility maximization problem as a constrained maximization problem:
max u(x1 , x2 )
x1 ,x2
such that p1 x1 + p2 x2 = m.
This problem asks that we choose values of x1 and x2 that do two things:
first, they have to satisfy the constraint, and second, they give a larger value for
u(x1 , x2 ) than any other values of x1 and x2 that satisfy the constraint.
There are two useful ways to solve this kind of problem. The first way is simply
to solve the constraint for one of the variables in terms of the other and then
substitute it into the objective function.
For example, for any given value of x1 , the amount of x2 that we need to
satisfy the budget constraint is given by the linear function
x2 (x1 ) =
m
p1
− x1 .
p2
p2
(5.7)
Now substitute x2 (x1 ) for x2 in the utility function to get the unconstrained
maximization problem
max u(x1 , m/p2 − (p1 /p2 )x1 ).
x1
This is an unconstrained maximization problem in x1 alone, since we have used
the function x2 (x1 ) to ensure that the value of x2 will always satisfy the budget
constraint, whatever the value of x1 is.
We can solve this kind of problem just by differentiating with respect to x1
and setting the result equal to zero in the usual way. This procedure will give us
a first-order condition of the form
∂u(x1 , x2 (x1 ))
∂u(x1 , x2 (x1 )) dx2
+
= 0.
∂x1
∂x2
dx1
(5.8)
92 CHOICE (Ch. 5)
Here the first term is the direct effect of how increasing x1 increases utility. The
second term consists of two parts: the rate of increase of utility as x2 increases,
∂u/∂x2 , times dx2 /dx1 , the rate of increase of x2 as x1 increases in order to
continue to satisfy the budget equation. We can differentiate (5.7) to calculate
this latter derivative
p1
dx2
=− .
dx1
p2
Substituting this into (5.8) gives us
∂u(x∗1 , x∗2 )/∂x1
p1
=
,
∂u(x∗1 , x∗2 )/∂x2
p2
which just says that the marginal rate of substitution between x1 and x2 must
equal the price ratio at the optimal choice (x∗1 , x∗2 ). This is exactly the condition
we derived above: the slope of the indifference curve must equal the slope of the
budget line. Of course the optimal choice must also satisfy the budget constraint
p1 x∗1 + p2 x∗2 = m, which again gives us two equations in two unknowns.
The second way that these problems can be solved is through the use of Lagrange multipliers. This method starts by defining an auxiliary function known
as the Lagrangian:
L = u(x1 , x2 ) − λ(p1 x1 + p2 x2 − m).
The new variable λ is called a Lagrange multiplier since it is multiplied by the
constraint.3 Then Lagrange’s theorem says that an optimal choice (x∗1 , x∗2 ) must
satisfy the three first-order conditions
∂u(x∗1 , x∗2 )
∂L
=
− λp1 = 0
∂x1
∂x1
∂u(x∗1 , x∗2 )
∂L
=
− λp2 = 0
∂x2
∂x2
∂L
= p1 x∗1 + p2 x∗2 − m = 0.
∂λ
There are several interesting things about these three equations. First, note
that they are simply the derivatives of the Lagrangian with respect to x1 , x2 ,
and λ, each set equal to zero. The last derivative, with respect to λ, is just the
budget constraint. Second, we now have three equations for the three unknowns,
x1 , x2 , and λ. We have a hope of solving for x1 and x2 in terms of p1 , p2 , and
m.
Lagrange’s theorem is proved in any advanced calculus book. It is used quite
extensively in advanced economics courses, but for our purposes we only need to
know the statement of the theorem and how to use it.
In our particular case, it is worthwhile noting that if we divide the first condition by the second one, we get
∂u(x∗1 , x∗2 )/∂x1
p1
=
,
∂u(x∗1 , x∗2 )/∂x2
p2
which simply says the MRS must equal the price ratio, just as before. The budget
constraint gives us the other equation, so we are back to two equations in two
unknowns.
3
The Greek letter λ is pronounced “lamb-da.”
APPENDIX
93
EXAMPLE: Cobb-Douglas Demand Functions
In Chapter 4 we introduced the Cobb-Douglas utility function
u(x1 , x2 ) = xc1 xd2 .
Since utility functions are only defined up to a monotonic transformation, it is
convenient to take logs of this expression and work with
ln u(x1 , x2 ) = c ln x1 + d ln x2 .
Let’s find the demand functions for x1 and x2 for the Cobb-Douglas utility
function. The problem we want to solve is
max c ln x1 + d ln x2
x1 ,x2
such that p1 x1 + p2 x2 = m.
There are at least three ways to solve this problem. One way is just to write
down the MRS condition and the budget constraint. Using the expression for the
MRS derived in Chapter 4, we have
p1
cx2
=
dx1
p2
p1 x1 + p2 x2 = m.
These are two equations in two unknowns that can be solved for the optimal
choice of x1 and x2 . One way to solve them is to substitute the second into the
first to get
c(m/p2 − x1 p1 /p2 )
p1
=
.
dx1
p2
Cross multiplying gives
c(m − x1 p1 ) = dp1 x1 .
Rearranging this equation gives
cm = (c + d)p1 x1
or
x1 =
c m
.
c + d p1
This is the demand function for x1 . To find the demand function for x2 , substitute
into the budget constraint to get
m
p1 c m
−
p2
p2 c + d p1
d m
=
.
c + d p2
x2 =
94 CHOICE (Ch. 5)
The second way is to substitute the budget constraint into the maximization
problem at the beginning. If we do this, our problem becomes
max c ln x1 + d ln(m/p2 − x1 p1 /p2 ).
x1
The first-order condition for this problem is
c
p1
p2
−d
= 0.
x1
m − p1 x1 p2
A little algebra—which you should do!—gives us the solution
x1 =
c m
.
c + d p1
Substitute this back into the budget constraint x2 = m/p2 − x1 p1 /p2 to get
x2 =
d m
.
c + d p2
These are the demand functions for the two goods, which, happily, are the same
as those derived earlier by the other method.
Now for Lagrange’s method. Set up the Lagrangian
L = c ln x1 + d ln x2 − λ(p1 x1 + p2 x2 − m)
and differentiate to get the three first-order conditions
c
∂L
=
− λp1 = 0
∂x1
x1
∂L
d
=
− λp2 = 0
∂x2
x2
∂L
= p1 x1 + p2 x2 − m = 0.
∂λ
Now the trick is to solve them! The best way to proceed is to first solve for λ and
then for x1 and x2 . So we rearrange and cross multiply the first two equations
to get
c = λp1 x1
d = λp2 x2 .
These equations are just asking to be added together:
c + d = λ(p1 x1 + p2 x2 ) = λm,
which gives us
c+d
.
m
Substitute this back into the first two equations and solve for x1 and x2 to get
λ=
c m
c + d p1
d m
x2 =
,
c + d p2
x1 =
just as before.
You have studied budgets, and you have studied preferences. Now is the
time to put these two ideas together and do something with them. In this
chapter you study the commodity bundle chosen by a utility-maximizing
consumer from a given budget.
Given prices and income, you know how to graph a consumer’s budget. If you also know the consumer’s preferences, you can graph some of
his indifference curves. The consumer will choose the “best” indifference
curve that he can reach given his budget. But when you try to do this, you
have to ask yourself, “How do I find the most desirable indifference curve
that the consumer can reach?” The answer to this question is “look in the
likely places.” Where are the likely places? As your textbook tells you,
there are three kinds of likely places. These are: (i) a tangency between
an indifference curve and the budget line; (ii) a kink in an indifference
curve; (iii) a “corner” where the consumer specializes in consuming just
one good.
Here is how you find a point of tangency if we are told the consumer’s
utility function, the prices of both goods, and the consumer’s income. The
budget line and an indifference curve are tangent at a point (x1 , x2 ) if they
have the same slope at that point. Now the slope of an indifference curve
at (x1 , x2 ) is the ratio −M U1 (x1 , x2 )/M U2 (x1 , x2 ). (This slope is also
known as the marginal rate of substitution.) The slope of the budget line
is −p1 /p2 . Therefore an indifference curve is tangent to the budget line
at the point (x1 , x2 ) when M U1 (x1 , x2 )/M U2 (x1 , x2 ) = p1 /p2 . This gives
us one equation in the two unknowns, x1 and x2 . If we hope to solve
for the x’s, we need another equation. That other equation is the budget
equation p1 x1 + p2 x2 = m. With these two equations you can solve for
(x1 , x2 ).∗
A consumer has the utility function U (x1 , x2 ) = x21 x2 . The price of good
1 is p1 = 1, the price of good 2 is p2 = 3, and his income is 180. Then,
M U1 (x1 , x2 ) = 2x1 x2 and M U2 (x1 , x2 ) = x21 . Therefore his marginal rate
of substitution is −M U1 (x1 , x2 )/M U2 (x1 , x2 ) = −2x1 x2 /x21 = −2x2 /x1 .
This implies that his indifference curve will be tangent to his budget line
when −2x2 /x1 = −p1 /p2 = −1/3. Simplifying this expression, we have
6x2 = x1 . This is one of the two equations we need to solve for the two
unknowns, x1 and x2 . The other equation is the budget equation. In this
case the budget equation is x1 + 3x2 = 180. Solving these two equations
in two unknowns, we find x1 = 120 and x2 = 20. Therefore we know that
∗
Some people have trouble remembering whether the marginal rate
of substitution is −M U1 /M U2 or −M U2 /M U1 . It isn’t really crucial to
remember which way this goes as long as you remember that a tangency
happens when the marginal utilities of any two goods are in the same
proportion as their prices.
the consumer chooses the bundle (x1 , x2 ) = (120, 20).
For equilibrium at kinks or at corners, we don’t need the slope of
the indifference curves to equal the slope of the budget line. So we don’t
have the tangency equation to work with. But we still have the budget
equation. The second equation that you can use is an equation that tells
you that you are at one of the kinky points or at a corner. You will see
exactly how this works when you work a few exercises.
A consumer has the utility function U (x1 , x2 ) = min{x1 , 3x2 }. The price
of x1 is 2, the price of x2 is 1, and her income is 140. Her indifference
curves are L-shaped. The corners of the L’s all lie along the line x1 = 3x2 .
She will choose a combination at one of the corners, so this gives us one
of the two equations we need for finding the unknowns x1 and x2 . The
second equation is her budget equation, which is 2x1 + x2 = 140. Solve
these two equations to find that x1 = 60 and x2 = 20. So we know that
the consumer chooses the bundle (x1 , x2 ) = (60, 20).
When you have finished these exercises, we hope that you will be
able to do the following:
• Calculate the best bundle a consumer can afford at given prices and
income in the case of simple utility functions where the best affordable bundle happens at a point of tangency.
• Find the best affordable bundle, given prices and income for a consumer with kinked indifference curves.
• Recognize standard examples where the best bundle a consumer can
afford happens at a corner of the budget set.
• Draw a diagram illustrating each of the above types of equilibrium.
• Apply the methods you have learned to choices made with some kinds
of nonlinear budgets that arise in real-world situations.
5.1 (0) We begin again with Charlie of the apples and bananas. Recall
that Charlie’s utility function is U (xA , xB ) = xA xB . Suppose that the
price of apples is 1, the price of bananas is 2, and Charlie’s income is 40.
(a) On the graph below, use blue ink to draw Charlie’s budget line. (Use
a ruler and try to make this line accurate.) Plot a few points on the
indifference curve that gives Charlie a utility of 150 and sketch this curve
with red ink. Now plot a few points on the indifference curve that gives
Charlie a utility of 300 and sketch this curve with black ink or pencil.
Bananas
40
30
20
10
0
10
20
30
40
Apples
(b) Can Charlie afford any bundles that give him a utility of 150?
.
(c) Can Charlie afford any bundles that give him a utility of 300?
(d) On your graph, mark a point that Charlie can afford and that gives
him a higher utility than 150. Label that point A.
(e) Neither of the indifference curves that you drew is tangent to Charlie’s
budget line. Let’s try to find one that is. At any point, (xA , xB ), Charlie’s
marginal rate of substitution is a function of xA and xB . In fact, if you
calculate the ratio of marginal utilities for Charlie’s utility function, you
will find that Charlie’s marginal rate of substitution is M RS(xA , xB ) =
−xB /xA . This is the slope of his indifference curve at (xA , xB ). The
slope of Charlie’s budget line is
(give a numerical answer).
(f ) Write an equation that implies that the budget line is tangent to
an indifference curve at (xA , xB ).
There are many
solutions to this equation. Each of these solutions corresponds to a point
on a different indifference curve. Use pencil to draw a line that passes
through all of these points.
(g) The best bundle that Charlie can afford must lie somewhere on the
line you just penciled in. It must also lie on his budget line. If the point
is outside of his budget line, he can’t afford it. If the point lies inside
of his budget line, he can afford to do better by buying more of both
goods. On your graph, label this best affordable bundle with an E. This
happens where xA =
and xB =
Verify your answer by
solving the two simultaneous equations given by his budget equation and
the tangency condition.
(h) What is Charlie’s utility if he consumes the bundle (20, 10)?
(i) On the graph above, use red ink to draw his indifference curve through
(20,10). Does this indifference curve cross Charlie’s budget line, just touch
it, or never touch it?
.
5.2 (0) Clara’s utility function is U (X, Y ) = (X + 2)(Y + 1), where X
is her consumption of good X and Y is her consumption of good Y .
(a) Write an equation for Clara’s indifference curve that goes through
the point (X, Y ) = (2, 8). Y =
Clara’s indifference curve for U = 36.
On the axes below, sketch
Y
16
12
8
4
0
4
8
12
16
X
(b) Suppose that the price of each good is 1 and that Clara has an income
of 11. Draw in her budget line. Can Clara achieve a utility of 36 with
this budget?
.
(c) At the commodity bundle, (X, Y ), Clara’s marginal rate of substitution is
.
(d) If we set the absolute value of the MRS equal to the price ratio, we
have the equation
.
(e) The budget equation is
.
(f ) Solving these two equations for the two unknowns, X and Y , we find
X=
and Y =
.
5.3 (0) Ambrose, the nut and berry consumer, has a utility function
√
U (x1 , x2 ) = 4 x1 + x2 , where x1 is his consumption of nuts and x2 is his
consumption of berries.
(a) The commodity bundle (25, 0) gives Ambrose a utility of 20. Other
points that give him the same utility are (16, 4), (9,
), (4,
), (1,
), and (0,
). Plot these points on the
axes below and draw a red indifference curve through them.
(b) Suppose that the price of a unit of nuts is 1, the price of a unit of
berries is 2, and Ambrose’s income is 24. Draw Ambrose’s budget line
with blue ink. How many units of nuts does he choose to buy?
(c) How many units of berries?
.
(d) Find some points on the indifference curve that gives him a utility of
25 and sketch this indifference curve (in red).
(e) Now suppose that the prices are as before, but Ambrose’s income is
34. Draw his new budget line (with pencil). How many units of nuts will
he choose?
How many units of berries?
.
Berries
20
15
10
5
0
5
10
15
20
25
30
Nuts
(f ) Now let us explore a case where there is a “boundary solution.” Suppose that the price of nuts is still 1 and the price of berries is 2, but
Ambrose’s income is only 9. Draw his budget line (in blue). Sketch the
indifference curve that passes through the point (9, 0). What is the slope
of his indifference curve at the point (9, 0)?
.
(g) What is the slope of his budget line at this point?
.
(h) Which is steeper at this point, the budget line or the indifference
curve?
.
(i) Can Ambrose afford any bundles that he likes better than the point
(9, 0)?
.
5.4 (1) Nancy Lerner is trying to decide how to allocate her time in
studying for her economics course. There are two examinations in this
course. Her overall score for the course will be the minimum of her scores
on the two examinations. She has decided to devote a total of 1,200
minutes to studying for these two exams, and she wants to get as high an
overall score as possible. She knows that on the first examination if she
doesn’t study at all, she will get a score of zero on it. For every 10 minutes
that she spends studying for the first examination, she will increase her
score by one point. If she doesn’t study at all for the second examination
she will get a zero on it. For every 20 minutes she spends studying for
the second examination, she will increase her score by one point.
(a) On the graph below, draw a “budget line” showing the various combinations of scores on the two exams that she can achieve with a total of
1,200 minutes of studying. On the same graph, draw two or three “indifference curves” for Nancy. On your graph, draw a straight line that goes
through the kinks in Nancy’s indifference curves. Label the point where
this line hits Nancy’s budget with the letter A. Draw Nancy’s indifference
curve through this point.
Score on Test 2
80
60
40
20
0
20
40
60
80
100
120
Score on Test 1
(b) Write an equation for the line passing through the kinks of Nancy’s
indifference curves.
.
(c) Write an equation for Nancy’s budget line.
.
(d) Solve these two equations in two unknowns to determine the intersection of these lines. This happens at the point (x1 , x2 ) =
.
(e) Given that she spends a total of 1,200 minutes studying, Nancy will
maximize her overall score by spending
first examination and
tion.
minutes studying for the
minutes studying for the second examina-
5.5 (1) In her communications course, Nancy also takes two examinations. Her overall grade for the course will be the maximum of her scores
on the two examinations. Nancy decides to spend a total of 400 minutes
studying for these two examinations. If she spends m1 minutes studying
for the first examination, her score on this exam will be x1 = m1 /5. If
she spends m2 minutes studying for the second examination, her score on
this exam will be x2 = m2 /10.
(a) On the graph below, draw a “budget line” showing the various combinations of scores on the two exams that she can achieve with a total of 400
minutes of studying. On the same graph, draw two or three “indifference
curves” for Nancy. On your graph, find the point on Nancy’s budget line
that gives her the best overall score in the course.
(b) Given that she spends a total of 400 minutes studying, Nancy will
maximize her overall score by achieving a score of
examination and
on the first
on the second examination.
(c) Her overall score for the course will then be
.
Score on Test 2
80
60
40
20
0
20
40
60
80
Score on Test 1
5.6 (0) Elmer’s utility function is U (x, y) = min{x, y 2 }.
(a) If Elmer consumes 4 units of x and 3 units of y, his utility is
.
(b) If Elmer consumes 4 units of x and 2 units of y, his utility is
.
(c) If Elmer consumes 5 units of x and 2 units of y, his utility is
.
(d) On the graph below, use blue ink to draw the indifference curve for
Elmer that contains the bundles that he likes exactly as well as the bundle
(4, 2).
(e) On the same graph, use blue ink to draw the indifference curve for
Elmer that contains bundles that he likes exactly as well as the bundle
(1, 1) and the indifference curve that passes through the point (16, 5).
(f ) On your graph, use black ink to show the locus of points at which
Elmer’s indifference curves have kinks. What is the equation for this
curve?
.
(g) On the same graph, use black ink to draw Elmer’s budget line when
the price of x is 1, the price of y is 2, and his income is 8. What bundle
does Elmer choose in this situation?
.
y
16
12
8
4
0
4
8
12
16
20
24
x
(h) Suppose that the price of x is 10 and the price of y is 15 and Elmer
buys 100 units of x. What is Elmer’s income?
(Hint: At first
you might think there is too little information to answer this question.
But think about how much y he must be demanding if he chooses 100
units of x.)
5.7 (0) Linus has the utility function U (x, y) = x + 3y.
(a) On the graph below, use blue ink to draw the indifference curve passing
through the point (x, y) = (3, 3). Use black ink to sketch the indifference
curve connecting bundles that give Linus a utility of 6.
y
16
12
8
4
0
4
8
12
16
x
(b) On the same graph, use red ink to draw Linus’s budget line if the
price of x is 1 and the price of y is 2 and his income is 8. What bundle
does Linus choose in this situation?
.
(c) What bundle would Linus choose if the price of x is 1, the price of y
is 4, and his income is 8?
.
5.8 (2) Remember our friend Ralph Rigid from Chapter 3? His favorite
diner, Food for Thought, has adopted the following policy to reduce the
crowds at lunch time: if you show up for lunch t hours before or after
12 noon, you get to deduct t dollars from your bill. (This holds for any
fraction of an hour as well.)
Money
20
15
10
5
10
11
12
1
2
Time
(a) Use blue ink to show Ralph’s budget set. On this graph, the horizontal
axis measures the time of day that he eats lunch, and the vertical axis
measures the amount of money that he will have to spend on things other
than lunch. Assume that he has $20 total to spend and that lunch at
noon costs $10. (Hint: How much money would he have left if he ate at
noon? at 1 P.M.? at 11 A.M.?)
(b) Recall that Ralph’s preferred lunch time is 12 noon, but that he is
willing to eat at another time if the food is sufficiently cheap. Draw
some red indifference curves for Ralph that would be consistent with his
choosing to eat at 11 A.M.
5.9 (0) Joe Grad has just arrived at the big U. He has a fellowship that
covers his tuition and the rent on an apartment. In order to get by, Joe
has become a grader in intermediate price theory, earning $100 a month.
Out of this $100 he must pay for his food and utilities in his apartment.
His utilities expenses consist of heating costs when he heats his apartment
and air-conditioning costs when he cools it. To raise the temperature of
his apartment by one degree, it costs $2 per month (or $20 per month
to raise it ten degrees). To use air-conditioning to cool his apartment by
a degree, it costs $3 per month. Whatever is left over after paying the
utilities, he uses to buy food at $1 per unit.
Food
120
100
80
60
40
20
0
10
20
30
40
50
60
70
80
90
100
Temperature
(a) When Joe first arrives in September, the temperature of his apartment
is 60 degrees. If he spends nothing on heating or cooling, the temperature
in his room will be 60 degrees and he will have $100 left to spend on food.
If he heated the room to 70 degrees, he would have
left to spend
on food. If he cooled the room to 50 degrees, he would have
left
to spend on food. On the graph below, show Joe’s September budget
constraint (with black ink). (Hint: You have just found three points that
Joe can afford. Apparently, his budget set is not bounded by a single
straight line.)
(b) In December, the outside temperature is 30 degrees and in August
poor Joe is trying to understand macroeconomics while the temperature
outside is 85 degrees. On the same graph you used above, draw Joe’s
budget constraints for the months of December (in blue ink) and August
(in red ink).
(c) Draw a few smooth (unkinky) indifference curves for Joe in such a way
that the following are true. (i) His favorite temperature for his apartment
would be 65 degrees if it cost him nothing to heat it or cool it. (ii) Joe
chooses to use the furnace in December, air-conditioning in August, and
neither in September. (iii) Joe is better off in December than in August.
(d) In what months is the slope of Joe’s budget constraint equal to the
slope of his indifference curve?
.
(e) In December Joe’s marginal rate of substitution between food and
degrees Fahrenheit is
In August, his MRS is
.
(f ) Since Joe neither heats nor cools his apartment in September, we
cannot determine his marginal rate of substitution exactly, but we do
know that it must be no smaller than
and no larger than
(Hint: Look carefully at your graph.)
5.10 (0) Central High School has $60,000 to spend on computers and
other stuff, so its budget equation is C + X = 60, 000, where C is expenditure on computers and X is expenditures on other things. C.H.S.
currently plans to spend $20,000 on computers.
The State Education Commission wants to encourage “computer literacy” in the high schools under its jurisdiction. The following plans have
been proposed.
Plan A: This plan would give a grant of $10,000 to each high school in
the state that the school could spend as it wished.
Plan B: This plan would give a $10,000 grant to any high school, so
long as the school spent at least $10,000 more than it currently spends on
computers. Any high school can choose not to participate, in which case it
does not receive the grant, but it doesn’t have to increase its expenditure
on computers.
Plan C: Plan C is a “matching grant.” For every dollar’s worth of
computers that a high school orders, the state will give the school 50
cents.
Plan D: This plan is like plan C, except that the maximum amount of
matching funds that any high school could get from the state would be
limited to $10,000.
(a) Write an equation for Central High School’s budget if plan A is
adopted.
Use black ink to draw the budget line for
Central High School if plan A is adopted.
(b) If plan B is adopted, the boundary of Central High School’s budget set
has two separate downward-sloping line segments. One of these segments
describes the cases where C.H.S. spends at least $30,000 on computers.
This line segment runs from the point (C, X) = (70, 000, 0) to the point
(C, X) =
.
(c) Another line segment corresponds to the cases where C.H.S. spends
less than $30,000 on computers. This line segment runs from (C, X) =
to the point (C, X) = (0, 60, 000). Use red ink to draw
these two line segments.
(d) If plan C is adopted and Central High School spends C dollars on
computers, then it will have X = 60, 000 − .5C dollars left to spend on
other things. Therefore its budget line has the equation
Use blue ink to draw this budget line.
(e) If plan D is adopted, the school district’s budget consists of two line
segments that intersect at the point where expenditure on computers is
and expenditure on other instructional materials is
.
(f ) The slope of the flatter line segment is
steeper segment is
The slope of the
Use pencil to draw this budget line.
Thousands of dollars worth of other things
60
50
40
30
20
10
0
10
20
30
40
50
60
Thousands of dollars worth of computers
5.11 (0) Suppose that Central High School has preferences that can
be represented by the utility function U (C, X) = CX 2 . Let us try to
determine how the various plans described in the last problem will affect
the amount that C.H.S. spends on computers.
(a) If the state adopts none of the new plans, find the expenditure on
computers that maximizes the district’s utility subject to its budget constraint.
.
(b) If plan A is adopted, find the expenditure on computers that maximizes the district’s utility subject to its budget constraint.
.
(c) On your graph, sketch the indifference curve that passes through the
point (30,000, 40,000) if plan B is adopted. At this point, which is steeper,
the indifference curve or the budget line?
.
(d) If plan B is adopted, find the expenditure on computers that maximizes the district’s utility subject to its budget constraint. (Hint: Look
at your graph.)
.
(e) If plan C is adopted, find the expenditure on computers that maximizes the district’s utility subject to its budget constraint.
.
(f ) If plan D is adopted, find the expenditure on computers that maximizes the district’s utility subject to its budget constraint.
.
5.12 (0) The telephone company allows one to choose between two
different pricing plans. For a fee of $12 per month you can make as
many local phone calls as you want, at no additional charge per call.
Alternatively, you can pay $8 per month and be charged 5 cents for each
local phone call that you make. Suppose that you have a total of $20 per
month to spend.
(a) On the graph below, use black ink to sketch a budget line for someone
who chooses the first plan. Use red ink to draw a budget line for someone
who chooses the second plan. Where do the two budget lines cross?
.
Other goods
16
12
8
4
0
20
40
60
80
100
120
Local phone calls
(b) On the graph above, use pencil to draw indifference curves for someone who prefers the second plan to the first. Use blue ink to draw an
indifference curve for someone who prefers the first plan to the second.
5.13 (1) This is a puzzle—just for fun. Lewis Carroll (1832-1898),
author of Alice in Wonderland and Through the Looking Glass, was a
mathematician, logician, and political scientist. Carroll loved careful reasoning about puzzling things. Here Carroll’s Alice presents a nice bit
of economic analysis. At first glance, it may seem that Alice is talking
nonsense, but, indeed, her reasoning is impeccable.
“I should like to buy an egg, please.” she said timidly. “How do you
sell them?”
“Fivepence farthing for one—twopence for two,” the Sheep replied.
“Then two are cheaper than one?” Alice said, taking out her purse.
“Only you must eat them both if you buy two,” said the Sheep.
“Then I’ll have one please,” said Alice, as she put the money down
on the counter. For she thought to herself, “They mightn’t be at all nice,
you know.”
(a) Let us try to draw a budget set and indifference curves that are
consistent with this story. Suppose that Alice has a total of 8 pence to
spend and that she can buy either 0, 1, or 2 eggs from the Sheep, but no
fractional eggs. Then her budget set consists of just three points. The
point where she buys no eggs is (0, 8). Plot this point and label it A. On
your graph, the point where she buys 1 egg is (1, 2 43 ). (A farthing is 1/4
of a penny.) Plot this point and label it B.
(b) The point where she buys 2 eggs is
Plot this point and
label it C. If Alice chooses to buy 1 egg, she must like the bundle B better
than either the bundle A or the bundle C. Draw indifference curves for
Alice that are consistent with this behavior.
Other goods
8
6
4
2
0
1
2
3
4
Eggs
5.1 In Problem 5.1, Charlie has a utility function U (xA , xB ) = xA xB ,
the price of apples is 1 and the price of bananas is 2. If Charlie’s income
were 240, how many units of bananas would he consume if he chose the
bundle that maximized his utility subject to his budget constraint?
(a) 60
(b) 30
(c) 120
(d) 12
(e) 180
5.2 In Problem 5.1, if Charlie’s income is 40, the price of apples is 5,
and the price of bananas is 6, how many apples are contained in the best
bundle that Charlie can afford?
(a) 8
(b) 15
(c) 10
(d) 11
(e) 4
5.3 In Problem 5.2, Clara’s utility function is U (X, Y ) = (X + 2)(Y + 1).
If Clara’s marginal rate of substitution is −2 and she is consuming 10
units of good X, how many units of good Y is she consuming?
(a) 2
(b) 24
(c) 12
(d) 23
(e) 5
1/2
5.4 In Problem 5.3, Ambrose’s utility function is U (x1 , x2 ) = 4x1 + x2 .
If the price of nuts is 1, the price of berries is 4, and his income is 72, how
many units of nuts will Ambrose choose?
(a) 2
(b) 64
(c) 128
(d) 67
(e) 32
1/2
5.5 Ambrose’s utility function is 4x1 + x2 . If the price of nuts is 1, the
price of berries is 4, and his income is 100, how many units of berries will
Ambrose choose?
(a) 65
(b) 9
(c) 18
(d) 8
(e) 12
5.6 In Problem 5.6, Elmer’s utility function is U (x, y) = min{x, y 2 }. If
the price of x is 15, the price of y is 10, and Elmer chooses to consume 7
units of y, what must Elmer’s income be?
(a) 1,610
(b) 175
(c) 905
(d) 805
(e) There is not enough information to tell.
CHAPTER
6
DEMAND
In the last chapter we presented the basic model of consumer choice: how
maximizing utility subject to a budget constraint yields optimal choices.
We saw that the optimal choices of the consumer depend on the consumer’s
income and the prices of the goods, and we worked a few examples to see
what the optimal choices are for some simple kinds of preferences.
The consumer’s demand functions give the optimal amounts of each
of the goods as a function of the prices and income faced by the consumer.
We write the demand functions as
x1 = x1 (p1 , p2 , m)
x2 = x2 (p1 , p2 , m).
The left-hand side of each equation stands for the quantity demanded. The
right-hand side of each equation is the function that relates the prices and
income to that quantity.
In this chapter we will examine how the demand for a good changes as
prices and income change. Studying how a choice responds to changes in the
economic environment is known as comparative statics, which we first
described in Chapter 1. “Comparative” means that we want to compare
96 DEMAND (Ch. 6)
two situations: before and after the change in the economic environment.
“Statics” means that we are not concerned with any adjustment process
that may be involved in moving from one choice to another; rather we will
only examine the equilibrium choice.
In the case of the consumer, there are only two things in our model
that affect the optimal choice: prices and income. The comparative statics
questions in consumer theory therefore involve investigating how demand
changes when prices and income change.
6.1 Normal and Inferior Goods
We start by considering how a consumer’s demand for a good changes
as his income changes. We want to know how the optimal choice at one
income compares to the optimal choice at another level of income. During
this exercise, we will hold the prices fixed and examine only the change in
demand due to the income change.
We know how an increase in money income affects the budget line when
prices are fixed—it shifts it outward in a parallel fashion. So how does this
affect demand?
We would normally think that the demand for each good would increase
when income increases, as shown in Figure 6.1. Economists, with a singular
lack of imagination, call such goods normal goods. If good 1 is a normal
good, then the demand for it increases when income increases, and decreases when income decreases. For a normal good the quantity demanded
always changes in the same way as income changes:
Δx1
> 0.
Δm
If something is called normal, you can be sure that there must be a
possibility of being abnormal. And indeed there is. Figure 6.2 presents
an example of nice, well-behaved indifference curves where an increase of
income results in a reduction in the consumption of one of the goods. Such
a good is called an inferior good. This may be “abnormal,” but when
you think about it, inferior goods aren’t all that unusual. There are many
goods for which demand decreases as income increases; examples might
include gruel, bologna, shacks, or nearly any kind of low-quality good.
Whether a good is inferior or not depends on the income level that we
are examining. It might very well be that very poor people consume more
bologna as their income increases. But after a point, the consumption of
bologna would probably decline as income continued to increase. Since in
real life the consumption of goods can increase or decrease when income
increases, it is comforting to know that economic theory allows for both
possibilities.
INCOME OFFER CURVES AND ENGEL CURVES
97
x2
Indifference
curves
Optimal choices
Budget lines
x1
Normal goods. The demand for both goods increases when
income increases, so both goods are normal goods.
6.2 Income Offer Curves and Engel Curves
We have seen that an increase in income corresponds to shifting the budget
line outward in a parallel manner. We can connect together the demanded
bundles that we get as we shift the budget line outward to construct the
income offer curve. This curve illustrates the bundles of goods that are
demanded at the different levels of income, as depicted in Figure 6.3A.
The income offer curve is also known as the income expansion path. If
both goods are normal goods, then the income expansion path will have a
positive slope, as depicted in Figure 6.3A.
For each level of income, m, there will be some optimal choice for each
of the goods. Let us focus on good 1 and consider the optimal choice at
each set of prices and income, x1 (p1 , p2 , m). This is simply the demand
function for good 1. If we hold the prices of goods 1 and 2 fixed and look
at how demand changes as we change income, we generate a curve known
as the Engel curve. The Engel curve is a graph of the demand for one of
the goods as a function of income, with all prices being held constant. For
an example of an Engel curve, see Figure 6.3B.
Figure
6.1
98 DEMAND (Ch. 6)
x2
Indifference
curves
Budget
lines
Optimal
choices
x1
Figure
6.2
An inferior good. Good 1 is an inferior good, which means
that the demand for it decreases when income increases.
x2
m
Income
offer
curve
Engel
curve
Indifference
curves
x1
A Income offer curve
Figure
6.3
x1
B Engel curve
How demand changes as income changes. The income offer curve (or income expansion path) shown in panel A depicts
the optimal choice at different levels of income and constant
prices. When we plot the optimal choice of good 1 against income, m, we get the Engel curve, depicted in panel B.
SOME EXAMPLES
99
6.3 Some Examples
Let’s consider some of the preferences that we examined in Chapter 5 and
see what their income offer curves and Engel curves look like.
Perfect Substitutes
The case of perfect substitutes is depicted in Figure 6.4. If p1 < p2 , so
that the consumer is specializing in consuming good 1, then if his income
increases he will increase his consumption of good 1. Thus the income offer
curve is the horizontal axis, as shown in Figure 6.4A.
m
x2
Indifference
curves
Engel
curve
Income
offer
curve
Typical
budget
line
Slope = p1
x1
A Income offer curve
x1
B Engel curve
Perfect substitutes. The income offer curve (A) and an Engel
curve (B) in the case of perfect substitutes.
Since the demand for good 1 is x1 = m/p1 in this case, the Engel curve
will be a straight line with a slope of p1 , as depicted in Figure 6.4B. (Since
m is on the vertical axis, and x1 on the horizontal axis, we can write
m = p1 x1 , which makes it clear that the slope is p1 .)
Perfect Complements
The demand behavior for perfect complements is shown in Figure 6.5. Since
the consumer will always consume the same amount of each good, no matter
Figure
6.4
100 DEMAND (Ch. 6)
what, the income offer curve is the diagonal line through the origin as
depicted in Figure 6.5A. We have seen that the demand for good 1 is
x1 = m/(p1 + p2 ), so the Engel curve is a straight line with a slope of
p1 + p2 as shown in Figure 6.5B.
x2
m
Indifference
curves
Income
offer
curve
Engel
curve
Slope = p1 + p2
Budget
lines
x1
A Income offer curve
Figure
6.5
x1
B Engel curve
Perfect complements. The income offer curve (A) and an
Engel curve (B) in the case of perfect complements.
Cobb-Douglas Preferences
For the case of Cobb-Douglas preferences it is easier to look at the algebraic
form of the demand functions to see what the graphs will look like. If
, the Cobb-Douglas demand for good 1 has the form
u(x1 , x2 ) = xa1 x1−a
2
x1 = am/p1 . For a fixed value of p1 , this is a linear function of m. Thus
doubling m will double demand, tripling m will triple demand, and so on.
In fact, multiplying m by any positive number t will just multiply demand
by the same amount.
The demand for good 2 is x2 = (1−a)m/p2 , and this is also clearly linear.
The fact that the demand functions for both goods are linear functions
of income means that the income expansion paths will be straight lines
through the origin, as depicted in Figure 6.6A. The Engel curve for good 1
will be a straight line with a slope of p1 /a, as depicted in Figure 6.6B.
SOME EXAMPLES
x2
101
m
Income
offer
curve
Engel
curve
Indifference
curves
Slope = p1 /a
Budget
lines
x1
A Income offer curve
x1
B Engel curve
Cobb-Douglas. An income offer curve (A) and an Engel curve
(B) for Cobb-Douglas utility.
Homothetic Preferences
All of the income offer curves and Engel curves that we have seen up to now
have been straightforward—in fact they’ve been straight lines! This has
happened because our examples have been so simple. Real Engel curves do
not have to be straight lines. In general, when income goes up, the demand
for a good could increase more or less rapidly than income increases. If the
demand for a good goes up by a greater proportion than income, we say
that it is a luxury good, and if it goes up by a lesser proportion than
income we say that it is a necessary good.
The dividing line is the case where the demand for a good goes up by
the same proportion as income. This is what happened in the three cases
we examined above. What aspect of the consumer’s preferences leads to
this behavior?
Suppose that the consumer’s preferences only depend on the ratio of
good 1 to good 2. This means that if the consumer prefers (x1 , x2 ) to
(y1 , y2 ), then she automatically prefers (2x1 , 2x2 ) to (2y1 , 2y2 ), (3x1 , 3x2 )
to (3y1 , 3y2 ), and so on, since the ratio of good 1 to good 2 is the same for
all of these bundles. In fact, the consumer prefers (tx1 , tx2 ) to (ty1 , ty2 ) for
any positive value of t. Preferences that have this property are known as
homothetic preferences. It is not hard to show that the three examples
of preferences given above—perfect substitutes, perfect complements, and
Cobb-Douglas—are all homothetic preferences.
Figure
6.6
102 DEMAND (Ch. 6)
If the consumer has homothetic preferences, then the income offer curves
are all straight lines through the origin, as shown in Figure 6.7. More
specifically, if preferences are homothetic, it means that when income is
scaled up or down by any amount t > 0, the demanded bundle scales up
or down by the same amount. This can be established rigorously, but it is
fairly clear from looking at the picture. If the indifference curve is tangent
to the budget line at (x∗1 , x∗2 ), then the indifference curve through (tx∗1 , tx∗2 )
is tangent to the budget line that has t times as much income and the same
prices. This implies that the Engel curves are straight lines as well. If you
double income, you just double the demand for each good.
x2
m
Indifference
curves
Engel
curve
Budget
lines
Income
offer curve
x1
A Income offer curve
Figure
6.7
x1
B Engel curve
Homothetic preferences. An income offer curve (A) and an
Engel curve (B) in the case of homothetic preferences.
Homothetic preferences are very convenient since the income effects are
so simple. Unfortunately, homothetic preferences aren’t very realistic for
the same reason! But they will often be of use in our examples.
Quasilinear Preferences
Another kind of preferences that generates a special form of income offer
curves and Engel curves is the case of quasilinear preferences. Recall the
definition of quasilinear preferences given in Chapter 4. This is the case
where all indifference curves are “shifted” versions of one indifference curve
SOME EXAMPLES
103
as in Figure 6.8. Equivalently, the utility function for these preferences
takes the form u(x1 , x2 ) = v(x1 ) + x2 . What happens if we shift the budget
line outward? In this case, if an indifference curve is tangent to the budget
line at a bundle (x∗1 , x∗2 ), then another indifference curve must also be
tangent at (x∗1 , x∗2 +k) for any constant k. Increasing income doesn’t change
the demand for good 1 at all, and all the extra income goes entirely to the
consumption of good 2. If preferences are quasilinear, we sometimes say
that there is a “zero income effect” for good 1. Thus the Engel curve for
good 1 is a vertical line—as you change income, the demand for good 1
remains constant.
x2
m
Income
offer
curve
Engel
curve
Indifference
curves
Budget
lines
x1
A Income offer curve
x1
B Engel curve
Quasilinear preferences. An income offer curve (A) and an
Engel curve (B) with quasilinear preferences.
What would be a real-life situation where this kind of thing might occur?
Suppose good 1 is pencils and good 2 is money to spend on other goods.
Initially I may spend my income only on pencils, but when my income
gets large enough, I stop buying additional pencils—all of my extra income
is spent on other goods. Other examples of this sort might be salt or
toothpaste. When we are examining a choice between all other goods and
some single good that isn’t a very large part of the consumer’s budget, the
quasilinear assumption may well be plausible, at least when the consumer’s
income is sufficiently large.
Figure
6.8
104 DEMAND (Ch. 6)
6.4 Ordinary Goods and Giffen Goods
Let us now consider price changes. Suppose that we decrease the price of
good 1 and hold the price of good 2 and money income fixed. Then what
can happen to the quantity demanded of good 1? Intuition tells us that
the quantity demanded of good 1 should increase when its price decreases.
Indeed this is the ordinary case, as depicted in Figure 6.9.
x2
Indifference
curves
Optimal
choices
Budget
lines
Price
decrease
x1
Figure
6.9
An ordinary good. Ordinarily, the demand for a good increases when its price decreases, as is the case here.
When the price of good 1 decreases, the budget line becomes flatter. Or
said another way, the vertical intercept is fixed and the horizontal intercept
moves to the right. In Figure 6.9, the optimal choice of good 1 moves to
the right as well: the quantity demanded of good 1 has increased. But we
might wonder whether this always happens this way. Is it always the case
that, no matter what kind of preferences the consumer has, the demand
for a good must increase when its price goes down?
As it turns out, the answer is no. It is logically possible to find wellbehaved preferences for which a decrease in the price of good 1 leads to a
reduction in the demand for good 1. Such a good is called a Giffen good,
ORDINARY GOODS AND GIFFEN GOODS
105
x2
Indifference
curves
Optimal
choices
Budget
lines
Price
decrease
Reduction
in demand
for good 1
x1
A Giffen good. Good 1 is a Giffen good, since the demand
for it decreases when its price decreases.
after the nineteenth-century economist who first noted the possibility. An
example is illustrated in Figure 6.10.
What is going on here in economic terms? What kind of preferences
might give rise to the peculiar behavior depicted in Figure 6.10? Suppose
that the two goods that you are consuming are gruel and milk and that
you are currently consuming 7 bowls of gruel and 7 cups of milk a week.
Now the price of gruel declines. If you consume the same 7 bowls of gruel
a week, you will have money left over with which you can purchase more
milk. In fact, with the extra money you have saved because of the lower
price of gruel, you may decide to consume even more milk and reduce your
consumption of gruel. The reduction in the price of gruel has freed up some
extra money to be spent on other things—but one thing you might want to
do with it is reduce your consumption of gruel! Thus the price change is to
some extent like an income change. Even though money income remains
constant, a change in the price of a good will change purchasing power,
and thereby change demand.
So the Giffen good is not implausible purely on logical grounds, although
Giffen goods are unlikely to be encountered in real-world behavior. Most
goods are ordinary goods—when their price increases, the demand for them
declines. We’ll see why this is the ordinary situation a little later.
Figure
6.10
106 DEMAND (Ch. 6)
Incidentally, it is no accident that we used gruel as an example of both
an inferior good and a Giffen good. It turns out that there is an intimate
relationship between the two which we will explore in a later chapter.
But for now our exploration of consumer theory may leave you with
the impression that nearly anything can happen: if income increases the
demand for a good can go up or down, and if price increases the demand can
go up or down. Is consumer theory compatible with any kind of behavior?
Or are there some kinds of behavior that the economic model of consumer
behavior rules out? It turns out that there are restrictions on behavior
imposed by the maximizing model. But we’ll have to wait until the next
chapter to see what they are.
6.5 The Price Offer Curve and the Demand Curve
Suppose that we let the price of good 1 change while we hold p2 and income
fixed. Geometrically this involves pivoting the budget line. We can think of
connecting together the optimal points to construct the price offer curve
as illustrated in Figure 6.11A. This curve represents the bundles that would
be demanded at different prices for good 1.
x2
p1
Indifference
curves
50
Price
offer
curve
40
Demand
curve
30
20
10
x1
A Price offer curve
Figure
6.11
2
4
6
8
10
B Demand curve
The price offer curve and demand curve. Panel A contains
a price offer curve, which depicts the optimal choices as the price
of good 1 changes. Panel B contains the associated demand
curve, which depicts a plot of the optimal choice of good 1 as a
function of its price.
12
x1
SOME EXAMPLES
107
We can depict this same information in a different way. Again, hold
the price of good 2 and money income fixed, and for each different value
of p1 plot the optimal level of consumption of good 1. The result is the
demand curve depicted in Figure 6.11B. The demand curve is a plot
of the demand function, x1 (p1 , p2 , m), holding p2 and m fixed at some
predetermined values.
Ordinarily, when the price of a good increases, the demand for that
good will decrease. Thus the price and quantity of a good will move in
opposite directions, which means that the demand curve will typically have
a negative slope. In terms of rates of change, we would normally have
Δx1
< 0,
Δp1
which simply says that demand curves usually have a negative slope.
However, we have also seen that in the case of Giffen goods, the demand
for a good may decrease when its price decreases. Thus it is possible, but
not likely, to have a demand curve with a positive slope.
6.6 Some Examples
Let’s look at a few examples of demand curves, using the preferences that
we discussed in Chapter 3.
Perfect Substitutes
The offer curve and demand curve for perfect substitutes—the red and blue
pencils example—are illustrated in Figure 6.12. As we saw in Chapter 5,
the demand for good 1 is zero when p1 > p2 , any amount on the budget
line when p1 = p2 , and m/p1 when p1 < p2 . The offer curve traces out
these possibilities.
In order to find the demand curve, we fix the price of good 2 at some
price p∗2 and graph the demand for good 1 versus the price of good 1 to get
the shape depicted in Figure 6.12B.
Perfect Complements
The case of perfect complements—the right and left shoes example—is
depicted in Figure 6.13. We know that whatever the prices are, a consumer
will demand the same amount of goods 1 and 2. Thus his offer curve will
be a diagonal line as depicted in Figure 6.13A.
We saw in Chapter 5 that the demand for good 1 is given by
m
.
x1 =
p1 + p2
If we fix m and p2 and plot the relationship between x1 and p1 , we get the
curve depicted in Figure 6.13B.
108 DEMAND (Ch. 6)
p1
x2
Indifference
curves
Demand
curve
Price
offer
curve
p1 = p*
2
x1
m/p1 = m/p*
2
A Price offer curve
Figure
6.12
x1
B Demand curve
Perfect substitutes. Price offer curve (A) and demand curve
(B) in the case of perfect substitutes.
x2
Indifference
curves
p1
Price
offer
curve
Demand
curve
Budget
lines
x1
A Price offer curve
Figure
6.13
x1
B Demand curve
Perfect complements. Price offer curve (A) and demand
curve (B) in the case of perfect complements.
A Discrete Good
Suppose that good 1 is a discrete good. If p1 is very high then the consumer
will strictly prefer to consume zero units; if p1 is low enough the consumer
will strictly prefer to consume one unit. At some price r1 , the consumer will
be indifferent between consuming good 1 or not consuming it. The price
SOME EXAMPLES
109
at which the consumer is just indifferent to consuming or not consuming
the good is called the reservation price.1 The indifference curves and
demand curve are depicted in Figure 6.14.
GOOD
2
PRICE
1
Slope = –r1
Optimal
bundles
at r2
Slope = –r2
Optimal
bundles
at r1
r1
r2
1
2
3
GOOD
1
A Optimal bundles at different prices
1
2
GOOD
1
B Demand curve
A discrete good. As the price of good 1 decreases there will
be some price, the reservation price, at which the consumer is
just indifferent between consuming good 1 or not consuming it.
As the price decreases further, more units of the discrete good
will be demanded.
Figure
6.14
It is clear from the diagram that the demand behavior can be described
by a sequence of reservation prices at which the consumer is just willing
to purchase another unit of the good. At a price of r1 the consumer is
willing to buy 1 unit of the good; if the price falls to r2 , he is willing to
buy another unit, and so on.
These prices can be described in terms of the original utility function.
For example, r1 is the price where the consumer is just indifferent between
consuming 0 or 1 unit of good 1, so it must satisfy the equation
u(0, m) = u(1, m − r1 ).
(6.1)
Similarly r2 satisfies the equation
u(1, m − r2 ) = u(2, m − 2r2 ).
1
(6.2)
The term reservation price comes from auction markets. When someone wanted to
sell something in an auction he would typically state a minimum price at which he
was willing to sell the good. If the best price offered was below this stated price, the
seller reserved the right to purchase the item himself. This price became known as
the seller’s reservation price and eventually came to be used to describe the price at
which someone was just willing to buy or sell some item.
110 DEMAND (Ch. 6)
The left-hand side of this equation is the utility from consuming one unit of
the good at a price of r2 . The right-hand side is the utility from consuming
two units of the good, each of which sells for r2 .
If the utility function is quasilinear, then the formulas describing the
reservation prices become somewhat simpler. If u(x1 , x2 ) = v(x1 ) + x2 ,
and v(0) = 0, then we can write equation (6.1) as
v(0) + m = m = v(1) + m − r1 .
Since v(0) = 0, we can solve for r1 to find
r1 = v(1).
(6.3)
Similarly, we can write equation (6.2) as
v(1) + m − r2 = v(2) + m − 2r2 .
Canceling terms and rearranging, this expression becomes
r2 = v(2) − v(1).
Proceeding in this manner, the reservation price for the third unit of consumption is given by
r3 = v(3) − v(2)
and so on.
In each case, the reservation price measures the increment in utility necessary to induce the consumer to choose an additional unit of the good.
Loosely speaking, the reservation prices measure the marginal utilities associated with different levels of consumption of good 1. Our assumption
of convex preferences implies that the sequence of reservation prices must
decrease: r1 > r2 > r3 · · ·.
Because of the special structure of the quasilinear utility function, the
reservation prices do not depend on the amount of good 2 that the consumer
has. This is certainly a special case, but it makes it very easy to describe
demand behavior. Given any price p, we just find where it falls in the list
of reservation prices. Suppose that p falls between r6 and r7 , for example.
The fact that r6 > p means that the consumer is willing to give up p dollars
per unit bought to get 6 units of good 1, and the fact that p > r7 means
that the consumer is not willing to give up p dollars per unit to get the
seventh unit of good 1.
This argument is quite intuitive, but let’s look at the math just to make
sure that it is clear. Suppose that the consumer demands 6 units of good 1.
We want to show that we must have r6 ≥ p ≥ r7 .
If the consumer is maximizing utility, then we must have
v(6) + m − 6p ≥ v(x1 ) + m − px1
SUBSTITUTES AND COMPLEMENTS
111
for all possible choices of x1 . In particular, we must have that
v(6) + m − 6p ≥ v(5) + m − 5p.
Rearranging this equation we have
r6 = v(6) − v(5) ≥ p,
which is half of what we wanted to show.
By the same logic,
v(6) + m − 6p ≥ v(7) + m − 7p.
Rearranging this gives us
p ≥ v(7) − v(6) = r7 ,
which is the other half of the inequality we wanted to establish.
6.7 Substitutes and Complements
We have already used the terms substitutes and complements, but it is now
appropriate to give a formal definition. Since we have seen perfect substitutes and perfect complements several times already, it seems reasonable
to look at the imperfect case.
Let’s think about substitutes first. We said that red pencils and blue
pencils might be thought of as perfect substitutes, at least for someone who
didn’t care about color. But what about pencils and pens? This is a case
of “imperfect” substitutes. That is, pens and pencils are, to some degree,
a substitute for each other, although they aren’t as perfect a substitute for
each other as red pencils and blue pencils.
Similarly, we said that right shoes and left shoes were perfect complements. But what about a pair of shoes and a pair of socks? Right shoes
and left shoes are nearly always consumed together, and shoes and socks
are usually consumed together. Complementary goods are those like shoes
and socks that tend to be consumed together, albeit not always.
Now that we’ve discussed the basic idea of complements and substitutes,
we can give a precise economic definition. Recall that the demand function
for good 1, say, will typically be a function of the price of both good 1 and
good 2, so we write x1 (p1 , p2 , m). We can ask how the demand for good 1
changes as the price of good 2 changes: does it go up or down?
If the demand for good 1 goes up when the price of good 2 goes up, then
we say that good 1 is a substitute for good 2. In terms of rates of change,
good 1 is a substitute for good 2 if
Δx1
> 0.
Δp2
112 DEMAND (Ch. 6)
The idea is that when good 2 gets more expensive the consumer switches to
consuming good 1: the consumer substitutes away from the more expensive
good to the less expensive good.
On the other hand, if the demand for good 1 goes down when the price
of good 2 goes up, we say that good 1 is a complement to good 2. This
means that
Δx1
< 0.
Δp2
Complements are goods that are consumed together, like coffee and sugar,
so when the price of one good rises, the consumption of both goods will
tend to decrease.
The cases of perfect substitutes and perfect complements illustrate these
points nicely. Note that Δx1 /Δp2 is positive (or zero) in the case of perfect
substitutes, and that Δx1 /Δp2 is negative in the case of perfect complements.
A couple of warnings are in order about these concepts. First, the twogood case is rather special when it comes to complements and substitutes.
Since income is being held fixed, if you spend more money on good 1, you’ll
have to spend less on good 2. This puts some restrictions on the kinds of
behavior that are possible. When there are more than two goods, these
restrictions are not so much of a problem.
Second, although the definition of substitutes and complements in terms
of consumer demand behavior seems sensible, there are some difficulties
with the definitions in more general environments. For example, if we use
the above definitions in a situation involving more than two goods, it is
perfectly possible that good 1 may be a substitute for good 3, but good 3
may be a complement for good 1. Because of this peculiar feature, more
advanced treatments typically use a somewhat different definition of substitutes and complements. The definitions given above describe concepts
known as gross substitutes and gross complements; they will be sufficient for our needs.
6.8 The Inverse Demand Function
If we hold p2 and m fixed and plot p1 against x1 we get the demand
curve. As suggested above, we typically think that the demand curve
slopes downwards, so that higher prices lead to less demand, although the
Giffen example shows that it could be otherwise.
As long as we do have a downward-sloping demand curve, as is usual,
it is meaningful to speak of the inverse demand function. The inverse
demand function is the demand function viewing price as a function of
quantity. That is, for each level of demand for good 1, the inverse demand
function measures what the price of good 1 would have to be in order for
the consumer to choose that level of consumption. So the inverse demand
THE INVERSE DEMAND FUNCTION
113
function measures the same relationship as the direct demand function, but
just from another point of view. Figure 6.15 depicts the inverse demand
function—or the direct demand function, depending on your point of view.
p1
Inverse demand
curve p1(x1)
x1
Inverse demand curve. If you view the demand curve as
measuring price as a function of quantity, you have an inverse
demand function.
Recall, for example, the Cobb-Douglas demand for good 1, x1 = am/p1 .
We could just as well write the relationship between price and quantity as
p1 = am/x1 . The first representation is the direct demand function; the
second is the inverse demand function.
The inverse demand function has a useful economic interpretation. Recall
that as long as both goods are being consumed in positive amounts, the
optimal choice must satisfy the condition that the absolute value of the
MRS equals the price ratio:
|MRS| =
p1
.
p2
This says that at the optimal level of demand for good 1, for example, we
must have
(6.4)
p1 = p2 |MRS|.
Thus, at the optimal level of demand for good 1, the price of good 1
is proportional to the absolute value of the MRS between good 1 and
good 2.
Figure
6.15
114 DEMAND (Ch. 6)
Suppose for simplicity that the price of good 2 is one. Then equation
(6.4) tells us that at the optimal level of demand, the price of good 1
measures how much the consumer is willing to give up of good 2 in order
to get a little more of good 1. In this case the inverse demand function is simply measuring the absolute value of the MRS. For any optimal level of x1 the inverse demand function tells how much of good 2
the consumer would want to have to compensate him for a small reduction in the amount of good 1. Or, turning this around, the inverse demand function measures how much the consumer would be willing to sacrifice of good 2 to make him just indifferent to having a little more of
good 1.
If we think of good 2 as being money to spend on other goods, then we
can think of the MRS as being how many dollars the individual would be
willing to give up to have a little more of good 1. We suggested earlier that
in this case, we can think of the MRS as measuring the marginal willingness
to pay. Since the price of good 1 is just the MRS in this case, this means
that the price of good 1 itself is measuring the marginal willingness to
pay.
At each quantity x1 , the inverse demand function measures how many
dollars the consumer is willing to give up for a little more of good 1; or,
said another way, how many dollars the consumer was willing to give up for
the last unit purchased of good 1. For a small enough amount of good 1,
they come down to the same thing.
Looked at in this way, the downward-sloping demand curve has a new
meaning. When x1 is very small, the consumer is willing to give up a lot of
money—that is, a lot of other goods, to acquire a little bit more of good 1.
As x1 is larger, the consumer is willing to give up less money, on the margin,
to acquire a little more of good 1. Thus the marginal willingness to pay,
in the sense of the marginal willingness to sacrifice good 2 for good 1, is
decreasing as we increase the consumption of good 1.
Summary
1. The consumer’s demand function for a good will in general depend on
the prices of all goods and income.
2. A normal good is one for which the demand increases when income
increases. An inferior good is one for which the demand decreases when
income increases.
3. An ordinary good is one for which the demand decreases when its price
increases. A Giffen good is one for which the demand increases when its
price increases.
APPENDIX
115
4. If the demand for good 1 increases when the price of good 2 increases,
then good 1 is a substitute for good 2. If the demand for good 1 decreases
in this situation, then it is a complement for good 2.
5. The inverse demand function measures the price at which a given quantity will be demanded. The height of the demand curve at a given level
of consumption measures the marginal willingness to pay for an additional
unit of the good at that consumption level.
REVIEW QUESTIONS
1. If the consumer is consuming exactly two goods, and she is always spending all of her money, can both of them be inferior goods?
2. Show that perfect substitutes are an example of homothetic preferences.
3. Show that Cobb-Douglas preferences are homothetic preferences.
4. The income offer curve is to the Engel curve as the price offer curve is
to . . .?
5. If the preferences are concave will the consumer ever consume both of
the goods together?
6. Are hamburgers and buns complements or substitutes?
7. What is the form of the inverse demand function for good 1 in the case
of perfect complements?
8. True or false? If the demand function is x1 = −p1 , then the inverse
demand function is x = −1/p1 .
APPENDIX
If preferences take a special form, this will mean that the demand functions that
come from those preferences will take a special form. In Chapter 4 we described
quasilinear preferences. These preferences involve indifference curves that are all
parallel to one another and can be represented by a utility function of the form
u(x1 , x2 ) = v(x1 ) + x2 .
The maximization problem for a utility function like this is
max v(x1 ) + x2
x1 ,x2
116 DEMAND (Ch. 6)
s.t. p1 x1 + p2 x2 = m.
Solving the budget constraint for x2 as a function of x1 and substituting into the
objective function, we have
max v(x1 ) + m/p2 − p1 x1 /p2 .
x1
Differentiating gives us the first-order condition
v (x∗1 ) =
p1
.
p2
This demand function has the interesting feature that the demand for good 1
must be independent of income—just as we saw by using indifference curves.
The inverse demand curve is given by
p1 (x1 ) = v (x1 )p2 .
That is, the inverse demand function for good 1 is the derivative of the utility
function times p2 . Once we have the demand function for good 1, the demand
function for good 2 comes from the budget constraint.
For example, let us calculate the demand functions for the utility function
u(x1 , x2 ) = ln x1 + x2 .
Applying the first-order condition gives
1
p1
=
,
x1
p2
so the direct demand function for good 1 is
x1 =
p2
,
p1
and the inverse demand function is
p1 (x1 ) =
p2
.
x1
The direct demand function for good 2 comes from substituting x1 = p2 /p1
into the budget constraint:
m
x2 =
− 1.
p2
A warning is in order concerning these demand functions. Note that the demand for good 1 is independent of income in this example. This is a general
feature of a quasilinear utility function—the demand for good 1 remains constant as income changes. However, this can only be true for some values of
income. A demand function can’t literally be independent of income for all values of income; after all, when income is zero, all demands are zero. It turns
APPENDIX
117
out that the quasilinear demand function derived above is only relevant when a
positive amount of each good is being consumed.
In this example, when m < p2 , the optimal consumption of good 2 will be zero.
As income increases the marginal utility of consumption of good 1 decreases.
When m = p2 , the marginal utility from spending additional income on good
1 just equals the marginal utility from spending additional income on good 2.
After that point, the consumer spends all additional income on good 2.
So a better way to write the demand for good 2 is:
x2 =
0
m/p2 − 1
when m ≤ p2
.
when m > p2
For more on the properties of quasilinear demand functions see Hal R. Varian,
Microeconomic Analysis, 3rd ed. (New York: Norton, 1992).
In the previous chapter, you found the commodity bundle that a consumer
with a given utility function would choose in a specific price-income situation. In this chapter, we take this idea a step further. We find demand
functions, which tell us for any prices and income you might want to
name, how much of each good a consumer would want. In general, the
amount of each good demanded may depend not only on its own price,
but also on the price of other goods and on income. Where there are two
goods, we write demand functions for Goods 1 and 2 as x1 (p1 , p2 , m) and
x2 (p1 , p2 , m).∗
When the consumer is choosing positive amounts of all commodities
and indifference curves have no kinks, the consumer chooses a point of
tangency between her budget line and the highest indifference curve that
it touches.
Consider a consumer with utility function U (x1 , x2 ) = (x1 + 2)(x2 + 10).
To find x1 (p1 , p2 , m) and x2 (p1 , p2 , m), we need to find a commodity bundle (x1 , x2 ) on her budget line at which her indifference curve is tangent to
her budget line. The budget line will be tangent to the indifference curve
at (x1 , x2 ) if the price ratio equals the marginal rate of substitution. For
this utility function, M U1 (x1 , x2 ) = x2 + 10 and M U2 (x1 , x2 ) = x1 + 2.
Therefore the “tangency equation” is p1 /p2 = (x2 + 10)/(x1 + 2). Crossmultiplying the tangency equation, one finds p1 x1 + 2p1 = p2 x2 + 10p2 .
The bundle chosen must also satisfy the budget equation, p1 x1 +
p2 x2 = m. This gives us two linear equations in the two unknowns, x1
and x2 . You can solve these equations yourself, using high school algebra.
You will find that the solution for the two “demand functions” is
m − 2p1 + 10p2
2p1
m + 2p1 − 10p2
x2 =
.
2p2
x1 =
There is one thing left to worry about with the “demand functions” we
just found. Notice that these expressions will be positive only if m−2p1 +
10p2 > 0 and m+2p1 −10p2 > 0. If either of these expressions is negative,
then it doesn’t make sense as a demand function. What happens in this
case is that the consumer will choose a “boundary solution” where she
∗
For some utility functions, demand for a good may not be affected by
all of these variables. For example, with Cobb-Douglas utility, demand
for a good depends on the good’s own price and on income but not on the
other good’s price. Still, there is no harm in writing demand for Good
1 as a function of p1 , p2 , and m. It just happens that the derivative of
x1 (p1 , p2 , m) with respect to p2 is zero.
consumes only one good. At this point, her indifference curve will not be
tangent to her budget line.
When a consumer has kinks in her indifference curves, she may choose
a bundle that is located at a kink. In the problems with kinks, you
will be able to solve for the demand functions quite easily by looking
at diagrams and doing a little algebra. Typically, instead of finding a
tangency equation, you will find an equation that tells you “where the
kinks are.” With this equation and the budget equation, you can then
solve for demand.
You might wonder why we pay so much attention to kinky indifference curves, straight line indifference curves, and other “funny cases.”
Our reason is this. In the funny cases, computations are usually pretty
easy. But often you may have to draw a graph and think about what
you are doing. That is what we want you to do. Think and fiddle with
graphs. Don’t just memorize formulas. Formulas you will forget, but the
habit of thinking will stick with you.
When you have finished this workout, we hope that you will be able
to do the following:
• Find demand functions for consumers with Cobb-Douglas and other
similar utility functions.
• Find demand functions for consumers with quasilinear utility functions.
• Find demand functions for consumers with kinked indifference curves
and for consumers with straight-line indifference curves.
• Recognize complements and substitutes from looking at a demand
curve.
• Recognize normal goods, inferior goods, luxuries, and necessities from
looking at information about demand.
• Calculate the equation of an inverse demand curve, given a simple
demand equation.
6.1 (0) Charlie is back—still consuming apples and bananas. His utility function is U (xA , xB ) = xA xB . We want to find his demand function for apples, xA (pA , pB , m), and his demand function for bananas,
xB (pA , pB , m).
(a) When the prices are pA and pB and Charlie’s income is m, the equation
for Charlie’s budget line is pA xA +pB xB = m. The slope of Charlie’s indifference curve at the bundle (xA , xB ) is −M U1 (xA , xB )/M U2 (xA , xB ) =
The slope of Charlie’s budget line is
Charlie’s indifference curve will be tangent to his budget line at the point
(xA , xB ) if the following equation is satisfied:
.
(b) You now have two equations, the budget equation and the tangency
equation, that must be satisfied by the bundle demanded. Solve these
two equations for xA and xB . Charlie’s demand function for apples
is xA (pA , pB , m) =
, and his demand function for bananas is
xB (pA , pB , m) =
.
(c) In general, the demand for both commodities will depend on the price
of both commodities and on income. But for Charlie’s utility function,
the demand function for apples depends only on income and the price
of apples. Similarly, the demand for bananas depends only on income
and the price of bananas. Charlie always spends the same fraction of his
income on bananas. What fraction is this?
.
6.2 (0) Douglas Cornfield’s preferences are represented by the utility
function u(x1 , x2 ) = x21 x32 . The prices of x1 and x2 are p1 and p2 .
(a) The slope of Cornfield’s indifference curve at the point (x1 , x2 ) is
.
(b) If Cornfield’s budget line is tangent to his indifference curve at (x1 , x2 ),
then pp21 xx21 =
(Hint: Look at the equation that equates the
slope of his indifference curve with the slope of his budget line.) When he
is consuming the best bundle he can afford, what fraction of his income
does Douglas spend on x1 ?
.
(c) Other members of Doug’s family have similar utility functions, but
the exponents may be different, or their utilities may be multiplied by a
positive constant. If a family member has a utility function U (x, y) =
cxa1 xb2 where a, b, and c are positive numbers, what fraction of his or her
income will that family member spend on x1 ?
.
6.3 (0) Our thoughts return to Ambrose and his nuts and berries. Am√
brose’s utility function is U (x1 , x2 ) = 4 x1 + x2 , where x1 is his consumption of nuts and x2 is his consumption of berries.
(a) Let us find his demand function for nuts. The slope of Ambrose’s
indifference curve at (x1 , x2 ) is
Setting this slope equal to
the slope of the budget line, you can solve for x1 without even using the
budget equation. The solution is x1 =
.
(b) Let us find his demand for berries. Now we need the budget equation.
In Part (a), you solved for the amount of x1 that he will demand. The
budget equation tells us that p1 x1 + p2 x2 = M . Plug the solution that
you found for x1 into the budget equation and solve for x2 as a function
of income and prices. The answer is x2 =
.
(c) When we visited Ambrose in Chapter 5, we looked at a “boundary
solution,” where Ambrose consumed only nuts and no berries. In that
example, p1 = 1, p2 = 2, and M = 9. If you plug these numbers into
the formulas we found in Parts (a) and (b), you find x1 =
, and
. Since we get a negative solution for x2 , it must be that
x2 =
the budget line x1 + 2x2 = 9 is not tangent to an indifference curve when
x2 ≥ 0. The best that Ambrose can do with this budget is to spend all
of his income on nuts. Looking at the formulas, we see that at the prices
p1 = 1 and p2 = 2, Ambrose will demand a positive amount of both goods
if and only if M >
.
6.4 (0) Donald Fribble is a stamp collector. The only things other
than stamps that Fribble consumes are Hostess Twinkies. It turns out
that Fribble’s preferences are represented by the utility function u(s, t) =
s + ln t where s is the number of stamps he collects and t is the number
of Twinkies he consumes. The price of stamps is ps and the price of
Twinkies is pt . Donald’s income is m.
(a) Write an expression that says that the ratio of Fribble’s marginal
utility for Twinkies to his marginal utility for stamps is equal to the ratio
of the price of Twinkies to the price of stamps.
(Hint:
The derivative of ln t with respect to t is 1/t, and the derivative of s with
respect to s is 1.)
(b) You can use the equation you found in the last part to show that if he
buys both goods, Donald’s demand function for Twinkies depends only
on the price ratio and not on his income. Donald’s demand function for
Twinkies is
.
(c) Notice that for this special utility function, if Fribble buys both goods,
then the total amount of money that he spends on Twinkies has the
peculiar property that it depends on only one of the three variables m,
(Hint: The amount of money
pt , and ps , namely the variable
that he spends on Twinkies is pt t(ps , pt , m).)
(d) Since there are only two goods, any money that is not spent on
Twinkies must be spent on stamps. Use the budget equation and Donald’s demand function for Twinkies to find an expression for the number
of stamps he will buy if his income is m, the price of stamps is ps and the
price of Twinkies is pt .
.
(e) The expression you just wrote down is negative if m < ps . Surely
it makes no sense for him to be demanding negative amounts of postage
stamps. If m < ps , what would Fribble’s demand for postage stamps be?
What would his demand for Twinkies be?
(Hint: Recall the discussion of boundary optimum.)
(f ) Donald’s wife complains that whenever Donald gets an extra dollar,
he always spends it all on stamps. Is she right? (Assume that m > ps .)
.
(g) Suppose that the price of Twinkies is $2 and the price of stamps is $1.
On the graph below, draw Fribble’s Engel curve for Twinkies in red ink
and his Engel curve for stamps in blue ink. (Hint: First draw the Engel
curves for incomes greater than $1, then draw them for incomes less than
$1.)
Income
8
6
4
2
0
2
4
6
8
Quantities
6.5 (0) Shirley Sixpack, as you will recall, thinks that two 8-ounce cans
of beer are exactly as good as one 16-ounce can of beer. Suppose that
these are the only sizes of beer available to her and that she has $30 to
spend on beer. Suppose that an 8-ounce beer costs $.75 and a 16-ounce
beer costs $1. On the graph below, draw Shirley’s budget line in blue ink,
and draw some of her indifference curves in red.
8-ounce cans
40
30
20
10
0
10
20
30
40
16-ounce cans
(a) At these prices, which size can will she buy, or will she buy some of
each?
.
(b) Suppose that the price of 16-ounce beers remains $1 and the price of
8-ounce beers falls to $.55. Will she buy more 8-ounce beers?
.
(c) What if the price of 8-ounce beers falls to $.40? How many 8-ounce
beers will she buy then?
.
(d) If the price of 16-ounce beers is $1 each and if Shirley chooses some
8-ounce beers and some 16-ounce beers, what must be the price of 8-ounce
beers?
.
(e) Now let us try to describe Shirley’s demand function for 16-ounce beers
as a function of general prices and income. Let the prices of 8-ounce and
16-ounce beers be p8 and p16 , and let her income be m. If p16 < 2p8 , then
the number of 16-ounce beers she will demand is
then the number of 16-ounce beers she will demand is
If p16 > 2p8 ,
If p16 =
p8 , she will be indifferent between any affordable combinations.
6.6 (0) Miss Muffet always likes to have things “just so.” In fact the
only way she will consume her curds and whey is in the ratio of 2 units of
whey per unit of curds. She has an income of $20. Whey costs $.75 per
unit. Curds cost $1 per unit. On the graph below, draw Miss Muffet’s
budget line, and plot some of her indifference curves. (Hint: Have you
noticed something kinky about Miss Muffet?)
(a) How many units of curds will Miss Muffet demand in this situation?
How many units of whey?
.
Whey
32
24
16
8
0
8
16
24
32
Curds
(b) Write down Miss Muffet’s demand function for whey as a function of
the prices of curds and whey and of her income, where pc is the price of
curds, pw is the price of whey, and m is her income. D(pc , pw , m) =
(Hint: You can solve for her demands by solving two equations in
two unknowns. One equation tells you that she consumes twice as much
whey as curds. The second equation is her budget equation.)
6.7 (1) Mary’s utility function is U (b, c) = b + 100c − c2 , where b is the
number of silver bells in her garden and c is the number of cockle shells.
She has 500 square feet in her garden to allocate between silver bells and
cockle shells. Silver bells each take up 1 square foot and cockle shells each
take up 4 square feet. She gets both kinds of seeds for free.
(a) To maximize her utility, given the size of her garden, Mary should
plant
silver bells and
cockle shells. (Hint: Write down
her “budget constraint” for space. Solve the problem as if it were an
ordinary demand problem.)
(b) If she suddenly acquires an extra 100 square feet for her garden, how
much should she increase her planting of silver bells?
How much should she increase her planting of cockle shells?
.
(c) If Mary had only 144 square feet in her garden, how many cockle
shells would she grow?
.
(d) If Mary grows both silver bells and cockle shells, then we know that
the number of square feet in her garden must be greater than
.
6.8 (0) Casper consumes cocoa and cheese. He has an income of $16.
Cocoa is sold in an unusual way. There is only one supplier and the more
cocoa one buys from him, the higher the price one has to pay per unit.
In fact, x units of cocoa will cost Casper a total of x2 dollars. Cheese is
sold in the usual way at a price of $2 per unit. Casper’s budget equation,
therefore, is x2 + 2y = 16 where x is his consumption of cocoa and y is
his consumption of cheese. Casper’s utility function is U (x, y) = 3x + y.
(a) On the graph below, draw the boundary of Casper’s budget set in
blue ink. Use red ink to sketch two or three of his indifference curves.
Cheese
16
12
8
4
0
4
8
12
16
Cocoa
(b) Write an equation that says that at the point (x, y), the slope of
Casper’s budget “line” equals the slope of his indifference “curve.”
Casper demands
units of cocoa and
units
of cheese.
6.9 (0) Perhaps after all of the problems with imaginary people and
places, you would like to try a problem based on actual fact. The U.S.
government’s Bureau of Labor Statistics periodically makes studies of
family budgets and uses the results to compile the consumer price index.
These budget studies and a wealth of other interesting economic data can
be found in the annually published Handbook of Labor Statistics. The
tables below report total current consumption expenditures and expenditures on certain major categories of goods for 5 different income groups
in the United States in 1961. People within each of these groups all had
similar incomes. Group A is the lowest income group and Group E is the
highest.
Table 6.1
Expenditures by Category for Various Income Groups in 1961
Income Group
Food Prepared at Home
Food Away from Home
Housing
Clothing
Transportation
Other
Total Expenditures
A
465
68
626
119
139
364
1781
B
783
171
1090
328
519
745
3636
C
1078
213
1508
508
826
1039
5172
D
1382
384
2043
830
1222
1554
7415
E
1848
872
4205
1745
2048
3490
14208
Table 6.2
Percentage Allocation of Family Budget
Income Group
A
B
C
D
E
Food Prepared at Home
26
22
21
19
13
Food Away from Home
3.8
4.7
4.1
5.2
6.1
Housing
35
30
Clothing
6.7
9.0
Transportation
7.8
14
(a) Complete Table 6.2.
(b) Which of these goods are normal goods?
.
(c) Which of these goods satisfy your textbook’s definition of luxury goods
at most income levels?
.
(d) Which of these goods satisfy your textbook’s definition of necessity
goods at most income levels?
.
(e) On the graph below, use the information from Table 6.1 to draw
“Engel curves.” (Use total expenditure on current consumption as income
for purposes of drawing this curve.) Use red ink to draw the Engel curve
for food prepared at home. Use blue ink to draw an Engel curve for
food away from home. Use pencil to draw an Engel curve for clothing.
How does the shape of an Engel curve for a luxury differ from the shape
of an Engel curve for a necessity?
.
Total expenditures (thousands of dollars)
12
9
6
3
0
750
1500 2250 3000
Expenditure on specific goods
6.10 (0) Percy consumes cakes and ale. His demand function for cakes
is qc = m − 30pc + 20pa , where m is his income, pa is the price of ale, pc
is the price of cakes, and qc is his consumption of cakes. Percy’s income
is $100, and the price of ale is $1 per unit.
(a) Is ale a substitute for cakes or a complement? Explain.
.
(b) Write an equation for Percy’s demand function for cakes where income
and the price of ale are held fixed at $100 and $1.
.
(c) Write an equation for Percy’s inverse demand function for cakes where
income is $100 and the price of ale remains at $1.
what price would Percy buy 30 cakes?
Percy’s inverse demand curve for cakes.
At
Use blue ink to draw
(d) Suppose that the price of ale rises to $2.50 per unit and remains there.
Write an equation for Percy’s inverse demand for cakes.
Use red ink to draw in Percy’s new inverse demand curve for
cakes.
Price
4
3
2
1
0
30
60
90
120
Number of cakes
6.11 (0) Richard and Mary Stout have fallen on hard times, but remain
rational consumers. They are making do on $80 a week, spending $40 on
food and $40 on all other goods. Food costs $1 per unit. On the graph
below, use black ink to draw a budget line. Label their consumption
bundle with the letter A.
(a) The Stouts suddenly become eligible for food stamps. This means
that they can go to the agency and buy coupons that can be exchanged
for $2 worth of food. Each coupon costs the Stouts $1. However, the
maximum number of coupons they can buy per week is 10. On the graph,
draw their new budget line with red ink.
(b) If the Stouts have homothetic preferences, how much more food will
they buy once they enter the food stamp program?
.
Dollars worth of other things
120
100
80
60
40
20
0
20
40
60
80
100
120
Food
6.12 (2) As you may remember, Nancy Lerner is taking an economics
course in which her overall score is the minimum of the number of correct
answers she gets on two examinations. For the first exam, each correct
answer costs Nancy 10 minutes of study time. For the second exam, each
correct answer costs her 20 minutes of study time. In the last chapter,
you found the best way for her to allocate 1200 minutes between the two
exams. Some people in Nancy’s class learn faster and some learn slower
than Nancy. Some people will choose to study more than she does, and
some will choose to study less than she does. In this section, we will find
a general solution for a person’s choice of study times and exam scores as
a function of the time costs of improving one’s score.
(a) Suppose that if a student does not study for an examination, he or
she gets no correct answers. Every answer that the student gets right
on the first examination costs P1 minutes of studying for the first exam.
Every answer that he or she gets right on the second examination costs
P2 minutes of studying for the second exam. Suppose that this student
spends a total of M minutes studying for the two exams and allocates
the time between the two exams in the most efficient possible way. Will
the student have the same number of correct answers on both exams?
Write a general formula for this student’s overall score for the
course as a function of the three variables, P1 , P2 , and M : S =
If this student wants to get an overall score of S, with the smallest possible
total amount of studying, this student must spend
studying for the first exam and
minutes
studying for the second exam.
(b) Suppose that a student has the utility function
U (S, M ) = S −
A 2
M ,
2
where S is the student’s overall score for the course, M is the number
of minutes the student spends studying, and A is a variable that reflects
how much the student dislikes studying. In Part (a) of this problem, you
found that a student who studies for M minutes and allocates this time
wisely between the two exams will get an overall score of S = P1M
+P2 .
M
Substitute P1 +P2 for S in the utility function and then differentiate with
respect to M to find the amount of study time, M , that maximizes the
student’s utility. M =
Your answer will be a function of the
variables P1 , P2 , and A. If the student chooses the utility-maximizing
amount of study time and allocates it wisely between the two exams, he
or she will have an overall score for the course of S =
.
(c) Nancy Lerner has a utility function like the one presented above. She
chose the utility-maximizing amount of study time for herself. For Nancy,
P1 = 10 and P2 = 20. She spent a total of M = 1, 200 minutes studying
for the two exams. This gives us enough information to solve for the
variable A in Nancy’s utility function. In fact, for Nancy, A =
.
(d) Ed Fungus is a student in Nancy’s class. Ed’s utility function is just
like Nancy’s, with the same value of A. But Ed learns more slowly than
Nancy. In fact it takes Ed exactly twice as long to learn anything as it
takes Nancy, so that for him, P1 = 20 and P2 = 40. Ed also chooses his
amount of study time so as to maximize his utility. Find the ratio of the
amount of time Ed spends studying to the amount of time Nancy spends
studying.
Will his score for the course be greater than half,
equal to half, or less than half of Nancy’s?
.
6.13 (1) Here is a puzzle for you. At first glance, it would appear that
there is not nearly enough information to answer this question. But when
you graph the indifference curve and think about it a little, you will see
that there is a neat, easily calculated solution.
Kinko spends all his money on whips and leather jackets. Kinko’s
utility function is U (x, y) = min{4x, 2x + y}, where x is his consumption
of whips and y is his consumption of leather jackets. Kinko is consuming
15 whips and 10 leather jackets. The price of whips is $10. You are to
find Kinko’s income.
(a) Graph the indifference curve for Kinko that passes through the point
(15, 10). What is the slope of this indifference curve at (15, 10)?
What must be the price of leather jackets if Kinko chooses this
point?
Now, what is Kinko’s income?
.
Leather jackets
40
30
20
10
0
10
20
30
40
Whips
4
6.1 (See Problem 6.1.) If Charlie’s utility function is XA
XB , apples cost
90 cents each, and bananas cost 10 cents each, then Charlie’s budget line
is tangent to one of his indifference curves whenever the following equation
is satisfied:
(a) 4XB = 9XA .
(b) XB = XA .
(c) XA = 4XB .
(d) XB = 4XA .
(e) 90XA + 10XB = M .
4
6.2 (See Problem 6.1.) If Charlie’s utility function is XA
XB , the price
of apples is pA , the price of bananas is pB , and his income is m, then
Charlie’s demand for apples is
(a) m/(2pA ).
(b) 0.25pA m.
(c) m/(pA + pB ).
(d) 0.80m/pA .
(e) 1.25pB m/pA .
6.3 Ambrose’s brother Bartholomew has a utility function U (x1 , x2 ) =
1/2
24x1 + x2 . His income is 51, the price of good 1 (nuts) is 4, and the
price of good 2 (berries) is 1. How many units of nuts will Bartholomew
demand?
(a) 19
(b) 5
(c) 7
(d) 9
(e) 16
6.4 Ambrose’s brother Bartholomew has a utility function U (x1 , x2 ) =
1/2
8x1 + x2 . His income is 23, the price of nuts is 2, and the price of berries
is 1. How many units of berries will Bartholomew demand?
(a) 15
(b) 4
(c) 30
(d) 10
(e) There is not enough information to determine the answer.
6.5 In Problem 6.6, recall that Miss Muffet insists on consuming 2 units
of whey per unit of curds. If the price of curds is 3 and the price of whey
is 6, then if Miss Muffett’s income is m, her demand for curds will be
(a) m/3.
(b) 6m/3.
(c) 3C + 6W = m.
(d) 3m.
(e) m/15.
6.6 In Problem 6.8, recall that Casper’s utility function is 3x + y, where
x is his consumption of cocoa and y is his consumption of cheese. If the
total cost of x units of cocoa is x2 , the price of a unit of cheese is $8, and
Casper’s income is $174, how many units of cocoa will he consume?
(a) 9
(b) 12
(c) 23
(d) 11
(e) 24
6.7 (See Problem 6.13.)
Kinko’s utility function is U (w, j) =
min{7w, 3w + 12j}, where w is the number of whips that he owns and j
is the number of leather jackets. If the price of whips is $20 and the price
of leather jackets is $60, Kinko will demand:
(a) 6 times as many whips as leather jackets.
(b) 5 times as many leather jackets as whips.
(c) 3 times as many whips as leather jackets.
(d) 4 times as many leather jackets as whips.
(e) only leather jackets.
CHAPTER
7
REVEALED
PREFERENCE
In Chapter 6 we saw how we can use information about the consumer’s
preferences and budget constraint to determine his or her demand. In
this chapter we reverse this process and show how we can use information about the consumer’s demand to discover information about his or
her preferences. Up until now, we were thinking about what preferences
could tell us about people’s behavior. But in real life, preferences are
not directly observable: we have to discover people’s preferences from
observing their behavior. In this chapter we’ll develop some tools to do
this.
When we talk of determining people’s preferences from observing their
behavior, we have to assume that the preferences will remain unchanged
while we observe the behavior. Over very long time spans, this is not very
reasonable. But for the monthly or quarterly time spans that economists
usually deal with, it seems unlikely that a particular consumer’s tastes
would change radically. Thus we will adopt a maintained hypothesis that
the consumer’s preferences are stable over the time period for which we
observe his or her choice behavior.
THE IDEA OF REVEALED PREFERENCE
119
7.1 The Idea of Revealed Preference
Before we begin this investigation, let’s adopt the convention that in this
chapter, the underlying preferences—whatever they may be—are known
to be strictly convex. Thus there will be a unique demanded bundle at
each budget. This assumption is not necessary for the theory of revealed
preference, but the exposition will be simpler with it.
Consider Figure 7.1, where we have depicted a consumer’s demanded
bundle, (x1 , x2 ), and another arbitrary bundle, (y1 , y2 ), that is beneath
the consumer’s budget line. Suppose that we are willing to postulate that
this consumer is an optimizing consumer of the sort we have been studying. What can we say about the consumer’s preferences between these two
bundles of goods?
x2
(x1, x 2 )
(y1 , y2 )
Budget line
x1
Revealed preference. The bundle (x1 , x2 ) that the consumer
chooses is revealed preferred to the bundle (y1 , y2 ), a bundle that
he could have chosen.
Well, the bundle (y1 , y2 ) is certainly an affordable purchase at the given
budget—the consumer could have bought it if he or she wanted to, and
would even have had money left over. Since (x1 , x2 ) is the optimal bundle,
it must be better than anything else that the consumer could afford. Hence,
in particular it must be better than (y1 , y2 ).
The same argument holds for any bundle on or underneath the budget
line other than the demanded bundle. Since it could have been bought at
Figure
7.1
120 REVEALED PREFERENCE (Ch. 7)
the given budget but wasn’t, then what was bought must be better. Here
is where we use the assumption that there is a unique demanded bundle
for each budget. If preferences are not strictly convex, so that indifference
curves have flat spots, it may be that some bundles that are on the budget
line might be just as good as the demanded bundle. This complication can
be handled without too much difficulty, but it is easier to just assume it
away.
In Figure 7.1 all of the bundles in the shaded area underneath the budget
line are revealed worse than the demanded bundle (x1 , x2 ). This is because
they could have been chosen, but were rejected in favor of (x1 , x2 ). We will
now translate this geometric discussion of revealed preference into algebra.
Let (x1 , x2 ) be the bundle purchased at prices (p1 , p2 ) when the consumer
has income m. What does it mean to say that (y1 , y2 ) is affordable at
those prices and income? It simply means that (y1 , y2 ) satisfies the budget
constraint
p1 y1 + p2 y2 ≤ m.
Since (x1 , x2 ) is actually bought at the given budget, it must satisfy the
budget constraint with equality
p1 x1 + p2 x2 = m.
Putting these two equations together, the fact that (y1 , y2 ) is affordable at
the budget (p1 , p2 , m) means that
p1 x1 + p2 x2 ≥ p1 y1 + p2 y2 .
If the above inequality is satisfied and (y1 , y2 ) is actually a different
bundle from (x1 , x2 ), we say that (x1 , x2 ) is directly revealed preferred
to (y1 , y2 ).
Note that the left-hand side of this inequality is the expenditure on the
bundle that is actually chosen at prices (p1 , p2 ). Thus revealed preference is
a relation that holds between the bundle that is actually demanded at some
budget and the bundles that could have been demanded at that budget.
The term “revealed preference” is actually a bit misleading. It does not
inherently have anything to do with preferences, although we’ve seen above
that if the consumer is making optimal choices, the two ideas are closely
related. Instead of saying “X is revealed preferred to Y ,” it would be better
to say “X is chosen over Y .” When we say that X is revealed preferred to
Y , all we are claiming is that X is chosen when Y could have been chosen;
that is, that p1 x1 + p2 x2 ≥ p1 y1 + p2 y2 .
7.2 From Revealed Preference to Preference
We can summarize the above section very simply. It follows from our model
of consumer behavior—that people are choosing the best things they can
FROM REVEALED PREFERENCE TO PREFERENCE
121
afford—that the choices they make are preferred to the choices that they
could have made. Or, in the terminology of the last section, if (x1 , x2 ) is
directly revealed preferred to (y1 , y2 ), then (x1 , x2 ) is in fact preferred to
(y1 , y2 ). Let us state this principle more formally:
The Principle of Revealed Preference. Let (x1 , x2 ) be the chosen
bundle when prices are (p1 , p2 ), and let (y1 , y2 ) be some other bundle such
that p1 x1 + p2 x2 ≥ p1 y1 + p2 y2 . Then if the consumer is choosing the most
preferred bundle she can afford, we must have (x1 , x2 ) (y1 , y2 ).
When you first encounter this principle, it may seem circular. If X is revealed preferred to Y , doesn’t that automatically mean that X is preferred
to Y ? The answer is no. “Revealed preferred” just means that X was chosen when Y was affordable; “preference” means that the consumer ranks
X ahead of Y . If the consumer chooses the best bundles she can afford,
then “revealed preference” implies “preference,” but that is a consequence
of the model of behavior, not the definitions of the terms.
This is why it would be better to say that one bundle is “chosen over”
another, as suggested above. Then we would state the principle of revealed
preference by saying: “If a bundle X is chosen over a bundle Y , then X
must be preferred to Y .” In this statement it is clear how the model of
behavior allows us to use observed choices to infer something about the
underlying preferences.
Whatever terminology you use, the essential point is clear: if we observe
that one bundle is chosen when another one is affordable, then we have
learned something about the preferences between the two bundles: namely,
that the first is preferred to the second.
Now suppose that we happen to know that (y1 , y2 ) is a demanded bundle
at prices (q1 , q2 ) and that (y1 , y2 ) is itself revealed preferred to some other
bundle (z1 , z2 ). That is,
q1 y 1 + q 2 y 2 ≥ q1 z1 + q 2 z2 .
Then we know that (x1 , x2 ) (y1 , y2 ) and that (y1 , y2 ) (z1 , z2 ). From
the transitivity assumption we can conclude that (x1 , x2 ) (z1 , z2 ).
This argument is illustrated in Figure 7.2. Revealed preference and transitivity tell us that (x1 , x2 ) must be better than (z1 , z2 ) for the consumer
who made the illustrated choices.
It is natural to say that in this case (x1 , x2 ) is indirectly revealed
preferred to (z1 , z2 ). Of course the “chain” of observed choices may be
longer than just three: if bundle A is directly revealed preferred to B, and
B to C, and C to D, . . . all the way to M , say, then bundle A is still
indirectly revealed preferred to M . The chain of direct comparisons can be
of any length.
If a bundle is either directly or indirectly revealed preferred to another
bundle, we will say that the first bundle is revealed preferred to the
122 REVEALED PREFERENCE (Ch. 7)
x2
(x1 , x2 )
(y , y )
1
2
Budget lines
(z1 , z2 )
x1
Figure
7.2
Indirect revealed preference. The bundle (x1 , x2 ) is indirectly revealed preferred to the bundle (z1 , z2 ).
second. The idea of revealed preference is simple, but it is surprisingly
powerful. Just looking at a consumer’s choices can give us a lot of information about the underlying preferences. Consider, for example, Figure
7.2. Here we have several observations on demanded bundles at different
budgets. We can conclude from these observations that since (x1 , x2 ) is
revealed preferred, either directly or indirectly, to all of the bundles in the
shaded area, (x1 , x2 ) is in fact preferred to those bundles by the consumer
who made these choices. Another way to say this is to note that the true indifference curve through (x1 , x2 ), whatever it is, must lie above the shaded
region.
7.3 Recovering Preferences
By observing choices made by the consumer, we can learn about his or her
preferences. As we observe more and more choices, we can get a better and
better estimate of what the consumer’s preferences are like.
Such information about preferences can be very important in making
policy decisions. Most economic policy involves trading off some goods for
others: if we put a tax on shoes and subsidize clothing, we’ll probably end
up having more clothes and fewer shoes. In order to evaluate the desirability of such a policy, it is important to have some idea of what consumer
preferences between clothes and shoes look like. By examining consumer
choices, we can extract such information through the use of revealed preference and related techniques.
RECOVERING PREFERENCES
123
If we are willing to add more assumptions about consumer preferences,
we can get more precise estimates about the shape of indifference curves.
For example, suppose we observe two bundles Y and Z that are revealed
preferred to X, as in Figure 7.3, and that we are willing to postulate
preferences are convex. Then we know that all of the weighted averages
of Y and Z are preferred to X as well. If we are willing to assume that
preferences are monotonic, then all the bundles that have more of both
goods than X, Y , and Z—or any of their weighted averages—are also
preferred to X.
x2
Better
bundles
Y
Possible
indifference
curve
X
Z
Budget
lines
Worse
bundles
x1
Trapping the indifference curve. The upper shaded area
consists of bundles preferred to X, and the lower shaded area
consists of bundles revealed worse than X. The indifference
curve through X must lie somewhere in the region between the
two shaded areas.
The region labeled “Worse bundles” in Figure 7.3 consists of all the
bundles to which X is revealed preferred. That is, this region consists of
all the bundles that cost less than X, along with all the bundles that cost
less than bundles that cost less than X, and so on.
Figure
7.3
124 REVEALED PREFERENCE (Ch. 7)
Thus, in Figure 7.3, we can conclude that all of the bundles in the upper
shaded area are better than X, and that all of the bundles in the lower
shaded area are worse than X, according to the preferences of the consumer who made the choices. The true indifference curve through X must
lie somewhere between the two shaded sets. We’ve managed to trap the
indifference curve quite tightly simply by an intelligent application of the
idea of revealed preference and a few simple assumptions about preferences.
7.4 The Weak Axiom of Revealed Preference
All of the above relies on the assumption that the consumer has preferences
and that she is always choosing the best bundle of goods she can afford. If
the consumer is not behaving this way, the “estimates” of the indifference
curves that we constructed above have no meaning. The question naturally
arises: how can we tell if the consumer is following the maximizing model?
Or, to turn it around: what kind of observation would lead us to conclude
that the consumer was not maximizing?
Consider the situation illustrated in Figure 7.4. Could both of these
choices be generated by a maximizing consumer? According to the logic
of revealed preference, Figure 7.4 allows us to conclude two things: (1)
(x1 , x2 ) is preferred to (y1 , y2 ); and (2) (y1 , y2 ) is preferred to (x1 , x2 ).
This is clearly absurd. In Figure 7.4 the consumer has apparently chosen
(x1 , x2 ) when she could have chosen (y1 , y2 ), indicating that (x1 , x2 ) was
preferred to (y1 , y2 ), but then she chose (y1 , y2 ) when she could have chosen
(x1 , x2 )—indicating the opposite!
Clearly, this consumer cannot be a maximizing consumer. Either the
consumer is not choosing the best bundle she can afford, or there is some
other aspect of the choice problem that has changed that we have not observed. Perhaps the consumer’s tastes or some other aspect of her economic
environment have changed. In any event, a violation of this sort is not consistent with the model of consumer choice in an unchanged environment.
The theory of consumer choice implies that such observations will not
occur. If the consumers are choosing the best things they can afford, then
things that are affordable, but not chosen, must be worse than what is
chosen. Economists have formulated this simple point in the following
basic axiom of consumer theory
Weak Axiom of Revealed Preference (WARP). If (x1 , x2 ) is directly
revealed preferred to (y1 , y2 ), and the two bundles are not the same, then it
cannot happen that (y1 , y2 ) is directly revealed preferred to (x1 , x2 ).
In other words, if a bundle (x1 , x2 ) is purchased at prices (p1 , p2 ) and a
different bundle (y1 , y2 ) is purchased at prices (q1 , q2 ), then if
p1 x1 + p2 x2 ≥ p1 y1 + p2 y2 ,
CHECKING WARP
125
x2
(x1, x 2 )
Budget lines
(y1, y 2 )
x1
Violation of the Weak Axiom of Revealed Preference.
A consumer who chooses both (x1 , x2 ) and (y1 , y2 ) violates the
Weak Axiom of Revealed Preference.
it must not be the case that
q1 y1 + q2 y2 ≥ q1 x1 + q2 x2 .
In English: if the y-bundle is affordable when the x-bundle is purchased,
then when the y-bundle is purchased, the x-bundle must not be affordable.
The consumer in Figure 7.4 has violated WARP. Thus we know that this
consumer’s behavior could not have been maximizing behavior.1
There is no set of indifference curves that could be drawn in Figure 7.4
that could make both bundles maximizing bundles. On the other hand,
the consumer in Figure 7.5 satisfies WARP. Here it is possible to find
indifference curves for which his behavior is optimal behavior. One possible
choice of indifference curves is illustrated.
Optional
7.5 Checking WARP
It is important to understand that WARP is a condition that must be satisfied by a consumer who is always choosing the best things he or she can
afford. The Weak Axiom of Revealed Preference is a logical implication
1
Could we say his behavior is WARPed? Well, we could, but not in polite company.
Figure
7.4
126 REVEALED PREFERENCE (Ch. 7)
x2
Possible
indifference
curves
(x 1 , x2 )
(y 1 , y2 )
Budget
lines
x1
Figure
7.5
Satisfying WARP. Consumer choices that satisfy the Weak
Axiom of Revealed Preference and some possible indifference
curves.
of that model and can therefore be used to check whether or not a particular consumer, or an economic entity that we might want to model as a
consumer, is consistent with our economic model.
Let’s consider how we would go about systematically testing WARP in
practice. Suppose that we observe several choices of bundles of goods at
different prices. Let us use (pt1 , pt2 ) to denote the tth observation of prices
and (xt1 , xt2 ) to denote the tth observation of choices. To use a specific
example, let’s take the data in Table 7.1.
Table
7.1
Some consumption data.
Observation
1
2
3
p1
1
2
1
p2
2
1
1
x1
1
2
2
x2
2
1
2
Given these data, we can compute how much it would cost the consumer
to purchase each bundle of goods at each different set of prices, as we’ve
CHECKING WARP
127
done in Table 7.2. For example, the entry in row 3, column 1, measures
how much money the consumer would have to spend at the third set of
prices to purchase the first bundle of goods.
Cost of each bundle at each set of prices.
Prices
1
2
3
1
5
4∗
3∗
Bundles
2
4∗
5
3∗
3
6
6
4
The diagonal terms in Table 7.2 measure how much money the consumer
is spending at each choice. The other entries in each row measure how much
she would have spent if she had purchased a different bundle. Thus we can
see whether bundle 3, say, is revealed preferred to bundle 1, by seeing if the
entry in row 3, column 1 (how much the consumer would have to spend at
the third set of prices to purchase the first bundle) is less than the entry in
row 3, column 3 (how much the consumer actually spent at the third set
of prices to purchase the third bundle). In this particular case, bundle 1
was affordable when bundle 3 was purchased, which means that bundle 3
is revealed preferred to bundle 1. Thus we put a star in row 3, column 1,
of the table.
From a mathematical point of view, we simply put a star in the entry in
row s, column t, if the number in that entry is less than the number in row
s, column s.
We can use this table to check for violations of WARP. In this framework,
a violation of WARP consists of two observations t and s such that row t,
column s, contains a star and row s, column t, contains a star. For this
would mean that the bundle purchased at s is revealed preferred to the
bundle purchased at t and vice versa.
We can use a computer (or a research assistant) to check and see whether
there are any pairs of observations like these in the observed choices. If
there are, the choices are inconsistent with the economic theory of the
consumer. Either the theory is wrong for this particular consumer, or
something else has changed in the consumer’s environment that we have
not controlled for. Thus the Weak Axiom of Revealed Preference gives
us an easily checkable condition for whether some observed choices are
consistent with the economic theory of the consumer.
In Table 7.2, we observe that row 1, column 2, contains a star and row 2,
column 1, contains a star. This means that observation 2 could have been
Table
7.2
128 REVEALED PREFERENCE (Ch. 7)
chosen when the consumer actually chose observation 1 and vice versa. This
is a violation of the Weak Axiom of Revealed Preference. We can conclude
that the data depicted in Tables 7.1 and 7.2 could not be generated by a
consumer with stable preferences who was always choosing the best things
he or she could afford.
7.6 The Strong Axiom of Revealed Preference
The Weak Axiom of Revealed Preference described in the last section gives
us an observable condition that must be satisfied by all optimizing consumers. But there is a stronger condition that is sometimes useful.
We have already noted that if a bundle of goods X is revealed preferred
to a bundle Y , and Y is in turn revealed preferred to a bundle Z, then X
must in fact be preferred to Z. If the consumer has consistent preferences,
then we should never observe a sequence of choices that would reveal that
Z was preferred to X.
The Weak Axiom of Revealed Preference requires that if X is directly
revealed preferred to Y , then we should never observe Y being directly
revealed preferred to X. The Strong Axiom of Revealed Preference
(SARP) requires that the same sort of condition hold for indirect revealed
preference. More formally, we have the following.
Strong Axiom of Revealed Preference (SARP). If (x1 , x2 ) is revealed preferred to (y1 , y2 ) (either directly or indirectly) and (y1 , y2 ) is different from (x1 , x2 ), then (y1 , y2 ) cannot be directly or indirectly revealed
preferred to (x1 , x2 ).
It is clear that if the observed behavior is optimizing behavior then it
must satisfy the SARP. For if the consumer is optimizing and (x1 , x2 )
is revealed preferred to (y1 , y2 ), either directly or indirectly, then we must
have (x1 , x2 ) (y1 , y2 ). So having (x1 , x2 ) revealed preferred to (y1 , y2 ) and
(y1 , y2 ) revealed preferred to (x1 , x2 ) would imply that (x1 , x2 ) (y1 , y2 )
and (y1 , y2 ) (x1 , x2 ), which is a contradiction. We can conclude that
either the consumer must not be optimizing, or some other aspect of the
consumer’s environment—such as tastes, other prices, and so on—must
have changed.
Roughly speaking, since the underlying preferences of the consumer must
be transitive, it follows that the revealed preferences of the consumer must
be transitive. Thus SARP is a necessary implication of optimizing behavior: if a consumer is always choosing the best things that he can afford,
then his observed behavior must satisfy SARP. What is more surprising is
that any behavior satisfying the Strong Axiom can be thought of as being
generated by optimizing behavior in the following sense: if the observed
choices satisfy SARP, we can always find nice, well-behaved preferences
HOW TO CHECK SARP
129
that could have generated the observed choices. In this sense SARP is a
sufficient condition for optimizing behavior: if the observed choices satisfy
SARP, then it is always possible to find preferences for which the observed
behavior is optimizing behavior. The proof of this claim is unfortunately
beyond the scope of this book, but appreciation of its importance is not.
What it means is that SARP gives us all of the restrictions on behavior
imposed by the model of the optimizing consumer. For if the observed
choices satisfy SARP, we can “construct” preferences that could have generated these choices. Thus SARP is both a necessary and a sufficient
condition for observed choices to be compatible with the economic model
of consumer choice.
Does this prove that the constructed preferences actually generated the
observed choices? Of course not. As with any scientific statement, we can
only show that observed behavior is not inconsistent with the statement.
We can’t prove that the economic model is correct; we can just determine
the implications of that model and see if observed choices are consistent
with those implications.
Optional
7.7 How to Check SARP
Let us suppose that we have a table like Table 7.2 that has a star in row t
and column s if observation t is directly revealed preferred to observation
s. How can we use this table to check SARP?
The easiest way is first to transform the table. An example is given in
Table 7.3. This is a table just like Table 7.2, but it uses a different set of
numbers. Here the stars indicate direct revealed preference. The star in
parentheses will be explained below.
Table
7.3
How to check SARP.
Prices
1
2
3
1
20
21
12
Bundles
2
10∗
20
15
3
22(∗)
15∗
10
Now we systematically look through the entries of the table and see
if there are any chains of observations that make some bundle indirectly
revealed preferred to that one. For example, bundle 1 is directly revealed
preferred to bundle 2 since there is a star in row 1, column 2. And bundle
130 REVEALED PREFERENCE (Ch. 7)
2 is directly revealed preferred to bundle 3, since there is a star in row 2,
column 3. Therefore bundle 1 is indirectly revealed preferred to bundle 3,
and we indicate this by putting a star (in parentheses) in row 1, column 3.
In general, if we have many observations, we will have to look for chains
of arbitrary length to see if one observation is indirectly revealed preferred
to another. Although it may not be exactly obvious how to do this, it
turns out that there are simple computer programs that can calculate the
indirect revealed preference relation from the table describing the direct
revealed preference relation. The computer can put a star in location st
of the table if observation s is revealed preferred to observation t by any
chain of other observations.
Once we have done this calculation, we can easily test for SARP. We just
see if there is a situation where there is a star in row t, column s, and also a
star in row s, column t. If so, we have found a situation where observation
t is revealed preferred to observation s, either directly or indirectly, and,
at the same time, observation s is revealed preferred to observation t. This
is a violation of the Strong Axiom of Revealed Preference.
On the other hand, if we do not find such violations, then we know that
the observations we have are consistent with the economic theory of the
consumer. These observations could have been made by an optimizing
consumer with well-behaved preferences. Thus we have a completely operational test for whether or not a particular consumer is acting in a way
consistent with economic theory.
This is important, since we can model several kinds of economic units as
behaving like consumers. Think, for example, of a household consisting of
several people. Will its consumption choices maximize “household utility”?
If we have some data on household consumption choices, we can use the
Strong Axiom of Revealed Preference to see. Another economic unit that
we might think of as acting like a consumer is a nonprofit organization
like a hospital or a university. Do universities maximize a utility function in making their economic choices? If we have a list of the economic
choices that a university makes when faced with different prices, we can,
in principle, answer this kind of question.
7.8 Index Numbers
Suppose we examine the consumption bundles of a consumer at two different times and we want to compare how consumption has changed from one
time to the other. Let b stand for the base period, and let t be some other
time. How does “average” consumption in year t compare to consumption
in the base period?
Suppose that at time t prices are (pt1 , pt2 ) and that the consumer chooses
t
(x1 , xt2 ). In the base period b, the prices are (pb1 , pb2 ), and the consumer’s
INDEX NUMBERS
131
choice is (xb1 , xb2 ). We want to ask how the “average” consumption of the
consumer has changed.
If we let w1 and w2 be some “weights” that go into making an average,
then we can look at the following kind of quantity index:
Iq =
w1 xt1 + w2 xt2
.
w1 xb1 + w2 xb2
If Iq is greater than 1, we can say that the “average” consumption has gone
up in the movement from b to t; if Iq is less than 1, we can say that the
“average” consumption has gone down.
The question is, what do we use for the weights? A natural choice is to
use the prices of the goods in question, since they measure in some sense
the relative importance of the two goods. But there are two sets of prices
here: which should we use?
If we use the base period prices for the weights, we have something called
a Laspeyres index, and if we use the t period prices, we have something
called a Paasche index. Both of these indices answer the question of what
has happened to “average” consumption, but they just use different weights
in the averaging process.
Substituting the t period prices for the weights, we see that the Paasche
quantity index is given by
Pq =
pt1 xt1 + pt2 xt2
,
pt1 xb1 + pt2 xb2
and substituting the b period prices shows that the Laspeyres quantity
index is given by
pb xt + pb2 xt2
Lq = 1b 1b
.
p1 x1 + pb2 xb2
It turns out that the magnitude of the Laspeyres and Paasche indices can
tell us something quite interesting about the consumer’s welfare. Suppose
that we have a situation where the Paasche quantity index is greater than 1:
Pq =
pt1 xt1 + pt2 xt2
> 1.
pt1 xb1 + pt2 xb2
What can we conclude about how well-off the consumer is at time t as
compared to his situation at time b?
The answer is provided by revealed preference. Just cross multiply this
inequality to give
pt1 xt1 + pt2 xt2 > pt1 xb1 + pt2 xb2 ,
which immediately shows that the consumer must be better off at t than at
b, since he could have consumed the b consumption bundle in the t situation
but chose not to do so.
132 REVEALED PREFERENCE (Ch. 7)
What if the Paasche index is less than 1? Then we would have
pt1 xt1 + pt2 xt2 < pt1 xb1 + pt2 xb2 ,
which says that when the consumer chose bundle (xt1 , xt2 ), bundle (xb1 , xb2 )
was not affordable. But that doesn’t say anything about the consumer’s
ranking of the bundles. Just because something costs more than you can
afford doesn’t mean that you prefer it to what you’re consuming now.
What about the Laspeyres index? It works in a similar way. Suppose
that the Laspeyres index is less than 1:
Lq =
pb1 xt1 + pb2 xt2
< 1.
pb1 xb1 + pb2 xb2
Cross multiplying yields
pb1 xb1 + pb2 xb2 > pb1 xt1 + pb2 xt2 ,
which says that (xb1 , xb2 ) is revealed preferred to (xt1 , xt2 ). Thus the consumer
is better off at time b than at time t.
7.9 Price Indices
Price indices work in much the same way. In general, a price index will be
a weighted average of prices:
Ip =
pt1 w1 + pt2 w2
.
pb1 w1 + pb2 w2
In this case it is natural to choose the quantities as the weights for computing the averages. We get two different indices, depending on our choice
of weights. If we choose the t period quantities for weights, we get the
Paasche price index:
Pp =
pt1 xt1 + pt2 xt2
,
pb1 xt1 + pb2 xt2
and if we choose the base period quantities we get the Laspeyres price
index:
pt xb + pt2 xb2
.
Lp = 1b 1b
p1 x1 + pb2 xb2
Suppose that the Paasche price index is less than 1; what does revealed
preference have to say about the welfare situation of the consumer in periods t and b?
PRICE INDICES
133
Revealed preference doesn’t say anything at all. The problem is that
there are now different prices in the numerator and in the denominator of
the fractions defining the indices, so the revealed preference comparison
can’t be made.
Let’s define a new index of the change in total expenditure by
M=
pt1 xt1 + pt2 xt2
.
pb1 xb1 + pb2 xb2
This is the ratio of total expenditure in period t to the total expenditure
in period b.
Now suppose that you are told that the Paasche price index was greater
than M . This means that
Pp =
pt1 xt1 + pt2 xt2
pt1 xt1 + pt2 xt2
>
.
pb1 xt1 + pb2 xt2
pb1 xb1 + pb2 xb2
Canceling the numerators from each side of this expression and cross multiplying, we have
pb1 xb1 + pb2 xb2 > pb1 xt1 + pb2 xt2 .
This statement says that the bundle chosen at year b is revealed preferred
to the bundle chosen at year t. This analysis implies that if the Paasche
price index is greater than the expenditure index, then the consumer must
be better off in year b than in year t.
This is quite intuitive. After all, if prices rise by more than income rises
in the movement from b to t, we would expect that would tend to make the
consumer worse off. The revealed preference analysis given above confirms
this intuition.
A similar statement can be made for the Laspeyres price index. If the
Laspeyres price index is less than M , then the consumer must be better off
in year t than in year b. Again, this simply confirms the intuitive idea that
if prices rise less than income, the consumer would become better off. In
the case of price indices, what matters is not whether the index is greater
or less than 1, but whether it is greater or less than the expenditure index.
EXAMPLE: Indexing Social Security Payments
Many elderly people have Social Security payments as their sole source
of income. Because of this, there have been attempts to adjust Social
Security payments in a way that will keep purchasing power constant even
when prices change. Since the amount of payments will then depend on the
movement of some price index or cost-of-living index, this kind of scheme
is referred to as indexing.
134 REVEALED PREFERENCE (Ch. 7)
One indexing proposal goes as follows. In some base year b, economists measure the average consumption bundle of senior citizens. In each
subsequent year the Social Security system adjusts payments so that the
“purchasing power” of the average senior citizen remains constant in the
sense that the average Social Security recipient is just able to afford the
consumption bundle available in year b, as depicted in Figure 7.6.
x2
Indifference
curves
Base period
optimal choice
x2b
Optimal choice
after indexing
Budget
line
before
indexing
Budget line
after indexing
x1b
Figure
7.6
Base
period
budget
(p 1b , p b2 )
x1
Social Security. Changing prices will typically make the consumer better off than in the base year.
One curious result of this indexing scheme is that the average senior
citizen will almost always be better off than he or she was in the base year
b. Suppose that year b is chosen as the base year for the price index. Then
the bundle (xb1 , xb2 ) is the optimal bundle at the prices (pb1 , pb2 ). This means
that the budget line at prices (pb1 , pb2 ) must be tangent to the indifference
curve through (xb1 , xb2 ).
Now suppose that prices change. To be specific, suppose that prices
increase so that the budget line, in the absence of Social Security, would
shift inward and tilt. The inward shift is due to the increase in prices; the
tilt is due to the change in relative prices. The indexing program would
then increase the Social Security payment so as to make the original bundle
(xb1 , xb2 ) affordable at the new prices. But this means that the budget line
would cut the indifference curve, and there would be some other bundle
REVIEW QUESTIONS
135
on the budget line that would be strictly preferred to (xb1 , xb2 ). Thus the
consumer would typically be able to choose a better bundle than he or she
chose in the base year.
Summary
1. If one bundle is chosen when another could have been chosen, we say
that the first bundle is revealed preferred to the second.
2. If the consumer is always choosing the most preferred bundles he or she
can afford, this means that the chosen bundles must be preferred to the
bundles that were affordable but weren’t chosen.
3. Observing the choices of consumers can allow us to “recover” or estimate the preferences that lie behind those choices. The more choices we
observe, the more precisely we can estimate the underlying preferences that
generated those choices.
4. The Weak Axiom of Revealed Preference (WARP) and the Strong Axiom of Revealed Preference (SARP) are necessary conditions that consumer
choices have to obey if they are to be consistent with the economic model
of optimizing choice.
REVIEW QUESTIONS
1. When prices are (p1 , p2 ) = (1, 2) a consumer demands (x1 , x2 ) = (1, 2),
and when prices are (q1 , q2 ) = (2, 1) the consumer demands (y1 , y2 ) = (2, 1).
Is this behavior consistent with the model of maximizing behavior?
2. When prices are (p1 , p2 ) = (2, 1) a consumer demands (x1 , x2 ) = (1, 2),
and when prices are (q1 , q2 ) = (1, 2) the consumer demands (y1 , y2 ) = (2, 1).
Is this behavior consistent with the model of maximizing behavior?
3. In the preceding exercise, which bundle is preferred by the consumer,
the x-bundle or the y-bundle?
4. We saw that the Social Security adjustment for changing prices would
typically make recipients at least as well-off as they were at the base year.
What kind of price changes would leave them just as well-off, no matter
what kind of preferences they had?
5. In the same framework as the above question, what kind of preferences
would leave the consumer just as well-off as he was in the base year, for all
price changes?
In the last section, you were given a consumer’s preferences and then you
solved for his or her demand behavior. In this chapter we turn this process
around: you are given information about a consumer’s demand behavior
and you must deduce something about the consumer’s preferences. The
main tool is the weak axiom of revealed preference. This axiom says the
following. If a consumer chooses commodity bundle A when she can afford
bundle B, then she will never choose bundle B from any budget in which
she can also afford A. The idea behind this axiom is that if you choose A
when you could have had B, you must like A better than B. But if you
like A better than B, then you will never choose B when you can have A.
If somebody chooses A when she can afford B, we say that for her, A is
directly revealed preferred to B. The weak axiom says that if A is directly
revealed preferred to B, then B is not directly revealed preferred to A.
Let us look at an example of how you check whether one bundle is revealed preferred to another. Suppose that a consumer buys the bundle
A
A
A A
A
(xA
1 , x2 ) = (2, 3) at prices (p1 , p2 ) = (1, 4). The cost of bundle (x1 , x2 )
at these prices is (2 × 1) + (3 × 4) = 14. Bundle (2, 3) is directly revealed
preferred to all the other bundles that she can afford at prices (1, 4), when
she has an income of 14. For example, the bundle (5, 2) costs only 13 at
prices (1, 4), so we can say that for this consumer (2, 3) is directly revealed
preferred to (1, 4).
You will also have some problems about price and quantity indexes.
A price index is a comparison of average price levels between two different
times or two different places. If there is more than one commodity, it is not
necessarily the case that all prices changed in the same proportion. Let us
suppose that we want to compare the price level in the “current year” with
the price level in some “base year.” One way to make this comparison
is to compare the costs in the two years of some “reference” commodity
bundle. Two reasonable choices for the reference bundle come to mind.
One possibility is to use the current year’s consumption bundle for the
reference bundle. The other possibility is to use the bundle consumed
in the base year. Typically these will be different bundles. If the baseyear bundle is the reference bundle, the resulting price index is called the
Laspeyres price index. If the current year’s consumption bundle is the
reference bundle, then the index is called the Paasche price index.
Suppose that there are just two goods. In 1980, the prices were (1, 3) and
a consumer consumed the bundle (4, 2). In 1990, the prices were (2, 4) and
the consumer consumed the bundle (3, 3). The cost of the 1980 bundle at
1980 prices is (1 × 4) + (3 × 2) = 10. The cost of this same bundle at 1990
prices is (2 × 4) + (4 × 2) = 16. If 1980 is treated as the base year and
1990 as the current year, the Laspeyres price ratio is 16/10. To calculate
the Paasche price ratio, you find the ratio of the cost of the 1990 bundle
at 1990 prices to the cost of the same bundle at 1980 prices. The 1990
bundle costs (2 × 3) + (4 × 3) = 18 at 1990 prices. The same bundle cost
(1 × 3) + (3 × 3) = 12 at 1980 prices. Therefore the Paasche price index
is 18/12. Notice that both price indexes indicate that prices rose, but
because the price changes are weighted differently, the two approaches
give different price ratios.
Making an index of the “quantity” of stuff consumed in the two
periods presents a similar problem. How do you weight changes in the
amount of good 1 relative to changes in the amount of good 2? This time
we could compare the cost of the two periods’ bundles evaluated at some
reference prices. Again there are at least two reasonable possibilities, the
Laspeyres quantity index and the Paasche quantity index. The Laspeyres
quantity index uses the base-year prices as the reference prices, and the
Paasche quantity index uses current prices as reference prices.
In the example above, the Laspeyres quantity index is the ratio of the
cost of the 1990 bundle at 1980 prices to the cost of the 1980 bundle at
1980 prices. The cost of the 1990 bundle at 1980 prices is 12 and the cost
of the 1980 bundle at 1980 prices is 10, so the Laspeyres quantity index
is 12/10. The cost of the 1990 bundle at 1990 prices is 18 and the cost
of the 1980 bundle at 1990 prices is 16. Therefore the Paasche quantity
index is 18/16.
When you have completed this section, we hope that you will be able
to do the following:
• Decide from given data about prices and consumption whether one
commodity bundle is preferred to another.
• Given price and consumption data, calculate Paasche and Laspeyres
price and quantity indexes.
• Use the weak axiom of revealed preferences to make logical deductions about behavior.
• Use the idea of revealed preference to make comparisons of well-being
across time and across countries.
7.1 (0) When prices are (4, 6), Goldie chooses the bundle (6, 6), and
when prices are (6, 3), she chooses the bundle (10, 0).
(a) On the graph below, show Goldie’s first budget line in red ink and
her second budget line in blue ink. Mark her choice from the first budget
with the label A, and her choice from the second budget with the label
B.
(b) Is Goldie’s behavior consistent with the weak axiom of revealed preference?
.
Good 2
20
15
10
5
0
5
10
15
20
Good 1
7.2 (0) Freddy Frolic consumes only asparagus and tomatoes, which are
highly seasonal crops in Freddy’s part of the world. He sells umbrellas for
a living, which provides a fluctuating income depending on the weather.
But Freddy doesn’t mind; he never thinks of tomorrow, so each week he
spends as much as he earns. One week, when the prices of asparagus and
tomatoes were each $1 a pound, Freddy consumed 15 pounds of each. Use
blue ink to show the budget line in the diagram below. Label Freddy’s
consumption bundle with the letter A.
(a) What is Freddy’s income?
.
(b) The next week the price of tomatoes rose to $2 a pound, but the price
of asparagus remained at $1 a pound. By chance, Freddy’s income had
changed so that his old consumption bundle of (15,15) was just affordable
at the new prices. Use red ink to draw this new budget line on the graph
below. Does your new budget line go through the point A?
What is the slope of this line?
.
(c) How much asparagus can he afford now if he spent all of his income
on asparagus?
.
(d) What is Freddy’s income now?
.
(e) Use pencil to shade the bundles of goods on Freddy’s new red budget
line that he definitely will not purchase with this budget. Is it possible
that he would increase his consumption of tomatoes when his budget
changes from the blue line to the red one?
.
Tomatoes
40
30
20
10
0
10
20
30
40
Asparagus
7.3 (0) Pierre consumes bread and wine. For Pierre, the price of bread
is 4 francs per loaf, and the price of wine is 4 francs per glass. Pierre has
an income of 40 francs per day. Pierre consumes 6 glasses of wine and 4
loaves of bread per day.
Bob also consumes bread and wine. For Bob, the price of bread is
1/2 dollar per loaf and the price of wine is 2 dollars per glass. Bob has
an income of $15 per day.
(a) If Bob and Pierre have the same tastes, can you tell whether Bob is
better off than Pierre or vice versa? Explain.
.
(b) Suppose prices and incomes for Pierre and Bob are as above and that
Pierre’s consumption is as before. Suppose that Bob spends all of his income. Give an example of a consumption bundle of wine and bread such
that, if Bob bought this bundle, we would know that Bob’s tastes are not
the same as Pierre’s tastes.
.
7.4 (0) Here is a table of prices and the demands of a consumer named
Ronald whose behavior was observed in 5 different price-income situations.
Situation
A
B
C
D
E
p1
1
1
1
3
1
p2
1
2
1
1
2
x1
5
35
10
5
10
x2
35
10
15
15
10
(a) Sketch each of his budget lines and label the point chosen in each case
by the letters A, B, C, D, and E.
(b) Is Ronald’s behavior consistent with the Weak Axiom of Revealed
Preference?
.
(c) Shade lightly in red ink all of the points that you are certain are worse
for Ronald than the bundle C.
(d) Suppose that you are told that Ronald has convex and monotonic
preferences and that he obeys the strong axiom of revealed preference.
Shade lightly in blue ink all of the points that you are certain are at least
as good as the bundle C.
x2
40
30
20
10
0
10
20
30
40
x1
7.5 (0) Horst and Nigel live in different countries. Possibly they have
different preferences, and certainly they face different prices. They each
consume only two goods, x and y. Horst has to pay 14 marks per unit of
x and 5 marks per unit of y. Horst spends his entire income of 167 marks
on 8 units of x and 11 units of y. Good x costs Nigel 9 quid per unit and
good y costs him 7 quid per unit. Nigel buys 10 units of x and 9 units of
y.
(a) Which prices and income would Horst prefer, Nigel’s income and
prices or his own, or is there too little information to tell? Explain your answer.
.
(b) Would Nigel prefer to have Horst’s income and prices or his own, or
is there too little information to tell?
.
7.6 (0) Here is a table that illustrates some observed prices and choices
for three different goods at three different prices in three different situations.
Situation
A
B
C
p1
1
4
3
p2
2
1
1
p3
8
8
2
x1
2
3
2
x2
1
4
6
x3
3
2
2
(a) We will fill in the table below as follows. Where i and j stand for any
of the letters A, B, and C in Row i and Column j of the matrix, write
the value of the Situation-j bundle at the Situation-i prices. For example,
in Row A and Column A, we put the value of the bundle purchased in
Situation A at Situation A prices. From the table above, we see that in
Situation A, the consumer bought bundle (2, 1, 3) at prices (1, 2, 8). The
cost of this bundle A at prices A is therefore (1×2)+(2×1)+(8×3) = 28,
so we put 28 in Row A, Column A. In Situation B the consumer bought
bundle (3, 4, 2). The value of the Situation-B bundle, evaluated at the
situation-A prices is (1 × 3) + (2 × 4) + (8 × 2) = 27, so put 27 in Row
A, Column B. We have filled in some of the boxes, but we leave a few for
you to do.
Prices/Quantities
A
A
B
28
27
B
C
32
13
C
30
17
(b) Fill in the entry in Row i and Column j of the table below with a D if
the Situation-i bundle is directly revealed preferred to the Situation-j bundle. For example, in Situation A the consumer’s expenditure is $28. We
see that at Situation-A prices, he could also afford the Situation-B bundle, which cost 27. Therefore the Situation-A bundle is directly revealed
preferred to the Situation-B bundle, so we put a D in Row A, Column
B. Now let us consider Row B, Column A. The cost of the Situation-B
bundle at Situation-B prices is 32. The cost of the Situation-A bundle
at Situation-B prices is 33. So, in Situation B, the consumer could not
afford the Situation-A bundle. Therefore Situation B is not directly revealed preferred to Situation A. So we leave the entry in Row B, Column
A blank. Generally, there is a D in Row i Column j if the number in the
ij entry of the table in part (a) is less than or equal to the entry in Row
i, Column i. There will be a violation of WARP if for some i and j, there
is a D in Row i Column j and also a D in Row j, Column i. Do these
.
observations violate WARP?
Situation
A
B
C
A
B
C
—
D
—
—
(c) Now fill in Row i, Column j with an I if observation i is indirectly
revealed preferred to j. Do these observations violate the Strong Axiom
of Revealed Preference?
.
7.7 (0) It is January, and Joe Grad, whom we met in Chapter 5, is
shivering in his apartment when the phone rings. It is Mandy Manana,
one of the students whose price theory problems he graded last term.
Mandy asks if Joe would be interested in spending the month of February
in her apartment. Mandy, who has switched majors from economics to
political science, plans to go to Aspen for the month and so her apartment
will be empty (alas). All Mandy asks is that Joe pay the monthly service
charge of $40 charged by her landlord and the heating bill for the month
of February. Since her apartment is much better insulated than Joe’s,
it only costs $1 per month to raise the temperature by 1 degree. Joe
thanks her and says he will let her know tomorrow. Joe puts his earmuffs
back on and muses. If he accepts Mandy’s offer, he will still have to pay
rent on his current apartment but he won’t have to heat it. If he moved,
heating would be cheaper, but he would have the $40 service charge. The
outdoor temperature averages 20 degrees Fahrenheit in February, and it
costs him $2 per month to raise his apartment temperature by 1 degree.
Joe is still grading homework and has $100 a month left to spend on food
and utilities after he has paid the rent on his apartment. The price of
food is still $1 per unit.
(a) Draw Joe’s budget line for February if he moves to Mandy’s apartment
and on the same graph, draw his budget line if he doesn’t move.
(b) After drawing these lines himself, Joe decides that he would be better
off not moving. From this, we can tell, using the principle of revealed
preference that Joe must plan to keep his apartment at a temperature of
less than
.
(c) Joe calls Mandy and tells her his decision. Mandy offers to pay half
the service charge. Draw Joe’s budget line if he accepts Mandy’s new
offer. Joe now accepts Mandy’s offer. From the fact that Joe accepted
this offer we can tell that he plans to keep the temperature in Mandy’s
apartment above
.
Food
120
100
80
60
40
20
0
10
20
30
40
50
60
70
80
Temperature
7.8 (0) Lord Peter Pommy is a distinguished criminologist, schooled
in the latest techniques of forensic revealed preference. Lord Peter is investigating the disappearance of Sir Cedric Pinchbottom who abandoned
his aging mother on a street corner in Liverpool and has not been seen
since. Lord Peter has learned that Sir Cedric left England and is living
under an assumed name somewhere in the Empire. There are three suspects, R. Preston McAfee of Brass Monkey, Ontario, Canada, Richard
Manning of North Shag, New Zealand, and Richard Stevenson of Gooey
Shoes, Falkland Islands. Lord Peter has obtained Sir Cedric’s diary, which
recorded his consumption habits in minute detail. By careful observation,
he has also discovered the consumption behavior of McAfee, Manning, and
Stevenson. All three of these gentlemen, like Sir Cedric, spend their entire
incomes on beer and sausage. Their dossiers reveal the following:
• Sir Cedric Pinchbottom — In the year before his departure, Sir
Cedric consumed 10 kilograms of sausage and 20 liters of beer per
week. At that time, beer cost 1 English pound per liter and sausage
cost 1 English pound per kilogram.
• R. Preston McAfee — McAfee is known to consume 5 liters of beer
and 20 kilograms of sausage. In Brass Monkey, Ontario beer costs 1
Canadian dollar per liter and sausage costs 2 Canadian dollars per
kilogram.
• Richard Manning — Manning consumes 5 kilograms of sausage
and 10 liters of beer per week. In North Shag, a liter of beer costs
2 New Zealand dollars and sausage costs 2 New Zealand dollars per
kilogram.
• Richard Stevenson — Stevenson consumes 5 kilograms of sausage
and 30 liters of beer per week. In Gooey Shoes, a liter of beer costs 10
Falkland Island pounds and sausage costs 20 Falkland Island pounds
per kilogram.
(a) Draw the budget line for each of the three fugitives, using a different
color of ink for each one. Label the consumption bundle that each chooses.
On this graph, superimpose Sir Cedric’s budget line and the bundle he
chose.
Sausage
40
30
20
10
0
10
20
30
40
Beer
(b) After pondering the dossiers for a few moments, Lord Peter announced. “Unless Sir Cedric has changed his tastes, I can eliminate one
of the suspects. Revealed preference tells me that one of the suspects is
innocent.” Which one?
.
(c) After thinking a bit longer, Lord Peter announced. “If Sir Cedric
left voluntarily, then he would have to be better off than he was before.
Therefore if Sir Cedric left voluntarily and if he has not changed his tastes,
he must be living in
.
7.9 (1) The McCawber family is having a tough time making ends meet.
They spend $100 a week on food and $50 on other things. A new welfare
program has been introduced that gives them a choice between receiving
a grant of $50 per week that they can spend any way they want, and
buying any number of $2 food coupons for $1 apiece. (They naturally
are not allowed to resell these coupons.) Food is a normal good for the
McCawbers. As a family friend, you have been asked to help them decide
on which option to choose. Drawing on your growing fund of economic
knowledge, you proceed as follows.
(a) On the graph below, draw their old budget line in red ink and label
their current choice C. Now use black ink to draw the budget line that
they would have with the grant. If they chose the coupon option, how
much food could they buy if they spent all their money on food coupons?
How much could they spend on other things if they bought
no food?
Use blue ink to draw their budget line if they
choose the coupon option.
Other things
180
150
120
90
60
30
0
30
60
90
120
150
180
210
240
Food
(b) Using the fact that food is a normal good for the McCawbers, and
knowing what they purchased before, darken the portion of the black
budget line where their consumption bundle could possibly be if they
chose the lump-sum grant option. Label the ends of this line segment A
and B.
(c) After studying the graph you have drawn, you report to the McCawbers. “I have enough information to be able to tell you which choice to
make. You should choose the
because
.
(d) Mr. McCawber thanks you for your help and then asks, “Would you
have been able to tell me what to do if you hadn’t known whether food
was a normal good for us?” On the axes below, draw the same budget
lines you drew on the diagram above, but draw indifference curves for
which food is not a normal good and for which the McCawbers would be
better off with the program you advised them not to take.
Other things
180
150
120
90
60
30
0
30
60
90
120
150
180
210
240
Food
7.10 (0) In 1933, the Swedish economist Gunnar Myrdal (who later won
a Nobel prize in economics) and a group of his associates at Stockholm
University collected a fantastically detailed historical series of prices and
price indexes in Sweden from 1830 until 1930. This was published in a
book called The Cost of Living in Sweden. In this book you can find
100 years of prices for goods such as oat groats, hard rye bread, salted
codfish, beef, reindeer meat, birchwood, tallow candles, eggs, sugar, and
coffee. There are also estimates of the quantities of each good consumed
by an average working-class family in 1850 and again in 1890.
The table below gives prices in 1830, 1850, 1890, and 1913, for flour,
meat, milk, and potatoes. In this time period, these four staple foods
accounted for about 2/3 of the Swedish food budget.
Prices of Staple Foods in Sweden
Prices are in Swedish kronor per kilogram, except for milk, which is in
Swedish kronor per liter.
Grain Flour
Meat
Milk
Potatoes
1830
.14
.28
.07
.032
1850
.14
.34
.08
.044
1890
.16
.66
.10
.051
1913
.19
.85
.13
.064
Based on the tables published in Myrdal’s book, typical consumption bundles for a working-class Swedish family in 1850 and 1890 are
listed below. (The reader should be warned that we have made some
approximations and simplifications to draw these simple tables from the
much more detailed information in the original study.)
Quantities Consumed by a Typical Swedish Family
Quantities are measured in kilograms per year, except for milk, which is
measured in liters per year.
Grain Flour
Meat
Milk
Potatoes
1850
165
22
120
200
1890
220
42
180
200
(a) Complete the table below, which reports the annual cost of the 1850
and 1890 bundles of staple foods at various years’ prices.
Cost of 1850 and 1890 Bundles at Various Years’ Prices
Cost
Cost at 1830 Prices
1850 bundle
1890 bundle
44.1
61.6
78.5
113.7
Cost at 1850 Prices
Cost at 1890 Prices
Cost at 1913 Prices
(b) Is the 1890 bundle revealed preferred to the 1850 bundle?
.
(c) The Laspeyres quantity index for 1890 with base year 1850 is the ratio
of the value of the 1890 bundle at 1850 prices to the value of the 1850
bundle at 1850 prices. Calculate the Laspeyres quantity index of staple
food consumption for 1890 with base year 1850.
.
(d) The Paasche quantity index for 1890 with base year 1850 is the ratio
of the value of the 1890 bundle at 1890 prices to the value of the 1850
bundle at 1890 prices. Calculate the Paasche quantity index for 1890 with
base year 1850.
.
(e) The Laspeyres price index for 1890 with base year 1850 is calculated
using 1850 quantities for weights. Calculate the Laspeyres price index for
1890 with base year 1850 for this group of four staple foods.
.
(f ) If a Swede were rich enough in 1850 to afford the 1890 bundle of staple
times as much on these
foods in 1850, he would have to spend
foods as does the typical Swedish worker of 1850.
(g) If a Swede in 1890 decided to purchase the same bundle of food staples
that was consumed by typical 1850 workers, he would spend the fraction
of the amount that the typical Swedish worker of 1890 spends
on these goods.
7.11 (0) This question draws from the tables in the previous question.
Let us try to get an idea of what it would cost an American family at
today’s prices to purchase the bundle consumed by an average Swedish
family in 1850. In the United States today, the price of flour is about $.40
per kilogram, the price of meat is about $3.75 per kilogram, the price of
milk is about $.50 per liter, and the price of potatoes is about $1 per
kilogram. We can also compute a Laspeyres price index across time and
across countries and use it to estimate the value of a current US dollar
relative to the value of an 1850 Swedish kronor.
(a) How much would it cost an American at today’s prices to buy the bundle of staple food commodities purchased by an average Swedish workingclass family in 1850?
.
(b) Myrdal estimates that in 1850, about 2/3 of the average family’s
budget was spent on food. In turn, the four staples discussed in the last
question constitute about 2/3 of the average family’s food budget. If the
prices of other goods relative to the price of the food staples are similar
in the United States today to what they were in Sweden in 1850, about
how much would it cost an American at current prices to consume the
same overall consumption bundle consumed by a Swedish working-class
family in 1850?
.
(c) Using the Swedish consumption bundle of staple foods in 1850 as
weights, calculate a Laspeyres price index to compare prices in current
American dollars relative to prices in 1850 Swedish kronor.
If we use this to estimate the value of current dollars relative to 1850
Swedish kronor, we would say that a U.S. dollar today is worth about
1850 Swedish kronor.
7.12 (0) Suppose that between 1960 and 1985, the price of all goods
exactly doubled while every consumer’s income tripled.
(a) Would the Laspeyres price index for 1985, with base year 1960 be
less than 2, greater than 2, or exactly equal to 2?
about the Paasche price index?
What
.
(b) If bananas are a normal good, will total banana consumption increase?
If everybody has homothetic preferences, can you
determine by what percentage total banana consumption must have increased? Explain.
.
7.13 (1) Norm and Sheila consume only meat pies and beer. Meat pies
used to cost $2 each and beer was $1 per can. Their gross income used
to be $60 per week, but they had to pay an income tax of $10. Use red
ink to sketch their old budget line for meat pies and beer.
Beer
60
50
40
30
20
10
0
10
20
30
40
50
60
Meat pies
(a) They used to buy 30 cans of beer per week and spent the rest of their
income on meat pies. How many meat pies did they buy?
.
(b) The government decided to eliminate the income tax and to put a
sales tax of $1 per can on beer, raising its price to $2 per can. Assuming
that Norm and Sheila’s pre-tax income and the price of meat pies did not
change, draw their new budget line in blue ink.
(c) The sales tax on beer induced Norm and Sheila to reduce their beer
consumption to 20 cans per week. What happened to their consumption
of meat pies?
How much revenue did this tax
raise from Norm and Sheila?
.
(d) This part of the problem will require some careful thinking. Suppose
that instead of just taxing beer, the government decided to tax both beer
and meat pies at the same percentage rate, and suppose that the price
of beer and the price of meat pies each went up by the full amount of
the tax. The new tax rate for both goods was set high enough to raise
exactly the same amount of money from Norm and Sheila as the tax on
beer used to raise. This new tax collects $
sold and $
for every bottle of beer
for every meat pie sold. (Hint: If both goods are
taxed at the same rate, the effect is the same as an income tax.) How
large an income tax would it take to raise the same revenue as the $1 tax
on beer?
Now you can figure out how big a tax on each good
is equivalent to an income tax of the amount you just found.
(e) Use black ink to draw the budget line for Norm and Sheila that corresponds to the tax in the last section. Are Norm and Sheila better off
having just beer taxed or having both beer and meat pies taxed if both
sets of taxes raise the same revenue?
principle of revealed preference.)
(Hint: Try to use the
7.1 In Problem 7.1, if the only information we had about Goldie were
that she chooses the bundle (6,6) when prices are (6,3) and she chooses
the bundle (10, 0) when prices are (5,5), then we could conclude that
(a) the bundle (6,6) is revealed preferred to (10,0) but there is no evidence
that she violates WARP.
(b) neither bundle is revealed preferred to the other.
(c) Goldie violates WARP.
(d) the bundle (10,0) is revealed preferred to (6,6) and she violates WARP.
(e) the bundle (10,0) is revealed preferred to (6,6) and there is no evidence
that she violates WARP.
7.2 In Problem 7.3, Pierre’s friend Henri lives in a town where he has
to pay 3 francs per glass of wine and 6 francs per loaf of bread. Henri
consumes 6 glasses of wine and 4 loaves of bread per day. Recall that Bob
has an income of $15 per day and pays $.50 per loaf of bread and $2 per
glass of wine. If Bob has the same tastes as Henri and if the only thing
that either of them cares about is consumption of bread and wine, we can
deduce
(a) nothing about whether one is better than the other.
(b) Henri is better off than Bob.
(c) Bob is better off than Henri.
(d) both of them violate the weak axiom of revealed preferences.
(e) Bob and Henri are equally well off.
7.3 Let us reconsider the case of Ronald in Problem 7.4. Let the prices
and consumptions in the base year be as in situation D, where p1 = 3,
p2 = 1, x1 = 5, and x2 = 15. If in the current year, the price of good 1 is
1 and the price of good 2 is 3, and his current consumptions of good 1 and
good 2 are 25 and 10 respectively, what is the Laspeyres price index of
current prices relative to base-year prices? (Pick the most nearly correct
answer.)
(a) 1.67
(b) 1.83
(c) 1
(d) 0.75
(e) 2.50
7.4 On the planet Homogenia, every consumer who has ever lived consumes only two goods x and y and has the utility function U (x, y) = xy.
The currency in Homogenia is the fragel. On this planet in 1900, the
price of good 1 was 1 fragel and the price of good 2 was 2 fragels. Per
capita income was 120 fragels. In 2000, the price of good 1 was 5 fragels
and the price of good 2 was 5 fragels. The Laspeyres price index for the
price level in 2000 relative to the price level in 1900 is
(a) 3.75.
(b) 5.
(c) 3.33.
(d) 6.25.
(e) not possible to determine from this information.
7.5 On the planet Hyperion, every consumer who has ever lived has a
utility function U (x, y) = min{x, 2y}. The currency of Hyperion is the
doggerel. In 1850 the price of x was 1 doggerel per unit, and the price of
y was 2 doggerels per unit. In 2000, the price of x was 10 doggerels per
unit and the price of y was 4 doggerels per unit. The Paasche price index
of prices in 2000 relative to prices in 1850 is
(a) 6.
(b) 4.67.
(c) 2.50.
(d) 3.50.
(e) not possible to determine without further information.
CHAPTER
8
SLUTSKY
EQUATION
Economists often are concerned with how a consumer’s behavior changes
in response to changes in the economic environment. The case we want
to consider in this chapter is how a consumer’s choice of a good responds
to changes in its price. It is natural to think that when the price of a
good rises the demand for it will fall. However, as we saw in Chapter 6
it is possible to construct examples where the optimal demand for a good
decreases when its price falls. A good that has this property is called a
Giffen good.
Giffen goods are pretty peculiar and are primarily a theoretical curiosity,
but there are other situations where changes in prices might have “perverse”
effects that, on reflection, turn out not to be so unreasonable. For example,
we normally think that if people get a higher wage they will work more.
But what if your wage went from $10 an hour to $1000 an hour? Would
you really work more? Might you not decide to work fewer hours and use
some of the money you’ve earned to do other things? What if your wage
were $1,000,000 an hour? Wouldn’t you work less?
For another example, think of what happens to your demand for apples
when the price goes up. You would probably consume fewer apples. But
THE SUBSTITUTION EFFECT
137
how about a family who grew apples to sell? If the price of apples went
up, their income might go up so much that they would feel that they could
now afford to consume more of their own apples. For the consumers in this
family, an increase in the price of apples might well lead to an increase in
the consumption of apples.
What is going on here? How is it that changes in price can have these
ambiguous effects on demand? In this chapter and the next we’ll try to
sort out these effects.
8.1 The Substitution Effect
When the price of a good changes, there are two sorts of effects: the rate
at which you can exchange one good for another changes, and the total
purchasing power of your income is altered. If, for example, good 1 becomes
cheaper, it means that you have to give up less of good 2 to purchase good
1. The change in the price of good 1 has changed the rate at which the
market allows you to “substitute” good 2 for good 1. The trade-off between
the two goods that the market presents the consumer has changed.
At the same time, if good 1 becomes cheaper it means that your money
income will buy more of good 1. The purchasing power of your money has
gone up; although the number of dollars you have is the same, the amount
that they will buy has increased.
The first part—the change in demand due to the change in the rate
of exchange between the two goods—is called the substitution effect.
The second effect—the change in demand due to having more purchasing
power—is called the income effect. These are only rough definitions of the
two effects. In order to give a more precise definition we have to consider
the two effects in greater detail.
The way that we will do this is to break the price movement into two
steps: first we will let the relative prices change and adjust money income
so as to hold purchasing power constant, then we will let purchasing power
adjust while holding the relative prices constant.
This is best explained by referring to Figure 8.1. Here we have a situation where the price of good 1 has declined. This means that the budget
line rotates around the vertical intercept m/p2 and becomes flatter. We
can break this movement of the budget line up into two steps: first pivot
the budget line around the original demanded bundle and then shift the
pivoted line out to the new demanded bundle.
This “pivot-shift” operation gives us a convenient way to decompose
the change in demand into two pieces. The first step—the pivot—is a
movement where the slope of the budget line changes while its purchasing
power stays constant, while the second step is a movement where the slope
stays constant and the purchasing power changes. This decomposition is
only a hypothetical construction—the consumer simply observes a change
138 SLUTSKY EQUATION (Ch. 8)
x2
Original
budget
x2
Indifference
curves
Original
choice
Final choice
Pivoted
budget
Final
budget
Shift
Pivot
x1
Figure
8.1
x1
Pivot and shift. When the price of good 1 changes and income
stays fixed, the budget line pivots around the vertical axis. We
will view this adjustment as occurring in two stages: first pivot
the budget line around the original choice, and then shift this
line outward to the new demanded bundle.
in price and chooses a new bundle of goods in response. But in analyzing
how the consumer’s choice changes, it is useful to think of the budget line
changing in two stages—first the pivot, then the shift.
What are the economic meanings of the pivoted and the shifted budget
lines? Let us first consider the pivoted line. Here we have a budget line with
the same slope and thus the same relative prices as the final budget line.
However, the money income associated with this budget line is different,
since the vertical intercept is different. Since the original consumption
bundle (x1 , x2 ) lies on the pivoted budget line, that consumption bundle
is just affordable. The purchasing power of the consumer has remained
constant in the sense that the original bundle of goods is just affordable at
the new pivoted line.
Let us calculate how much we have to adjust money income in order to
keep the old bundle just affordable. Let m be the amount of money income
that will just make the original consumption bundle affordable; this will
be the amount of money income associated with the pivoted budget line.
Since (x1 , x2 ) is affordable at both (p1 , p2 , m) and (p1 , p2 , m ), we have
m = p1 x1 + p2 x2
m = p1 x1 + p2 x2 .
Subtracting the second equation from the first gives
m − m = x1 [p1 − p1 ].
THE SUBSTITUTION EFFECT
139
This equation says that the change in money income necessary to make
the old bundle affordable at the new prices is just the original amount of
consumption of good 1 times the change in prices.
Letting Δp1 = p1 − p1 represent the change in price 1, and Δm =
m − m represent the change in income necessary to make the old bundle
just affordable, we have
(8.1)
Δm = x1 Δp1 .
Note that the change in income and the change in price will always move
in the same direction: if the price goes up, then we have to raise income to
keep the same bundle affordable.
Let’s use some actual numbers. Suppose that the consumer is originally
consuming 20 candy bars a week, and that candy bars cost 50 cents a piece.
If the price of candy bars goes up by 10 cents—so that Δp1 = .60 − .50 =
.10—how much would income have to change to make the old consumption
bundle affordable?
We can apply the formula given above. If the consumer had $2.00 more
income, he would just be able to consume the same number of candy bars,
namely, 20. In terms of the formula:
Δm = Δp1 × x1 = .10 × 20 = $2.00.
Now we have a formula for the pivoted budget line: it is just the budget
line at the new price with income changed by Δm. Note that if the price of
good 1 goes down, then the adjustment in income will be negative. When
a price goes down, a consumer’s purchasing power goes up, so we will have
to decrease the consumer’s income in order to keep purchasing power fixed.
Similarly, when a price goes up, purchasing power goes down, so the change
in income necessary to keep purchasing power constant must be positive.
Although (x1 , x2 ) is still affordable, it is not generally the optimal purchase at the pivoted budget line. In Figure 8.2 we have denoted the optimal
purchase on the pivoted budget line by Y . This bundle of goods is the optimal bundle of goods when we change the price and then adjust dollar
income so as to keep the old bundle of goods just affordable. The movement from X to Y is known as the substitution effect. It indicates how
the consumer “substitutes” one good for the other when a price changes
but purchasing power remains constant.
More precisely, the substitution effect, Δxs1 , is the change in the demand
for good 1 when the price of good 1 changes to p1 and, at the same time,
money income changes to m :
Δxs1 = x1 (p1 , m ) − x1 (p1 , m).
In order to determine the substitution effect, we must use the consumer’s
demand function to calculate the optimal choices at (p1 , m ) and (p1 , m).
The change in the demand for good 1 may be large or small, depending
140 SLUTSKY EQUATION (Ch. 8)
x2
Indifference curves
m/p2
m'/p2
Z
X
Y
Shift
Pivot
x1
Substitution
effect
Figure
8.2
Income
effect
Substitution effect and income effect. The pivot gives the
substitution effect, and the shift gives the income effect.
on the shape of the consumer’s indifference curves. But given the demand
function, it is easy to just plug in the numbers to calculate the substitution
effect. (Of course the demand for good 1 may well depend on the price of
good 2; but the price of good 2 is being held constant during this exercise,
so we’ve left it out of the demand function so as not to clutter the notation.)
The substitution effect is sometimes called the change in compensated
demand. The idea is that the consumer is being compensated for a price
rise by having enough income given back to him to purchase his old bundle. Of course if the price goes down he is “compensated” by having money
taken away from him. We’ll generally stick with the “substitution” terminology, for consistency, but the “compensation” terminology is also widely
used.
EXAMPLE: Calculating the Substitution Effect
Suppose that the consumer has a demand function for milk of the form
x1 = 10 +
m
.
10p1
Originally his income is $120 per week and the price of milk is $3 per quart.
Thus his demand for milk will be 10 + 120/(10 × 3) = 14 quarts per week.
THE INCOME EFFECT
141
Now suppose that the price of milk falls to $2 per quart. Then his
demand at this new price will be 10 + 120/(10 × 2) = 16 quarts of milk per
week. The total change in demand is +2 quarts a week.
In order to calculate the substitution effect, we must first calculate how
much income would have to change in order to make the original consumption of milk just affordable when the price of milk is $2 a quart. We apply
the formula (8.1):
Δm = x1 Δp1 = 14 × (2 − 3) = −$14.
Thus the level of income necessary to keep purchasing power constant
is m = m + Δm = 120 − 14 = 106. What is the consumer’s demand for
milk at the new price, $2 per quart, and this level of income? Just plug
the numbers into the demand function to find
106
= 15.3.
x1 (p1 , m ) = x1 (2, 106) = 10 +
10 × 2
Thus the substitution effect is
Δxs1 = x1 (2, 106) − x1 (3, 120) = 15.3 − 14 = 1.3.
8.2 The Income Effect
We turn now to the second stage of the price adjustment—the shift movement. This is also easy to interpret economically. We know that a parallel
shift of the budget line is the movement that occurs when income changes
while relative prices remain constant. Thus the second stage of the price
adjustment is called the income effect. We simply change the consumer’s
income from m to m, keeping the prices constant at (p1 , p2 ). In Figure
8.2 this change moves us from the point (y1 , y2 ) to (z1 , z2 ). It is natural to
call this last movement the income effect since all we are doing is changing
income while keeping the prices fixed at the new prices.
More precisely, the income effect, Δxn1 , is the change in the demand for
good 1 when we change income from m to m, holding the price of good 1
fixed at p1 :
Δxn1 = x1 (p1 , m) − x1 (p1 , m ).
We have already considered the income effect earlier in section 6.1. There
we saw that the income effect can operate either way: it will tend to increase
or decrease the demand for good 1 depending on whether we have a normal
good or an inferior good.
When the price of a good decreases, we need to decrease income in order
to keep purchasing power constant. If the good is a normal good, then
this decrease in income will lead to a decrease in demand. If the good is
an inferior good, then the decrease in income will lead to an increase in
demand.
142 SLUTSKY EQUATION (Ch. 8)
EXAMPLE: Calculating the Income Effect
In the example given earlier in this chapter we saw that
x1 (p1 , m) = x1 (2, 120) = 16
x1 (p1 , m ) = x1 (2, 106) = 15.3.
Thus the income effect for this problem is
Δxn1 = x1 (2, 120) − x1 (2, 106) = 16 − 15.3 = 0.7.
Since milk is a normal good for this consumer, the demand for milk increases when income increases.
8.3 Sign of the Substitution Effect
We have seen above that the income effect can be positive or negative, depending on whether the good is a normal good or an inferior good. What
about the substitution effect? If the price of a good goes down, as in
Figure 8.2, then the change in the demand for the good due to the substitution effect must be nonnegative. That is, if p1 > p1 , then we must have
x1 (p1 , m ) ≥ x1 (p1 , m), so that Δxs1 ≥ 0.
The proof of this goes as follows. Consider the points on the pivoted
budget line in Figure 8.2 where the amount of good 1 consumed is less
than at the bundle X. These bundles were all affordable at the old prices
(p1 , p2 ) but they weren’t purchased. Instead the bundle X was purchased.
If the consumer is always choosing the best bundle he can afford, then X
must be preferred to all of the bundles on the part of the pivoted line that
lies inside the original budget set.
This means that the optimal choice on the pivoted budget line must not
be one of the bundles that lies underneath the original budget line. The
optimal choice on the pivoted line would have to be either X or some point
to the right of X. But this means that the new optimal choice must involve
consuming at least as much of good 1 as originally, just as we wanted to
show. In the case illustrated in Figure 8.2, the optimal choice at the pivoted
budget line is the bundle Y , which certainly involves consuming more of
good 1 than at the original consumption point, X.
The substitution effect always moves opposite to the price movement.
We say that the substitution effect is negative, since the change in demand
due to the substitution effect is opposite to the change in price: if the price
increases, the demand for the good due to the substitution effect decreases.
THE TOTAL CHANGE IN DEMAND
143
8.4 The Total Change in Demand
The total change in demand, Δx1 , is the change in demand due to the
change in price, holding income constant:
Δx1 = x1 (p1 , m) − x1 (p1 , m).
We have seen above how this change can be broken up into two changes: the
substitution effect and the income effect. In terms of the symbols defined
above,
x1 (p1 , m)
Δx1 = Δxs1 + Δxn1
− x1 (p1 , m) = [x1 (p1 , m ) − x1 (p1 , m)]
+ [x1 (p1 , m) − x1 (p1 , m )].
In words this equation says that the total change in demand equals the
substitution effect plus the income effect. This equation is called the Slutsky identity.1 Note that it is an identity: it is true for all values of p1 ,
p1 , m, and m . The first and fourth terms on the right-hand side cancel
out, so the right-hand side is identically equal to the left-hand side.
The content of the Slutsky identity is not just the algebraic identity—
that is a mathematical triviality. The content comes in the interpretation
of the two terms on the right-hand side: the substitution effect and the
income effect. In particular, we can use what we know about the signs of
the income and substitution effects to determine the sign of the total effect.
While the substitution effect must always be negative—opposite the
change in the price—the income effect can go either way. Thus the total effect may be positive or negative. However, if we have a normal good,
then the substitution effect and the income effect work in the same direction. An increase in price means that demand will go down due to the
substitution effect. If the price goes up, it is like a decrease in income,
which, for a normal good, means a decrease in demand. Both effects reinforce each other. In terms of our notation, the change in demand due to a
price increase for a normal good means that
Δx1 = Δxs1 + Δxn1 .
(−)
(−)
(−)
(The minus signs beneath each term indicate that each term in this expression is negative.)
1
Named for Eugen Slutsky (1880–1948), a Russian economist who investigated demand
theory.
144 SLUTSKY EQUATION (Ch. 8)
Note carefully the sign on the income effect. Since we are considering
a situation where the price rises, this implies a decrease in purchasing
power—for a normal good this will imply a decrease in demand.
On the other hand, if we have an inferior good, it might happen that the
income effect outweighs the substitution effect, so that the total change in
demand associated with a price increase is actually positive. This would
be a case where
Δx1 = Δxs1 + Δxn1 .
(+)
(?)
(−)
If the second term on the right-hand side—the income effect—is large
enough, the total change in demand could be positive. This would mean
that an increase in price could result in an increase in demand. This is the
perverse Giffen case described earlier: the increase in price has reduced the
consumer’s purchasing power so much that he has increased his consumption of the inferior good.
But the Slutsky identity shows that this kind of perverse effect can only
occur for inferior goods: if a good is a normal good, then the income and
substitution effects reinforce each other, so that the total change in demand
is always in the “right” direction.
Thus a Giffen good must be an inferior good. But an inferior good is
not necessarily a Giffen good: the income effect not only has to be of the
“wrong” sign, it also has to be large enough to outweigh the “right” sign
of the substitution effect. This is why Giffen goods are so rarely observed
in real life: they would not only have to be inferior goods, but they would
have to be very inferior.
This is illustrated graphically in Figure 8.3. Here we illustrate the usual
pivot-shift operation to find the substitution effect and the income effect.
In both cases, good 1 is an inferior good, and the income effect is therefore
negative. In Figure 8.3A, the income effect is large enough to outweigh
the substitution effect and produce a Giffen good. In Figure 8.3B, the
income effect is smaller, and thus good 1 responds in the ordinary way to
the change in its price.
8.5 Rates of Change
We have seen that the income and substitution effects can be described
graphically as a combination of pivots and shifts, or they can be described
algebraically in the Slutsky identity
Δx1 = Δxs1 + Δxn1 ,
which simply says that the total change in demand is the substitution
effect plus the income effect. The Slutsky identity here is stated in terms
RATES OF CHANGE
x2
145
x2
Indifference
curves
Indifference
curves
Original
budget
line
Original
budget
line
Final
budget
line
Final
budget
line
Income
Substitution
Total
A The Giffen case
x1
Substitution
Total
Income
x1
B Non-Giffen inferior good
Inferior goods. Panel A shows a good that is inferior enough
to cause the Giffen case. Panel B shows a good that is inferior,
but the effect is not strong enough to create a Giffen good.
Figure
8.3
of absolute changes, but it is more common to express it in terms of rates
of change.
When we express the Slutsky identity in terms of rates of change it turns
out to be convenient to define Δxm
1 to be the negative of the income effect:
n
Δxm
1 = x1 (p1 , m ) − x1 (p1 , m) = −Δx1 .
Given this definition, the Slutsky identity becomes
Δx1 = Δxs1 − Δxm
1 .
If we divide each side of the identity by Δp1 , we have
Δxs1
Δxm
Δx1
1
=
−
.
Δp1
Δp1
Δp1
(8.2)
The first term on the right-hand side is the rate of change of demand
when price changes and income is adjusted so as to keep the old bundle
affordable—the substitution effect. Let’s work on the second term. Since
we have an income change in the numerator, it would be nice to get an
income change in the denominator.
146 SLUTSKY EQUATION (Ch. 8)
Remember that the income change, Δm, and the price change, Δp1 , are
related by the formula
Δm = x1 Δp1 .
Solving for Δp1 we find
Δp1 =
Δm
.
x1
Now substitute this expression into the last term in (8.2) to get our final
formula:
Δxs1
Δxm
Δx1
1
x1 .
=
−
Δp1
Δp1
Δm
This is the Slutsky identity in terms of rates of change. We can interpret
each term as follows:
x1 (p1 , m) − x1 (p1 , m)
Δx1
=
Δp1
Δp1
is the rate of change in demand as price changes, holding income fixed;
x1 (p1 , m ) − x1 (p1 , m)
Δxs1
=
Δp1
Δp1
is the rate of change in demand as the price changes, adjusting income so
as to keep the old bundle just affordable, that is, the substitution effect;
and
x1 (p1 , m ) − x1 (p1 , m)
Δxm
1
x1 =
x1
(8.3)
Δm
m − m
is the rate of change of demand holding prices fixed and adjusting income,
that is, the income effect.
The income effect is itself composed of two pieces: how demand changes
as income changes, times the original level of demand. When the price
changes by Δp1 , the change in demand due to the income effect is
Δxm
1 =
x1 (p1 , m ) − x1 (p1 , m)
x1 Δp1 .
Δm
But this last term, x1 Δp1 , is just the change in income necessary to keep
the old bundle feasible. That is, x1 Δp1 = Δm, so the change in demand
due to the income effect reduces to
Δxm
1 =
just as we had before.
x1 (p1 , m ) − x1 (p1 , m)
Δm,
Δm
EXAMPLES OF INCOME AND SUBSTITUTION EFFECTS
147
8.6 The Law of Demand
In Chapter 5 we voiced some concerns over the fact that consumer theory
seemed to have no particular content: demand could go up or down when a
price increased, and demand could go up or down when income increased.
If a theory doesn’t restrict observed behavior in some fashion it isn’t much
of a theory. A model that is consistent with all behavior has no real content.
However, we know that consumer theory does have some content—we’ve
seen that choices generated by an optimizing consumer must satisfy the
Strong Axiom of Revealed Preference. Furthermore, we’ve seen that any
price change can be decomposed into two changes: a substitution effect
that is sure to be negative—opposite the direction of the price change—
and an income effect whose sign depends on whether the good is a normal
good or an inferior good.
Although consumer theory doesn’t restrict how demand changes when
price changes or how demand changes when income changes, it does restrict how these two kinds of changes interact. In particular, we have the
following.
The Law of Demand. If the demand for a good increases when income
increases, then the demand for that good must decrease when its price increases.
This follows directly from the Slutsky equation: if the demand increases
when income increases, we have a normal good. And if we have a normal
good, then the substitution effect and the income effect reinforce each other,
and an increase in price will unambiguously reduce demand.
8.7 Examples of Income and Substitution Effects
Let’s now consider some examples of price changes for particular kinds of
preferences and decompose the demand changes into the income and the
substitution effects.
We start with the case of perfect complements. The Slutsky decomposition is illustrated in Figure 8.4. When we pivot the budget line around the
chosen point, the optimal choice at the new budget line is the same as at
the old one—this means that the substitution effect is zero. The change in
demand is due entirely to the income effect.
What about the case of perfect substitutes, illustrated in Figure 8.5?
Here when we tilt the budget line, the demand bundle jumps from the
vertical axis to the horizontal axis. There is no shifting left to do! The
entire change in demand is due to the substitution effect.
148 SLUTSKY EQUATION (Ch. 8)
x2
Indifference
curves
Original
budget
line
Final budget line
Shift
Pivot
x1
Income effect = total effect
Figure
8.4
Perfect complements.
complements.
Slutsky decomposition with perfect
As a third example, let us consider the case of quasilinear preferences.
This situation is somewhat peculiar. We have already seen that a shift
in income causes no change in demand for good 1 when preferences are
quasilinear. This means that the entire change in demand for good 1 is due
to the substitution effect, and that the income effect is zero, as illustrated
in Figure 8.6.
EXAMPLE: Rebating a Tax
In 1974 the Organization of Petroleum Exporting Countries (OPEC) instituted an oil embargo against the United States. OPEC was able to stop oil
shipments to U.S. ports for several weeks. The vulnerability of the United
States to such disruptions was very disturbing to Congress and the president, and there were many plans proposed to reduce the United States’s
dependence on foreign oil.
One such plan involved increasing the gasoline tax. Increasing the cost
of gasoline to the consumers would make them reduce their consumption
of gasoline, and the reduced demand for gasoline would in turn reduce the
demand for foreign oil.
But a straight increase in the tax on gasoline would hit consumers where
it hurts—in the pocketbook—and by itself such a plan would be politically
EXAMPLES OF INCOME AND SUBSTITUTION EFFECTS
149
x2
Indifference
curves
Original
choice
Final budget line
Final choice
Original
budget
line
x1
Substitution effect = total effect
Perfect substitutes. Slutsky decomposition with perfect substitutes.
infeasible. So it was suggested that the revenues raised from consumers by
this tax would be returned to the consumers in the form of direct money
payments, or via the reduction of some other tax.
Critics of this proposal argued that paying the revenue raised by the tax
back to the consumers would have no effect on demand since they could
just use the rebated money to purchase more gasoline. What does economic
analysis say about this plan?
Let us suppose, for simplicity, that the tax on gasoline would end up
being passed along entirely to the consumers of gasoline so that the price
of gasoline will go up by exactly the amount of the tax. (In general, only
part of the tax would be passed along, but we will ignore that complication
here.) Suppose that the tax would raise the price of gasoline from p to
p = p + t, and that the average consumer would respond by reducing
his demand from x to x . The average consumer is paying t dollars more
for gasoline, and he is consuming x gallons of gasoline after the tax is
imposed, so the amount of revenue raised by the tax from the average
consumer would be
R = tx = (p − p)x .
Note that the revenue raised by the tax will depend on how much gasoline the consumer ends up consuming, x , not how much he was initially
Figure
8.5
150 SLUTSKY EQUATION (Ch. 8)
x2
Indifference
curves
Final budget line
Original
budget
line
Pivot
x1
Substitution effect = total effect
Figure
8.6
Quasilinear preferences. In the case of quasilinear preferences, the entire change in demand is due to the substitution
effect.
consuming, x.
If we let y be the expenditure on all other goods and set its price to be
1, then the original budget constraint is
px + y = m,
(8.4)
and the budget constraint in the presence of the tax-rebate plan is
(p + t)x + y = m + tx .
(8.5)
In budget constraint (8.5) the average consumer is choosing the left-hand
side variables—the consumption of each good—but the right-hand side—
his income and the rebate from the government—are taken as fixed. The
rebate depends on what all consumers do, not what the average consumer
does. In this case, the rebate turns out to be the taxes collected from the
average consumer—but that’s because he is average, not because of any
causal connection.
If we cancel tx from each side of equation (8.5), we have
px + y = m.
Thus (x , y ) is a bundle that was affordable under the original budget
constraint and rejected in favor of (x, y). Thus it must be that (x, y)
EXAMPLES OF INCOME AND SUBSTITUTION EFFECTS
151
is preferred to (x , y ): the consumers are made worse off by this plan.
Perhaps that is why it was never put into effect!
The equilibrium with a rebated tax is depicted in Figure 8.7. The tax
makes good 1 more expensive, and the rebate increases money income.
The original bundle is no longer affordable, and the consumer is definitely
made worse off. The consumer’s choice under the tax-rebate plan involves
consuming less gasoline and more of “all other goods.”
y
m + t x'
Indifference
curves
m
(x', y')
(x, y)
Budget line
after tax
and rebate
slope = – (p + t )
Budget line
before tax
slope = – p
x
Rebating a tax. Taxing a consumer and rebating the tax
revenues makes the consumer worse off.
What can we say about the amount of consumption of gasoline? The
average consumer could afford his old consumption of gasoline, but because
of the tax, gasoline is now more expensive. In general, the consumer would
choose to consume less of it.
EXAMPLE: Voluntary Real Time Pricing
Electricity production suffers from an extreme capacity problem: it is relatively cheap to produce up to capacity, at which point it is, by definition,
impossible to produce more. Building capacity is extremely expensive, so
Figure
8.7
152 SLUTSKY EQUATION (Ch. 8)
finding ways to reduce the use of electricity during periods of peak demand
is very attractive from an economic point of view.
In states with warm climates, such as Georgia, roughly 30 percent of
usage during periods of peak demand is due to air conditioning. Furthermore, it is relatively easy to forecast temperature one day ahead so that
potential users will have time to adjust their demand by setting their air
conditioning to a higher temperature, wearing light clothes, and so on.
The challenge is to set up a pricing system so that those users who are able
to cut back on their electricity use will have an incentive to reduce their
consumption.
One way to accomplish this is through the use of Real Time Pricing
(RTP). In a Real Time Pricing program, large industrial users are equipped
with special meters that allow the price of electricity to vary from minute to
minute, depending on signals sent from the electricity generating company.
As the demand for electricity approaches capacity, the generating company
increases the price so as to encourage users to cut back on their usage.
The price schedule is determined as a function of the total demand for
electricity.
Georgia Power Company claims that it runs the largest real time pricing program in the world. In 1999 it was able to reduce demand by 750
megawatts on high-price days by inducing some large customers to cut their
demand by as much as 60 percent.
Georgia Power has devised several interesting variations on the basic real
time pricing model. In one pricing plan, customers are assigned a baseline
quantity, which represents their normal usage. When electricity is in short
supply and the real time price increases, these users face a higher price for
electricity use in excess of their baseline quantity. But they also receive a
rebate if they can manage to cut their electricity use below their baseline
amount.
Figure 8.8 shows how this affects the budget line of the users. The
vertical axis is “money to spend on things other than electricity” and the
horizontal axis is “electricity use.” In normal times, users choose their
electricity consumption to maximize utility subject to a budget constraint
which is determined by the baseline price of electricity. The resulting choice
is their baseline consumption.
When the temperature rises, the real time price increases, making electricity more expensive. But this increase in price is a good thing for users
who can cut back their consumption, since they receive a rebate based on
the high real time price for every kilowatt of reduced usage. If usage stays
at the baseline amount, then the user’s bill will not change.
It is not hard to see that this pricing plan is a Slutsky pivot around the
baseline consumption. Thus we can be confident that electricity usage will
decline, and that users will be at least as well off at the real time price as
at the baseline price. Indeed, the program has been quite popular, with
over 1,600 voluntary participants.
ANOTHER SUBSTITUTION EFFECT
153
OTHER
GOODS
Consumption
under RTP
RTP budget
constraint
Baseline
consumption
Baseline budget
constraint
ELECTRICITY
Voluntary real time pricing. Users pay higher rates for
additional electricity when the real time price rises, but they
also get rebates at the same price if they cut back their use.
This results in a pivot around the baseline use and tends to
make the customers better off.
8.8 Another Substitution Effect
The substitution effect is the name that economists give to the change in
demand when prices change but a consumer’s purchasing power is held
constant, so that the original bundle remains affordable. At least this is
one definition of the substitution effect. There is another definition that is
also useful.
The definition we have studied above is called the Slutsky substitution
effect. The definition we will describe in this section is called the Hicks
substitution effect.2
Suppose that instead of pivoting the budget line around the original
consumption bundle, we now roll the budget line around the indifference
curve through the original consumption bundle, as depicted in Figure 8.9.
In this way we present the consumer with a new budget line that has the
same relative prices as the final budget line but has a different income. The
purchasing power he has under this budget line will no longer be sufficient to
2
The concept is named for Sir John Hicks, an English recipient of the Nobel Prize in
Economics.
Figure
8.8
154 SLUTSKY EQUATION (Ch. 8)
purchase his original bundle of goods—but it will be sufficient to purchase
a bundle that is just indifferent to his original bundle.
x2
Indifference
curves
Original
budget
Final
budget
Original
choice
Final
choice
x1
Substitution
effect
Figure
8.9
Income
effect
The Hicks substitution effect. Here we pivot the budget line
around the indifference curve rather than around the original
choice.
Thus the Hicks substitution effect keeps utility constant rather than keeping purchasing power constant. The Slutsky substitution effect gives the
consumer just enough money to get back to his old level of consumption,
while the Hicks substitution effect gives the consumer just enough money
to get back to his old indifference curve. Despite this difference in definition, it turns out that the Hicks substitution effect must be negative—in
the sense that it is in a direction opposite that of the price change—just
like the Slutsky substitution effect.
The proof is again by revealed preference. Let (x1 , x2 ) be a demanded
bundle at some prices (p1 , p2 ), and let (y1 , y2 ) be a demanded bundle at
some other prices (q1 , q2 ). Suppose that income is such that the consumer
is indifferent between (x1 , x2 ) and (y1 , y2 ). Since the consumer is indifferent
between (x1 , x2 ) and (y1 , y2 ), neither bundle can be revealed preferred to
the other.
Using the definition of revealed preference, this means that the following
COMPENSATED DEMAND CURVES
155
two inequalities are not true:
p1 x1 + p2 x2 > p1 y1 + p2 y2
q1 y1 + q2 y2 > q1 x1 + q2 x2 .
It follows that these inequalities are true:
p1 x1 + p2 x2 ≤ p1 y1 + p2 y2
q1 y1 + q2 y2 ≤ q1 x1 + q2 x2 .
Adding these inequalities together and rearranging them we have
(q1 − p1 )(y1 − x1 ) + (q2 − p2 )(y2 − x2 ) ≤ 0.
This is a general statement about how demands change when prices
change if income is adjusted so as to keep the consumer on the same indifference curve. In the particular case we are concerned with, we are only
changing the first price. Therefore q2 = p2 , and we are left with
(q1 − p1 )(y1 − x1 ) ≤ 0.
This equation says that the change in the quantity demanded must have
the opposite sign from that of the price change, which is what we wanted
to show.
The total change in demand is still equal to the substitution effect plus
the income effect—but now it is the Hicks substitution effect. Since the
Hicks substitution effect is also negative, the Slutsky equation takes exactly
the same form as we had earlier and has exactly the same interpretation.
Both the Slutsky and Hicks definitions of the substitution effect have their
place, and which is more useful depends on the problem at hand. It can
be shown that for small changes in price, the two substitution effects are
virtually identical.
8.9 Compensated Demand Curves
We have seen how the quantity demanded changes as a price changes in
three different contexts: holding income fixed (the standard case), holding
purchasing power fixed (the Slutsky substitution effect), and holding utility
fixed (the Hicks substitution effect). We can draw the relationship between
price and quantity demanded holding any of these three variables fixed.
This gives rise to three different demand curves: the standard demand
curve, the Slutsky demand curve, and the Hicks demand curve.
The analysis of this chapter shows that the Slutsky and Hicks demand
curves are always downward sloping curves. Furthermore the ordinary
156 SLUTSKY EQUATION (Ch. 8)
demand curve is a downward sloping curve for normal goods. However,
the Giffen analysis shows that it is theoretically possible that the ordinary
demand curve may slope upwards for an inferior good.
The Hicksian demand curve—the one with utility held constant—is sometimes called the compensated demand curve. This terminology arises
naturally if you think of constructing the Hicksian demand curve by adjusting income as the price changes so as to keep the consumer’s utility
constant. Hence the consumer is “compensated” for the price changes, and
his utility is the same at every point on the Hicksian demand curve. This
is in contrast to the situation with an ordinary demand curve. In this case
the consumer is worse off facing higher prices than lower prices since his
income is constant.
The compensated demand curve turns out to be very useful in advanced
courses, especially in treatments of benefit-cost analysis. In this sort of
analysis it is natural to ask what size payments are necessary to compensate consumers for some policy change. The magnitude of such payments
gives a useful estimate of the cost of the policy change. However, actual
calculation of compensated demand curves requires more mathematical machinery than we have developed in this text.
Summary
1. When the price of a good decreases, there will be two effects on consumption. The change in relative prices makes the consumer want to consume
more of the cheaper good. The increase in purchasing power due to the
lower price may increase or decrease consumption, depending on whether
the good is a normal good or an inferior good.
2. The change in demand due to the change in relative prices is called the
substitution effect; the change due to the change in purchasing power is
called the income effect.
3. The substitution effect is how demand changes when prices change and
purchasing power is held constant, in the sense that the original bundle
remains affordable. To hold real purchasing power constant, money income
will have to change. The necessary change in money income is given by
Δm = x1 Δp1 .
4. The Slutsky equation says that the total change in demand is the sum
of the substitution effect and the income effect.
5. The Law of Demand says that normal goods must have downwardsloping demand curves.
APPENDIX
157
REVIEW QUESTIONS
1. Suppose a consumer has preferences between two goods that are perfect
substitutes. Can you change prices in such a way that the entire demand
response is due to the income effect?
2. Suppose that preferences are concave. Is it still the case that the substitution effect is negative?
3. In the case of the gasoline tax, what would happen if the rebate to the
consumers were based on their original consumption of gasoline, x, rather
than on their final consumption of gasoline, x ?
4. In the case described in the preceding question, would the government
be paying out more or less than it received in tax revenues?
5. In this case would the consumers be better off or worse off if the tax
with rebate based on original consumption were in effect?
APPENDIX
Let us derive the Slutsky equation using calculus. Consider the Slutsky definition of the substitution effect, in which the income is adjusted so as to give the
consumer just enough to buy the original consumption bundle, which we will now
denote by (x1 , x2 ). If the prices are (p1 , p2 ), then the consumer’s actual choice
with this adjustment will depend on (p1 , p2 ) and (x1 , x2 ). Let’s call this relationship the Slutsky demand function for good 1, and write it as xs1 (p1 , p2 , x1 , x2 ).
Suppose the original demanded bundle is (x1 , x2 ) at prices (p1 , p2 ) and income
m. The Slutsky demand function tells us what the consumer would demand
facing some different prices (p1 , p2 ) and having income p1 x1 + p2 x2 . Thus the
Slutsky demand function at (p1 , p2 , x1 , x2 ) is the ordinary demand at (p1 , p2 ) and
income p1 x1 + p2 x2 . That is,
xs1 (p1 , p2 , x1 , x2 ) ≡ x1 (p1 , p2 , p1 x1 + p2 x2 ).
This equation says that the Slutsky demand at prices (p1 , p2 ) is that amount
which the consumer would demand if he had enough income to purchase his
original bundle of goods (x1 , x2 ). This is just the definition of the Slutsky demand
function.
Differentiating this identity with respect to p1 , we have
∂xs1 (p1 , p2 , x1 , x2 )
∂x1 (p1 , p2 , m)
∂x1 (p1 , p2 , m)
=
+
x1 .
∂p1
∂p1
∂m
Rearranging we have
∂x1 (p1 , p2 , m)
∂xs1 (p1 , p2 , x1 , x2 )
∂x1 (p1 , p2 , m)
=
−
x1 .
∂p1
∂p1
∂m
158 SLUTSKY EQUATION (Ch. 8)
Note the use of the chain rule in this calculation.
This is a derivative form of the Slutsky equation. It says that the total effect
of a price change is composed of a substitution effect (where income is adjusted
to keep the bundle (x1 , x2 ) feasible) and an income effect. We know from the
text that the substitution effect is negative and that the sign of the income effect
depends on whether the good in question is inferior or not. As you can see, this
is just the form of the Slutsky equation considered in the text, except that we
have replaced the Δ’s with derivative signs.
What about the Hicks substitution effect? It is also possible to define a Slutsky
equation for it. We let xh1 (p1 , p2 , u) be the Hicksian demand function, which
measures how much the consumer demands of good 1 at prices (p1 , p2 ) if income
is adjusted to keep the level of utility constant at the original level u. It turns
out that in this case the Slutsky equation takes the form
∂xh1 (p1 , p2 , u)
∂x1 (p1 , p2 , m)
∂x1 (p1 , p2 , m)
=
−
x1 .
∂p1
∂p1
∂m
The proof of this equation hinges on the fact that
∂xs1 (p1 , p2 , x1 , x2 )
∂xh1 (p1 , p2 , u)
=
∂p1
∂p1
for infinitesimal changes in price. That is, for derivative size changes in price, the
Slutsky substitution and the Hicks substitution effect are the same. The proof
of this is not terribly difficult, but it involves some concepts that are beyond
the scope of this book. A relatively simple proof is given in Hal R. Varian,
Microeconomic Analysis, 3rd ed. (New York: Norton, 1992).
EXAMPLE: Rebating a Small Tax
We can use the calculus version of the Slutsky equation to see how consumption
choices would react to a small change in a tax when the tax revenues are rebated
to the consumers.
Assume, as before, that the tax causes the price to rise by the full amount of
the tax. Let x be the amount of gasoline, p its original price, and t the amount
of the tax. Then the change in consumption will be given by
dx =
∂x
∂x
t+
tx.
∂p
∂m
The first term measures how demand responds to the price change times the
amount of the price change—which gives us the price effect of the tax. The
second terms tells us how demand responds to a change in income times the
amount that income has changed—income has gone up by the amount of the tax
revenues rebated to the consumer.
Now use Slutsky’s equation to expand the first term on the right-hand side to
get the substitution and income effects of the price change itself:
dx =
∂x
∂x
∂xs
∂xs
t−
tx +
tx =
t.
∂p
∂m
∂m
∂p
APPENDIX
159
The income effect cancels out, and all that is left is the pure substitution effect.
Imposing a small tax and rebating the revenues of the tax is just like imposing a price change and adjusting income so that the old consumption bundle is
feasible—as long as the tax is small enough so that the derivative approximation
is valid.
It is useful to think of a price change as having two distinct effects, a
substitution effect and an income effect. The substitution effect of a
price change is the change that would have happened if income changed
at the same time in such a way that the consumer could exactly afford
her old consumption bundle. The rest of the change in the consumer’s
demand is called the income effect. Why do we bother with breaking
a real change into the sum of two hypothetical changes? Because we
know things about the pieces that we wouldn’t know about the whole
without taking it apart. In particular, we know that the substitution
effect of increasing the price of a good must reduce the demand for it.
We also know that the income effect of an increase in the price of a good
is equivalent to the effect of a loss of income. Therefore if the good whose
price has risen is a normal good, then both the income and substitution
effect operate to reduce demand. But if the good is an inferior good,
income and substitution effects act in opposite directions.
A consumer has the utility function U (x1 , x2 ) = x1 x2 and an income of
$24. Initially the price of good 1 was $1 and the price of good 2 was $2.
Then the price of good 2 rose to $3 and the price of good 1 stayed at $1.
Using the methods you learned in Chapters 5 and 6, you will find that
this consumer’s demand function for good 1 is D1 (p1 , p2 , m) = m/2p1
and her demand function for good 2 is D2 (p1 , p2 , m) = m/2p2 . Therefore
initially she will demand 12 units of good 1 and 6 units of good 2. If,
when the price of good 2 rose to $3, her income had changed enough so
that she could exactly afford her old bundle, her new income would have
to be (1 × 12) + (3 × 6) = $30. At an income of $30, at the new prices, she
would demand D2 (1, 3, 30) = 5 units of good 2. Before the change she
bought 6 units of 2, so the substitution effect of the price change on her
demand for good 2 is 5 − 6 = −1 units. Our consumer’s income didn’t
really change. Her income stayed at $24. Her actual demand for good 2
after the price change was D2 (1, 3, 24) = 4. The difference between what
she actually demanded after the price change and what she would have
demanded if her income had changed to let her just afford the old bundle
is the income effect. In this case the income effect is 4 − 5 = −1 units
of good 2. Notice that in this example, both the income effect and the
substitution effect of the price increase worked to reduce the demand for
good 2.
When you have completed this workout, we hope that you will be
able to do the following:
• Find Slutsky income effect and substitution effect of a specific price
change if you know the demand function for a good.
• Show the Slutsky income and substitution effects of a price change
on an indifference curve diagram.
• Show the Hicks income and substitution effects of a price change on
an indifference curve diagram.
• Find the Slutsky income and substitution effects for special utility functions such as perfect substitutes, perfect complements, and
Cobb-Douglas.
• Use an indifference-curve diagram to show how the case of a Giffen
good might arise.
• Show that the substitution effect of a price increase unambiguously
decreases demand for the good whose price rose.
• Apply income and substitution effects to draw some inferences about
behavior.
8.1 (0) Gentle Charlie, vegetarian that he is, continues to consume
apples and bananas. His utility function is U (xA , xB ) = xA xB . The price
of apples is $1, the price of bananas is $2, and Charlie’s income is $40 a
day. The price of bananas suddenly falls to $1.
(a) Before the price change, Charlie consumed
apples and
bananas per day. On the graph below, use black ink to draw
Charlie’s original budget line and put the label A on his chosen consumption bundle.
(b) If, after the price change, Charlie’s income had changed so that he
could exactly afford his old consumption bundle, his new income would
have been
With this income and the new prices, Charlie would
consume
apples and
bananas. Use red ink to draw
the budget line corresponding to this income and these prices. Label the
bundle that Charlie would choose at this income and the new prices with
the letter B.
(c) Does the substitution effect of the fall in the price of bananas make
him buy more bananas or fewer bananas?
How many
more or fewer?
(d) After the price change, Charlie actually buys
.
apples and
bananas. Use blue ink to draw Charlie’s actual budget line
after the price change. Put the label C on the bundle that he actually
chooses after the price change. Draw 3 horizontal lines on your graph, one
from A to the vertical axis, one from B to the vertical axis, and one from
C to the vertical axis. Along the vertical axis, label the income effect, the
substitution effect, and the total effect on the demand for bananas. Is the
blue line parallel to the red line or the black line that you drew before?
.
Bananas
40
30
20
10
0
10
20
30
40
Apples
(e) The income effect of the fall in the price of bananas on Charlie’s
demand for bananas is the same as the effect of an (increase, decrease)
in his income of $
per day. Does the income
effect make him consume more bananas or fewer?
more or how many fewer?
How many
.
(f ) Does the substitution effect of the fall in the price of bananas make
Charlie consume more apples or fewer?
fewer?
How many more or
Does the income effect of the fall in the price of
bananas make Charlie consume more apples or fewer?
What
is the total effect of the change in the price of bananas on the demand for
apples?
.
8.2 (0) Neville’s passion is fine wine. When the prices of all other
goods are fixed at current levels, Neville’s demand function for highquality claret is q = .02m − 2p, where m is his income, p is the price
of claret (in British pounds), and q is the number of bottles of claret that
he demands. Neville’s income is 7,500 pounds, and the price of a bottle
of suitable claret is 30 pounds.
(a) How many bottles of claret will Neville buy?
.
(b) If the price of claret rose to 40 pounds, how much income would Neville
have to have in order to be exactly able to afford the amount of claret
and the amount of other goods that he bought before the price change?
At this income, and a price of 40 pounds, how many
bottles would Neville buy?
.
(c) At his original income of 7,500 and a price of 40, how much claret
would Neville demand?
.
(d) When the price of claret rose from 30 to 40, the number of bottles
that Neville demanded decreased by
creased, reduced)
The substitution effect (in-
his demand by
income effect (increased, reduced)
bottles and the
his demand by
.
8.3 (0) Note: Do this problem only if you have read the section entitled
“Another Substitution Effect” that describes the “Hicks substitution effect”. Consider the figure below, which shows the budget constraint and
the indifference curves of good King Zog. Zog is in equilibrium with an
income of $300, facing prices pX = $4 and pY = $10.
Y
30
22.5
2.5
C
E
F
30
35 43 75
(a) How much X does Zog consume?
90
120
X
.
(b) If the price of X falls to $2.50, while income and the price of Y stay
constant, how much X will Zog consume?
.
(c) How much income must be taken away from Zog to isolate the Hicksian
income and substitution effects (i.e., to make him just able to afford to
reach his old indifference curve at the new prices)?
.
(d) The total effect of the price change is to change consumption from
the point
to the point
.
(e) The income effect corresponds to the movement from the point
to the point
to the movement from the point
while the substitution effect corresponds
to the point
(f ) Is X a normal good or an inferior good?
.
.
(g) On the axes below, sketch an Engel curve and a demand curve for
Good X that would be reasonable given the information in the graph
above. Be sure to label the axes on both your graphs.
8.4 (0) Maude spends all of her income on delphiniums and hollyhocks.
She thinks that delphiniums and hollyhocks are perfect substitutes; one
delphinium is just as good as one hollyhock. Delphiniums cost $4 a unit
and hollyhocks cost $5 a unit.
(a) If the price of delphiniums decreases to $3 a unit, will Maude buy
more of them?
What part of the change in consumption is
due to the income effect and what part is due to the substitution effect?
.
(b) If the prices of delphiniums and hollyhocks are respectively pd = $4
and ph = $5 and if Maude has $120 to spend, draw her budget line in
blue ink. Draw the highest indifference curve that she can attain in red
ink, and label the point that she chooses as A.
Delphiniums
40
30
20
10
0
10
20
30
40
Hollyhocks
(c) Now let the price of hollyhocks fall to $3 a unit, while the price of
delphiniums does not change. Draw her new budget line in black ink.
Draw the highest indifference curve that she can now reach with red ink.
Label the point she chooses now as B.
(d) How much would Maude’s income have to be after the price of hollyhocks fell, so that she could just exactly afford her old commodity bundle
A?
.
(e) When the price of hollyhocks fell to $3, what part of the change in
Maude’s demand was due to the income effect and what part was due to
the substitution effect?
.
8.5 (1) Suppose that two goods are perfect complements. If the price
of one good changes, what part of the change in demand is due to the
substitution effect, and what part is due to the income effect?
.
8.6 (0) Douglas Cornfield’s demand function for good x is x(px , py , m) =
2m/5px . His income is $1,000, the price of x is $5, and the price of y is
$20. If the price of x falls to $4, then his demand for x will change from
to
.
(a) If his income were to change at the same time so that he could exactly
afford his old commodity bundle at px = 4 and py = 20, what would his
new income be?
What would be his demand for x at this
new level of income, at prices px = 4 and py = 20?
(b) The substitution effect is a change in demand from
.
to
The income effect of the price change is a change in demand
from
to
.
(c) On the axes below, use blue ink to draw Douglas Cornfield’s budget
line before the price change. Locate the bundle he chooses at these prices
on your graph and label this point A. Use black ink to draw Douglas
Cornfield’s budget line after the price change. Label his consumption
bundle after the change by B.
y
80
60
40
20
0
40
80
120
160
200
240
280
320
x
(d) On the graph above, use black ink to draw a budget line with the new
prices but with an income that just allows Douglas to buy his old bundle,
A. Find the bundle that he would choose with this budget line and label
this bundle C.
8.7 (1) Mr. Consumer allows himself to spend $100 per month on
cigarettes and ice cream. Mr. C’s preferences for cigarettes and ice cream
are unaffected by the season of the year.
(a) In January, the price of cigarettes was $1 per pack, while ice cream
cost $2 per pint. Faced with these prices, Mr. C bought 30 pints of ice
cream and 40 packs of cigarettes. Draw Mr. C’s January budget line with
blue ink and label his January consumption bundle with the letter J.
Ice cream
100
90
80
70
60
50
40
30
20
10
0
10
20
30
40
50
60
70
80
(b) In February, Mr. C again had $100 to spend and ice cream still cost
$2 per pint, but the price of cigarettes rose to $1.25 per pack. Mr. C
consumed 30 pints of ice cream and 32 packs of cigarettes. Draw Mr. C’s
February budget line with red ink and mark his February bundle with the
letter F . The substitution effect of this price change would make him buy
(less, more, the same amount of)
cigarettes and (less, more,
the same amount of)
ice cream. Since this is true and the
total change in his ice cream consumption was zero, it must be that the
income effect of this price change on his consumption of ice cream makes
him buy (more, less, the same amount of)
ice cream. The
90
100
Cigarettes
income effect of this price change is like the effect of an (increase, decrease)
in his income. Therefore the information we have suggests
that ice cream is a(n) (normal, inferior, neutral)
good.
(c) In March, Mr. C again had $100 to spend. Ice cream was on sale for $1
per pint. Cigarette prices, meanwhile, increased to $1.50 per pack. Draw
his March budget line with black ink. Is he better off than in January,
worse off, or can you not make such a comparison?
How
does your answer to the last question change if the price of cigarettes had
increased to $2 per pack?
.
8.8 (1) This problem continues with the adventures of Mr. Consumer
from the previous problem.
(a) In April, cigarette prices rose to $2 per pack and ice cream was still
on sale for $1 per pint. Mr. Consumer bought 34 packs of cigarettes and
32 pints of ice cream. Draw his April budget line with pencil and label
his April bundle with the letter A. Was he better off or worse off than in
January?
Was he better off or worse off than in February,
or can’t one tell?
.
(b) In May, cigarettes stayed at $2 per pack and as the sale on ice cream
ended, the price returned to $2 per pint. On the way to the store, however, Mr. C found $30 lying in the street. He then had $130 to spend
on cigarettes and ice cream. Draw his May budget with a dashed line.
Without knowing what he purchased, one can determine whether he is
better off than he was in at least one previous month. Which month or
months?
.
(c) In fact, Mr. C buys 40 packs of cigarettes and 25 pints of ice cream
in May. Does he satisfy WARP?
.
8.9 (2) In the last chapter, we studied a problem involving food prices
and consumption in Sweden in 1850 and 1890.
(a) Potato consumption was the same in both years. Real income must
have gone up between 1850 and 1890, since the amount of food staples purchased, as measured by either the Laspeyres or the Paasche
quantity index, rose. The price of potatoes rose less rapidly than the
price of either meat or milk, and at about the same rate as the price
of grain flour. So real income went up and the price of potatoes
went down relative to other goods. From this information, determine
whether potatoes were most likely a normal or an inferior good. Explain your answer.
.
(b) Can one also tell from these data whether it is likely that potatoes were
a Giffen good?
.
8.10 (1) Agatha must travel on the Orient Express from Istanbul to
Paris. The distance is 1,500 miles. A traveler can choose to make any
fraction of the journey in a first-class carriage and travel the rest of the
way in a second-class carriage. The price is 10 cents a mile for a secondclass carriage and 20 cents a mile for a first-class carriage. Agatha much
prefers first-class to second-class travel, but because of a misadventure in
an Istanbul bazaar, she has only $200 left with which to buy her tickets.
Luckily, she still has her toothbrush and a suitcase full of cucumber sandwiches to eat on the way. Agatha plans to spend her entire $200 on her
tickets for her trip. She will travel first class as much as she can afford
to, but she must get all the way to Paris, and $200 is not enough money
to get her all the way to Paris in first class.
(a) On the graph below, use red ink to show the locus of combinations
of first- and second-class tickets that Agatha can just afford to purchase
with her $200. Use blue ink to show the locus of combinations of firstand second-class tickets that are sufficient to carry her the entire distance
from Istanbul to Paris. Locate the combination of first- and second-class
miles that Agatha will choose on your graph and label it A.
First-class miles
1600
1200
800
400
0
400
800
1200 1600
Second-class miles
(b) Let m1 be the number of miles she travels by first-class coach and m2
be the number of miles she travels by second-class coach. Write down two
equations that you can solve to find the number of miles she chooses to
travel by first-class coach and the number of miles she chooses to travel
by second-class coach.
.
(c) The number of miles that she travels by second-class coach is
.
(d) Just before she was ready to buy her tickets, the price of second-class
tickets fell to $.05 while the price of first-class tickets remained at $.20.
On the graph that you drew above, use pencil to show the combinations
of first-class and second-class tickets that she can afford with her $200
at these prices. On your graph, locate the combination of first-class and
second-class tickets that she would now choose. (Remember, she is going
to travel as much first-class as she can afford to and still make the 1,500
mile trip on $200.) Label this point B. How many miles does she travel by
second class now?
(Hint: For an exact solution you will have
to solve two linear equations in two unknowns.) Is second-class travel a
normal good for Agatha?
Is it a Giffen good for her?
.
8.11 (0) We continue with the adventures of Agatha, from the previous
problem. Just after the price change from $.10 per mile to $.05 per mile
for second-class travel, and just before she had bought any tickets, Agatha
misplaced her handbag. Although she kept most of her money in her sock,
the money she lost was just enough so that at the new prices, she could
exactly afford the combination of first- and second-class tickets that she
would have purchased at the old prices. How much money did she lose?
On the graph you started in the previous problem, use black
ink to draw the locus of combinations of first- and second-class tickets
that she can just afford after discovering her loss. Label the point that
she chooses with a C. How many miles will she travel by second class
now?
.
(a) Finally, poor Agatha finds her handbag again. How many miles will
she travel by second class now (assuming she didn’t buy any tickets before
she found her lost handbag)?
When the price of second-class
tickets fell from $.10 to $.05, how much of a change in Agatha’s demand
for second-class tickets was due to a substitution effect?
much of a change was due to an income effect?
How
.
8.1 In Problem 8.1, Charlie’s utility function is xA xB . The price of
apples used to be $1 per unit and the price of bananas was $2 per unit.
His income was $40 per day. If the price of apples increased to $1.25 and
the price of bananas fell to $1.25, then in order to be able to just afford
his old bundle, Charlie would have to have a daily income of
(a) $37.50.
(b) $76.
(c) $18.75.
(d) $56.25.
(e) $150.
8.2 In Problem 8.1, Charlie’s utility function is xA xB . The price of apples
used to be $1 and the price of bananas used to be $2, and his income used
to be $40. If the price of apples increased to 8 and the price of bananas
stayed constant, the substitution effect on Charlie’s apple consumption
reduces his consumption by
(a) 17.50 apples.
(b) 7 apples.
(c) 8.75 apples.
(d) 13.75 apples.
(e) None of the other options are correct.
8.3 Neville, in Problem 8.2, has a friend named Colin. Colin has the same
demand function for claret as Neville, namely q = .02m − 2p, where m
is income and p is price. Colin’s income is 6,000 and he initially had to
pay a price of 30 per bottle of claret. The price of claret rose to 40. The
substitution effect of the price change
(a) reduced his demand by 20.
(b) increased his demand by 20.
(c) reduced his demand by 8.
(d) reduced his demand by 32.
(e) reduced his demand by 18.
8.4 Goods 1 and 2 are perfect complements and a consumer always consumes them in the ratio of 2 units of good 2 per unit of good 1. If a
consumer has income 120 and if the price of good 2 changes from 3 to 4,
while the price of good 1 stays at 1, then the income effect of the price
change
(a) is 4 times as strong as the substitution effect.
(b) does not change the demand for good 1.
(c) accounts for the entire change in demand.
(d) is exactly twice as strong as the substitution effect.
(e) is 3 times as strong as the substitution effect.
8.5 Suppose that Agatha in Problem 8.10 had $570 to spend on tickets
for her trip. She needs to travel a total of 1,500 miles. Suppose that the
price of first-class tickets is $0.50 per mile and the price of second-class
tickets is $0.30 per mile. How many miles will she travel by second class?
(a) 900
(b) 1,050
(c) 450
(d) 1,000
(e) 300
8.6 In Problem 8.4, Maude thinks delphiniums and hollyhocks are perfect
substitutes, one for one. If delphiniums currently cost $5 per unit and
hollyhocks cost $6 per unit and if the price of delphiniums rises to $9 per
unit,
(a) the income effect of the change in demand for delphiniums will be
bigger than the substitution effect.
(b) there will be no change in the demand for hollyhocks.
(c) the entire change in demand for delphiniums will be due to the substitution effect.
(d) 1/4 of the change will be due to the income effect.
(e) 3/4 of the change will be due to the income effect.
CHAPTER
9
BUYING AND
SELLING
In the simple model of the consumer that we considered in the preceding
chapters, the income of the consumer was given. In reality people earn their
income by selling things that they own: items that they have produced,
assets that they have accumulated, or, most commonly, their own labor.
In this chapter we will examine how the earlier model must be modified so
as to describe this kind of behavior.
9.1 Net and Gross Demands
As before, we will limit ourselves to the two-good model. We now suppose that the consumer starts off with an endowment of the two goods,
which we will denote by (ω1 , ω2 ).1 This is how much of the two goods the
consumer has before he enters the market. Think of a farmer who goes
to market with ω1 units of carrots and ω2 units of potatoes. The farmer
inspects the prices available at the market and decides how much he wants
to buy and sell of the two goods.
1
The Greek letter ω, omega, is pronounced “o–may–gah.”
THE BUDGET CONSTRAINT
161
Let us make a distinction here between the consumer’s gross demands
and his net demands. The gross demand for a good is the amount of the
good that the consumer actually ends up consuming: how much of each of
the goods he or she takes home from the market. The net demand for a
good is the difference between what the consumer ends up with (the gross
demand) and the initial endowment of goods. The net demand for a good
is simply the amount that is bought or sold of the good.
If we let (x1 , x2 ) be the gross demands, then (x1 − ω1 , x2 − ω2 ) are the
net demands. Note that while the gross demands are typically positive
numbers, the net demands may be positive or negative. If the net demand
for good 1 is negative, it means that the consumer wants to consume less
of good 1 than she has; that is, she wants to supply good 1 to the market.
A negative net demand is simply an amount supplied.
For purposes of economic analysis, the gross demands are the more important, since that is what the consumer is ultimately concerned with. But
the net demands are what are actually exhibited in the market and thus
are closer to what the layman means by demand or supply.
9.2 The Budget Constraint
The first thing we should do is to consider the form of the budget constraint.
What constrains the consumer’s final consumption? It must be that the
value of the bundle of goods that she goes home with must be equal to the
value of the bundle of goods that she came with. Or, algebraically:
p1 x1 + p2 x2 = p1 ω1 + p2 ω2 .
We could just as well express this budget line in terms of net demands as
p1 (x1 − ω1 ) + p2 (x2 − ω2 ) = 0.
If (x1 − ω1 ) is positive we say that the consumer is a net buyer or net
demander of good 1; if it is negative we say that she is a net seller or
net supplier. Then the above equation says that the value of what the
consumer buys must equal the value of what she sells, which seems sensible
enough.
We could also express the budget line when the endowment is present
in a form similar to the way we described it before. Now it takes two
equations:
p1 x1 + p2 x2 = m
m = p 1 ω 1 + p2 ω 2 .
Once the prices are fixed, the value of the endowment, and hence the
consumer’s money income, is fixed.
162 BUYING AND SELLING (Ch. 9)
What does the budget line look like graphically? When we fix the prices,
money income is fixed, and we have a budget equation just like we had
before. Thus the slope must be given by −p1 /p2 , just as before, so the only
problem is to determine the location of the line.
The location of the line can be determined by the following simple observation: the endowment bundle is always on the budget line. That is, one
value of (x1 , x2 ) that satisfies the budget line is x1 = ω1 and x2 = ω2 . The
endowment is always just affordable, since the amount you have to spend
is precisely the value of the endowment.
Putting these facts together shows that the budget line has a slope of
−p1 /p2 and passes through the endowment point. This is depicted in Figure 9.1.
x2
Indifference curves
ω2
x2*
Budget line
slope = –p1 /p2
ω1
Figure
9.1
x1*
x1
The budget line. The budget line passes through the endowment and has a slope of −p1 /p2 .
Given this budget constraint, the consumer can choose the optimal consumption bundle just as before. In Figure 9.1 we have shown an example
of an optimal consumption bundle (x∗1 , x∗2 ). Just as before, it will satisfy
the optimality condition that the marginal rate of substitution is equal to
the price ratio.
CHANGING THE ENDOWMENT
163
In this particular case, x∗1 > ω1 and x∗2 < ω2 , so the consumer is a net
buyer of good 1 and a net seller of good 2. The net demands are simply the
net amounts that the consumer buys or sells of the two goods. In general
the consumer may decide to be either a buyer or a seller depending on the
relative prices of the two goods.
9.3 Changing the Endowment
In our previous analysis of choice we examined how the optimal consumption changed as the money income changed while the prices remained fixed.
We can do a similar analysis here by asking how the optimal consumption
changes as the endowment changes while the prices remain fixed.
For example, suppose that the endowment changes from (ω1 , ω2 ) to some
other value (ω1 , ω2 ) such that
p1 ω1 + p2 ω2 > p1 ω1 + p2 ω2 .
This inequality means that the new endowment (ω1 , ω2 ) is worth less than
the old endowment—the money income that the consumer could achieve
by selling her endowment is less.
This is depicted graphically in Figure 9.2A: the budget line shifts inward. Since this is exactly the same as a reduction in money income, we
can conclude the same two things that we concluded in our examination of
that case. First, the consumer is definitely worse off with the endowment
(ω1 , ω2 ) than she was with the old endowment, since her consumption possibilities have been reduced. Second, her demand for each good will change
according to whether that good is a normal good or an inferior good.
For example, if good 1 is a normal good and the consumer’s endowment
changes in a way that reduces its value, we can conclude that the consumer’s
demand for good 1 will decrease.
The case where the value of the endowment increases is depicted in Figure 9.2B. Following the above argument we conclude that if the budget
line shifts outward in a parallel way, the consumer must be made better
off. Algebraically, if the endowment changes from (ω1 , ω2 ) to (ω1 , ω2 ) and
p1 ω1 + p2 ω2 < p1 ω1 + p2 ω2 , then the consumer’s new budget set must contain her old budget set. This in turn implies that the optimal choice of the
consumer with the new budget set must be preferred to the optimal choice
given the old endowment.
It is worthwhile pondering this point a moment. In Chapter 7 we argued
that just because a consumption bundle had a higher cost than another
didn’t mean that it would be preferred to the other bundle. But that
only holds for a bundle that must be consumed. If a consumer can sell a
bundle of goods on a free market at constant prices, then she will always
prefer a higher-valued bundle to a lower-valued bundle, simply because a
164 BUYING AND SELLING (Ch. 9)
x2
x2
(ω 1, ω 2 )
(ω 1, ω 2 )
Budget
lines
Budget
lines
(ω'1, ω'2 )
(ω'1, ω'2 )
x1
A A decrease in the value
of the endowment
Figure
9.2
x1
B An increase in the value
of the endowment
Changes in the value of the endowment. In case A the
value of the endowment decreases, and in case B it increases.
higher-valued bundle gives her more income, and thus more consumption
possibilities. Therefore, an endowment that has a higher value will always
be preferred to an endowment with a lower value. This simple observation
will turn out to have some important implications later on.
There’s one more case to consider: what happens if p1 ω1 +p2 ω2 = p1 ω1 +
p2 ω2 ? Then the budget set doesn’t change at all: the consumer is just
as well-off with (ω1 , ω2 ) as with (ω1 , ω2 ), and her optimal choice should
be exactly the same. The endowment has just shifted along the original
budget line.
9.4 Price Changes
Earlier, when we examined how demand changed when price changed, we
conducted our investigation under the hypothesis that money income remained constant. Now, when money income is determined by the value
of the endowment, such a hypothesis is unreasonable: if the value of a
good you are selling changes, your money income will certainly change.
Thus in the case where the consumer has an endowment, changing prices
automatically implies changing income.
Let us first think about this geometrically. If the price of good 1 decreases, we know that the budget line becomes flatter. Since the endowment bundle is always affordable, this means that the budget line must
pivot around the endowment, as depicted in Figure 9.3.
PRICE CHANGES
x2
165
Indifference
curves
Original
consumption
bundle
New
consumption
bundle
x *2
ω2
Endowment
Budget lines
x *1
ω1
x1
Decreasing the price of good 1. Lowering the price of good
1 makes the budget line pivot around the endowment. If the
consumer remains a supplier she must be worse off.
In this case, the consumer is initially a seller of good 1 and remains a
seller of good 1 even after the price has declined. What can we say about
this consumer’s welfare? In the case depicted, the consumer is on a lower
indifference curve after the price change than before, but will this be true
in general? The answer comes from applying the principle of revealed
preference.
If the consumer remains a supplier, then her new consumption bundle
must be on the colored part of the new budget line. But this part of the new
budget line is inside the original budget set: all of these choices were open to
the consumer before the price changed. Therefore, by revealed preference,
all of these choices are worse than the original consumption bundle. We can
therefore conclude that if the price of a good that a consumer is selling goes
down, and the consumer decides to remain a seller, then the consumer’s
welfare must have declined.
What if the price of a good that the consumer is selling decreases and
the consumer decides to switch to being a buyer of that good? In this case,
the consumer may be better off or she may be worse off—there is no way
to tell.
Let us now turn to the situation where the consumer is a net buyer of a
good. In this case everything neatly turns around: if the consumer is a net
Figure
9.3
166 BUYING AND SELLING (Ch. 9)
buyer of a good, its price increases, and the consumer optimally decides
to remain a buyer, then she must definitely be worse off. But if the price
increase leads her to become a seller, it could go either way—she may be
better off, or she may be worse off. These observations follow from a simple
application of revealed preference just like the cases described above, but it
is good practice for you to draw a graph just to make sure you understand
how this works.
Revealed preference also allows us to make some interesting points about
the decision of whether to remain a buyer or to become a seller when prices
change. Suppose, as in Figure 9.4, that the consumer is a net buyer of good
1, and consider what happens if the price of good 1 decreases. Then the
budget line becomes flatter as in Figure 9.4.
x2
Original
budget
Endowment
ω2
Must consume here
x 2*
Original
choice
New
budget
ω1
Figure
9.4
x1*
x1
Decreasing the price of good 1. If a person is a buyer and
the price of what she is buying decreases, she remains a buyer.
As usual we don’t know for certain whether the consumer will buy more
or less of good 1—it depends on her tastes. However, we can say something
for sure: the consumer will continue to be a net buyer of good 1—she will
not switch to being a seller.
How do we know this? Well, consider what would happen if the consumer
did switch. Then she would be consuming somewhere on the colored part
of the new budget line in Figure 9.4. But those consumption bundles were
feasible for her when she faced the original budget line, and she rejected
OFFER CURVES AND DEMAND CURVES
167
them in favor of (x∗1 , x∗2 ). So (x∗1 , x∗2 ) must be better than any of those
points. And under the new budget line, (x∗1 , x∗2 ) is a feasible consumption
bundle. So whatever she consumes under the new budget line, it must be
better than (x∗1 , x∗2 )—and thus better than any points on the colored part
of the new budget line. This implies that her consumption of x1 must
be to the right of her endowment point—that is, she must remain a net
demander of good 1.
Again, this kind of observation applies equally well to a person who is
a net seller of a good: if the price of what she is selling goes up, she will
not switch to being a net buyer. We can’t tell for sure if the consumer will
consume more or less of the good she is selling—but we know that she will
keep selling it if the price goes up.
9.5 Offer Curves and Demand Curves
Recall from Chapter 6 that price offer curves depict those combinations of
both goods that may be demanded by a consumer and that demand curves
depict the relationship between the price and the quantity demanded of
some good. Exactly the same constructions work when the consumer has
an endowment of both goods.
Consider, for example, Figure 9.5, which illustrates the price offer curve
and the demand curve for a consumer. The offer curve will always pass
through the endowment, because at some price the endowment will be
a demanded bundle; that is, at some prices the consumer will optimally
choose not to trade.
As we’ve seen, the consumer may decide to be a buyer of good 1 for
some prices and a seller of good 1 for other prices. Thus the offer curve
will generally pass to the left and to the right of the endowment point.
The demand curve illustrated in Figure 9.5B is the gross demand curve—
it measures the total amount the consumer chooses to consume of good 1.
We have illustrated the net demand curve in Figure 9.6.
Note that the net demand for good 1 will typically be negative for some
prices. This will be when the price of good 1 becomes so high that the
consumer chooses to become a seller of good 1. At some price the consumer
switches between being a net demander to being a net supplier of good 1.
It is conventional to plot the supply curve in the positive orthant, although it actually makes more sense to think of supply as just a negative
demand. We’ll bow to tradition here and plot the net supply curve in the
normal way—as a positive amount, as in Figure 9.6.
Algebraically the net demand for good 1, d1 (p1 , p2 ), is the difference
between the gross demand x1 (p1 , p2 ) and the endowment of good 1, when
this difference is positive; that is, when the consumer wants more of the
good than he or she has:
d1 (p1 , p2 ) = x1 (p1 , p2 ) − ω1 if this is positive;
0
otherwise.
168 BUYING AND SELLING (Ch. 9)
x2
p1
Indifference
curve
Endowment
of good 1
Offer curve
ω2
Endowment
Slope = –p1*/p2*
ω1
x1
A Offer curve
Figure
9.5
Demand curve
for good 1
p1*
ω1
x1
B Demand curve
The offer curve and the demand curve. These are two
ways of depicting the relationship between the demanded bundle
and the prices when an endowment is present.
The net supply curve is the difference between how much the consumer
has of good 1 and how much he or she wants when this difference is positive:
s1 (p1 , p2 ) =
ω1 − x1 (p1 , p2 ) if this is positive;
0
otherwise.
Everything that we’ve established about the properties of demand behavior applies directly to the supply behavior of a consumer—because supply
is just negative demand. If the gross demand curve is always downward
sloping, then the net demand curve will be downward sloping and the supply curve will be upward sloping. Think about it: if an increase in the
price makes the net demand more negative, then the net supply will be
more positive.
9.6 The Slutsky Equation Revisited
The above applications of revealed preference are handy, but they don’t
really answer the main question: how does the demand for a good react to
a change in its price? We saw in Chapter 8 that if money income was held
constant, and the good was a normal good, then a reduction in its price
must lead to an increase in demand.
The catch is the phrase “money income was held constant.” The case we
are examining here necessarily involves a change in money income, since
the value of the endowment will necessarily change when a price changes.
THE SLUTSKY EQUATION REVISITED
p1
p1
169
p1
Gross supply
Same curve
but flipped
p1*
Same
curve
d1
A Net demand
ω1
B Gross demand
s1
x1
C Net supply
Gross demand, net demand, and net supply. Using the
gross demand and net demand to depict the demand and supply
behavior.
In Chapter 8 we described the Slutsky equation that decomposed the
change in demand due to a price change into a substitution effect and an
income effect. The income effect was due to the change in purchasing power
when prices change. But now, purchasing power has two reasons to change
when a price changes. The first is the one involved in the definition of the
Slutsky equation: when a price falls, for example, you can buy just as much
of a good as you were consuming before and have some extra money left
over. Let us refer to this as the ordinary income effect. But the second
effect is new. When the price of a good changes, it changes the value of
your endowment and thus changes your money income. For example, if
you are a net supplier of a good, then a fall in its price will reduce your
money income directly since you won’t be able to sell your endowment for
as much money as you could before. We will have the same effects that
we had before, plus an extra income effect from the influence of the prices
on the value of the endowment bundle. We’ll call this the endowment
income effect.
In the earlier form of the Slutsky equation, the amount of money income
you had was fixed. Now we have to worry about how your money income
changes as the value of your endowment changes. Thus, when we calculate
the effect of a change in price on demand, the Slutsky equation will take
the form:
total change in demand = change due to substitution effect + change in demand due to ordinary income effect + change in demand due to endowment
income effect.
Figure
9.6
170 BUYING AND SELLING (Ch. 9)
The first two effects are familiar. As before, let us use Δx1 to stand for
the total change in demand, Δxs1 to stand for the change in demand due
to the substitution effect, and Δxm
1 to stand for the change in demand due
to the ordinary income effect. Then we can substitute these terms into the
above “verbal equation” to get the Slutsky equation in terms of rates of
change:
Δxs1
Δxm
Δx1
1
+ endowment income effect.
=
− x1
Δp1
Δp1
Δm
(9.1)
What will the last term look like? We’ll derive an explicit expression
below, but let us first think about what is involved. When the price of the
endowment changes, money income will change, and this change in money
income will induce a change in demand. Thus the endowment income effect
will consist of two terms:
endowment income effect = change in demand when income changes
× the change in income when price changes.
(9.2)
Let’s look at the second effect first. Since income is defined to be
m = p 1 ω 1 + p2 ω 2 ,
we have
Δm
= ω1 .
Δp1
This tells us how money income changes when the price of good 1 changes:
if you have 10 units of good 1 to sell, and its price goes up by $1, your
money income will go up by $10.
The first term in equation (9.2) is just how demand changes when income
changes. We already have an expression for this: it is Δxm
1 /Δm: the change
in demand divided by the change in income. Thus the endowment income
effect is given by
endowment income effect =
Δxm
Δxm
1 Δm
1
ω1 .
=
Δm Δp1
Δm
(9.3)
Inserting equation (9.3) into equation (9.1) we get the final form of the
Slutsky equation:
Δxs1
Δxm
Δx1
1
.
=
+ (ω1 − x1 )
Δp1
Δp1
Δm
This equation can be used to answer the question posed above. We know
that the sign of the substitution effect is always negative—opposite the
direction of the change in price. Let us suppose that the good is a normal
THE SLUTSKY EQUATION REVISITED
171
good, so that Δxm
1 /Δm > 0. Then the sign of the combined income effect
depends on whether the person is a net demander or a net supplier of
the good in question. If the person is a net demander of a normal good,
and its price increases, then the consumer will necessarily buy less of it.
If the consumer is a net supplier of a normal good, then the sign of the
total effect is ambiguous: it depends on the magnitude of the (positive)
combined income effect as compared to the magnitude of the (negative)
substitution effect.
As before, each of these changes can be depicted graphically, although
the graph gets rather messy. Refer to Figure 9.7, which depicts the Slutsky
decomposition of a price change. The total change in the demand for good 1
is indicated by the movement from A to C. This is the sum of three separate
movements: the substitution effect, which is the movement from A to B,
and two income effects. The ordinary income effect, which is the movement
from B to D, is the change in demand holding money income fixed—that
is, the same income effect that we examined in Chapter 8. But since the
value of the endowment changes when prices change, there is now an extra
income effect: because of the change in the value of the endowment, money
income changes. This change in money income shifts the budget line back
inward so that it passes through the endowment bundle. The change in
demand from D to C measures this endowment income effect.
x2
Endowment
Final choice
Original
choice
Indifference
curves
A
B C D
x1
The Slutsky equation revisited. Breaking up the effect
of the price change into the substitution effect (A to B), the
ordinary income effect (B to D), and the endowment income
effect (D to C).
Figure
9.7
172 BUYING AND SELLING (Ch. 9)
9.7 Use of the Slutsky Equation
Suppose that we have a consumer who sells apples and oranges that he
grows on a few trees in his backyard, like the consumer we described at the
beginning of Chapter 8. We said there that if the price of apples increased,
then this consumer might actually consume more apples. Using the Slutsky
equation derived in this chapter, it is not hard to see why. If we let xa stand
for the consumer’s demand for apples, and let pa be the price of apples,
then we know that
Δxsa
Δxm
Δxa
a
=
−
x
)
+
(ω
a
a
Δp
Δpa
Δm .
a
(+)
(+)
(−)
This says that the total change in the demand for apples when the price
of apples changes is the substitution effect plus the income effect. The substitution effect works in the right direction—increasing the price decreases
the demand for apples. But if apples are a normal good for this consumer,
the income effect works in the wrong direction. Since the consumer is a net
supplier of apples, the increase in the price of apples increases his money
income so much that he wants to consume more apples due to the income
effect. If the latter term is strong enough to outweigh the substitution
effect, we can easily get the “perverse” result.
EXAMPLE: Calculating the Endowment Income Effect
Let’s try a little numerical example. Suppose that a dairy farmer produces
40 quarts of milk a week. Initially the price of milk is $3 a quart. His
demand function for milk, for his own consumption, is
x1 = 10 +
m
.
10p1
Since he is producing 40 quarts at $3 a quart, his income is $120 a week.
His initial demand for milk is therefore x1 = 14. Now suppose that the
price of milk changes to $2 a quart. His money income will then change to
m = 2 × 40 = $80, and his demand will be x1 = 10 + 80/20 = 14.
If his money income had remained fixed at m = $120, he would have
purchased x1 = 10 + 120/10 × 2 = 16 quarts of milk at this price. Thus the
endowment income effect—the change in his demand due to the change
in the value of his endowment—is −2. The substitution effect and the
ordinary income effect for this problem were calculated in Chapter 8.
LABOR SUPPLY
173
9.8 Labor Supply
Let us apply the idea of an endowment to analyzing a consumer’s labor
supply decision. The consumer can choose to work a lot and have relatively high consumption, or can choose to work a little and have a small
consumption. The amount of consumption and labor will be determined
by the interaction of the consumer’s preferences and the budget constraint.
The Budget Constraint
Let us suppose that the consumer initially has some money income M that
she receives whether she works or not. This might be income from investments or from relatives, for example. We call this amount the consumer’s
nonlabor income. (The consumer could have zero nonlabor income, but
we want to allow for the possibility that it is positive.)
Let us use C to indicate the amount of consumption the consumer has,
and use p to denote the price of consumption. Then letting w be the wage
rate, and L the amount of labor supplied, we have the budget constraint:
pC = M + wL.
This says that the value of what the consumer consumes must be equal to
her nonlabor income plus her labor income.
Let us try to compare the above formulation to the previous examples
of budget constraints. The major difference is that we have something
that the consumer is choosing—labor supply—on the right-hand side of
the equation. We can easily transpose it to the left-hand side to get
pC − wL = M.
This is better, but we have a minus sign where we normally have a
plus sign. How can we remedy this? Let us suppose that there is some
maximum amount of labor supply possible—24 hours a day, 7 days a week,
or whatever is compatible with the units of measurement we are using. Let
L denote this amount of labor time. Then adding wL to each side and
rearranging we have
pC + w(L − L) = M + wL.
Let us define C = M/p, the amount of consumption that the consumer
would have if she didn’t work at all. That is, C is her endowment of
consumption, so we write
pC + w(L − L) = pC + wL.
174 BUYING AND SELLING (Ch. 9)
Now we have an equation very much like those we’ve seen before. We
have two choice variables on the left-hand side and two endowment variables
on the right-hand side. The variable L−L can be interpreted as the amount
of “leisure”—that is, time that isn’t labor time. Let us use the variable
R (for relaxation!) to denote leisure, so that R = L − L. Then the total
amount of time you have available for leisure is R = L and the budget
constraint becomes
pC + wR = pC + wR.
The above equation is formally identical to the very first budget constraint that we wrote in this chapter. However, it has a much more interesting interpretation. It says that the value of a consumer’s consumption
plus her leisure has to equal the value of her endowment of consumption
and her endowment of time, where her endowment of time is valued at her
wage rate. The wage rate is not only the price of labor, it is also the price
of leisure.
After all, if your wage rate is $10 an hour and you decide to consume
an extra hour’s leisure, how much does it cost you? The answer is that
it costs you $10 in forgone income—that’s the price of that extra hour’s
consumption of leisure. Economists sometimes say that the wage rate is
the opportunity cost of leisure.
The right-hand side of this budget constraint is sometimes called the
consumer’s full income or implicit income. It measures the value of
what the consumer owns—her endowment of consumption goods, if any,
and her endowment of her own time. This is to be distinguished from the
consumer’s measured income, which is simply the income she receives
from selling off some of her time.
The nice thing about this budget constraint is that it is just like the ones
we’ve seen before. It passes through the endowment point (L, C) and has a
slope of −w/p. The endowment would be what the consumer would get if
she did not engage in market trade at all, and the slope of the budget line
tells us the rate at which the market will exchange one good for another.
The optimal choice occurs where the marginal rate of substitution—the
tradeoff between consumption and leisure—equals w/p, the real wage, as
depicted in Figure 9.8. The value of the extra consumption to the consumer
from working a little more has to be just equal to the value of the lost leisure
that it takes to generate that consumption. The real wage is the amount
of consumption that the consumer can purchase if she gives up an hour of
leisure.
9.9 Comparative Statics of Labor Supply
First let us consider how a consumer’s labor supply changes as money
income changes with the price and wage held fixed. If you won the state
COMPARATIVE STATICS OF LABOR SUPPLY
175
CONSUMPTION
Indifference
curve
Optimal choice
C
Endowment
C
R
R
Leisure
LEISURE
Labor
Labor supply. The optimal choice describes the demand for
leisure measured from the origin to the right, and the supply of
labor measured from the endowment to the left.
lottery and got a big increase in nonlabor income, what would happen to
your supply of labor? What would happen to your demand for leisure?
For most people, the supply of labor would drop when their money income increased. In other words, leisure is probably a normal good for most
people: when their money income rises, people choose to consume more
leisure. There seems to be a fair amount of evidence for this observation,
so we will adopt it as a maintained hypothesis: we will assume that leisure
is a normal good.
What does this imply about the response of the consumer’s labor supply
to changes in the wage rate? When the wage rate increases there are two
effects: the return to working more increase and the cost of consuming
leisure increases. By using the ideas of income and substitution effects and
the Slutsky equation we can isolate these individual effects and analyze
them.
When the wage rate increases, leisure becomes more expensive, which by
itself leads people to want less of it (the substitution effect). Since leisure
is a normal good, we would then predict that an increase in the wage rate
would necessarily lead to a decrease in the demand for leisure—that is, an
increase in the supply of labor. This follows from the Slutsky equation
given in Chapter 8. A normal good must have a negatively sloped demand
curve. If leisure is a normal good, then the supply curve of labor must be
positively sloped.
Figure
9.8
176 BUYING AND SELLING (Ch. 9)
But there is a problem with this analysis. First, at an intuitive level, it
does not seem reasonable that increasing the wage would always result in
an increased supply of labor. If my wage becomes very high, I might well
“spend” the extra income in consuming leisure. How can we reconcile this
apparently plausible behavior with the economic theory given above?
If the theory gives the wrong answer, it is probably because we’ve misapplied the theory. And indeed in this case we have. The Slutsky example
described earlier gave the change in demand holding money income constant. But if the wage rate changes, then money income must change as
well. The change in demand resulting from a change in money income is
an extra income effect—the endowment income effect. It occurs on top of
the ordinary income effect.
If we apply the appropriate version of the Slutsky equation given earlier
in this chapter, we get the following expression:
ΔR = substitution effect + (R − R) ΔR .
Δm
Δw
(+)
(−)
(+)
(9.4)
In this expression the substitution effect is definitely negative, as it always is, and ΔR/Δm is positive since we are assuming that leisure is a
normal good. But (R − R) is positive as well, so the sign of the whole
expression is ambiguous. Unlike the usual case of consumer demand, the
demand for leisure will have an ambiguous sign, even if leisure is a normal
good. As the wage rate increases, people may work more or less.
Why does this ambiguity arise? When the wage rate increases, the substitution effect says work more in order to substitute consumption for leisure.
But when the wage rate increases, the value of the endowment goes up as
well. This is just like extra income, which may very well be consumed in
taking extra leisure. Which is the larger effect is an empirical matter and
cannot be decided by theory alone. We have to look at people’s actual
labor supply decisions to determine which effect dominates.
The case where an increase in the wage rate results in a decrease in the
supply of labor is represented by a backward-bending labor supply
curve. The Slutsky equation tells us that this effect is more likely to occur
the larger is (R − R), that is, the larger is the supply of labor. When
R = R, the consumer is consuming only leisure, so an increase in the wage
will result in a pure substitution effect and thus an increase in the supply
of labor. But as the labor supply increases, each increase in the wage gives
the consumer additional income for all the hours he is working, so that
after some point he may well decide to use this extra income to “purchase”
additional leisure—that is, to reduce his supply of labor.
A backward-bending labor supply curve is depicted in Figure 9.9. When
the wage rate is small, the substitution effect is larger than the income
effect, and an increase in the wage will decrease the demand for leisure and
hence increase the supply of labor. But for larger wage rates the income
COMPARATIVE STATICS OF LABOR SUPPLY
177
effect may outweigh the substitution effect, and an increase in the wage
will reduce the supply of labor.
CONSUMPTION
WAGE
Supply
of labor
Endowment
C
L1
LEISURE
L1
L2
LABOR
L2
A Indifference curves
B Labor supply curve
Backward-bending labor supply. As the wage rate increases, the supply of labor increases from L1 to L2 . But a
further increase in the wage rate reduces the supply of labor
back to L1 .
EXAMPLE: Overtime and the Supply of Labor
Consider a worker who has chosen to supply a certain amount of labor
L∗ = R − R∗ when faced with the wage rate w as depicted in Figure 9.10.
Now suppose that the firm offers him a higher wage, w > w, for extra time
that he chooses to work. Such a payment is known as an overtime wage.
In terms of Figure 9.10, this means that the slope of the budget line will
be steeper for labor supplied in excess of L∗ . But then we know that the
worker will optimally choose to supply more labor, by the usual sort of
revealed preference argument: the choices involving working less than L∗
were available before the overtime was offered and were rejected.
Note that we get an unambiguous increase in labor supply with an overtime wage, whereas just offering a higher wage for all hours worked has an
ambiguous effect—as discussed above, labor supply may increase or it may
decrease. The reason is that the response to an overtime wage is essentially
a pure substitution effect—the change in the optimal choice resulting from
Figure
9.9
178 BUYING AND SELLING (Ch. 9)
CONSUMPTION
Overtime wage
budget line
Optimal choice with
higher wage
Optimal
choice
with
overtime
Higher wage for all
hours budget line
C*
Indifference
curves
C
Endowment
R*
Figure
9.10
Original wage
budget line
R
LEISURE
Overtime versus an ordinary wage increase. An increase
in the overtime wage definitely increases the supply of labor,
while an increase in the straight wage could decrease the supply
of labor.
pivoting the budget line around the chosen point. Overtime gives a higher
payment for the extra hours worked, whereas a straight increase in the wage
gives a higher payment for all hours worked. Thus a straight-wage increase
involves both a substitution and an income effect while an overtime-wage
increase results in a pure substitution effect. An example of this is shown in
Figure 9.10. There an increase in the straight wage results in a decrease in
labor supply, while an increase in the overtime wage results in an increase
in labor supply.
Summary
1. Consumers earn income by selling their endowment of goods.
2. The gross demand for a good is the amount that the consumer ends up
consuming. The net demand for a good is the amount the consumer buys.
Thus the net demand is the difference between the gross demand and the
endowment.
APPENDIX
179
3. The budget constraint has a slope of −p1 /p2 and passes through the
endowment bundle.
4. When a price changes, the value of what the consumer has to sell will
change and thereby generate an additional income effect in the Slutsky
equation.
5. Labor supply is an interesting example of the interaction of income and
substitution effects. Due to the interaction of these two effects, the response
of labor supply to a change in the wage rate is ambiguous.
REVIEW QUESTIONS
1. If a consumer’s net demands are (5, −3) and her endowment is (4, 4),
what are her gross demands?
2. The prices are (p1 , p2 ) = (2, 3), and the consumer is currently consuming
(x1 , x2 ) = (4, 4). There is a perfect market for the two goods in which they
can be bought and sold costlessly. Will the consumer necessarily prefer
consuming the bundle (y1 , y2 ) = (3, 5)? Will she necessarily prefer having
the bundle (y1 , y2 )?
3. The prices are (p1 , p2 ) = (2, 3), and the consumer is currently consuming
(x1 , x2 ) = (4, 4). Now the prices change to (q1 , q2 ) = (2, 4). Could the
consumer be better off under these new prices?
4. The U.S. currently imports about half of the petroleum that it uses. The
rest of its needs are met by domestic production. Could the price of oil rise
so much that the U.S. would be made better off?
5. Suppose that by some miracle the number of hours in the day increased
from 24 to 30 hours (with luck this would happen shortly before exam
week). How would this affect the budget constraint?
6. If leisure is an inferior good, what can you say about the slope of the
labor supply curve?
APPENDIX
The derivation of the Slutsky equation in the text contained one bit of hand
waving. When we considered how changing the monetary value of the endowment
affects demand, we said that it was equal to Δxm
1 /Δm. In our old version of the
Slutsky equation this was the rate of change in demand when income changed
so as to keep the original consumption bundle affordable. But that will not
180 BUYING AND SELLING (Ch. 9)
necessarily be equal to the rate of change of demand when the value of the
endowment changes. Let’s examine this point in a little more detail.
Let the price of good 1 change from p1 to p1 , and use m to denote the new
money income at the price p1 due to the change in the value of the endowment.
Suppose that the price of good 2 remains fixed so we can omit it as an argument
of the demand function.
By definition of m , we know that
m − m = Δp1 ω1 .
Note that it is identically true that
x1 (p1 , m ) − x1 (p1 , m)
=
Δp1
x1 (p1 , m ) − x1 (p1 , m)
(substitution effect)
Δp1
x1 (p1 , m ) − x1 (p1 , m)
−
(ordinary income effect)
Δp1
x1 (p1 , m ) − x1 (p1 , m)
+
(endowment income effect).
Δp1
+
(Just cancel out identical terms with opposite signs on the right-hand side.)
By definition of the ordinary income effect,
Δp1 =
m − m
x1
and by definition of the endowment income effect,
Δp1 =
m − m
.
ω1
Making these replacements gives us a Slutsky equation of the form
x1 (p1 , m ) − x1 (p1 , m)
=
Δp1
x1 (p1 , m ) − x1 (p1 , m)
(substitution effect)
Δp1
x1 (p1 , m ) − x1 (p1 , m)
−
x1 (ordinary income effect)
m − m
x1 (p1 , m ) − x1 (p1 , m)
+
ω1 (endowment income effect).
m − m
+
Writing this in terms of Δs, we have
Δx1
Δxs1
Δxm
Δxw
1
1
=
−
x1 +
ω1 .
Δp1
Δp1
Δm
Δm
APPENDIX
181
The only new term here is the last one. It tells how the demand for good 1
changes as income changes, times the endowment of good 1. This is precisely the
endowment income effect.
Suppose that we are considering a very small price change, and thus a small
associated income change. Then the fractions in the two income effects will be
virtually the same, since the rate of change of good 1 when income changes from
m to m should be about the same as when income changes from m to m . For
such small changes we can collect terms and write the last two terms—the income
effects—as
Δxm
1
(ω1 − x1 ),
Δm
which yields a Slutsky equation of the same form as that derived earlier:
Δx1
Δxs1
Δxm
1
=
+ (ω1 − x1 )
.
Δp1
Δp1
Δm
If we want to express the Slutsky equation in calculus terms, we can just take
limits in this expression. Or, if you prefer, we can calculate the correct equation
directly, just by taking partial derivatives. Let x1 (p1 , m(p1 )) be the demand
function for good 1 where we hold price 2 fixed and recognize that money income
depends on the price of good 1 via the relationship m(p1 ) = p1 ω1 + p2 ω2 . Then
we can write
dx1 (p1 , m(p1 ))
∂x1 (p1 , m)
∂x1 (p1 , m) dm(p1 )
=
+
.
dp1
∂p1
∂m
dp1
(9.5)
By the definition of m(p1 ) we know how income changes when price changes:
∂m(p1 )
= ω1 ,
∂p1
(9.6)
and by the Slutsky equation we know how demand changes when price changes,
holding money income fixed:
∂x1 (p1 , m)
∂xs1 (p1 )
∂x(p1 , m)
=
−
x1 .
∂p1
∂p1
∂m
Inserting equations (9.6) and (9.7) into equation (9.5) we have
dx1 (p1 , m(p1 ))
∂xs1 (p1 )
∂x(p1 , m)
=
+
(ω1 − x1 ),
dp1
∂p1
∂m
which is the form of the Slutsky equation that we want.
(9.7)
In previous chapters, we studied the behavior of consumers who start out
without owning any goods, but who had some money with which to buy
goods. In this chapter, the consumer has an initial endowment, which is
the bundle of goods the consumer owns before any trades are made. A
consumer can trade away from his initial endowment by selling one good
and buying the other.
The techniques that you have already learned will serve you well here.
To find out how much a consumer demands at given prices, you find his
budget line and then find a point of tangency between his budget line and
an indifference curve. To determine a budget line for a consumer who
is trading from an initial endowment and who has no source of income
other than his initial endowment, notice two things. First, the initial
endowment must lie on the consumer’s budget line. This is true because,
no matter what the prices are, the consumer can always afford his initial
endowment. Second, if the prices are p1 and p2 , the slope of the budget
line must be −p1 /p2 . This is true, since for every unit of good 1 the
consumer gives up, he can get exactly p1 /p2 units of good 2. Therefore
if you know the prices and you know the consumer’s initial endowment,
then you can always write an equation for the consumer’s budget line.
After all, if you know one point on a line and you know its slope, you
can either draw the line or write down its equation. Once you have the
budget equation, you can find the bundle the consumer chooses, using the
same methods you learned in Chapter 5.
A peasant consumes only rice and fish. He grows some rice and some
fish, but not necessarily in the same proportion in which he wants to
consume them. Suppose that if he makes no trades, he will have 20 units
of rice and 5 units of fish. The price of rice is 1 yuan per unit, and the
price of fish is 2 yuan per unit. The value of the peasant’s endowment is
(1 × 20) + (2 × 5) = 30. Therefore the peasant can consume any bundle
(R, F ) such that (1 × R) + (2 × F ) = 30.
Perhaps the most interesting application of trading from an initial
endowment is the theory of labor supply. To study labor supply, we
consider the behavior of a consumer who is choosing between leisure and
other goods. The only thing that is at all new or “tricky” is finding
the appropriate budget constraint for the problem at hand. To study
labor supply, we think of the consumer as having an initial endowment of
leisure, some of which he may trade away for goods.
In most applications we set the price of “other goods” at 1. The
wage rate is the price of leisure. The role that is played by income in
the ordinary consumer-good model is now played by “full income.” A
worker’s full income is the income she would have if she chose to take no
leisure.
Sherwin has 18 hours a day which he divides between labor and leisure.
He can work as many hours a day as he wishes for a wage of $5 per hour.
He also receives a pension that gives him $10 a day whether he works
or not. The price of other goods is $1 per unit. If Sherwin makes no
trades at all, he will have 18 hours of leisure and 10 units of other goods.
Therefore Sherwin’s initial endowment is 18 hours of leisure a day and
$10 a day for other goods. Let R be the amount of leisure that he has per
day, and let C be the number of dollars he has to spend per day on other
goods. If his wage is $5 an hour, he can afford to consume bundle (R, C)
if it costs no more per day than the value of his initial endowment. The
value of his initial endowment (his full income) is $10 + ($5 × 18) = $100
per day. Therefore Sherwin’s budget equation is 5R + C = 100.
9.1 (0) Abishag Appleby owns 20 quinces and 5 kumquats. She has no
income from any other source, but she can buy or sell either quinces or
kumquats at their market prices. The price of kumquats is four times the
price of quinces. There are no other commodities of interest.
(a) How many quinces could she have if she was willing to do without
kumquats?
How many kumquats could she have if she was
willing to do without quinces?
.
Kumquats
40
30
20
10
0
10
20
30
40
Quinces
(b) Draw Abishag’s budget set, using blue ink, and label the endowment bundle with the letter E. If the price of quinces is 1 and the
price of kumquats is 4, write Abishag’s budget equation.
If the price of quinces is 2 and the price of kumquats is
8, write Abishag’s budget equation.
What effect does
doubling both prices have on the set of commodity bundles that Abishag
can afford?
.
(c) Suppose that Abishag decides to sell 10 quinces. Label her final
consumption bundle in your graph with the letter C.
(d) Now, after she has sold 10 quinces and owns the bundle labeled C,
suppose that the price of kumquats falls so that kumquats cost the same
as quinces. On the diagram above, draw Abishag’s new budget line, using
red ink.
(e) If Abishag obeys the weak axiom of revealed preference, then there are
some points on her red budget line that we can be sure Abishag will not
choose. On the graph, make a squiggly line over the portion of Abishag’s
red budget line that we can be sure she will not choose.
9.2 (0) Mario has a small garden where he raises eggplant and tomatoes.
He consumes some of these vegetables, and he sells some in the market.
Eggplants and tomatoes are perfect complements for Mario, since the only
recipes he knows use them together in a 1:1 ratio. One week his garden
yielded 30 pounds of eggplant and 10 pounds of tomatoes. At that time
the price of each vegetable was $5 per pound.
(a) What is the monetary value of Mario’s endowment of vegetables?
.
(b) On the graph below, use blue ink to draw Mario’s budget line. Mario
ends up consuming
pounds of tomatoes and
pounds
of eggplant. Draw the indifference curve through the consumption bundle
that Mario chooses and label this bundle A.
(c) Suppose that before Mario makes any trades, the price of tomatoes
rises to $15 a pound, while the price of eggplant stays at $5 a pound.
What is the value of Mario’s endowment now?
Draw his
new budget line, using red ink. He will now choose a consumption bundle
consisting of
tomatoes and
eggplants.
(d) Suppose that Mario had sold his entire crop at the market for a total
of $200, intending to buy back some tomatoes and eggplant for his own
consumption. Before he had a chance to buy anything back, the price of
tomatoes rose to $15, while the price of eggplant stayed at $5. Draw his
budget line, using pencil or black ink. Mario will now consume
pounds of tomatoes and
pounds of eggplant.
(e) Assuming that the price of tomatoes rose to $15 from $5 before Mario
made any transactions, the change in the demand for tomatoes due to the
substitution effect was
The change in the demand for tomatoes
due to the ordinary income effect was
The change in the
demand for tomatoes due to the endowment income effect was
The total change in the demand for tomatoes was
.
Eggplant
40
30
20
10
0
10
20
30
40
Tomatoes
9.3 (0) Lucetta consumes only two goods, A and B. Her only source of
income is gifts of these commodities from her many admirers. She doesn’t
always get these goods in the proportions in which she wants to consume
them, but she can always buy or sell A at the price pA = 1 and B at the
price pB = 2. Lucetta’s utility function is U (a, b) = ab, where a is the
amount of A she consumes and b is the amount of B she consumes.
(a) Suppose that Lucetta’s admirers give her 100 units of A and 200 units
of B. In the graph below, use red ink to draw her budget line. Label her
initial endowment E.
(b) What are Lucetta’s gross demands for A?
And for B?
.
(c) What are Lucetta’s net demands?
.
(d) Suppose that before Lucetta has made any trades, the price of good
B falls to 1, and the price of good A stays at 1. Draw Lucetta’s budget
line at these prices on your graph, using blue ink.
(e) Does Lucetta’s consumption of good B rise or fall?
how much?
By
What happens to Lucetta’s consumption of
good A?
.
Good B
600
500
400
300
200
100
0
75
150
225
300
Good A
(f ) Suppose that before the price of good B fell, Lucetta had exchanged all
of her gifts for money, planning to use the money to buy her consumption
bundle later. How much of good B will she choose to consume?
How much of good A?
.
(g) Explain why her consumption is different depending on whether she
was holding goods or money at the time of the price change.
.
9.4 (0) Priscilla finds it optimal not to engage in trade at the going prices
and just consumes her endowment. Priscilla has no kinks in her indifference curves, and she is endowed with positive amounts of both goods. Use
pencil or black ink to draw a budget line and an indifference curve for
Priscilla that would be consistent with these facts. Suppose that the price
of good 2 stays the same, but the price of good 1 falls. Use blue ink to show
her new budget line. Priscilla satisfies the weak axiom of revealed preference. Could it happen that Priscilla will consume less of good 1 than before? Explain.
.
9.5 (0) Potatoes are a Giffen good for Paddy, who has a small potato
farm. The price of potatoes fell, but Paddy increased his potato consumption. At first this astonished the village economist, who thought that a
decrease in the price of a Giffen good was supposed to reduce demand.
But then he remembered that Paddy was a net supplier of potatoes. With
the help of a graph, he was able to explain Paddy’s behavior. In the axes
below, show how this could have happened. Put “potatoes” on the horizontal axis and “all other goods” on the vertical axis. Label the old
equilibrium A and the new equilibrium B. Draw a point C so that the
Slutsky substitution effect is the movement from A to C and the Slutsky
income effect is the movement from C to B. On this same graph, you are
also going to have to show that potatoes are a Giffen good. To do this,
draw a budget line showing the effect of a fall in the price of potatoes if
Paddy didn’t own any potatoes, and only had money income. Label the
new consumption point under these circumstances by D. (Warning: You
probably will need to make a few dry runs on some scratch paper to get
the whole story straight.)
9.6 (0) Recall the travails of Agatha, from the previous chapter. She had
to travel 1,500 miles from Istanbul to Paris. She had only $200 with which
to buy first-class and second-class tickets on the Orient Express when the
price of first-class tickets was $.20 a mile and the price of second-class
tickets was $.10 a mile. She bought tickets that enabled her to travel all
the way to Paris, with as many miles of first class as she could afford.
After she boarded the train, she discovered to her amazement that the
price of second-class tickets had fallen to $.05 a mile while the price of
first-class tickets remained at $.20 a mile. She also discovered that on the
train it was possible to buy or sell first-class tickets for $.20 a mile and
to buy or sell second-class tickets for $.05 a mile. Agatha had no money
left to buy either kind of ticket, but she did have the tickets that she had
already bought.
(a) On the graph below, use pencil to show the combinations of tickets
that she could afford at the old prices. Use blue ink to show the combinations of tickets that would take her exactly 1,500 miles. Mark the point
that she chooses with the letter A.
First-class miles
1600
1200
800
400
0
400
800
1200 1600
Second-class miles
(b) Use red ink to draw a line showing all of the combinations of first-class
and second-class travel that she can afford when she is on the train, by
trading her endowment of tickets at the new prices that apply on board
the train.
(c) On your graph, show the point that she chooses after finding out
about the price change. Does she choose more, less, or the same amount
of second-class tickets?
.
9.7 (0) Mr. Cog works in a machine factory. He can work as many
hours per day as he wishes at a wage rate of w. Let C be the number of
dollars he spends on consumer goods and let R be the number of hours
of leisure that he chooses.
(a) Mr. Cog earns $8 an hour and has 18 hours per day to devote to
labor or leisure, and he has $16 of nonlabor income per day. Write an
equation for his budget between consumption and leisure.
Use blue ink to draw his budget line in the graph below. His
initial endowment is the point where he does no work and enjoys 18 hours
of leisure per day. Mark this point on the graph below with the letter A.
(Remember that although Cog can choose to work and thereby “sell” some
of his endowment of leisure, he cannot “buy leisure” by paying somebody
else to loaf for him.) If Mr. Cog has the utility function U (R, C) = CR,
how many hours of leisure per day will he choose?
hours per day will he work?
How many
.
Consumption
240
200
160
120
80
40
0
4
8
12
16
20
24
Leisure
(b) Suppose that Mr. Cog’s wage rate rises to $12 an hour. Use red ink
to draw his new budget line. (He still has $16 a day in nonlabor income.)
If he continued to work exactly as many hours as he did before the wage
increase, how much more money would he have each day to spend on
consumption?
work
But with his new budget line, he chooses to
hours, and so his consumption increases by
.
(c) Suppose that Mr. Cog still receives $8 an hour but that his nonlabor
income rises to $48 per day. Use black ink to draw his budget line. How
many hours does he choose to work?
.
(d) Suppose that Mr. Cog has a wage of $w per hour, a nonlabor income
of $m, and that he has 18 hours a day to divide between labor and leisure.
Cog’s budget line has the equation C + wR = m + 18w. Using the same
methods you used in the chapter on demand functions, find the amount
of leisure that Cog will demand as a function of wages and of nonlabor
income. (Hint: Notice that this is the same as finding the demand for R
when the price of R is w, the price of C is 1, and income is m + 18w.) Mr.
Cog’s demand function for leisure is R(w, m) =
supply function for labor is therefore 18 − R(w, m) =
Mr. Cog’s
.
9.8 (0) Fred has just arrived at college and is trying to figure out how to
supplement the meager checks that he gets from home. “How can anyone
live on $50 a week for spending money?” he asks. But he asks to no
avail. “If you want more money, get a job,” say his parents. So Fred
glumly investigates the possibilities. The amount of leisure time that he
has left after allowing for necessary activities like sleeping, brushing teeth,
and studying for economics classes is 50 hours a week. He can work as
many hours per week at a nearby Taco Bell for $5 an hour. Fred’s utility
function for leisure and money to spend on consumption is U (C, L) = CL.
(a) Fred has an endowment that consists of $50 of money to spend on
hours of leisure, some of which he might “sell”
consumption and
for money. The money value of Fred’s endowment bundle, including both
his money allowance and the market value of his leisure time is therefore
Fred’s “budget line” for leisure and consumption is like a
budget line for someone who can buy these two goods at a price of $1 per
unit of consumption and a price of
per unit of leisure. The only
difference is that this budget line doesn’t run all the way to the horizontal
axis.
(b) On the graph below, use black ink to show Fred’s budget line. (Hint:
Find the combination of leisure and consumption expenditures that he
could have if he didn’t work at all. Find the combination he would have
if he chose to have no leisure at all. What other points are on your graph?)
On the same graph, use blue ink to sketch the indifference curves that
give Fred utility levels of 3,000, 4,500, and 7,500.
(c) If you maximized Fred’s utility subject to the above budget, how
much consumption would he choose?
(Hint: Remember how
to solve for the demand function of someone with a Cobb-Douglas utility
function?)
(d) The amount of leisure that Fred will choose to consume is
hours. This means that his optimal labor supply will be
hours.
Consumption
300
250
200
150
100
50
0
10
20
30
40
50
60
Leisure
9.9 (0) George Johnson earns $5 per hour in his job as a truffle sniffer. After allowing time for all of the activities necessary for bodily upkeep, George has 80 hours per week to allocate between leisure and labor.
Sketch the budget constraints for George resulting from the following
government programs.
(a) There is no government subsidy or taxation of labor income. (Use
blue ink on the graph below.)
Consumption
400
300
200
100
0
20
40
60
80
Leisure
(b) All individuals receive a lump sum payment of $100 per week from the
government. There is no tax on the first $100 per week of labor income.
But all labor income above $100 per week is subject to a 50% income tax.
(Use red ink on the graph above.)
(c) If an individual is not working, he receives a payment of $100. If he
works he does not receive the $100, and all wages are subject to a 50%
income tax. (Use blue ink on the graph below.)
Consumption
400
300
200
100
0
20
40
60
80
Leisure
(d) The same conditions as in Part (c) apply, except that the first 20 hours
of labor are exempt from the tax. (Use red ink on the graph above.)
(e) All wages are taxed at 50%, but as an incentive to encourage work,
the government gives a payment of $100 to anyone who works more than
20 hours a week. (Use blue ink on the graph below.)
Consumption
400
300
200
100
0
20
40
60
80
Leisure
9.10 (0) In the United States, real wage rates in manufacturing have
risen steadily from 1890 to the present. In the period from 1890 to 1930,
the length of the workweek was reduced dramatically. But after 1930,
despite the continuing growth of real wage rates, the length of the work
week has stayed remarkably constant at about 40 hours per week.
Hourly Wages and Length of Work Week
in U.S. Manufacturing, 1890–1983
Sources: Handbook of Labor Statistics, 1983 and U.S. Economic History,
by Albert Niemi (p. 274). Wages are in 1983 dollars.
Year
1890
1909
1920
1930
1940
1950
1960
1970
1983
Wage
1.89
2.63
3.11
3.69
5.27
6.86
8.56
9.66
10.74
Hours Worked
59.0
51.0
47.4
42.1
38.1
40.5
39.7
39.8
40.1
(a) Use these data to plot a “labor supply curve” on the graph below.
Hourly wage rate (in 1983 dollars)
12
10
8
6
4
2
0
10
20
30
40
50
60
Hours of work per week
(b) At wage rates below $4 an hour, does the workweek get longer or
shorter as the wage rate rises?
.
(c) The data in this table could be consistent with workers choosing
various hours a week to work, given the wage rate. An increase in
wages has both an endowment income effect and a substitution effect.
The substitution effect alone would make for a (longer, shorter)
workweek. If leisure is a normal good, the endowment income
effect tends to make people choose (more, less)
a (longer, shorter)
leisure and
workweek. At wage rates below $4 an
hour, the (substitution, endowment income)
effect
appears to dominate. How would you explain what happens at wages
above $4 an hour?
.
(d) Between 1890 and 1909, wage rates rose by
weekly earnings rose by only
percent, but
percent. For this period, the gain
in earnings (overstates, understates)
wealth, since they chose to take (more, less)
than they took in 1890.
the gain in workers’
leisure in 1909
9.11 (0) Professor Mohamed El Hodiri of the University of Kansas, in
a classic tongue-in-cheek article “The Economics of Sleeping,” Manifold,
17 (1975), offered the following analysis. “Assume there are 24 hours in
a day. Daily consumption being x and hours of sleep s, the consumer
maximizes a utility function of the form u = x2 s, where x = w(24 − s),
with w being the wage rate.”
(a) In El Hodiri’s model, does the optimal amount of sleeping increase,
decrease, or stay the same as wages increase?
.
(b) How many hours of sleep per day is best in El Hodiri’s model?
.
9.12 (0) Wendy and Mac work in fast food restaurants. Wendy gets $4
an hour for the first 40 hours that she works and $6 an hour for every
hour beyond 40 hours a week. Mac gets $5 an hour no matter how many
hours he works. Each has 80 hours a week to allocate between work and
leisure and neither has any income from sources other than labor. Each
has a utility function U = cr, where c is consumption and r is leisure.
Each can choose the number of hours to work.
(a) How many hours will Mac choose to work?
.
(b) Wendy’s budget “line” has a kink in it at the point where r =
and c =
Use blue ink for the part of her budget line where she
would be if she does not work overtime. Use red ink for the part where
she would be if she worked overtime.
Consumption
400
300
200
100
0
20
40
60
80
Leisure
(c) The blue line segment that you drew lies on a line with equation
The red line that you drew lies on a line with equation
(Hint: For the red line, you know one point on the line
and you know its slope.)
(d) If Wendy was paid $4 an hour no matter how many hours she worked,
she would work
hours and earn a total of
a week.
On your graph, use black ink to draw her indifference curve through this
point.
(e) Will Wendy choose to work overtime?
choice for Wendy from the red budget line? (c, r) =
many hours a week will she work?
What is the best
How
.
(f ) Suppose that the jobs are equally agreeable in all other respects. Since
Wendy and Mac have the same preferences, they will be able to agree
about who has the better job. Who has the better job?
(Hint:
Calculate Wendy’s utility when she makes her best choice. Calculate what
her utility would be if she had Mac’s job and chose the best amount of
time to work.)
9.13 (1) Wally Piper is a plumber. He charges $10 per hour for his work
and he can work as many hours as he likes. Wally has no source of income
other than his labor. He has 168 hours per week to allocate between labor
and leisure. On the graph below, draw Wally’s budget set, showing the
various combinations of weekly leisure and income that Wally can afford.
Income
2400
2000
1600
1200
800
400
0
40
80
120
160
(a) Write down Wally’s budget equation.
200
240
Leisure
.
(b) While self-employed, Wally chose to work 40 hours per week. The
construction firm, Glitz and Drywall, had a rush job to complete. They
offered Wally $20 an hour and said that he could work as many hours as
he liked. Wally still chose to work only 40 hours per week. On the graph
you drew above, draw in Wally’s new budget line.
(c) Wally has convex preferences and no kinks in his indifference curves.
On the graph, draw indifference curves that are consistent with his choice
of working hours when he was self-employed and when he worked for Glitz
and Drywall.
(d) Glitz and Drywall were in a great hurry to complete their project and
wanted Wally to work more than 40 hours. They decided that instead of
paying him $20 per hour, they would pay him only $10 an hour for the
first 40 hours that he worked per week and $20 an hour for every hour of
“overtime” that he worked beyond 40 hours per week. On the graph that
you drew above, use red ink to sketch in Wally’s budget line with this
pay schedule. Draw the indifference curve through the point that Wally
chooses with this pay schedule. Will Wally work more than 40 hours or
less than 40 hours per week with this pay schedule?
.
9.14 (1) Felicity loves her job. She is paid $10 an hour and can work
as many hours a day as she wishes. She chooses to work only 5 hours
a day. She says the job is so interesting that she is happier working at
this job than she would be if she made the same income without working
at all. A skeptic asks, “If you like the job better than not working at
all, why don’t you work more hours and earn more money?” Felicity,
who is entirely rational, patiently explains that work may be desirable on
average but undesirable on the margin. The skeptic insists that she show
him her indifference curves and her budget line.
(a) On the axes below, draw a budget line and indifference curves that are
consistent with Felicity’s behavior and her remarks. Put leisure on the
horizontal axis and income on the vertical axis. (Hint: Where does the
indifference curve through her actual choice hit the vertical line l = 24?)
9.15 (2) Dudley’s utility function is U (C, R) = C − (12 − R)2 , where R
is the amount of leisure he has per day. He has 16 hours a day to divide
between work and leisure. He has an income of $20 a day from nonlabor
sources. The price of consumption goods is $1 per unit.
(a) If Dudley can work as many hours a day as he likes but gets zero
wages for his labor, how many hours of leisure will he choose?
.
(b) If Dudley can work as many hours a day as he wishes for a wage
rate of $10 an hour, how many hours of leisure will he choose?
How many hours will he work?
(Hint: Write down Dudley’s
budget constraint. Solve for the amount of leisure that maximizes his
utility subject to this constraint. Remember that the amount of labor he
wishes to supply is 16 minus his demand for leisure.)
(c) If Dudley’s nonlabor income decreased to $5 a day, while his wage rate
remained at $10, how many hours would he choose to work?
.
(d) Suppose that Dudley has to pay an income tax of 20 percent on all
of his income, and suppose that his before-tax wage remained at $10 an
hour and his before-tax nonlabor income was $20 per day. How many
hours would he choose to work?
.
9.1 In Problem 9.1, if Abishag owned 9 quinces and 10 kumquats and
if the price of kumquats were 3 times the price of quinces, how many
kumquats could she afford if she spent all of her money on kumquats?
(a) 26
(b) 19
(c) 10
(d) 13
(e) 10
9.2 Suppose that Mario in Problem 9.2 consumes eggplants and tomatoes
in the ratio of 1 bushel of eggplant per bushel of tomatoes. His garden
yields 30 bushels of eggplants and 10 bushels of tomatoes. He initially
faced prices of $10 per bushel for each vegetable, but the price of eggplants
rose to $30 per bushel, while the price of tomatoes stayed unchanged.
After the price change, he would
(a) increase his eggplant consumption by 5 bushels.
(b) decrease his eggplant consumption by at least 5 bushels.
(c) increase his eggplant consumption by 7 bushels.
(d) decrease his eggplant consumption by 7 bushels.
(e) decrease his tomato consumption by at least 1 bushel.
9.3 (See Problem 9.9(b).) Dr. Johnson earns $5 per hour for his labor
and has 80 hours to allocate between labor and leisure. His only other
income besides his earnings from labor is a lump sum payment of $50 per
week. Suppose that the first $200 per week of his labor income is untaxed,
but all of his labor income above $200 is taxed at a rate of 40 percent.
(a) Dr. Johnson’s budget line has a kink in it at the point where he takes
50 units of leisure.
(b) Dr. Johnson’s budget line has a kink where his income is $250 and
his leisure is 40 units.
(c) The slope of Dr. Johnson’s budget line is everywhere −3.
(d) Dr. Johnson’s budget line has no kinks in the part of it that corresponds to a positive labor supply.
(e) Dr. Johnson’s budget line has a piece that is a horizontal straight
line.
9.4 Dudley, in Problem 9.15, has a utility function U (C, R) = C − (12 −
R)2 , where R is leisure and C is consumption per day. He has 16 hours
per day to divide between work and leisure. If Dudley has a nonlabor
income of $40 per day and is paid a wage of $6 per hour, how many hours
of leisure will he choose per day?
(a) 6
(b) 7
(c) 8
(d) 10
(e) 9
9.5 Mr. Cog in Problem 9.7 has 18 hours a day to divide between labor
and leisure. His utility function is U (C, R) = CR, where C is the number
of dollars per day that he spends on consumption and R is the number of
hours per day that he spends at leisure. If he has 16 dollars of nonlabor
income per day and gets a wage rate of 13 dollars per hour when he works,
his budget equation, expressing combinations of consumption and leisure
that he can afford to have, is:
(a) 13R + C = 16.
(b) 13R + C = 250.
(c) R + C/13 = 328.
(d) C = 250 + 13R.
(e) C = 298 + 13R.
9.6 Mr. Cog in Problem 9.7 has 18 hours per day to divide between labor
and leisure. His utility function is U (C, R) = CR, where C is the number
of dollars per day that he spends on consumption and R is the number of
hours per day that he spends at leisure. If he has a nonlabor income of
42 dollars per day and a wage rate of 13 dollars per hour, he will choose
a combination of labor and leisure that allows him to spend
(a) 276 dollars per day on consumption.
(b) 128 dollars per day on consumption.
(c) 159 dollars per day on consumption.
(d) 138 dollars per day on consumption.
(e) 207 dollars per day on consumption.
CHAPTER
10
INTERTEMPORAL
CHOICE
In this chapter we continue our examination of consumer behavior by considering the choices involved in saving and consuming over time. Choices
of consumption over time are known as intertemporal choices.
10.1 The Budget Constraint
Let us imagine a consumer who chooses how much of some good to consume
in each of two time periods. We will usually want to think of this good
as being a composite good, as described in Chapter 2, but you can think
of it as being a specific commodity if you wish. We denote the amount
of consumption in each period by (c1 , c2 ) and suppose that the prices of
consumption in each period are constant at 1. The amount of money the
consumer will have in each period is denoted by (m1 , m2 ).
Suppose initially that the only way the consumer has of transferring
money from period 1 to period 2 is by saving it without earning interest.
Furthermore let us assume for the moment that he has no possibility of
THE BUDGET CONSTRAINT
183
C2
Budget line; slope = –1
m2
Endowment
m1
C1
Budget constraint. This is the budget constraint when the
rate of interest is zero and no borrowing is allowed. The less
the individual consumes in period 1, the more he can consume
in period 2.
borrowing money, so that the most he can spend in period 1 is m1 . His
budget constraint will then look like the one depicted in Figure 10.1.
We see that there will be two possible kinds of choices. The consumer
could choose to consume at (m1 , m2 ), which means that he just consumes
his income each period, or he can choose to consume less than his income
during the first period. In this latter case, the consumer is saving some of
his first-period consumption for a later date.
Now, let us allow the consumer to borrow and lend money at some
interest rate r. Keeping the prices of consumption in each period at 1 for
convenience, let us derive the budget constraint. Suppose first that the
consumer decides to be a saver so his first period consumption, c1 , is less
than his first-period income, m1 . In this case he will earn interest on the
amount he saves, m1 − c1 , at the interest rate r. The amount that he can
consume next period is given by
c2 = m2 + (m1 − c1 ) + r(m1 − c1 )
= m2 + (1 + r)(m1 − c1 ).
(10.1)
This says that the amount that the consumer can consume in period 2 is
his income plus the amount he saved from period 1, plus the interest that
he earned on his savings.
Now suppose that the consumer is a borrower so that his first-period
consumption is greater than his first-period income. The consumer is a
Figure
10.1
184 INTERTEMPORAL CHOICE (Ch. 10)
borrower if c1 > m1 , and the interest he has to pay in the second period
will be r(c1 − m1 ). Of course, he also has to pay back the amount that he
borrowed, c1 − m1 . This means his budget constraint is given by
c2 = m2 − r(c1 − m1 ) − (c1 − m1 )
= m2 + (1 + r)(m1 − c1 ),
which is just what we had before. If m1 − c1 is positive, then the consumer
earns interest on this savings; if m1 − c1 is negative, then the consumer
pays interest on his borrowings.
If c1 = m1 , then necessarily c2 = m2 , and the consumer is neither a
borrower nor a lender. We might say that this consumption position is the
“Polonius point.”1
We can rearrange the budget constraint for the consumer to get two
alternative forms that are useful:
(1 + r)c1 + c2 = (1 + r)m1 + m2
and
c1 +
c2
m2
= m1 +
.
1+r
1+r
(10.2)
(10.3)
Note that both equations have the form
p1 x1 + p2 x2 = p1 m1 + p2 m2 .
In equation (10.2), p1 = 1 + r and p2 = 1. In equation (10.3), p1 = 1 and
p2 = 1/(1 + r).
We say that equation (10.2) expresses the budget constraint in terms of
future value and that equation (10.3) expresses the budget constraint in
terms of present value. The reason for this terminology is that the first
budget constraint makes the price of future consumption equal to 1, while
the second budget constraint makes the price of present consumption equal
to 1. The first budget constraint measures the period-1 price relative to
the period-2 price, while the second equation does the reverse.
The geometric interpretation of present value and future value is given in
Figure 10.2. The present value of an endowment of money in two periods is
the amount of money in period 1 that would generate the same budget set
as the endowment. This is just the horizontal intercept of the budget line,
which gives the maximum amount of first-period consumption possible.
1
“Neither a borrower, nor a lender be; For loan oft loses both itself and friend, And
borrowing dulls the edge of husbandry.” Hamlet, Act I, scene iii; Polonius giving
advice to his son.
PREFERENCES FOR CONSUMPTION
185
C2
(1 + r ) m1 + m 2
(future value)
m2
Endowment
Budget line;
slope = – (1 + r )
m1
m1 + m 2 /(1 + r )
(present value)
C1
Present and future values. The vertical intercept of the
budget line measures future value, and the horizontal intercept
measures the present value.
Examining the budget constraint, this amount is c1 = m1 + m2 /(1 + r),
which is the present value of the endowment.
Similarly, the vertical intercept is the maximum amount of second-period
consumption, which occurs when c1 = 0. Again, from the budget constraint, we can solve for this amount c2 = (1 + r)m1 + m2 , the future value
of the endowment.
The present-value form is the more important way to express the intertemporal budget constraint since it measures the future relative to the
present, which is the way we naturally look at it.
It is easy from any of these equations to see the form of this budget
constraint. The budget line passes through (m1 , m2 ), since that is always
an affordable consumption pattern, and the budget line has a slope of
−(1 + r).
10.2 Preferences for Consumption
Let us now consider the consumer’s preferences, as represented by his indifference curves. The shape of the indifference curves indicates the consumer’s tastes for consumption at different times. If we drew indifference
curves with a constant slope of −1, for example, they would represent tastes
of a consumer who didn’t care whether he consumed today or tomorrow.
His marginal rate of substitution between today and tomorrow is −1.
Figure
10.2
186 INTERTEMPORAL CHOICE (Ch. 10)
If we drew indifference curves for perfect complements, this would indicate that the consumer wanted to consume equal amounts today and
tomorrow. Such a consumer would be unwilling to substitute consumption
from one time period to the other, no matter what it might be worth to
him to do so.
As usual, the intermediate case of well-behaved preferences is the more
reasonable situation. The consumer is willing to substitute some amount of
consumption today for consumption tomorrow, and how much he is willing
to substitute depends on the particular pattern of consumption that he
has.
Convexity of preferences is very natural in this context, since it says that
the consumer would rather have an “average” amount of consumption each
period rather than have a lot today and nothing tomorrow or vice versa.
10.3 Comparative Statics
Given a consumer’s budget constraint and his preferences for consumption
in each of the two periods, we can examine the optimal choice of consumption (c1 , c2 ). If the consumer chooses a point where c1 < m1 , we will say
that she is a lender, and if c1 > m1 , we say that she is a borrower. In
Figure 10.3A we have depicted a case where the consumer is a borrower,
and in Figure 10.3B we have depicted a lender.
C2
C2
Endowment
c2
m2
Indifference
curve
Indifference
curve
m2
Choice
c2
Choice
Endowment
m1
c1
A Borrower
Figure
10.3
C1
c1 m1
C1
B Lender
Borrower and lender. Panel A depicts a borrower, since
c1 > m1 , and panel B depicts a lender, since c1 < m1 .
Let us now consider how the consumer would react to a change in the
THE SLUTSKY EQUATION AND INTERTEMPORAL CHOICE
187
interest rate. From equation (10.1) we see that increasing the rate of interest must tilt the budget line to a steeper position: for a given reduction in
c1 you will get more consumption in the second period if the interest rate
is higher. Of course the endowment always remains affordable, so the tilt
is really a pivot around the endowment.
We can also say something about how the choice of being a borrower
or a lender changes as the interest rate changes. There are two cases,
depending on whether the consumer is initially a borrower or initially a
lender. Suppose first that he is a lender. Then it turns out that if the
interest rate increases, the consumer must remain a lender.
This argument is illustrated in Figure 10.4. If the consumer is initially a
lender, then his consumption bundle is to the left of the endowment point.
Now let the interest rate increase. Is it possible that the consumer shifts
to a new consumption point to the right of the endowment?
No, because that would violate the principle of revealed preference:
choices to the right of the endowment point were available to the consumer when he faced the original budget set and were rejected in favor of
the chosen point. Since the original optimal bundle is still available at the
new budget line, the new optimal bundle must be a point outside the old
budget set—which means it must be to the left of the endowment. The
consumer must remain a lender when the interest rate increases.
There is a similar effect for borrowers: if the consumer is initially a
borrower, and the interest rate declines, he or she will remain a borrower.
(You might sketch a diagram similar to Figure 10.4 and see if you can spell
out the argument.)
Thus if a person is a lender and the interest rate increases, he will remain
a lender. If a person is a borrower and the interest rate decreases, he will
remain a borrower. On the other hand, if a person is a lender and the
interest rate decreases, he may well decide to switch to being a borrower;
similarly, an increase in the interest rate may induce a borrower to become
a lender. Revealed preference tells us nothing about these last two cases.
Revealed preference can also be used to make judgments about how the
consumer’s welfare changes as the interest rate changes. If the consumer
is initially a borrower, and the interest rate rises, but he decides to remain
a borrower, then he must be worse off at the new interest rate. This argument is illustrated in Figure 10.5; if the consumer remains a borrower, he
must be operating at a point that was affordable under the old budget set
but was rejected, which implies that he must be worse off.
10.4 The Slutsky Equation and Intertemporal Choice
The Slutsky equation can be used to decompose the change in demand due
to an interest rate change into income effects and substitution effects, just
188 INTERTEMPORAL CHOICE (Ch. 10)
C2
Indifference
curves
New consumption
Original
consumption
m2
Endowment
Slope = – (1 + r )
m1
Figure
10.4
C1
If a person is a lender and the interest rate rises, he or
she will remain a lender. Increasing the interest rate pivots
the budget line around the endowment to a steeper position;
revealed preference implies that the new consumption bundle
must lie to the left of the endowment.
as in Chapter 9. Suppose that the interest rate rises. What will be the
effect on consumption in each period?
This is a case that is easier to analyze by using the future-value budget
constraint, rather than the present-value constraint. In terms of the futurevalue budget constraint, raising the interest rate is just like raising the price
of consumption today as compared to consumption tomorrow. Writing out
the Slutsky equation we have
Δcs1
Δct1
Δcm
1
Δp1 = Δp1 + (m1 − c1 ) Δm .
(?)
(+)
(?)
(−)
The substitution effect, as always, works opposite the direction of price.
In this case the price of period-1 consumption goes up, so the substitution
effect says the consumer should consume less first period. This is the
meaning of the minus sign under the substitution effect. Let’s assume that
consumption this period is a normal good, so that the very last term—how
consumption changes as income changes—will be positive. So we put a
plus sign under the last term. Now the sign of the whole expression will
depend on the sign of (m1 − c1 ). If the person is a borrower, this term
will be negative and the whole expression will therefore unambiguously be
INFLATION
189
C2
Indifference
curves
m2
Original consumption
New
consumption
m1
C1
A borrower is made worse off by an increase in the interest rate. When the interest rate facing a borrower increases
and the consumer chooses to remain a borrower, he or she is
certainly worse off.
negative—for a borrower, an increase in the interest rate must lower today’s
consumption.
Why does this happen? When the interest rate rises, there is always
a substitution effect towards consuming less today. For a borrower, an
increase in the interest rate means that he will have to pay more interest
tomorrow. This effect induces him to borrow less, and thus consume less,
in the first period.
For a lender the effect is ambiguous. The total effect is the sum of a negative substitution effect and a positive income effect. From the viewpoint
of a lender an increase in the interest rate may give him so much extra
income that he will want to consume even more first period.
The effects of changing interest rates are not terribly mysterious. There
is an income effect and a substitution effect as in any other price change.
But without a tool like the Slutsky equation to separate out the various
effects, the changes may be hard to disentangle. With such a tool, the
sorting out of the effects is quite straightforward.
10.5 Inflation
The above analysis has all been conducted in terms of a general “consump-
Figure
10.5
190 INTERTEMPORAL CHOICE (Ch. 10)
tion” good. Giving up Δc units of consumption today buys you (1 + r)Δc
units of consumption tomorrow. Implicit in this analysis is the assumption
that the “price” of consumption doesn’t change—there is no inflation or
deflation.
However, the analysis is not hard to modify to deal with the case of inflation. Let us suppose that the consumption good now has a different price
in each period. It is convenient to choose today’s price of consumption as
1 and to let p2 be the price of consumption tomorrow. It is also convenient
to think of the endowment as being measured in units of the consumption
goods as well, so that the monetary value of the endowment in period 2 is
p2 m2 . Then the amount of money the consumer can spend in the second
period is given by
p2 c2 = p2 m2 + (1 + r)(m1 − c1 ),
and the amount of consumption available second period is
c 2 = m2 +
1+r
(m1 − c1 ).
p2
Note that this equation is very similar to the equation given earlier—we
just use (1 + r)/p2 rather than 1 + r.
Let us express this budget constraint in terms of the rate of inflation.
The inflation rate, π, is just the rate at which prices grow. Recalling that
p1 = 1, we have
p2 = 1 + π,
which gives us
c2 = m2 +
1+r
(m1 − c1 ).
1+π
Let’s create a new variable ρ, the real interest rate, and define it by2
1+ρ=
1+r
1+π
so that the budget constraint becomes
c2 = m2 + (1 + ρ)(m1 − c1 ).
One plus the real interest rate measures how much extra consumption you
can get in period 2 if you give up some consumption in period 1. That
is why it is called the real rate of interest: it tells you how much extra
consumption you can get, not how many extra dollars you can get.
2
The Greek letter ρ, rho, is pronounced “row.”
PRESENT VALUE: A CLOSER LOOK
191
The interest rate on dollars is called the nominal rate of interest. As
we’ve seen above, the relationship between the two is given by
1+ρ=
1+r
.
1+π
In order to get an explicit expression for ρ, we write this equation as
1+r
1+π
1+r
−1=
−
1+π
1+π 1+π
r−π
.
=
1+π
ρ=
This is an exact expression for the real interest rate, but it is common to
use an approximation. If the inflation rate isn’t too large, the denominator
of the fraction will be only slightly larger than 1. Thus the real rate of
interest will be approximately given by
ρ ≈ r − π,
which says that the real rate of interest is just the nominal rate minus the
rate of inflation. (The symbol ≈ means “approximately equal to.”) This
makes perfectly good sense: if the interest rate is 18 percent, but prices
are rising at 10 percent, then the real interest rate—the extra consumption
you can buy next period if you give up some consumption now—will be
roughly 8 percent.
Of course, we are always looking into the future when making consumption plans. Typically, we know the nominal rate of interest for the next
period, but the rate of inflation for next period is unknown. The real interest rate is usually taken to be the current interest rate minus the expected
rate of inflation. To the extent that people have different estimates about
what the next year’s rate of inflation will be, they will have different estimates of the real interest rate. If inflation can be reasonably well forecast,
these differences may not be too large.
10.6 Present Value: A Closer Look
Let us return now to the two forms of the budget constraint described
earlier in section 10.1 in equations (10.2) and (10.3):
(1 + r)c1 + c2 = (1 + r)m1 + m2
and
c1 +
c2
m2
= m1 +
.
1+r
1+r
192 INTERTEMPORAL CHOICE (Ch. 10)
Consider just the right-hand sides of these two equations. We said that
the first one expresses the value of the endowment in terms of future value
and that the second one expresses it in terms of present value.
Let us examine the concept of future value first. If we can borrow and
lend at an interest rate of r, what is the future equivalent of $1 today?
The answer is (1 + r) dollars. That is, $1 today can be turned into (1 + r)
dollars next period simply by lending it to the bank at an interest rate r.
In other words, (1 + r) dollars next period is equivalent to $1 today since
that is how much you would have to pay next period to purchase—that is,
borrow—$1 today. The value (1 + r) is just the price of $1 today, relative
to $1 next period. This can be easily seen from the first budget constraint:
it is expressed in terms of future dollars—the second-period dollars have a
price of 1, and first-period dollars are measured relative to them.
What about present value? This is just the reverse: everything is measured in terms of today’s dollars. How much is a dollar next period worth
in terms of a dollar today? The answer is 1/(1 + r) dollars. This is because
1/(1 + r) dollars can be turned into a dollar next period simply by saving
it at the rate of interest r. The present value of a dollar to be delivered
next period is 1/(1 + r).
The concept of present value gives us another way to express the budget
for a two-period consumption problem: a consumption plan is affordable if
the present value of consumption equals the present value of income.
The idea of present value has an important implication that is closely
related to a point made in Chapter 9: if the consumer can freely buy and sell
goods at constant prices, then the consumer would always prefer a highervalued endowment to a lower-valued one. In the case of intertemporal
decisions, this principle implies that if a consumer can freely borrow and
lend at a constant interest rate, then the consumer would always prefer a
pattern of income with a higher present value to a pattern with a lower
present value.
This is true for the same reason that the statement in Chapter 9 was
true: an endowment with a higher value gives rise to a budget line that is
farther out. The new budget set contains the old budget set, which means
that the consumer would have all the consumption opportunities she had
with the old budget set plus some more. Economists sometimes say that
an endowment with a higher present value dominates one with a lower
present value in the sense that the consumer can have larger consumption
in every period by selling the endowment with the higher present value
that she could get by selling the endowment with the lower present value.
Of course, if the present value of one endowment is higher than another,
then the future value will be higher as well. However, it turns out that the
present value is a more convenient way to measure the purchasing power
of an endowment of money over time, and it is the measure to which we
will devote the most attention.
ANALYZING PRESENT VALUE FOR SEVERAL PERIODS
193
10.7 Analyzing Present Value for Several Periods
Let us consider a three-period model. We suppose that we can borrow or
lend money at an interest rate r each period and that this interest rate will
remain constant over the three periods. Thus the price of consumption in
period 2 in terms of period-1 consumption will be 1/(1 + r), just as before.
What will the price of period-3 consumption be? Well, if I invest $1
today, it will grow into (1 + r) dollars next period; and if I leave this money
invested, it will grow into (1 + r)2 dollars by the third period. Thus if I
start with 1/(1 + r)2 dollars today, I can turn this into $1 in period 3. The
price of period-3 consumption relative to period-1 consumption is therefore
1/(1 + r)2 . Each extra dollar’s worth of consumption in period 3 costs me
1/(1 + r)2 dollars today. This implies that the budget constraint will have
the form
c1 +
c3
m3
c2
m2
+
+
= m1 +
.
1 + r (1 + r)2
1 + r (1 + r)2
This is just like the budget constraints we’ve seen before, where the price
of period-t consumption in terms of today’s consumption is given by
pt =
1
.
(1 + r)t−1
As before, moving to an endowment that has a higher present value at
these prices will be preferred by any consumer, since such a change will
necessarily shift the budget set farther out.
We have derived this budget constraint under the assumption of constant
interest rates, but it is easy to generalize to the case of changing interest
rates. Suppose, for example, that the interest earned on savings from period
1 to 2 is r1 , while savings from period 2 to 3 earn r2 . Then $1 in period 1
will grow to (1 + r1 )(1 + r2 ) dollars in period 3. The present value of $1 in
period 3 is therefore 1/(1 + r1 )(1 + r2 ). This implies that the correct form
of the budget constraint is
c1 +
c2
c3
m2
m3
= m1 +
.
+
+
1 + r1
(1 + r1 )(1 + r2 )
1 + r1
(1 + r1 )(1 + r2 )
This expression is not so hard to deal with, but we will typically be content
to examine the case of constant interest rates.
Table 10.1 contains some examples of the present value of $1 T years in
the future at different interest rates. The notable fact about this table is
how quickly the present value goes down for “reasonable” interest rates.
For example, at an interest rate of 10 percent, the value of $1 20 years from
now is only 15 cents.
194 INTERTEMPORAL CHOICE (Ch. 10)
Table
10.1
The present value of $1 t years in the future.
Rate
.05
.10
.15
.20
1
.95
.91
.87
.83
2
.91
.83
.76
.69
5
.78
.62
.50
.40
10
.61
.39
.25
.16
15
.48
.24
.12
.06
20
.37
.15
.06
.03
25
.30
.09
.03
.01
30
.23
.06
.02
.00
10.8 Use of Present Value
Let us start by stating an important general principle: present value is the
only correct way to convert a stream of payments into today’s dollars. This
principle follows directly from the definition of present value: the present
value measures the value of a consumer’s endowment of money. As long as
the consumer can borrow and lend freely at a constant interest rate, an endowment with higher present value can always generate more consumption
in every period than an endowment with lower present value. Regardless
of your own tastes for consumption in different periods, you should always
prefer a stream of money that has a higher present value to one with lower
present value—since that always gives you more consumption possibilities
in every period.
This argument is illustrated in Figure 10.6. In this figure, (m1 , m2 )
is a worse consumption bundle than the consumer’s original endowment,
(m1 , m2 ), since it lies beneath the indifference curve through her endowment. Nevertheless, the consumer would prefer (m1 , m2 ) to (m1 , m2 ) if
she is able to borrow and lend at the interest rate r. This is true because
with the endowment (m1 , m2 ) she can afford to consume a bundle such
as (c1 , c2 ), which is unambiguously better than her current consumption
bundle.
One very useful application of present value is in valuing the income
streams offered by different kinds of investments. If you want to compare
two different investments that yield different streams of payments to see
which is better, you simply compute the two present values and choose the
larger one. The investment with the larger present value always gives you
more consumption possibilities.
Sometimes it is necessary to purchase an income stream by making a
stream of payments over time. For example, one could purchase an apartment building by borrowing money from a bank and making mortgage payments over a number of years. Suppose that the income stream (M1 , M2 )
can be purchased by making a stream of payments (P1 , P2 ).
In this case we can evaluate the investment by comparing the present
USE OF PRESENT VALUE
195
C2
Indifference
curves
Possible consumption (c1, c 2 )
Endowment with higher
present value
m2
Original
endowment
m'2
m1
m'1
C1
Higher present value. An endowment with higher present
value gives the consumer more consumption possibilities in each
period if she can borrow and lend at the market interest rates.
value of the income stream to the present value of the payment stream. If
M1 +
M2
P2
> P1 +
,
1+r
1+r
(10.4)
the present value of the income stream exceeds the present value of its
cost, so this is a good investment—it will increase the present value of our
endowment.
An equivalent way to value the investment is to use the idea of net
present value. In order to calculate this number we calculate at the net
cash flow in each period and then discount this stream back to the present.
In this example, the net cash flow is (M1 −P1 , M2 −P2 ), and the net present
value is
M 2 − P2
.
N P V = M 1 − P1 +
1+r
Comparing this to equation (10.4) we see that the investment should be
purchased if and only if the net present value is positive.
The net present value calculation is very convenient since it allows us to
add all of the positive and negative cash flows together in each period and
then discount the resulting stream of cash flows.
EXAMPLE: Valuing a Stream of Payments
Suppose that we are considering two investments, A and B. Investment A
Figure
10.6
196 INTERTEMPORAL CHOICE (Ch. 10)
pays $100 now and will also pay $200 next year. Investment B pays $0
now, and will generate $310 next year. Which is the better investment?
The answer depends on the interest rate. If the interest rate is zero, the
answer is clear—just add up the payments. For if the interest rate is zero,
then the present-value calculation boils down to summing up the payments.
If the interest rate is zero, the present value of investment A is
P VA = 100 + 200 = 300,
and the present value of investment B is
P VB = 0 + 310 = 310,
so B is the preferred investment.
But we get the opposite answer if the interest rate is high enough. Suppose, for example, that the interest rate is 20 percent. Then the presentvalue calculation becomes
200
= 266.67
1.20
310
= 258.33.
P VB = 0 +
1.20
P VA = 100 +
Now A is the better investment. The fact that A pays back more money
earlier means that it will have a higher present value when the interest rate
is large enough.
EXAMPLE: The True Cost of a Credit Card
Borrowing money on a credit card is expensive: many companies quote
yearly interest charges of 15 to 21 percent. However, because of the way
these finance charges are computed, the true interest rate on credit card
debt is much higher than this.
Suppose that a credit card owner charges a $2000 purchase on the first
day of the month and that the finance charge is 1.5 percent a month. If
the consumer pays the entire balance by the end of the month, he does not
have to pay the finance charge. If the consumer pays none of the $2,000,
he has to pay a finance charge of $2000 × .015 = $30 at the beginning of
the next month.
What happens if the consumer pays $1,800 towards the $2000 balance
on the last day of the month? In this case, the consumer has borrowed
only $200, so the finance charge should be $3. However, many credit card
companies charge the consumers much more than this. The reason is that
many companies base their charges on the “average monthly balance,” even
if part of that balance is paid by the end of the month. In this example,
USE OF PRESENT VALUE
197
the average monthly balance would be about $2000 (30 days of the $2000
balance and 1 day of the $200 balance). The finance charge would therefore
be slightly less than $30, even though the consumer has only borrowed $200.
Based on the actual amount of money borrowed, this is an interest rate of
15 percent a month!
EXAMPLE: Extending Copyright
Article I, Section 8 of the U.S. Constitution enables Congress to grant
patents and copyrights using this language: “To promote the Progress
of Science and useful Arts, by securing for limited Times to Authors and
Inventors the exclusive Right to their respective Writings and Discoveries.”
But what does “limited Times” mean? The lifetime of a patent in the
United States is fixed at 20 years; the lifetime for copyright is quite different.
The first copyright act, passed by Congress in 1790, offered a 14-year
term along with a 14-year renewal. Subsequently, the copyright term was
lengthened to 28 years in 1831, with a 28-year renewal option added in
1909. In 1962 the term became 47 years, and 67 years in 1978. In 1967
the term was defined as the life of the author plus 50 years, or 75 years
for “works for hire.” The 1998 Sonny Bono Copyright Term Extension Act
lengthened this term to the life of the author plus 70 years for individuals
and 75–95 years for works for hire.
It is questionable whether “the life of the author plus 70 years” should
be considered a limited time. One might ask what additional incentive the
1998 extension creates for authors to create works?
Let us look at a simple example. Suppose that the interest rate is 7%.
Then the increase in present value of extending the copyright term from
80 to 100 years is about 0.33% of the present value of the first 80 years.
Those extra 20 years have almost no impact on the present value of the
copyright at time of creation since they come so far in the future. Hence
they likely provide miniscule incremental incentive to create the works in
the first place.
Given this tiny increase in value from extending the copyright term why
would it pay anybody to lobby for such a change? The answer is that the
1998 act extended the copyright term retroactively so that works that were
near expiration were given a new lease on life.
For example, it has been widely claimed that Disney lobbied heavily
for the copyright term extension, since the original Mickey Mouse film,
Steamboat Willie, was about to go out of copyright.
Retroactive copyright extensions of this sort make no economic sense,
since what matters for the authors are the incentives present at the time
the work is created. If there were no such retroactive extension, it is unlikely
198 INTERTEMPORAL CHOICE (Ch. 10)
that anyone would have bothered to ask for copyright extensions given the
low economic value of the additional years of protection.
10.9 Bonds
Securities are financial instruments that promise certain patterns of payment schedules. There are many kinds of financial instruments because
there are many kinds of payment schedules that people want. Financial
markets give people the opportunity to trade different patterns of cash
flows over time. These cash flows are typically used to finance consumption at some time or other.
The particular kind of security that we will examine here is a bond.
Bonds are issued by governments and corporations. They are basically a
way to borrow money. The borrower—the agent who issues the bond—
promises to pay a fixed number of dollars x (the coupon) each period
until a certain date T (the maturity date), at which point the borrower
will pay an amount F (the face value) to the holder of the bond.
Thus the payment stream of a bond looks like (x, x, x, . . . , F ). If the
interest rate is constant, the present discounted value of such a bond is
easy to compute. It is given by
PV =
x
F
x
+
+ ··· +
.
(1 + r) (1 + r)2
(1 + r)T
Note that the present value of a bond will decline if the interest rate
increases. Why is this? When the interest rate goes up the price now for
$1 delivered in the future goes down. So the future payments of the bond
will be worth less now.
There is a large and developed market for bonds. The market value
of outstanding bonds will fluctuate as the interest rate fluctuates since
the present value of the stream of payments represented by the bond will
change.
An interesting special kind of a bond is a bond that makes payments
forever. These are called consols or perpetuities. Suppose that we consider a consol that promises to pay $x dollars a year forever. To compute
the value of this consol we have to compute the infinite sum:
PV =
x
x
+
+ ···.
1 + r (1 + r)2
The trick to computing this is to factor out 1/(1 + r) to get
x
x
1
x+
+
PV =
+ ··· .
1+r
(1 + r) (1 + r)2
BONDS
199
But the term in the brackets is just x plus the present value! Substituting
and solving for P V :
1
[x + P V ]
PV =
(1 + r)
x
= .
r
This wasn’t hard to do, but there is an easy way to get the answer right
off. How much money, V , would you need at an interest rate r to get x
dollars forever? Just write down the equation
V r = x,
which says that the interest on V must equal x. But then the value of such
an investment is given by
x
V = .
r
Thus it must be that the present value of a consol that promises to pay x
dollars forever must be given by x/r.
For a consol it is easy to see directly how increasing the interest rate
reduces the value of a bond. Suppose, for example, that a consol is issued
when the interest rate is 10 percent. Then if it promises to pay $10 a year
forever, it will be worth $100 now—since $100 would generate $10 a year
in interest income.
Now suppose that the interest rate goes up to 20 percent. The value of
the consol must fall to $50, since it only takes $50 to earn $10 a year at a
20 percent interest rate.
The formula for the consol can be used to calculate an approximate value
of a long-term bond. If the interest rate is 10 percent, for example, the
value of $1 30 years from now is only 6 cents. For the size of interest rates
we usually encounter, 30 years might as well be infinity.
EXAMPLE: Installment Loans
Suppose that you borrow $1000 that you promise to pay back in 12 monthly
installments of $100 each. What rate of interest are you paying?
At first glance it seems that your interest rate is 20 percent: you have
borrowed $1000, and you are paying back $1200. But this analysis is incorrect. For you haven’t really borrowed $1000 for an entire year. You have
borrowed $1000 for a month, and then you pay back $100. Then you only
have borrowed $900, and you owe only a month’s interest on the $900. You
borrow that for a month and then pay back another $100. And so on.
The stream of payments that we want to value is
(1000, −100, −100, . . . , −100).
We can find the interest rate that makes the present value of this stream
equal to zero by using a calculator or a computer. The actual interest rate
that you are paying on the installment loan is about 35 percent!
200 INTERTEMPORAL CHOICE (Ch. 10)
10.10 Taxes
In the United States, interest payments are taxed as ordinary income. This
means that you pay the same tax on interest income as on labor income.
Suppose that your marginal tax bracket is t, so that each extra dollar of
income, Δm, increases your tax liability by tΔm. Then if you invest X
dollars in an asset, you’ll receive an interest payment of rX. But you’ll
also have to pay taxes of trX on this income, which will leave you with
only (1 − t)rX dollars of after-tax income. We call the rate (1 − t)r the
after-tax interest rate.
What if you decide to borrow X dollars, rather than lend them? Then
you’ll have to make an interest payment of rX. In the United States, some
interest payments are tax deductible and some are not. For example, the
interest payments for a mortgage are tax deductable, but interest payments
on ordinary consumer loans are not. On the other hand, businesses can
deduct most kinds of the interest payments that they make.
If a particular interest payment is tax deductible, you can subtract your
interest payment from your other income and only pay taxes on what’s left.
Thus the rX dollars you pay in interest will reduce your tax payments by
trX. The total cost of the X dollars you borrowed will be rX − trX =
(1 − t)rX.
Thus the after-tax interest rate is the same whether you are borrowing
or lending, for people in the same tax bracket. The tax on saving will
reduce the amount of money that people want to save, but the subsidy on
borrowing will increase the amount of money that people want to borrow.
EXAMPLE: Scholarships and Savings
Many students in the United States receive some form of financial aid to
help defray college costs. The amount of financial aid a student receives
depends on many factors, but one important factor is the family’s ability to
pay for college expenses. Most U.S. colleges and universities use a standard
measure of ability to pay calculated by the College Entrance Examination
Board (CEEB).
If a student wishes to apply for financial aid, his or her family must fill
out a questionnaire describing their financial circumstances. The CEEB
uses the information on the income and assets of the parents to construct
a measure of “adjusted available income.” The fraction of their adjusted
available income that parents are expected to contribute varies between
22 and 47 percent, depending on income. In 1985, parents with a total
before-tax income of around $35,000 dollars were expected to contribute
about $7000 toward college expenses.
CHOICE OF THE INTEREST RATE
201
Each additional dollar of assets that the parents accumulate increases
their expected contribution and decreases the amount of financial aid that
their child can hope to receive. The formula used by the CEEB effectively
imposes a tax on parents who save for their children’s college education.
Martin Feldstein, President of the National Bureau of Economic Research
(NBER) and Professor of Economics at Harvard University, calculated the
magnitude of this tax.3
Consider the situation of some parents contemplating saving an additional dollar just as their daughter enters college. At a 6 percent rate of
interest, the future value of a dollar 4 years from now is $1.26. Since federal
and state taxes must be paid on interest income, the dollar yields $1.19 in
after-tax income in 4 years. However, since this additional dollar of savings
increases the total assets of the parents, the amount of aid received by the
daughter goes down during each of her four college years. The effect of this
“education tax” is to reduce the future value of the dollar to only 87 cents
after 4 years. This is equivalent to an income tax of 150 percent!
Feldstein also examined the savings behavior of a sample of middle-class
households with pre-college children. He estimates that a household with
income of $40,000 a year and two college-age children saves about 50 percent less than they would otherwise due to the combination of federal, state,
and “education” taxes that they face.
10.11 Choice of the Interest Rate
In the above discussion, we’ve talked about “the interest rate.” In real life
there are many interest rates: there are nominal rates, real rates, before-tax
rates, after-tax rates, short-term rates, long-term rates, and so on. Which
is the “right” rate to use in doing present-value analysis?
The way to answer this question is to think about the fundamentals.
The idea of present discounted value arose because we wanted to be able
to convert money at one point in time to an equivalent amount at another
point in time. “The interest rate” is the return on an investment that
allows us to transfer funds in this way.
If we want to apply this analysis when there are a variety of interest
rates available, we need to ask which one has the properties most like the
stream of payments we are trying to value. If the stream of payments
is not taxed, we should use an after-tax interest rate. If the stream of
payments will continue for 30 years, we should use a long-term interest
rate. If the stream of payments is risky, we should use the interest rate
on an investment with similar risk characteristics. (We’ll have more to say
later about what this last statement actually means.)
3
Martin Feldstein, “College Scholarship Rules and Private Savings,” American Economic Review, 85, 3 (June 1995).
202 INTERTEMPORAL CHOICE (Ch. 10)
The interest rate measures the opportunity cost of funds—the value
of alternative uses of your money. So every stream of payments should be
compared to your best alternative that has similar characteristics in terms
of tax treatment, risk, and liquidity.
Summary
1. The budget constraint for intertemporal consumption can be expressed
in terms of present value or future value.
2. The comparative statics results derived earlier for general choice problems can be applied to intertemporal consumption as well.
3. The real rate of interest measures the extra consumption that you can
get in the future by giving up some consumption today.
4. A consumer who can borrow and lend at a constant interest rate should
always prefer an endowment with a higher present value to one with a lower
present value.
REVIEW QUESTIONS
1. How much is $1 million to be delivered 20 years in the future worth
today if the interest rate is 20 percent?
2. As the interest rate rises, does the intertemporal budget constraint become steeper or flatter?
3. Would the assumption that goods are perfect substitutes be valid in a
study of intertemporal food purchases?
4. A consumer, who is initially a lender, remains a lender even after a
decline in interest rates. Is this consumer better off or worse off after the
change in interest rates? If the consumer becomes a borrower after the
change is he better off or worse off?
5. What is the present value of $100 one year from now if the interest rate
is 10%? What is the present value if the interest rate is 5%?
The theory of consumer saving uses techniques that you have already
learned. In order to focus attention on consumption over time, we will
usually consider examples where there is only one consumer good, but this
good can be consumed in either of two time periods. We will be using two
“tricks.” One trick is to treat consumption in period 1 and consumption in
period 2 as two distinct commodities. If you make period-1 consumption
the numeraire, then the “price” of period-2 consumption is the amount
of period-1 consumption that you have to give up to get an extra unit of
period-2 consumption. This price turns out to be 1/(1 + r), where r is
the interest rate.
The second trick is in the way you treat income in the two different
periods. Suppose that a consumer has an income of m1 in period 1 and
m2 in period 2 and that there is no inflation. The total amount of period1 consumption that this consumer could buy, if he borrowed as much
m2
money as he could possibly repay in period 2, is m1 + 1+r
. As you
work the exercises and study the text, it should become clear that the
consumer’s budget equation for choosing consumption in the two periods
is always
c2
m2
c1 +
= m1 +
.
1+r
1+r
This budget constraint looks just like the standard budget constraint that
you studied in previous chapters, where the price of “good 1” is 1, the
m2
price of “good 2” is 1/(1 + r), and “income” is m1 + (1+r)
. Therefore
if you are given a consumer’s utility function, the interest rate, and the
consumer’s income in each period, you can find his demand for consumption in periods 1 and 2 using the methods you already know. Having
solved for consumption in each period, you can also find saving, since the
consumer’s saving is just the difference between his period-1 income and
his period-1 consumption.
A consumer has the utility function U (c1 , c2 ) = c1 c2 . There is no inflation,
the interest rate is 10%, and the consumer has income 100 in period 1
and 121 in period 2. Then the consumer’s budget constraint c1 + c2 /1.1 =
100 + 121/1.1 = 210. The ratio of the price of good 1 to the price of good
2 is 1 + r = 1.1. The consumer will choose a consumption bundle so that
M U1 /M U2 = 1.1. But M U1 = c2 and M U2 = c1 , so the consumer must
choose a bundle such that c2 /c1 = 1.1. Take this equation together with
the budget equation to solve for c1 and c2 . The solution is c1 = 105 and
c2 = 115.50. Since the consumer’s period-1 income is only 100, he must
borrow 5 in order to consume 105 in period 1. To pay back principal and
interest in period 2, he must pay 5.50 out of his period-2 income of 121.
This leaves him with 115.50 to consume.
You will also be asked to determine the effects of inflation on consumer behavior. The key to understanding the effects of inflation is to
see what happens to the budget constraint.
Suppose that in the previous example, there happened to be an inflation
rate of 6%, and suppose that the price of period-1 goods is 1. Then if you
save $1 in period 1 and get it back with 10% interest, you will get back
$1.10 in period 2. But because of the inflation, goods in period 2 cost 1.06
dollars per unit. Therefore the amount of period-1 consumption that you
have to give up to get a unit of period-2 consumption is 1.06/1.10 = .964
units of period-2 consumption. If the consumer’s money income in each
period is unchanged, then his budget equation is c1 + .964c2 = 210. This
budget constraint is the same as the budget constraint would be if there
were no inflation and the interest rate were r, where .964 = 1/(1 + r).
The value of r that solves this equation is known as the real rate of
interest. In this case the real rate of interest is about .038. When the
interest rate and inflation rate are both small, the real rate of interest
is closely approximated by the difference between the nominal interest
rate, (10% in this case) and the inflation rate (6% in this case), that is,
.038 ∼ .10 − .06. As you will see, this is not such a good approximation
if inflation rates and interest rates are large.
10.1 (0) Peregrine Pickle consumes (c1 , c2 ) and earns (m1 , m2 ) in periods
1 and 2 respectively. Suppose the interest rate is r.
(a) Write down Peregrine’s intertemporal budget constraint in present
value terms.
.
(b) If Peregrine does not consume anything in period 1, what is the most
he can consume in period 2?
present value) of his endowment.
This is the (future value,
.
(c) If Peregrine does not consume anything in period 2, what is the most
he can consume in period 1?
present value) of his endowment.
of Peregrine’s budget line?
This is the (future value,
What is the slope
.
10.2 (0) Molly has a Cobb-Douglas utility function U (c1 , c2 ) = ca1 c1−a
,
2
where 0 < a < 1 and where c1 and c2 are her consumptions in periods 1
and 2 respectively. We saw earlier that if utility has the form u(x1 , x2 ) =
xa1 x1−a
and the budget constraint is of the “standard” form p1 x1 +p2 x2 =
2
m, then the demand functions for the goods are x1 = am/p1 and x2 =
(1 − a)m/p2 .
(a) Suppose that Molly’s income is m1 in period 1 and m2 in period 2.
Write down her budget constraint in terms of present values.
.
(b) We want to compare this budget constraint to one of the standard
form. In terms of Molly’s budget constraint, what is p1 ?
What is m?
is p2 ?
What
.
(c) If a = .2, solve for Molly’s demand functions for consumption in each
period as a function of m1 , m2 , and r. Her demand function for consumption in period 1 is c1 =
Her demand function
for consumption in period 2 is c2 =
(d) An increase in the interest rate will
.
her period-1 con-
her period-2 consumption and
sumption. It will
her savings in period 1.
10.3 (0) Nickleby has an income of $2,000 this year, and he expects an
income of $1,100 next year. He can borrow and lend money at an interest
rate of 10%. Consumption goods cost $1 per unit this year and there is
no inflation.
Consumption next year in 1,000s
4
3
2
1
0
1
2
3
4
Consumption this year in 1,000s
(a) What is the present value of Nickleby’s endowment?
What is the future value of his endowment?
With blue
ink, show the combinations of consumption this year and consumption
next year that he can afford. Label Nickelby’s endowment with the letter
E.
(b) Suppose that Nickleby has the utility function U (C1 , C2 ) = C1 C2 .
Write an expression for Nickleby’s marginal rate of substitution between
consumption this year and consumption next year. (Your answer will be
a function of the variables C1 , C2 .)
.
(c) What is the slope of Nickleby’s budget line?
Write
an equation that states that the slope of Nickleby’s indifference curve
is equal to the slope of his budget line when the interest rate is 10%.
Also write down Nickleby’s budget equation.
.
(d) Solve these two equations. Nickleby will consume
in period 1 and
diagram.
units
units in period 2. Label this point A on your
(e) Will he borrow or save in the first period?
How much?
.
(f ) On your graph use red ink to show what Nickleby’s budget line would
be if the interest rate rose to 20%. Knowing that Nickleby chose the
point A at a 10% interest rate, even without knowing his utility function,
you can determine that his new choice cannot be on certain parts of his
new budget line. Draw a squiggly mark over the part of his new budget
line where that choice can not be. (Hint: Close your eyes and think of
WARP.)
(g) Solve for Nickleby’s optimal choice when the interest rate is 20%.
Nickleby will consume
in period 2.
units in period 1 and
(h) Will he borrow or save in the first period?
units
How much?
.
10.4 (0) Decide whether each of the following statements is true or
false. Then explain why your answer is correct, based on the Slutsky
decomposition into income and substitution effects.
(a) “If both current and future consumption are normal goods, an increase
in the interest rate will necessarily make a saver save more.”
.
(b) “If both current and future consumption are normal goods, an increase
in the interest rate will necessarily make a saver choose more consumption
in the second period.”
.
10.5 (1) Laertes has an endowment of $20 each period. He can borrow
money at an interest rate of 200%, and he can lend money at a rate of
0%. (Note: If the interest rate is 0%, for every dollar that you save, you
get back $1 in the next period. If the interest rate is 200%, then for every
dollar you borrow, you have to pay back $3 in the next period.)
(a) Use blue ink to illustrate his budget set in the graph below. (Hint:
The boundary of the budget set is not a single straight line.)
C2
40
30
20
10
0
10
20
30
40
C1
(b) Laertes could invest in a project that would leave him with m1 = 30
and m2 = 15. Besides investing in the project, he can still borrow at
200% interest or lend at 0% interest. Use red ink to draw the new
budget set in the graph above. Would Laertes be better off or worse
off by investing in this project given his possibilities for borrowing or
lending? Or can’t one tell without knowing something about his preferences? Explain.
.
(c) Consider an alternative project that would leave Laertes with the
endowment m1 = 15, m2 = 30. Again suppose he can borrow
and lend as above. But if he chooses this project, he can’t do the
first project. Use pencil or black ink to draw the budget set available to Laertes if he chooses this project. Is Laertes better off or
worse off by choosing this project than if he didn’t choose either
project? Or can’t one tell without knowing more about his preferences? Explain.
.
10.6 (0) The table below reports the inflation rate and the annual rate
of return on treasury bills in several countries for the years 1984 and 1985.
Inflation Rate and Interest Rate for Selected Countries
Country
United States
Israel
Switzerland
W. Germany
Italy
Argentina
Japan
% Inflation
Rate, 1984
3.6
304.6
3.1
2.2
9.2
90.0
0.6
% Inflation
Rate, 1985
1.9
48.1
0.8
−0.2
5.8
672.2
2.0
% Interest
Rate, 1984
9.6
217.3
3.6
5.3
15.3
NA
NA
% Interest
Rate, 1985
7.5
210.1
4.1
4.2
13.9
NA
NA
(a) In the table below, use the formula that your textbook gives for the
exact real rate of interest to compute the exact real rates of interest.
(b) What would the nominal rate of return on a bond in Argentina have
to be to give a real rate of return of 5% in 1985?
What would
the nominal rate of return on a bond in Japan have to be to give a real
rate of return of 5% in 1985?
.
(c) Subtracting the inflation rate from the nominal rate of return gives
a good approximation to the real rate for countries with a low rate of
inflation. For the United States in 1984, the approximation gives you
while the more exact method suggested by the text gives you
But for countries with very high inflation this is a poor approximation. The approximation gives you
for Israel in 1984,
while the more exact formula gives you
For Argentina in
1985, the approximation would tell us that a bond yielding a nominal rate
of
would yield a real interest rate of 5%. This contrasts with
the answer
that you found above.
Real Rates of Interest in 1984 and 1985
Country
1984
1985
United States
Israel
Switzerland
W. Germany
Italy
10.7 (0) We return to the planet Mungo. On Mungo, macroeconomists
and bankers are jolly, clever creatures, and there are two kinds of money,
red money and blue money. Recall that to buy something in Mungo you
have to pay for it twice, once with blue money and once with red money.
Everything has a blue-money price and a red-money price, and nobody
is ever allowed to trade one kind of money for the other. There is a bluemoney bank where you can borrow and lend blue money at a 50% annual
interest rate. There is a red-money bank where you can borrow and lend
red money at a 25% annual interest rate.
A Mungoan named Jane consumes only one commodity, ambrosia,
but it must decide how to allocate its consumption between this year and
next year. Jane’s income this year is 100 blue currency units and no red
currency units. Next year, its income will be 100 red currency units and
no blue currency units. The blue currency price of ambrosia is one b.c.u.
per flagon this year and will be two b.c.u.’s per flagon next year. The red
currency price of ambrosia is one r.c.u. per flagon this year and will be
the same next year.
(a) If Jane spent all of its blue income in the first period, it would be
enough to pay the blue price for
flagons of ambrosia. If Jane
saved all of this year’s blue income at the blue-money bank, it would
have
b.c.u.’s next year. This would give it enough blue currency
to pay the blue price for
flagons of ambrosia. On the graph
below, draw Jane’s blue budget line, depicting all of those combinations
of current and next period’s consumption that it has enough blue income
to buy.
Ambrosia next period
100
75
50
25
0
25
50
75
100
Ambrosia this period
(b) If Jane planned to spend no red income in the next period and to
borrow as much red currency as it can pay back with interest with next
period’s red income, how much red currency could it borrow?
(c) The (exact) real rate of interest on blue money is
real rate of interest on red money is
.
The
.
(d) On the axes below, draw Jane’s blue budget line and its red budget
line. Shade in all of those combinations of current and future ambrosia
consumption that Jane can afford given that it has to pay with both
currencies.
Ambrosia next period
100
75
50
25
0
25
50
75
100
Ambrosia this period
(e) It turns out that Jane finds it optimal to operate on its blue budget
line and beneath its red budget line. Find such a point on your graph and
mark it with a C.
(f ) On the following graph, show what happens to Jane’s original budget
set if the blue interest rate rises and the red interest rate does not change.
On your graph, shade in the part of the new budget line where Jane’s
new demand could possibly be. (Hint: Apply the principle of revealed
preference. Think about what bundles were available but rejected when
Jane chose to consume at C before the change in blue interest rates.)
Ambrosia next period
100
75
50
25
0
25
50
75
100
Ambrosia this period
10.8 (0) Mr. O. B. Kandle will only live for two periods. In the first
period he will earn $50,000. In the second period he will retire and live
on his savings. His utility function is U (c1 , c2 ) = c1 c2 , where c1 is consumption in period 1 and c2 is consumption in period 2. He can borrow
and lend at the interest rate r = .10.
(a) If the interest rate rises, will his period-1 consumption increase, decrease, or stay the same?
.
(b) Would an increase in the interest rate make him consume more or less
in the second period?
.
(c) If Mr. Kandle’s income is zero in period 1, and $ 55,000 in period 2,
would an increase in the interest rate make him consume more, less, or
the same amount in period 1?
.
10.9 (1) Harvey Habit’s utility function is U (c1 , c2 ) = min{c1 , c2 }, where
c1 is his consumption of bread in period 1 and c2 is his consumption of
bread in period 2. The price of bread is $1 per loaf in period 1. The
interest rate is 21%. Harvey earns $2,000 in period 1 and he will earn
$1,100 in period 2.
(a) Write Harvey’s budget constraint in terms of future value, assuming
no inflation.
.
(b) How much bread does Harvey consume in the first period and
how much money does he save?
(The answer is not necessarily an integer.)
.
(c) Suppose that Harvey’s money income in both periods is the same as
before, the interest rate is still 21%, but there is a 10% inflation rate.
Then in period 2, a loaf of bread will cost $
Write down
Harvey’s budget equation for period-1 and period-2 bread, given this new
information.
.
10.10 (2) In an isolated mountain village, the only crop is corn. Good
harvests alternate with bad harvests. This year the harvest will be 1,000
bushels. Next year it will be 150 bushels. There is no trade with the
outside world. Corn can be stored from one year to the next, but rats
will eat 25% of what is stored in a year. The villagers have Cobb-Douglas
utility functions, U (c1 , c2 ) = c1 c2 where c1 is consumption this year, and
c2 is consumption next year.
(a) Use red ink to draw a “budget line,” showing consumption possibilities
for the village, with this year’s consumption on the horizontal axis and
next year’s consumption on the vertical axis. Put numbers on your graph
to show where the budget line hits the axes.
(b) How much corn will the villagers consume this year?
How much will the rats eat?
How much corn
will the villagers consume next year?
.
(c) Suppose that a road is built to the village so that now the village is
able to trade with the rest of the world. Now the villagers are able to buy
and sell corn at the world price, which is $1 per bushel. They are also
able to borrow and lend money at an interest rate of 10%. On your graph,
use blue ink to draw the new budget line for the villagers. Solve for the
amount they would now consume in the first period
the second period
and
.
(d) Suppose that all is as in the last part of the question except that there
is a transportation cost of $.10 per bushel for every bushel of grain hauled
into or out of the village. On your graph, use black ink or pencil to draw
the budget line for the village under these circumstances.
10.11 (0) The table below records percentage interest rates and inflation
rates for the United States in some recent years. Complete this table.
Inflation and Interest in the United States, 1965-1985
Year
1965
1970
1975
1978
1980
1985
CPI, Start of Year
38.3
47.1
66.3
79.2
100.0
130.0
CPI, End of Year
39.4
49.2
69.1
88.1
110.4
133
% Inflation Rate
2.9
4.3
4.2
11.3
Nominal Int. Rate
4.0
6.4
5.8
7.2
11.6
7.5
Real Int. Rate
1.1
2.1
1.6
(a) People complained a great deal about the high interest rates in the
late 70s. In fact, interest rates had never reached such heights in modern
times. Explain why such complaints are misleading.
.
(b) If you gave up a unit of consumption goods at the beginning of 1985
and saved your money at interest, you could use the proceeds of your
saving to buy
units of consumption goods at the beginning
of 1986. If you gave up a unit of consumption goods at the beginning
of 1978 and saved your money at interest, you would be able to use the
proceeds of your saving to buy
the beginning of 1979.
units of consumption goods at
10.12 (1) Marsha Mellow doesn’t care whether she consumes in period
1 or in period 2. Her utility function is simply U (c1 , c2 ) = c1 + c2 . Her
initial endowment is $20 in period 1 and $40 in period 2. In an antique
shop, she discovers a cookie jar that is for sale for $12 in period 1 and that
she is certain she can sell for $20 in period 2. She derives no consumption
benefits from the cookie jar, and it costs her nothing to store it for one
period.
(a) On the graph below, label her initial endowment, E, and use blue ink
to draw the budget line showing combinations of period-1 and period-2
consumption that she can afford if she doesn’t buy the cookie jar. On the
same graph, label the consumption bundle, A, that she would have if she
did not borrow or lend any money but bought the cookie jar in period 1,
sold it in period 2, and used the proceeds to buy period-2 consumption.
If she cannot borrow or lend, should Marsha invest in the cookie jar?
.
(b) Suppose that Marsha can borrow and lend at an interest rate of 50%.
On the graph where you labelled her initial endowment, draw the budget
line showing all of the bundles she can afford if she invests in the cookie
jar and borrows or lends at the interest rate of 50%. On the same graph
use red ink to draw one or two of Marsha’s indifference curves.
Period-2 consumption
80
60
40
20
0
20
40
60
80
Period-1 consumption
(c) Suppose that instead of consumption in the two periods being perfect substitutes, they are perfect complements, so that Marsha’s utility
function is min{c1 , c2 }. If she cannot borrow or lend, should she buy the
cookie jar?
If she can borrow and lend at an interest rate of
50%, should she invest in the cookie jar?
If she can borrow or
lend as much at an interest rate of 100%, should she invest in the cookie
jar?
.
10.1 If Peregrine in Problem 10.1 consumes (1,000, 1,155) and earns
(800,1365) and if the interest rate is 0.05, the present value of his endowment is
(a) 2,165.
(b) 2,100.
(c) 2,155.
(d) 4,305.
(e) 5,105.
10.2 Suppose that Molly from Problem 10.2 had an income of $400 in
period 1 and an income of $550 in period 2. Suppose that her utility
, where a = 0.40 and the interest rate were 0.10. If
function were ca1 c1−a
2
her income in period 1 doubled and her income in period 2 stayed the
same, her consumption in period 1 would
(a) double.
(b) increase by $160.
(c) increase by $80
(d) stay constant.
(e) increase by $400.
10.3 Mr. O. B. Kandle, of Problem 10.8, has a utility function c1 c2 where
c1 is his consumption in period 1 and c2 is his consumption in period 2.
He will have no income in period 2. If he had an income of 30,000 in
period 1 and the interest rate increased from 10% to 12%,
(a) his savings would increase by 2% and his consumption in period 2
would also increase.
(b) his savings would not change but his consumption in period 2 would
increase by 300.
(c) his consumption in both periods would increase.
(d) his consumption in both periods would decrease.
(e) his consumption in period 1 would decrease by 12% and his consumption in period 2 would also decrease.
10.4 Harvey Habit in Problem 10.9 has a utility function U (c1 , c2 ) =
min{c1 , c2 }. If he had an income of 1,025 in period 1, and 410 in period
2, and if the interest rate were 0.05, how much would Harvey choose to
spend on bread in period 1?
(a) 1,087.50
(b) 241.67
(c) 362.50
(d) 1,450
(e) 725
10.5 In the village in Problem 10.10, if the harvest this year is 3,000 and
the harvest next year will be 1,100 bushels of grain, and if rats eat 50% of
any grain that is stored for a year, how many bushels of grain could the
villagers consume next year if they consume 1,000 bushels of grain this
year?
(a) 2,100.
(b) 1,000.
(c) 4,100.
(d) 3,150.
(e) 1,200.
1/2
1/2
10.6 Patience has a utility function U (c1 , c2 ) = c1 + 0.83c2 , c1 is her
consumption in period 1 and c2 is her consumption in period 2. Her
income in period 1 is 2 times as large as her income in period 2. At what
interest rate will she choose to consume the same amount in period 1 as
in period 2?
(a) 0.40
(b) 0.10
(c) 0.20
(d) 0
(e) 0.30
CHAPTER
12
UNCERTAINTY
Uncertainty is a fact of life. People face risks every time they take a shower,
walk across the street, or make an investment. But there are financial institutions such as insurance markets and the stock market that can mitigate
at least some of these risks. We will study the functioning of these markets in the next chapter, but first we must study individual behavior with
respect to choices involving uncertainty.
12.1 Contingent Consumption
Since we now know all about the standard theory of consumer choice, let’s
try to use what we know to understand choice under uncertainty. The first
question to ask is what is the basic “thing” that is being chosen?
The consumer is presumably concerned with the probability distribution of getting different consumption bundles of goods. A probability
distribution consists of a list of different outcomes—in this case, consumption bundles—and the probability associated with each outcome. When a
consumer decides how much automobile insurance to buy or how much to
218 UNCERTAINTY (Ch. 12)
invest in the stock market, he is in effect deciding on a pattern of probability
distribution across different amounts of consumption.
For example, suppose that you have $100 now and that you are contemplating buying lottery ticket number 13. If number 13 is drawn in the
lottery, the holder will be paid $200. This ticket costs, say, $5. The two
outcomes that are of interest are the event that the ticket is drawn and the
event that it isn’t.
Your original endowment of wealth—the amount that you would have if
you did not purchase the lottery ticket—is $100 if 13 is drawn, and $100
if it isn’t drawn. But if you buy the lottery ticket for $5, you will have
a wealth distribution consisting of $295 if the ticket is a winner, and $95
if it is not a winner. The original endowment of probabilities of wealth
in different circumstances has been changed by the purchase of the lottery
ticket. Let us examine this point in more detail.
In this discussion we’ll restrict ourselves to examining monetary gambles
for convenience of exposition. Of course, it is not money alone that matters; it is the consumption that money can buy that is the ultimate “good”
being chosen. The same principles apply to gambles over goods, but restricting ourselves to monetary outcomes makes things simpler. Second,
we will restrict ourselves to very simple situations where there are only a
few possible outcomes. Again, this is only for reasons of simplicity.
Above we described the case of gambling in a lottery; here we’ll consider
the case of insurance. Suppose that an individual initially has $35,000
worth of assets, but there is a possibility that he may lose $10,000. For
example, his car may be stolen, or a storm may damage his house. Suppose
that the probability of this event happening is p = .01. Then the probability
distribution the person is facing is a 1 percent probability of having $25,000
of assets, and a 99 percent probability of having $35,000.
Insurance offers a way to change this probability distribution. Suppose
that there is an insurance contract that will pay the person $100 if the loss
occurs in exchange for a $1 premium. Of course the premium must be paid
whether or not the loss occurs. If the person decides to purchase $10,000
dollars of insurance, it will cost him $100. In this case he will have a 1
percent chance of having $34,900 ($35,000 of other assets − $10,000 loss +
$10,000 payment from the insurance payment – $100 insurance premium)
and a 99 percent chance of having $34,900 ($35,000 of assets − $100 insurance premium). Thus the consumer ends up with the same wealth no
matter what happens. He is now fully insured against loss.
In general, if this person purchases K dollars of insurance and has to pay
a premium γK, then he will face the gamble:1
probability .01 of getting $25, 000 + K − γK
1
The Greek letter γ, gamma, is pronounced “gam-ma.”
CONTINGENT CONSUMPTION
219
and
probability .99 of getting $35, 000 − γK.
What kind of insurance will this person choose? Well, that depends on
his preferences. He might be very conservative and choose to purchase a lot
of insurance, or he might like to take risks and not purchase any insurance
at all. People have different preferences over probability distributions in
the same way that they have different preferences over the consumption of
ordinary goods.
In fact, one very fruitful way to look at decision making under uncertainty
is just to think of the money available under different circumstances as
different goods. A thousand dollars after a large loss has occurred may
mean a very different thing from a thousand dollars when it hasn’t. Of
course, we don’t have to apply this idea just to money: an ice cream cone
if it happens to be hot and sunny tomorrow is a very different good from
an ice cream cone if it is rainy and cold. In general, consumption goods will
be of different value to a person depending upon the circumstances under
which they become available.
Let us think of the different outcomes of some random event as being
different states of nature. In the insurance example given above there
were two states of nature: the loss occurs or it doesn’t. But in general
there could be many different states of nature. We can then think of
a contingent consumption plan as being a specification of what will
be consumed in each different state of nature—each different outcome of
the random process. Contingent means depending on something not yet
certain, so a contingent consumption plan means a plan that depends on the
outcome of some event. In the case of insurance purchases, the contingent
consumption was described by the terms of the insurance contract: how
much money you would have if a loss occurred and how much you would
have if it didn’t. In the case of the rainy and sunny days, the contingent
consumption would just be the plan of what would be consumed given the
various outcomes of the weather.
People have preferences over different plans of consumption, just like
they have preferences over actual consumption. It certainly might make
you feel better now to know that you are fully insured. People make choices
that reflect their preferences over consumption in different circumstances,
and we can use the theory of choice that we have developed to analyze
those choices.
If we think about a contingent consumption plan as being just an ordinary consumption bundle, we are right back in the framework described in
the previous chapters. We can think of preferences as being defined over
different consumption plans, with the “terms of trade” being given by the
budget constraint. We can then model the consumer as choosing the best
consumption plan he or she can afford, just as we have done all along.
220 UNCERTAINTY (Ch. 12)
Let’s describe the insurance purchase in terms of the indifference-curve
analysis we’ve been using. The two states of nature are the event that the
loss occurs and the event that it doesn’t. The contingent consumptions are
the values of how much money you would have in each circumstance. We
can plot this on a graph as in Figure 12.1.
Cg
$35,000
Endowment
Slope = –
Choice
$35,000 – γK
$25,000
Figure
12.1
γ
1–γ
$25,000 + K – γK
Cb
Insurance. The budget line associated with the purchase of
insurance. The insurance premium γ allows us to give up some
consumption in the good outcome (Cg ) in order to have more
consumption in the bad outcome (Cb ).
Your endowment of contingent consumption is $25,000 in the “bad”
state—if the loss occurs—and $35,000 in the “good” state—if it doesn’t
occur. Insurance offers you a way to move away from this endowment
point. If you purchase K dollars’ worth of insurance, you give up γK dollars of consumption possibilities in the good state in exchange for K − γK
dollars of consumption possibilities in the bad state. Thus the consumption
you lose in the good state, divided by the extra consumption you gain in
the bad state, is
γ
γK
ΔCg
=−
.
=−
ΔCb
K − γK
1−γ
This is the slope of the budget line through your endowment. It is just
as if the price of consumption in the good state is 1 − γ and the price in
the bad state is γ.
CONTINGENT CONSUMPTION
221
We can draw in the indifference curves that a person might have for contingent consumption. Here again it is very natural for indifference curves
to have a convex shape: this means that the person would rather have a
constant amount of consumption in each state than a large amount in one
state and a low amount in the other.
Given the indifference curves for consumption in each state of nature,
we can look at the choice of how much insurance to purchase. As usual,
this will be characterized by a tangency condition: the marginal rate of
substitution between consumption in each state of nature should be equal
to the price at which you can trade off consumption in those states.
Of course, once we have a model of optimal choice, we can apply all of
the machinery developed in early chapters to its analysis. We can examine
how the demand for insurance changes as the price of insurance changes,
as the wealth of the consumer changes, and so on. The theory of consumer
behavior is perfectly adequate to model behavior under uncertainty as well
as certainty.
EXAMPLE: Catastrophe Bonds
We have seen that insurance is a way to transfer wealth from good states
of nature to bad states of nature. Of course there are two sides to these
transactions: those who buy insurance and those who sell it. Here we focus
on the sell side of insurance.
The sell side of the insurance market is divided into a retail component,
which deals directly with end buyers, and a wholesale component, in which
insurers sell risks to other parties. The wholesale part of the market is
known as the reinsurance market.
Typically, the reinsurance market has relied on large investors such as
pension funds to provide financial backing for risks. However, some reinsurers rely on large individual investors. Lloyd’s of London, one of the most
famous reinsurance consortia, generally uses private investors.
Recently, the reinsurance industry has been experimenting with catastrophe bonds, which, according to some, are a more flexible way to provide reinsurance. These bonds, generally sold to large institutions, have
typically been tied to natural disasters, like earthquakes or hurricanes.
A financial intermediary, such as a reinsurance company or an investment bank, issues a bond tied to a particular insurable event, such as an
earthquake involving, say, at least $500 million in insurance claims. If
there is no earthquake, investors are paid a generous interest rate. But if
the earthquake occurs and the claims exceed the amount specified in the
bond, investors sacrifice their principal and interest.
Catastrophe bonds have some attractive features. They can spread risks
widely and can be subdivided indefinitely, allowing each investor to bear
222 UNCERTAINTY (Ch. 12)
only a small part of the risk. The money backing up the insurance is paid
in advance, so there is no default risk to the insured.
From the economist’s point of view, “cat bonds” are a form of state
contingent security, that is, a security that pays off if and only if some
particular event occurs. This concept was first introduced by Nobel laureate Kenneth J. Arrow in a paper published in 1952 and was long thought
to be of only theoretical interest. But it turned out that all sorts of options
and other derivatives could be best understood using contingent securities. Now Wall Street rocket scientists draw on this 50-year-old work when
creating exotic new derivatives such as catastrophe bonds.
12.2 Utility Functions and Probabilities
If the consumer has reasonable preferences about consumption in different
circumstances, then we will be able to use a utility function to describe these
preferences, just as we have done in other contexts. However, the fact that
we are considering choice under uncertainty does add a special structure
to the choice problem. In general, how a person values consumption in one
state as compared to another will depend on the probability that the state
in question will actually occur. In other words, the rate at which I am
willing to substitute consumption if it rains for consumption if it doesn’t
should have something to do with how likely I think it is to rain. The
preferences for consumption in different states of nature will depend on the
beliefs of the individual about how likely those states are.
For this reason, we will write the utility function as depending on the
probabilities as well as on the consumption levels. Suppose that we are
considering two mutually exclusive states such as rain and shine, loss or
no loss, or whatever. Let c1 and c2 represent consumption in states 1 and
2, and let π1 and π2 be the probabilities that state 1 or state 2 actually
occurs.
If the two states are mutually exclusive, so that only one of them can
happen, then π2 = 1 − π1 . But we’ll generally write out both probabilities
just to keep things looking symmetric.
Given this notation, we can write the utility function for consumption in
states 1 and 2 as u(c1 , c2 , π1 , π2 ). This is the function that represents the
individual’s preference over consumption in each state.
EXAMPLE: Some Examples of Utility Functions
We can use nearly any of the examples of utility functions that we’ve seen
up until now in the context of choice under uncertainty. One nice example is the case of perfect substitutes. Here it is natural to weight each
EXPECTED UTILITY
223
consumption by the probability that it will occur. This gives us a utility
function of the form
u(c1 , c2 , π1 , π2 ) = π1 c1 + π2 c2 .
In the context of uncertainty, this kind of expression is known as the expected value. It is just the average level of consumption that you would
get.
Another example of a utility function that might be used to examine
choice under uncertainty is the Cobb–Douglas utility function:
.
u(c1 , c2 , π, 1 − π) = cπ1 c1−π
2
Here the utility attached to any combination of consumption bundles depends on the pattern of consumption in a nonlinear way.
As usual, we can take a monotonic transformation of utility and still
represent the same preferences. It turns out that the logarithm of the
Cobb-Douglas utility will be very convenient in what follows. This will
give us a utility function of the form
ln u(c1 , c2 , π1 , π2 ) = π1 ln c1 + π2 ln c2 .
12.3 Expected Utility
One particularly convenient form that the utility function might take is the
following:
u(c1 , c2 , π1 , π2 ) = π1 v(c1 ) + π2 v(c2 ).
This says that utility can be written as a weighted sum of some function
of consumption in each state, v(c1 ) and v(c2 ), where the weights are given
by the probabilities π1 and π2 .
Two examples of this were given above. The perfect substitutes, or
expected value utility function, had this form where v(c) = c. The CobbDouglas didn’t have this form originally, but when we expressed it in terms
of logs, it had the linear form with v(c) = ln c.
If one of the states is certain, so that π1 = 1, say, then v(c1 ) is the utility
of certain consumption in state 1. Similarly, if π2 = 1, v(c2 ) is the utility
of consumption in state 2. Thus the expression
π1 v(c1 ) + π2 v(c2 )
represents the average utility, or the expected utility, of the pattern of
consumption (c1 , c2 ).
224 UNCERTAINTY (Ch. 12)
For this reason, we refer to a utility function with the particular form
described here as an expected utility function, or, sometimes, a von
Neumann-Morgenstern utility function.2
When we say that a consumer’s preferences can be represented by an
expected utility function, or that the consumer’s preferences have the expected utility property, we mean that we can choose a utility function that
has the additive form described above. Of course we could also choose a different form; any monotonic transformation of an expected utility function
is a utility function that describes the same preferences. But the additive
form representation turns out to be especially convenient. If the consumer’s
preferences are described by π1 ln c1 + π2 ln c2 they will also be described
by cπ1 1 cπ2 2 . But the latter representation does not have the expected utility
property, while the former does.
On the other hand, the expected utility function can be subjected to
some kinds of monotonic transformation and still have the expected utility
property. We say that a function v(u) is a positive affine transformation if it can be written in the form: v(u) = au + b where a > 0. A
positive affine transformation simply means multiplying by a positive number and adding a constant. It turns out that if you subject an expected
utility function to a positive affine transformation, it not only represents
the same preferences (this is obvious since an affine transformation is just a
special kind of monotonic transformation) but it also still has the expected
utility property.
Economists say that an expected utility function is “unique up to an
affine transformation.” This just means that you can apply an affine transformation to it and get another expected utility function that represents
the same preferences. But any other kind of transformation will destroy
the expected utility property.
12.4 Why Expected Utility Is Reasonable
The expected utility representation is a convenient one, but is it a reasonable one? Why would we think that preferences over uncertain choices
would have the particular structure implied by the expected utility function? As it turns out there are compelling reasons why expected utility is
a reasonable objective for choice problems in the face of uncertainty.
The fact that outcomes of the random choice are consumption goods
that will be consumed in different circumstances means that ultimately
only one of those outcomes is actually going to occur. Either your house
2
John von Neumann was one of the major figures in mathematics in the twentieth
century. He also contributed several important insights to physics, computer science,
and economic theory. Oscar Morgenstern was an economist at Princeton who, along
with von Neumann, helped to develop mathematical game theory.
WHY EXPECTED UTILITY IS REASONABLE
225
will burn down or it won’t; either it will be a rainy day or a sunny day. The
way we have set up the choice problem means that only one of the many
possible outcomes is going to occur, and hence only one of the contingent
consumption plans will actually be realized.
This turns out to have a very interesting implication. Suppose you are
considering purchasing fire insurance on your house for the coming year. In
making this choice you will be concerned about wealth in three situations:
your wealth now (c0 ), your wealth if your house burns down (c1 ), and your
wealth if it doesn’t (c2 ). (Of course, what you really care about are your
consumption possibilities in each outcome, but we are simply using wealth
as a proxy for consumption here.) If π1 is the probability that your house
burns down and π2 is the probability that it doesn’t, then your preferences
over these three different consumptions can generally be represented by a
utility function u(π1 , π2 , c0 , c1 , c2 ).
Suppose that we are considering the tradeoff between wealth now and
one of the possible outcomes—say, how much money we would be willing
to sacrifice now to get a little more money if the house burns down. Then
this decision should be independent of how much consumption you will have
in the other state of nature—how much wealth you will have if the house
is not destroyed. For the house will either burn down or it won’t. If it
happens to burn down, then the value of extra wealth shouldn’t depend
on how much wealth you would have if it didn’t burn down. Bygones are
bygones—so what doesn’t happen shouldn’t affect the value of consumption
in the outcome that does happen.
Note that this is an assumption about an individual’s preferences. It may
be violated. When people are considering a choice between two things, the
amount of a third thing they have typically matters. The choice between
coffee and tea may well depend on how much cream you have. But this
is because you consume coffee together with cream. If you considered a
choice where you rolled a die and got either coffee, or tea, or cream, then
the amount of cream that you might get shouldn’t affect your preferences
between coffee and tea. Why? Because you are either getting one thing or
the other: if you end up with cream, the fact that you might have gotten
either coffee or tea is irrelevant.
Thus in choice under uncertainty there is a natural kind of “independence” between the different outcomes because they must be consumed
separately—in different states of nature. The choices that people plan to
make in one state of nature should be independent from the choices that
they plan to make in other states of nature. This assumption is known as
the independence assumption. It turns out that this implies that the
utility function for contingent consumption will take a very special structure: it has to be additive across the different contingent consumption
bundles.
That is, if c1 , c2 , and c3 are the consumptions in different states of nature,
and π1 , π2 , and π3 are the probabilities that these three different states of
226 UNCERTAINTY (Ch. 12)
nature materialize, then if the independence assumption alluded to above
is satisfied, the utility function must take the form
U (c1 , c2 , c3 ) = π1 u(c1 ) + π2 u(c2 ) + π3 u(c3 ).
This is what we have called an expected utility function. Note that the
expected utility function does indeed satisfy the property that the marginal
rate of substitution between two goods is independent of how much there
is of the third good. The marginal rate of substitution between goods 1
and 2, say, takes the form
ΔU (c1 , c2 , c3 )/Δc1
ΔU (c1 , c2 , c3 )/Δc2
π1 Δu(c1 )/Δc1
=−
.
π2 Δu(c2 )/Δc2
MRS12 = −
This MRS depends only on how much you have of goods 1 and 2, not
how much you have of good 3.
12.5 Risk Aversion
We claimed above that the expected utility function had some very convenient properties for analyzing choice under uncertainty. In this section
we’ll give an example of this.
Let’s apply the expected utility framework to a simple choice problem.
Suppose that a consumer currently has $10 of wealth and is contemplating
a gamble that gives him a 50 percent probability of winning $5 and a
50 percent probability of losing $5. His wealth will therefore be random:
he has a 50 percent probability of ending up with $5 and a 50 percent
probability of ending up with $15. The expected value of his wealth is $10,
and the expected utility is
1
1
u($15) + u($5).
2
2
This is depicted in Figure 12.2. The expected utility of wealth is the
average of the two numbers u($15) and u($5), labeled .5u(5) + .5u(15) in
the graph. We have also depicted the utility of the expected value of wealth,
which is labeled u($10). Note that in this diagram the expected utility of
wealth is less than the utility of the expected wealth. That is,
u
1
1
15 + 5
2
2
= u (10) >
1
1
u (15) + u (5) .
2
2
232 UNCERTAINTY (Ch. 12)
Similarly, the later shareholders of a company can use the stock market
to reallocate their risks. If a company you hold shares in is adopting a
policy that is too risky for your taste—or too conservative—you can sell
those shares and purchase others.
In the case of insurance, an individual was able to reduce his risk to
zero by purchasing insurance. For a flat fee of $100, the individual could
purchase full insurance against the $10,000 loss. This was true because
there was basically no risk in the aggregate: if the probability of the loss
occurring was 1 percent, then on average 10 of the 1000 people would face
a loss—we just didn’t know which ones.
In the case of the stock market, there is risk in the aggregate. One year
the stock market as a whole might do well, and another year it might do
poorly. Somebody has to bear that kind of risk. The stock market offers a
way to transfer risky investments from people who don’t want to bear risk
to people who are willing to bear risk.
Of course, few people outside of Las Vegas like to bear risk: most people
are risk averse. Thus the stock market allows people to transfer risk from
people who don’t want to bear it to people who are willing to bear it if
they are sufficiently compensated for it. We’ll explore this idea further in
the next chapter.
Summary
1. Consumption in different states of nature can be viewed as consumption
goods, and all the analysis of previous chapters can be applied to choice
under uncertainty.
2. However, the utility function that summarizes choice behavior under
uncertainty may have a special structure. In particular, if the utility function is linear in the probabilities, then the utility assigned to a gamble will
just be the expected utility of the various outcomes.
3. The curvature of the expected utility function describes the consumer’s
attitudes toward risk. If it is concave, the consumer is a risk averter; and
if it is convex, the consumer is a risk lover.
4. Financial institutions such as insurance markets and the stock market
provide ways for consumers to diversify and spread risks.
REVIEW QUESTIONS
1. How can one reach the consumption points to the left of the endowment
in Figure 12.1?
APPENDIX
233
2. Which of the following utility functions have the expected utility property? (a) u(c1 , c2 , π1 , π2 ) = a(π1 c1 + π2 c2 ), (b) u(c1 , c2 , π1 , π2 ) = π1 c1 +
π2 c22 , (c) u(c1 , c2 , π1 , π2 ) = π1 ln c1 + π2 ln c2 + 17.
3. A risk-averse individual is offered a choice between a gamble that pays
$1000 with a probability of 25% and $100 with a probability of 75%, or a
payment of $325. Which would he choose?
4. What if the payment was $320?
5. Draw a utility function that exhibits risk-loving behavior for small gambles and risk-averse behavior for larger gambles.
6. Why might a neighborhood group have a harder time self insuring for
flood damage versus fire damage?
APPENDIX
Let us examine a simple problem to demonstrate the principles of expected utility
maximization. Suppose that the consumer has some wealth w and is considering
investing some amount x in a risky asset. This asset could earn a return of rg in
the “good” outcome, or it could earn a return of rb in the “bad” outcome. You
should think of rg as being a positive return—the asset increases in value, and
rb being a negative return—a decrease in asset value.
Thus the consumer’s wealth in the good and bad outcomes will be
Wg = (w − x) + x(1 + rg ) = w + xrg
Wb = (w − x) + x(1 + rb ) = w + xrb .
Suppose that the good outcome occurs with probability π and the bad outcome
with probability (1 − π). Then the expected utility if the consumer decides to
invest x dollars is
EU (x) = πu(w + xrg ) + (1 − π)u(w + xrb ).
The consumer wants to choose x so as to maximize this expression.
Differentiating with respect to x, we find the way in which utility changes as
x changes:
(12.3)
EU (x) = πu (w + xrg )rg + (1 − π)u (w + xrb )rb .
The second derivative of utility with respect to x is
EU (x) = πu (w + xrg )rg2 + (1 − π)u (w + xrb )rb2 .
(12.4)
If the consumer is risk averse his utility function will be concave, which implies
that u (w) < 0 for every level of wealth. Thus the second derivative of expected
utility is unambiguously negative. Expected utility will be a concave function
of x.
234 UNCERTAINTY (Ch. 12)
Consider the change in expected utility for the first dollar invested in the risky
asset. This is just equation (12.3) with the derivative evaluated at x = 0:
EU (0) = πu (w)rg + (1 − π)u (w)rb
= u (w)[πrg + (1 − π)rb ].
The expression inside the brackets is the expected return on the asset. If
the expected return on the asset is negative, then expected utility must decrease
when the first dollar is invested in the asset. But since the second derivative
of expected utility is negative due to concavity, then utility must continue to
decrease as additional dollars are invested.
Hence we have found that if the expected value of a gamble is negative, a risk
averter will have the highest expected utility at x∗ = 0: he will want no part of a
losing proposition.
On the other hand, if the expected return on the asset is positive, then increasing x from zero will increase expected utility. Thus he will always want to
invest a little bit in the risky asset, no matter how risk averse he is.
Expected utility as a function of x is illustrated in Figure 12.4. In Figure 12.4A
the expected return is negative, and the optimal choice is x∗ = 0. In Figure 12.4B
the expected return is positive over some range, so the consumer wants to invest
some positive amount x∗ in the risky asset.
EXPECTED
UTILITY
EXPECTED
UTILITY
INVESTMENT
x* = 0
A
Figure
12.4
INVESTMENT
x*
B
How much to invest in the risky asset. In panel A, the optimal
investment is zero, but in panel B the consumer wants to invest a
positive amount.
The optimal amount for the consumer to invest will be determined by the
condition that the derivative of expected utility with respect to x be equal to zero.
Since the second derivative of utility is automatically negative due to concavity,
this will be a global maximum.
Setting (12.3) equal to zero we have
EU (x) = πu (w + xrg )rg + (1 − π)u (w + xrb )rb = 0.
(12.5)
This equation determines the optimal choice of x for the consumer in question.
APPENDIX
235
EXAMPLE: The Effect of Taxation on Investment in Risky Assets
How does the level of investment in a risky asset behave when you tax its return?
If the individual pays taxes at rate t, then the after-tax returns will be (1 − t)rg
and (1 − t)rb . Thus the first-order condition determining his optimal investment,
x, will be
EU (x) = πu (w + x(1 − t)rg )(1 − t)rg + (1 − π)u (w + x(1 − t)rb )(1 − t)rb = 0.
Canceling the (1 − t) terms, we have
EU (x) = πu (w + x(1 − t)rg )rg + (1 − π)u (w + x(1 − t)rb )rb = 0.
(12.6)
Let us denote the solution to the maximization problem without taxes—when
t = 0—by x∗ and denote the solution to the maximization problem with taxes
by x̂. What is the relationship between x∗ and x̂?
Your first impulse is probably to think that x∗ > x̂—that taxation of a risky
asset will tend to discourage investment in it. But that turns out to be exactly
wrong! Taxing a risky asset in the way we described will actually encourage
investment in it!
In fact, there is an exact relation between x∗ and x̂. It must be the case that
x̂ =
x∗
.
1−t
The proof is simply to note that this value of x̂ satisfies the first-order condition
for the optimal choice in the presence of the tax. Substituting this choice into
equation (12.6) we have
x∗
(1 − t)rg )rg
1−t
x∗
+ (1 − π)u (w +
(1 − t)rb )rb
1−t
∗
= πu (w + x rg )rg + (1 − π)u (w + x∗ rb )rb = 0,
EU (x̂) = πu (w +
where the last equality follows from the fact that x∗ is the optimal solution when
there is no tax.
What is going on here? How can imposing a tax increase the amount of
investment in the risky asset? Here is what is happening. When the tax is
imposed, the individual will have less of a gain in the good state, but he will
also have less of a loss in the bad state. By scaling his original investment up
by 1/(1 − t) the consumer can reproduce the same after-tax returns that he had
before the tax was put in place. The tax reduces his expected return, but it also
reduces his risk: by increasing his investment the consumer can get exactly the
same pattern of returns he had before and thus completely offset the effect of the
tax. A tax on a risky investment represents a tax on the gain when the return is
positive—but it represents a subsidy on the loss when the return is negative.
In Chapter 11, you learned some tricks that allow you to use techniques
you already know for studying intertemporal choice. Here you will learn
some similar tricks, so that you can use the same methods to study risk
taking, insurance, and gambling.
One of these new tricks is similar to the trick of treating commodities at different dates as different commodities. This time, we invent
new commodities, which we call contingent commodities. If either of two
events A or B could happen, then we define one contingent commodity
as consumption if A happens and another contingent commodity as consumption if B happens. The second trick is to find a budget constraint
that correctly specifies the set of contingent commodity bundles that a
consumer can afford.
This chapter presents one other new idea, and that is the notion
of von Neumann-Morgenstern utility. A consumer’s willingness to take
various gambles and his willingness to buy insurance will be determined
by how he feels about various combinations of contingent commodities.
Often it is reasonable to assume that these preferences can be expressed
by a utility function that takes the special form known as von NeumannMorgenstern utility. The assumption that utility takes this form is called
the expected utility hypothesis. If there are two events, 1 and 2 with
probabilities π1 and π2 , and if the contingent consumptions are c1 and
c2 , then the von Neumann-Morgenstern utility function has the special
functional form, U (c1 , c2 ) = π1 u(c1 ) + π2 u(c2 ). The consumer’s behavior
is determined by maximizing this utility function subject to his budget
constraint.
You are thinking of betting on whether the Cincinnati Reds will make it
to the World Series this year. A local gambler will bet with you at odds
of 10 to 1 against the Reds. You think the probability that the Reds will
make it to the World Series is π = .2. If you don’t bet, you are certain to
have $1,000 to spend on consumption goods. Your behavior satisfies the
expected utility hypothesis and your von Neumann-Morgenstern utility
√
√
function is π1 c1 + π2 c2 .
The contingent commodities are dollars if the Reds make the World
Series and dollars if the Reds don’t make the World Series. Let cW be
your consumption contingent on the Reds making the World Series and
cN W be your consumption contingent on their not making the Series.
Betting on the Reds at odds of 10 to 1 means that if you bet $x on the
Reds, then if the Reds make it to the Series, you make a net gain of $10x,
but if they don’t, you have a net loss of $x. Since you had $1,000 before
betting, if you bet $x on the Reds and they made it to the Series, you
would have cW = 1, 000 + 10x to spend on consumption. If you bet $x
on the Reds and they didn’t make it to the Series, you would lose $x,
and you would have cN W = 1, 000 − x. By increasing the amount $x that
you bet, you can make cW larger and cN W smaller. (You could also bet
against the Reds at the same odds. If you bet $x against the Reds and
they fail to make it to the Series, you make a net gain of .1x and if they
make it to the Series, you lose $x. If you work through the rest of this
discussion for the case where you bet against the Reds, you will see that
the same equations apply, with x being a negative number.) We can use
the above two equations to solve for a budget equation. From the second
equation, we have x = 1, 000 − cN W . Substitute this expression for x into
the first equation and rearrange terms to find cW + 10cN W = 11, 000, or
equivalently, .1cW + cN W = 1, 100. (The same budget equation can be
written in many equivalent ways by multiplying both sides by a positive
constant.)
Then you will choose your contingent consumption bundle (cW , cN W )
√
√
to maximize U (cW , cN W ) = .2 cW + .8 cN W subject to the budget
constraint, .1cW + cN W = 1, 100. Using techniques that are now familiar,
you can solve this consumer problem. From the budget constraint, you
see that consumption contingent on the Reds making the World Series
costs 1/10 as much as consumption contingent on their not making it. If
you set the marginal rate of substitution between cW and cN W equal to
the price ratio and simplify the resulting expression, you will find that
cN W = .16cW . This equation, together with the budget equation implies
that cW = $4, 230.77 and cN W = $676.92. You achieve this bundle by
betting $323.08 on the Reds. If the Reds make it to the Series, you will
have $1, 000 + 10 × 323.08 = $4, 230.80. If not, you will have $676.92.
(We rounded the solutions to the nearest penny.)
12.1 (0) In the next few weeks, Congress is going to decide whether
or not to develop an expensive new weapons system. If the system is
approved, it will be very profitable for the defense contractor, General
Statics. Indeed, if the new system is approved, the value of stock in
General Statics will rise from $10 per share to $15 a share, and if the
project is not approved, the value of the stock will fall to $5 a share. In
his capacity as a messenger for Congressman Kickback, Buzz Condor has
discovered that the weapons system is much more likely to be approved
than is generally thought. On the basis of what he knows, Condor has
decided that the probability that the system will be approved is 3/4 and
the probability that it will not be approved is 1/4. Let cA be Condor’s
consumption if the system is approved and cN A be his consumption if
the system is not approved. Condor’s von Neumann-Morgenstern utility
function is U (cA , cN A ) = .75 ln cA + .25 ln cN A . Condor’s total wealth is
$50,000, all of which is invested in perfectly safe assets. Condor is about
to buy stock in General Statics.
(a) If Condor buys x shares of stock, and if the weapons system is approved, he will make a profit of $5 per share. Thus the amount he can
consume, contingent on the system being approved, is cA = $50, 000 + 5x.
If Condor buys x shares of stock, and if the weapons system is not approved, then he will make a loss of $
per share. Thus the amount
he can consume, contingent on the system not being approved, is cN A =
.
(b) You can solve for Condor’s budget constraint on contingent commodity bundles (cA , cN A ) by eliminating x from these two equations. His budget constraint can be written as
cA +
cN A = 50, 000.
(c) Buzz Condor has no moral qualms about trading on inside information, nor does he have any concern that he will be caught and punished.
To decide how much stock to buy, he simply maximizes his von NeumannMorgenstern utility function subject to his budget. If he sets his marginal
rate of substitution between the two contingent commodities equal to
their relative prices and simplifies the equation, he finds that cA /cN A =
(Reminder: Where a is any constant, the derivative of a ln x
with respect to x is a/x.)
(d) Condor finds that his optimal contingent commodity bundle is
(cA , cN A ) =
bundle, he must buy
To acquire this contingent commodity
shares of stock in General Statics.
12.2 (0) Willy owns a small chocolate factory, located close to a river
that occasionally floods in the spring, with disastrous consequences. Next
summer, Willy plans to sell the factory and retire. The only income he
will have is the proceeds of the sale of his factory. If there is no flood,
the factory will be worth $500,000. If there is a flood, then what is left
of the factory will be worth only $50,000. Willy can buy flood insurance
at a cost of $.10 for each $1 worth of coverage. Willy thinks that the
probability that there will be a flood this spring is 1/10. Let cF denote the
contingent commodity dollars if there is a flood and cN F denote dollars
if there is no flood. Willy’s von Neumann-Morgenstern utility function is
√
√
U (cF , cN F ) = .1 cF + .9 cN F .
(a) If he buys no insurance, then in each contingency, Willy’s consumption
will equal the value of his factory, so Willy’s contingent commodity bundle
will be (cF , cN F ) =
.
(b) To buy insurance that pays him $x in case of a flood, Willy must
pay an insurance premium of .1x. (The insurance premium must be paid
whether or not there is a flood.) If Willy insures for $x, then if there is a
flood, he gets $x in insurance benefits. Suppose that Willy has contracted
for insurance that pays him $x in the event of a flood. Then after paying
his insurance premium, he will be able to consume cF =
If Willy has this amount of insurance and there is no flood, then he will
be able to consume cN F =
.
(c) You can eliminate x from the two equations for cF and cN F that
you found above. This gives you a budget equation for Willy. Of course
there are many equivalent ways of writing the same budget equation,
since multiplying both sides of a budget equation by a positive constant
yields an equivalent budget equation. The form of the budget equation
in which the “price” of cN F is 1 can be written as .9cN F +
cF =
.
(d) Willy’s marginal rate of substitution between the two contingent commodities, dollars if there
is no flood and dollars if there is a flood, is
√
.1 cN F
√
M RS(cF , cN F ) = − .9 cF . To find his optimal bundle of contingent
commodities, you must set this marginal rate of substitution equal to the
number
Solving this equation, you find that Willy will choose
to consume the two contingent commodities in the ratio
.
(e) Since you know the ratio in which he will consume cF and cN F , and
you know his budget equation, you can solve for his optimal consumption
bundle, which is (cF , cN F )=
surance policy that will pay him
Willy will buy an inif there is a flood. The
amount of insurance premium that he will have to pay is
.
12.3 (0) Clarence Bunsen is an expected utility maximizer. His preferences among contingent commodity bundles are represented by the expected utility function
√
√
u(c1 , c2 , π1 , π2 ) = π1 c1 + π2 c2 .
Clarence’s friend, Hjalmer Ingqvist, has offered to bet him $1,000 on the
outcome of the toss of a coin. That is, if the coin comes up heads, Clarence
must pay Hjalmer $1,000 and if the coin comes up tails, Hjalmer must
pay Clarence $1,000. The coin is a fair coin, so that the probability of
heads and the probability of tails are both 1/2. If he doesn’t accept the
bet, Clarence will have $10,000 with certainty. In the privacy of his car
dealership office over at Bunsen Motors, Clarence is making his decision.
(Clarence uses the pocket calculator that his son, Elmer, gave him last
Christmas. You will find that it will be helpful for you to use a calculator
too.) Let Event 1 be “coin comes up heads” and let Event 2 be “coin
comes up tails.”
(a) If Clarence accepts the bet, then in Event 1, he will have
dollars and in Event 2, he will have
dollars.
(b) Since the probability of each event is 1/2, Clarence’s expected utility
for a gamble in which he gets c1 in Event 1 and c2 in Event 2 can be
described by the formula
Therefore Clarence’s expected
utility if he accepts the bet with Hjalmer will be
that calculator.)
(Use
(c) If Clarence decides not to bet, then in Event 1, he will have
dollars and in Event 2, he will have
dollars. Therefore if he
doesn’t bet, his expected utility will be
.
(d) Having calculated his expected utility if he bets and if he does not bet,
Clarence determines which is higher and makes his decision accordingly.
Does Clarence take the bet?
.
12.4 (0) It is a slow day at Bunsen Motors, so since he has his calculator warmed up, Clarence Bunsen (whose preferences toward risk were
described in the last problem) decides to study his expected utility function more closely.
(a) Clarence first thinks about really big gambles. What if he bet his
entire $10,000 on the toss of a coin, where he loses if heads and wins if
tails? Then if the coin came up heads, he would have 0 dollars and if it
came up tails, he would have $20,000. His expected utility if he took the
bet would be
, while his expected utility if he didn’t take the
bet would be
such a bet.
Therefore he concludes that he would not take
(b) Clarence then thinks, “Well, of course, I wouldn’t want to take a
chance on losing all of my money on just an ordinary bet. But, what
if somebody offered me a really good deal. Suppose I had a chance to
bet where if a fair coin came up heads, I lost my $10,000, but if it came
up tails, I would win $50,000. Would I take the bet? If I took the bet,
my expected utility would be
expected utility would be
bet.”
If I didn’t take the bet, my
Therefore I should
the
(c) Clarence later asks himself, “If I make a bet where I lose my $10,000
if the coin comes up heads, what is the smallest amount that I would have
to win in the event of tails in order to make the bet a good one for me
to take?” After some trial and error, Clarence found the answer. You,
too, might want to find the answer by trial and error, but it is easier to
find the answer by solving an equation. On the left side of your equation,
you would write down Clarence’s utility if he doesn’t bet. On the right
side of the equation, you write down an expression for Clarence’s utility
if he makes a bet such that he is left with zero consumption in Event 1
and x in Event 2. Solve this equation for x. The answer to Clarence’s
question is where x = 10, 000. The equation that you should write is
The solution is x =
.
(d) Your answer to the last part gives you two points on Clarence’s indifference curve between the contingent commodities, money in Event 1
and money in Event 2. (Poor Clarence has never heard of indifference
curves or contingent commodities, so you will have to work this part for
him, while he heads over to the Chatterbox Cafe for morning coffee.) One
of these points is where money in both events is $10,000. On the graph
below, label this point A. The other is where money in Event 1 is zero
and money in Event 2 is
B.
On the graph below, label this point
Money in Event 2 (×1, 000)
40
30
20
10
0
10
20
30
40
Money in Event 1 (×1, 000)
(e) You can quickly find a third point on this indifference curve. The
coin is a fair coin, and Clarence cares whether heads or tails turn up only
because that determines his prize. Therefore Clarence will be indifferent
between two gambles that are the same except that the assignment of
prizes to outcomes are reversed. In this example, Clarence will be indifferent between point B on the graph and a point in which he gets zero if
if Event 1 happens. Find this point on
Event 2 happens and
the Figure above and label it C.
(f ) Another gamble that is on the same indifference curve for Clarence
as not gambling at all is the gamble where he loses $5,000 if heads turn
up and where he wins
dollars if tails turn up. (Hint: To
solve this problem, put the utility of not betting on the left side of an
equation and on the right side of the equation, put the utility of having
$10, 000 − $5, 000 in Event 1 and $10, 000 + x in Event 2. Then solve the
resulting equation for x.) On the axes above, plot this point and label it
D. Now sketch in the entire indifference curve through the points that
you have labeled.
12.5 (0) Hjalmer Ingqvist’s son-in-law, Earl, has not worked out very
well. It turns out that Earl likes to gamble. His preferences over contingent commodity bundles are represented by the expected utility function
u(c1 , c2 , π1 , π2 ) = π1 c21 + π2 c22 .
(a) Just the other day, some of the boys were down at Skoog’s tavern
when Earl stopped in. They got to talking about just how bad a bet they
could get him to take. At the time, Earl had $100. Kenny Olson shuffled
a deck of cards and offered to bet Earl $20 that Earl would not cut a spade
from the deck. Assuming that Earl believed that Kenny wouldn’t cheat,
the probability that Earl would win the bet was 1/4 and the probability
that Earl would lose the bet was 3/4. If he won the bet, Earl would have
dollars and if he lost the bet, he would have
dollars.
Earl’s expected utility if he took the bet would be
, and his
expected utility if he did not take the bet would be
he refused the bet.
Therefore
(b) Just when they started to think Earl might have changed his ways,
Kenny offered to make the same bet with Earl except that they would
bet $100 instead of $20. What is Earl’s expected utility if he takes that
bet?
Would Earl be willing to take this bet?
.
(c) Let Event 1 be the event that a card drawn from a fair deck of cards is
a spade. Let Event 2 be the event that the card is not a spade. Earl’s preferences between income contingent on Event 1, c1 , and income contingent
Use
on Event 2, c2 , can be represented by the equation
blue ink on the graph below to sketch Earl’s indifference curve passing
through the point (100, 100).
Money in Event 2
200
150
100
50
0
50
100
150
200
Money in Event 1
(d) On the same graph, let us draw Hjalmer’s son-in-law Earl’s indifference curves between contingent commodities where the probabilities
are different. Suppose that a card is drawn from a fair deck of cards.
Let Event 1 be the event that the card is black. Let event 2 be the event
that the card drawn is red. Suppose each event has probability 1/2. Then
Earl’s preferences between income contingent on Event 1 and income contingent on Event 2 are represented by the formula
On
the graph, use red ink to show two of Earl’s indifference curves, including
the one that passes through (100, 100).
12.6 (1) Sidewalk Sam makes his living selling sunglasses at the boardwalk in Atlantic City. If the sun shines Sam makes $30, and if it rains
Sam only makes $10. For simplicity, we will suppose that there are only
two kinds of days, sunny ones and rainy ones.
(a) One of the casinos in Atlantic City has a new gimmick. It is accepting
bets on whether it will be sunny or rainy the next day. The casino sells
dated “rain coupons” for $1 each. If it rains the next day, the casino will
give you $2 for every rain coupon you bought on the previous day. If it
doesn’t rain, your rain coupon is worthless. In the graph below, mark
Sam’s “endowment” of contingent consumption if he makes no bets with
the casino, and label it E.
Cr
40
30
20
10
0
10
20
30
40
Cs
(b) On the same graph, mark the combination of consumption contingent
on rain and consumption contingent on sun that he could achieve by
buying 10 rain coupons from the casino. Label it A.
(c) On the same graph, use blue ink to draw the budget line representing
all of the other patterns of consumption that Sam can achieve by buying
rain coupons. (Assume that he can buy fractional coupons, but not negative amounts of them.) What is the slope of Sam’s budget line at points
above and to the left of his initial endowment?
.
(d) Suppose that the casino also sells sunshine coupons. These tickets
also cost $1. With these tickets, the casino gives you $2 if it doesn’t rain
and nothing if it does. On the graph above, use red ink to sketch in the
budget line of contingent consumption bundles that Sam can achieve by
buying sunshine tickets.
(e) If the price of a dollar’s worth of consumption when it rains is set
equal to 1, what is the price of a dollar’s worth of consumption if it
shines?
.
12.7 (0) Sidewalk Sam, from the previous problem, has the utility function for consumption in the two states of nature
u(cs , cr , π) = c1−π
cπr ,
s
where cs is the dollar value of his consumption if it shines, cr is the dollar
value of his consumption if it rains, and π is the probability that it will
rain. The probability that it will rain is π = .5.
(a) How many units of consumption is it optimal for Sam to consume
conditional on rain?
.
(b) How many rain coupons is it optimal for Sam to buy?
.
12.8 (0) Sidewalk Sam’s brother Morgan von Neumanstern is an expected utility maximizer. His von Neumann-Morgenstern utility function
for wealth is u(c) = ln c. Sam’s brother also sells sunglasses on another
beach in Atlantic City and makes exactly the same income as Sam does.
He can make exactly the same deal with the casino as Sam can.
(a) If Morgan believes that there is a 50% chance of rain and a 50% chance
of sun every day, what would his expected utility of consuming (cs , cr )
be?
.
(b) How does Morgan’s utility function compare to Sam’s? Is one a
monotonic transformation of the other?
.
(c) What will Morgan’s optimal pattern of consumption be? Answer:
Morgan will consume
on the sunny days and
on the
rainy days. How does this compare to Sam’s consumption?
.
12.9 (0) Billy John Pigskin of Mule Shoe, Texas,
√ has a von NeumannMorgenstern utility function of the form u(c) = c. Billy John also weighs
about 300 pounds and can outrun jackrabbits and pizza delivery trucks.
Billy John is beginning his senior year of college football. If he is not
seriously injured, he will receive a $1,000,000 contract for playing professional football. If an injury ends his football career, he will receive a
$10,000 contract as a refuse removal facilitator in his home town. There
is a 10% chance that Billy John will be injured badly enough to end his
career.
(a) What is Billy John’s expected utility?
.
(b) If Billy John pays $p for an insurance policy that would give him
$1,000,000 if he suffered a career-ending injury while in college, then he
would be sure to have an income of $1, 000, 000 − p no matter what happened to him. Write an equation that can be solved to find the largest
price that Billy John would be willing to pay for such an insurance policy.
.
(c) Solve this equation for p.
.
12.10 (1) You have $200 and are thinking about betting on the Big
Game next Saturday. Your team, the Golden Boars, are scheduled to
play their traditional rivals the Robber Barons. It appears that the going
odds are 2 to 1 against the Golden Boars. That is to say if you want
to bet $10 on the Boars, you can find someone who will agree to pay
you $20 if the Boars win in return for your promise to pay him $10 if
the Robber Barons win. Similarly if you want to bet $10 on the Robber
Barons, you can find someone who will pay you $10 if the Robber Barons
win, in return for your promise to pay him $20 if the Robber Barons lose.
Suppose that you are able to make as large a bet as you like, either on
the Boars or on the Robber Barons so long as your gambling losses do
not exceed $200. (To avoid tedium, let us ignore the possibility of ties.)
(a) If you do not bet at all, you will have $200 whether or not the Boars
win. If you bet $50 on the Boars, then after all gambling obligations are
settled, you will have a total of
dollars if the Boars win and
dollars if they lose. On the graph below, use blue ink to draw a
line that represents all of the combinations of “money if the Boars win”
and “money if the Robber Barons win” that you could have by betting
from your initial $200 at these odds.
Money if the Boars lose
400
300
200
100
0
100
200
300
400
Money if the Boars win
(b) Label the point on this graph where you would be if you did not bet
at all with an E.
(c) After careful thought you decide to bet $50 on the Boars. Label the
point you have chosen on the graph with a C. Suppose that after you have
made this bet, it is announced that the star Robber Baron quarterback
suffered a sprained thumb during a tough economics midterm examination
and will miss the game. The market odds shift from 2 to 1 against the
Boars to “even money” or 1 to 1. That is, you can now bet on either
team and the amount you would win if you bet on the winning team is
the same as the amount that you would lose if you bet on the losing team.
You cannot cancel your original bet, but you can make new bets at the
new odds. Suppose that you keep your first bet, but you now also bet
$50 on the Robber Barons at the new odds. If the Boars win, then after
you collect your winnings from one bet and your losses from the other,
how much money will you have left?
If the Robber Barons
win, how much money will you have left after collecting your winnings
and paying off your losses?
.
(d) Use red ink to draw a line on the diagram you made above, showing
the combinations of “money if the Boars win” and “money if the Robber
Barons win” that you could arrange for yourself by adding possible bets
at the new odds to the bet you made before the news of the quarterback’s
misfortune. On this graph, label the point D that you reached by making
the two bets discussed above.
12.11 (2) The certainty equivalent of a lottery is the amount of money
you would have to be given with certainty to be just as well-off with that
lottery. Suppose that your von Neumann-Morgenstern utility function
over lotteries that give you an amount x√ if Event 1 happens and y if
√
Event 1 does not happen is U (x, y, π) = π x + (1 − π) y, where π is the
probability that Event 1 happens and 1 − π is the probability that Event
1 does not happen.
(a) If π = .5, calculate the utility of a lottery that gives you $10,000 if
Event 1 happens and $100 if Event 1 does not happen.
.
(b) If you were sure to receive $4,900, what would your utility be?
(Hint: If you receive $4,900 with certainty, then you receive $4,900 in both
events.)
(c) Given this utility function and π = .5, write a general formula for the
certainty equivalent of a lottery that gives you $x if Event 1 happens and
$y if Event 1 does not happen.
.
(d) Calculate the certainty equivalent of receiving $10,000 if Event 1 happens and $100 if Event 1 does not happen.
.
12.12 (0) Dan Partridge
is a risk averter who tries to maximize the
√
expected value of c, where c is his wealth. Dan has $50,000 in safe
assets and he also owns a house that is located in an area where there
are lots of forest fires. If his house burns down, the remains of his house
and the lot it is built on would be worth only $40,000, giving him a total
wealth of $90,000. If his home doesn’t burn, it will be worth $200,000
and his total wealth will be $250,000. The probability that his home will
burn down is .01.
(a) Calculate his expected utility if he doesn’t buy fire insurance.
.
(b) Calculate the certainty equivalent of the lottery he faces if he doesn’t
.
buy fire insurance.
(c) Suppose that he can buy insurance at a price of $1 per $100 of insurance. For example if he buys $100,000 worth of insurance, he will pay
$1,000 to the company no matter what happens, but if his house burns,
he will also receive $100,000 from the company. If Dan buys $160,000
worth of insurance, he will be fully insured in the sense that no matter
what happens his after-tax wealth will be
.
(d) Therefore if he buys full insurance, the certainty equivalent of his
wealth is
, and his expected utility is
.
12.13 (1) Portia has been waiting a long time for her ship to come in
and has concluded that there is a 25% chance that it will arrive today.
If it does come in today, she will receive $1,600. If it does not come
in today, it will never come and her wealth will be zero. Portia has a
von Neumann-Morgenstern
utility such that she wants to maximize the
√
expected value of c, where c is total wealth. What is the minimum price
at which she will sell the rights to her ship?
.
12.1 In Problem 12.9, Billy has a von Neumann-Morgenstern utility function U (c) = c1/2 . If Billy is not injured this season, he will receive an
income of 25 million dollars. If he is injured, his income will be only 10,000
dollars. The probability that he will be injured is .1 and the probability
that he will not be injured is .9. His expected utility is
(a) 4,510.
(b) between 24 million and 25 million dollars.
(c) 100,000.
(d) 9,020.
(e) 18,040.
12.2 (See Problem 12.2.) Willy’s only source of wealth is his chocolate
factory, which may be damaged by a flood. Let cf and cnf be his wealth
contingent on a flood and on no flood, respectively. His utility function is
1/2
1/2
pcf + (1 − p)cnf , where p is the probability of a flood and 1 − p is the
probability of no flood. The probability of a flood is p = 1/15. The value
of Willy’s factory is $600,000 if there is no flood and 0 if there is a flood.
Willy can buy insurance where if he buys $x worth of insurance, he must
pay the insurance company $3x/17 whether there is a flood or not, but
he gets back $x from the company if there is a flood. Willy should buy
(a) no insurance since the cost per dollar of insurance exceeds the probability of a flood.
(b) enough insurance so that if there were a flood, after he collected his
insurance his wealth would be 1/9 of what it would be if there were no
flood.
(c) enough insurance so that if there were a flood, after he collected his
insurance, his wealth would be the same whether there were a flood or
not.
(d) enough insurance so that if there were a flood, after he collected his
insurance, his wealth would be 1/4 of what it would be if there were no
flood.
(e) enough insurance so that if there were a flood, after he collects his
insurance his wealth would be 1/7 of what it would be if there were no
flood.
12.3 Sally Kink is an expected utility maximizer with utility function
pu(c1 ) + (1 − p)u(c2 ), where for any x < 4, 000, u(x) = 2x and where
u(x) = 4, 000 + x for x greater than or equal to 4,000. (Hint: Draw a
graph of u(x).)
(a) Sally will be risk averse if her income is less than 4,000 but risk loving
if her income is more than 4,000.
(b) Sally will be risk neutral if her income is less than 4,000 and risk
averse if her income is more than 4,000.
(c) For bets that involve no chance of her wealth’s exceeding 4,000, Sally
will take any bet that has a positive expected net payoff.
(d) Sally will never take a bet if there is a chance that it leaves her with
wealth less than 8,000.
(e) None of the above are true.
1/2
12.4 (See Problem 12.11.) Martin’s expected utility function is pc1 +
1/2
(1 − p)c2 , where p is the probability that he consumes c1 and 1 − p is
the probability that he consumes c2 . Wilbur is offered a choice between
getting a sure payment of $Z or a lottery in which he receives $2,500 with
probability .40 and $900 with probability .60. Wilbur will choose the sure
payment if
(a) Z > 1, 444 and the lottery if Z < 1, 444.
(b) Z > 1, 972 and the lottery if Z < 1, 972.
(c) Z > 900 and the lottery if Z < 900.
(d) Z > 1, 172 and the lottery if Z < 1, 172.
(e) Z > 1, 540 and the lottery if Z < 1, 540.
12.5 Clancy has $4,800. He plans to bet on a boxing match between
Sullivan and Flanagan. He finds that he can buy coupons for $6 that
will pay off $10 each if Sullivan wins. He also finds in another store some
coupons that will pay off $10 if Flanagan wins. The Flanagan tickets cost
$4 each. Clancy believes that the two fighters each have a probability
of 1/2 of winning. Clancy is a risk averter who tries to maximize the
expected value of the natural log of his wealth. Which of the following
strategies would maximize his expected utility?
(a) Don’t gamble at all.
(b) Buy 400 Sullivan tickets and 600 Flanagan tickets.
(c) Buy exactly as many Flanagan tickets as Sullivan tickets.
(d) Buy 200 Sullivan tickets and 300 Flanagan tickets.
(e) Buy 200 Sullivan tickets and 600 Flanagan tickets.
ANSWERS
1 The Market
1.1. It would be constant at $500 for 25 apartments and then drop to $200.
1.2. In the first case, $500, and in the second case, $200. In the third case,
the equilibrium price would be any price between $200 and $500.
1.3. Because if we want to rent one more apartment, we have to offer a lower
price. The number of people who have reservation prices greater than p
must always increase as p decreases.
1.4. The price of apartments in the inner ring would go up since demand
for apartments would not change but supply would decrease.
1.5. The price of apartments in the inner ring would rise.
1.6. A tax would undoubtedly reduce the number of apartments supplied
in the long run.
1.7. He would set a price of 25 and rent 50 apartments. In the second case
he would rent all 40 apartments at the maximum price the market would
bear. This would be given by the solution to D(p) = 100 − 2p = 40, which
is p∗ = 30.
1.8. Everyone who had a reservation price higher than the equilibrium
price in the competitive market, so that the final outcome would be Pareto
efficient. (Of course in the long run there would probably be fewer new
apartments built, which would lead to another kind of inefficiency.)
2 Budget Constraint
2.1. The new budget line is given by 2p1 x1 + 8p2 x2 = 4m.
2.2. The vertical intercept (x2 axis) decreases and the horizontal intercept
(x1 axis) stays the same. Thus the budget line becomes flatter.
A12 ANSWERS
2.3. Flatter. The slope is −2p1 /3p2 .
2.4. A good whose price has been set to 1; all other goods’ prices are
measured relative to the numeraire good’s price.
2.5. A tax of 8 cents a gallon.
2.6. (p1 + t)x1 + (p2 − s)x2 = m − u.
2.7. Yes, since all of the bundles the consumer could afford before are
affordable at the new prices and income.
3 Preferences
3.1. No. It might be that the consumer was indifferent between the two
bundles. All we are justified in concluding is that (x1 , x2 ) (y1 , y2 ).
3.2. Yes to both.
3.3. It is transitive, but it is not complete—two people might be the same
height. It is not reflexive since it is false that a person is strictly taller than
himself.
3.4. It is transitive, but not complete. What if A were bigger but slower
than B? Which one would he prefer?
3.5. Yes. An indifference curve can cross itself, it just can’t cross another
distinct indifference curve.
3.6. No, because there are bundles on the indifference curve that have
strictly more of both goods than other bundles on the (alleged) indifference
curve.
3.7. A negative slope. If you give the consumer more anchovies, you’ve
made him worse off, so you have to take away some pepperoni to get him
back on his indifference curve. In this case the direction of increasing utility
is toward the origin.
3.8. Because the consumer weakly prefers the weighted average of two bundles to either bundle.
3.9. If you give up one $5 bill, how many $1 bills do you need to compensate you? Five $1 bills will do nicely. Hence the answer is −5 or −1/5,
depending on which good you put on the horizontal axis.
3.10. Zero—if you take away some of good 1, the consumer needs zero units
of good 2 to compensate him for his loss.
ANSWERS
A13
3.11. Anchovies and peanut butter, scotch and Kool Aid, and other similar
repulsive combinations.
4 Utility
4.1. The function f (u) = u2 is a monotonic transformation for positive u,
but not for negative u.
4.2. (1) Yes. (2) No (works for v positive). (3) No (works for v negative).
(4) Yes (only defined for v positive). (5) Yes. (6) No. (7) Yes. (8) No.
4.3. Suppose that the diagonal intersected a given indifference curve at
two points, say (x, x) and (y, y). Then either x > y or y > x, which
means that one of the bundles has more of both goods. But if preferences
are monotonic, then one of the bundles would have to be preferred to the
other.
4.4. Both represent perfect substitutes.
4.5. Quasilinear preferences. Yes.
4.6. The utility function represents Cobb-Douglas preferences. No. Yes.
4.7. Because the MRS is measured along an indifference curve, and utility
remains constant along an indifference curve.
5 Choice
5.1. x2 = 0 when p2 > p1 , x2 = m/p2 when p2 < p1 , and anything between
0 and m/p2 when p1 = p2 .
5.2. The optimal choices will be x1 = m/p1 and x2 = 0 if p1 /p2 < b,
x1 = 0 and x2 = m/p2 if p1 /p2 > b, and any amount on the budget line if
p1 /p2 = b.
5.3. Let z be the number of cups of coffee the consumer buys. Then we
know that 2z is the number of teaspoons of sugar he or she buys. We must
satisfy the budget constraint
2p1 z + p2 z = m.
Solving for z we have
z=
m
.
2p1 + p2
A14 ANSWERS
5.4. We know that you’ll either consume all ice cream or all olives. Thus
the two choices for the optimal consumption bundles will be x1 = m/p1 ,
x2 = 0, or x1 = 0, x2 = m/p2 .
5.5. This is a Cobb-Douglas utility function, so she will spend 4/(1 + 4) =
4/5 of her income on good 2.
5.6. For kinked preferences, such as perfect complements, where the change
in price doesn’t induce any change in demand.
6 Demand
6.1. No. If her income increases, and she spends it all, she must be purchasing more of at least one good.
6.2. The utility function for perfect substitutes is u(x1 , x2 ) = x1 + x2 .
Thus if u(x1 , x2 ) > u(y1 , y2 ), we have x1 + x2 > y1 + y2 . It follows that
tx1 + tx2 > ty1 + ty2 , so that u(tx1 , tx2 ) > u(ty1 , ty2 ).
6.3. The Cobb-Douglas utility function has the property that
u(tx1 , tx2 ) = (tx1 )a (tx2 )1−a = ta t1−a xa1 x1−a
= txa1 x1−a
= tu(x1 , x2 ).
2
2
Thus if u(x1 , x2 ) > u(y1 , y2 ), we know that u(tx1 , tx2 ) > u(ty1 , ty2 ), so
that Cobb-Douglas preferences are indeed homothetic.
6.4. The demand curve.
6.5. No. Concave preferences can only give rise to optimal consumption
bundles that involve zero consumption of one of the goods.
6.6. Normally they would be complements, at least for non-vegetarians.
6.7. We know that x1 = m/(p1 + p2 ). Solving for p1 as a function of the
other variables, we have
m
− p2 .
p1 =
x1
6.8. False.
7 Revealed Preference
7.1. No. This consumer violates the Weak Axiom of Revealed Preference
since when he bought (x1 , x2 ) he could have bought (y1 , y2 ) and vice versa.
In symbols:
p1 x1 + p2 x2 = 1 × 1 + 2 × 2 = 5 > 4 = 1 × 2 + 2 × 1 = p1 y1 + p2 y2
ANSWERS
A15
and
q1 y1 + q2 y2 = 2 × 2 + 1 × 1 = 5 > 4 = 2 × 1 + 1 × 2 = q1 x1 + q2 x2 .
7.2. Yes. No violations of WARP are present, since the y-bundle is not
affordable when the x-bundle was purchased and vice versa.
7.3. Since the y-bundle was more expensive than the x-bundle when the
x-bundle was purchased and vice versa, there is no way to tell which bundle
is preferred.
7.4. If both prices changed by the same amount. Then the base-year bundle
would still be optimal.
7.5. Perfect complements.
8 Slutsky Equation
8.1. Yes. To see this, use our favorite example of red pencils and blue
pencils. Suppose red pencils cost 10 cents a piece, and blue pencils cost
5 cents a piece, and the consumer spends $1 on pencils. She would then
consume 20 blue pencils. If the price of blue pencils falls to 4 cents a piece,
she would consume 25 blue pencils, a change which is entirely due to the
income effect.
8.2. Yes.
8.3. Then the income effect would cancel out. All that would be left would
be the pure substitution effect, which would automatically be negative.
8.4. They are receiving tx in revenues and paying out tx, so they are losing
money.
8.5. Since their old consumption is affordable, the consumers would have to
be at least as well-off. This happens because the government is giving them
back more money than they are losing due to the higher price of gasoline.
9 Buying and Selling
9.1. Her gross demands are (9, 1).
9.2. The bundle (y1 , y2 ) = (3, 5) costs more than the bundle (4, 4) at the
current prices. The consumer will not necessarily prefer consuming this
A16 ANSWERS
bundle, but would certainly prefer to own it, since she could sell it and
purchase a bundle that she would prefer.
9.3. Sure. It depends on whether she was a net buyer or a net seller of the
good that became more expensive.
9.4. Yes, but only if the U.S. switched to being a net exporter of oil.
9.5. The new budget line would shift outward and remain parallel to the
old one, since the increase in the number of hours in the day is a pure
endowment effect.
9.6. The slope will be positive.
10 Intertemporal Choice
10.1. According to Table 10.1, $1 20 years from now is worth 3 cents today
at a 20 percent interest rate. Thus $1 million is worth .03 × 1, 000, 000 =
$30, 000 today.
10.2. The slope of the intertemporal budget constraint is equal to −(1 + r).
Thus as r increases the slope becomes more negative (steeper).
10.3. If goods are perfect substitutes, then consumers will only purchase the
cheaper good. In the case of intertemporal food purchases, this implies that
consumers only buy food in one period, which may not be very realistic.
10.4. In order to remain a lender after the change in interest rates, the
consumer must be choosing a point that he could have chosen under the
old interest rates, but decided not to. Thus the consumer must be worse
off. If the consumer becomes a borrower after the change, then he is choosing a previously unavailable point that cannot be compared to the initial
point (since the initial point is no longer available under the new budget
constraint), and therefore the change in the consumer’s welfare is unknown.
10.5. At an interest rate of 10%, the present value of $100 is $90.91. At a
rate of 5% the present value is $95.24.
11 Asset Markets
11.1. Asset A must be selling for 11/(1 + .10) = $10.
11.2. The rate of return is equal to (10, 000 + 10, 000)/100, 000 = 20%.
ANSWERS
A17
11.3. We know that the rate of return on the nontaxable bonds, r, must be
such that (1 − t)rt = r, therefore (1 − .40).10 = .06 = r.
11.4. The price today must be 40/(1 + .10)10 = $15.42.
12 Uncertainty
12.1. We need a way to reduce consumption in the bad state and increase
consumption in the good state. To do this you would have to sell insurance
against the loss rather than buy it.
12.2. Functions (a) and (c) have the expected utility property (they are
affine transformations of the functions discussed in the chapter), while (b)
does not.
12.3. Since he is risk-averse, he prefers the expected value of the gamble,
$325, to the gamble itself, and therefore he would take the payment.
12.4. If the payment is $320 the decision will depend on the form of the
utility function; we can’t say anything in general.
12.5. Your picture should show a function that is initially convex, but then
becomes concave.
12.6. In order to self-insure, the risks must be independent. However, this
does not hold in the case of flood damage. If one house in the neighborhood
is damaged by a flood it is likely that all of the houses will be damaged.
13 Risky Assets
13.1. To achieve a standard deviation of 2% you will need to invest x =
σx /σm = 2/3 of your wealth in the risky asset. This will result in a rate of
return equal to (2/3).09 + (1 − 2/3).06 = 8%.
13.2. The price of risk is equal to (rm − rf )/σm = (9 − 6)/3 = 1. That
is, for every additional percent of standard deviation you can gain 1% of
return.
13.3. According to the CAPM pricing equation, the stock should offer an
expected rate of return of rf + β(rm − rf ) = .05 + 1.5(.10 − .05) = .125 or
12.5%. The stock should be selling for its expected present value, which is
equal to 100/1.125 = $88.89.
Choices Involving Risk
11
Learning Objectives
After reading this chapter, students should be able to:
} Define and measure economic risk.
} Illustrate an individual’s risk preferences graphically.
} Explain why people purchase insurance policies.
} Analyze why people take certain risks and avoid others.
} Identify and explain several strategies for managing risk.
E
arly in 1996, two graduate students at Stanford University, Larry Page and Sergey
Brin, began developing a new technology for retrieving information from huge databases. With no business experience to speak of, they tried but failed to convince
investors of their technology’s commercial potential. Reluctantly, they set up operations in
Page’s dorm room, where they cobbled together a patchwork data center made of surplus
computer memory acquired at bargain prices. Over the next two years, they managed to
raise just under $1 million from interested investors,
which they used to launch a company. By September
1998, Google Inc. was officially open for business. Six
years later, on August 19, 2004, the company held an initial public offering (IPO) in which for the first time members of the general public were allowed to buy shares.
Page and Brin both collected more than $40 million in
cash, retaining shares valued at more than $3.2 billion
each! They (and their investors) had taken a big chance—
one that had paid off beyond their wildest expectations.
During the dot-com mania of the late 1990s, thousands of bright young entrepreneurs with clever ideas
launched risky new ventures. Unlike Page and Brin,
most came up empty handed. A grocery delivery service Google, Inc. founders Larry Page and Sergey Brin
365
366
Part II Economic Decision Making
called Webvan went belly-up in 2001 with accumulated losses of $830
million. Clothing outlet Boo.com blew through $225 million of investors’ money, only to be acquired for well under $1 million by Fashionmall.com. Web sites like “The Museum of E-Failure” were created just
to keep track of the duds. Some of them, like “Dot-Com Flop Tracker,”
flopped themselves. For an investor, the trick was to distinguish the
Googles from the Webvans. Many of those who guessed right became
wealthy; others lost their shirts. Still others watched from the sidelines,
unwilling to gamble their hard-earned savings on such risky prospects.
Though most people aren’t dot-com entrepreneurs, everyone takes
risks. Obviously, buying a lottery ticket is a risky proposition, as are
going to college (since jobs aren’t guaranteed), lending money to a
© The New Yorker Collection 2001 Mick Stevens from
friend (since default is possible), and driving a car (since accidents do
cartoonbank.com. All Rights Reserved.
happen). Risk is the rule, not the exception.
In this chapter, we’ll examine four topics related to risky economic decisions.
1. What is risk? Common sense tells us when a decision involves risk. Economics and
statistics show us how to gauge the amount of risk with some precision.
2. Risk preferences. In Chapter 4, we learned how to describe an individual’s consumption
preferences. Here, we’ll apply the same concepts to situations in which the outcome
is uncertain.
3. Insurance. Much of life is unavoidably risky. People address a wide range of financial risks by purchasing insurance policies. We’ll see why insurance makes them
better off.
4. Other methods of managing risk. Most people avoid some risks but voluntarily accept
others. For example, they reduce risk by purchasing insurance, but then introduce
new risks by investing in the stock market. We’ll see why doing so makes sense. We’ll
also study the steps people take to moderate the risks they accept.
11.1
WHAT IS RISK?
Risk exists whenever the consequences of a decision are uncertain. In this section, we’ll
explain how economists analyze risks and gauge their magnitude.
Possibilities
A state of nature is one
possible way in which
events relevant to a risky
decision can unfold.
ber00279_c11_365-401.indd 366
The consequences of any risky decision depend on events outside the decision maker’s
control. Usually, events can unfold in many different ways. Economists and statisticians
refer to each possible unfolding of events as a state of nature, or state for short.
To illustrate, suppose that Alberto is trying to decide whether to buy tickets to a
baseball game. His enjoyment of the game will depend on two uncertain events that are
beyond his control: whether it rains and whether his team wins. He will be happiest if it
doesn’t rain and his team wins. That is one possible state of nature.
To analyze a risky decision, economists begin by describing every possible state of
nature. In Alberto’s case, events can unfold in four different ways: it rains and his team
wins; it rains and his team loses; it doesn’t rain and his team wins; or it doesn’t rain and
his team loses. Each item on this list is a state of nature.
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Chapter 11 Choices Involving Risk
367
Once someone makes a choice, he experiences one and only one state of nature.
For example, Alberto will experience one of the four states listed in the last paragraph,
because we haven’t left out any possibilities. He can’t experience more than one state,
however, because we’ve described each one in a way that rules out the others.
Probability
Some states of nature are more likely than others. For example, if Alberto lives in San
Diego (where it rarely rains during baseball season), states of nature involving rain are
relatively unlikely.
Probability is a measure of the likelihood that a state of nature will occur. It’s usually
written either as a number between 0 and 1 or as a percentage. A probability of 0 (or 0%)
means that a state is impossible; a probability of 1 (or 100%) means that it’s certain. A
probability of, say, 3/4 (or 75%) means that the odds of the state in question occurring are
three out of four.
By adding the probabilities of two states of nature, we obtain the probability that one
of those two states will occur. For example, let’s suppose the odds are 3 in 10 (30%) that
it will rain and Alberto’s team will win, and 4 in 10 (40%) that it will rain and Alberto’s
team will lose. Then the odds of it raining (with Alberto’s team either winning or losing)
must be 7 in 10 (70% 30% 40%). The probabilities of all states of nature always add
up to 1 (or 100%), because it’s certain that something will happen.
Sometimes we can measure the probability of a state of nature by determining the
frequency with which it has occurred in the past, under comparable conditions. Such
measures are known as objective probabilities. For example, if we flip a coin thousands
of times, we’ll find that it comes up heads and tails with roughly equal frequency. Thus,
we conclude that the probability of each outcome is the same—in each case, 1/2 (or 50%).
Likewise, Alberto might estimate the probability of each state of nature based on the historical frequency of rain and his team’s won-lost record.
Sometimes we can measure the probability of a state of nature, at least in part, by
exercising our own judgment. While this type of assessment, known as subjective probability, may be informed by facts, it may also reflect “instinct” or a “gut feeling.” Naturally, different people may come to very different subjective judgments. For example, if
the Yankees play the Giants in the World Series, their fans may harbor radically different
views concerning the likely outcome, even based on the same information (see Application 11.2 on page 380).
Probability is a measure of
the likelihood that a state of
nature will occur.
An objective probability is
a measure of the likelihood
that a state of nature
will occur based on the
frequency with which it has
occurred in the past, under
comparable conditions.
A subjective probability is
a measure of the likelihood
that an event will occur
based on subjective
judgment.
Uncertain Payoffs
Risky choices often have financial consequences, also known as payoffs. Payoffs can be
either positive (gains) or negative (losses). To evaluate a choice, we need to know the
probability distribution of the payoffs—that is, the likelihood of each possible payoff
occurring. Like other consequences, payoffs depend on unfolding events, in other words,
on the state of nature. As long as we know the probability of each possible state of nature,
we can determine the probability of each possible payoff.
Here’s an example. The managers of New Stuff, Inc., are thinking about building a
factory to make a new product called a thingamajig. A competitor has already started producing a similar product and has filed for patent protection. If approved, the patent would
prevent New Stuff from making thingamajigs. In addition, the market for thingamajigs is
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The probability distribution
of a set of payoffs tells us
the likelihood that each
possible payoff will occur.
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Part II Economic Decision Making
untested. Eventually, demand will turn out to be either high or low. If it’s low, New Stuff
will have to shut its factory down, whether or not the competitor’s patent is approved.
In this example, there are four possible states of nature: the competitor’s patent is
approved and demand is low; the competitor’s patent is approved and demand is high;
the competitor’s patent is not approved and demand is low; or the competitor’s patent is
not approved and demand is high. Based on both objective evidence and subjective judgment, New Stuff ’s managers arrive at the probabilities shown in the second column of
Table 11.1.
Unsure of their best course of action, the managers have drawn up two sets of plans,
one for a large factory and the other for a small one. If the competitor’s patent is not
approved and demand is high, New Stuff expects to make $8.5 million from the large
factory and $4.5 million from the small one, net of investment costs. With all other states
of nature, they expect to lose their investment ($1.5 million for the large factory and $0.5
million for the small one). All of these figures refer to net present values, defined in Section 10.3. New Stuff ’s payoffs are shown in the third and fourth columns of Table 11.1.
Table 11.2 shows the probability distributions of the payoffs from the two factories.
(For the moment, ignore the last column in the table, as well as the rows labeled expected
Table 11.1
States of Nature, Probabilities, and Payoffs for New Stuff’s
Thingamajig
Payoff (NPV, $ million)
State of Nature
Probability
Large Factory
Small Factory
Patent approved, low demand
Patent approved, high demand
Patent not approved, low demand
Patent not approved, high demand
30%
20
30
20
1.5
1.5
1.5
8.5
0.5
0.5
0.5
4.5
Table 11.2
Expected Value of New Stuff’s Profits on the Thingamajig
Large Factory
Payoff ($)
8,500,000
1,500,000
Probability
20%
80
Expected payoff
Probability Payoff
1,700,000
1,200,000
500,000
Small Factory
Payoff ($)
4,500,000
500,000
Expected payoff
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Probability
20%
80
Probability Payoff
900,000
400,000
500,000
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Chapter 11 Choices Involving Risk
369
payoff.) With the large factory, New Stuff receives a payoff of $8.5 million with 20 percent probability, which is the probability that the competitor’s patent is not approved and
demand is high, and a payoff of $1.5 million with 80 percent probability, which is the
total probability of all other states of nature. Similarly, with the small factory, New Stuff
receives a payoff of $4.5 million with 20 percent probability and a payoff of $0.5 million with 80 percent probability (as shown in the lower half of Table 11.2).
Histograms can help us to visualize probability distributions. The one in Figure
11.1(a) summarizes the possible consequences of New Stuff ’s large factory. The horizontal axis measures the payoff, and the vertical axis measures probability. The taller of the
two red bars tells us that New Stuff loses $1.5 million (the horizontal coordinate) with
80 percent probability (the vertical coordinate). The shorter bar tells us that the company
gains $8.5 million (the horizontal coordinate) with 20 percent probability (the vertical
coordinate). The histogram in Figure 11.1(b) summarizes the smaller factory’s possible
consequences. The two blue bars tell us that New Stuff loses $0.5 million with 80 percent
probability and gains $4.5 million with 20 percent probability.
To evaluate a choice with risky financial prospects, we usually begin with two simple
questions. First, what do we expect to gain or lose, on average? Second, do we expect the
actual gain or loss to be close to that average or far from it? We’ll tackle these two questions in turn.
Figure 11.1
(a) Large factory
Probability (%)
80
20
–1.5
0
8.5
Possible Consequences of
New Stuff’s Investment in the
Thingamajig. New Stuff’s large factory [figure (a)] results in a loss of $1.5
million with 80 percent probability and
a gain of $8.5 million with 20 percent
probability. The small factory [figure (b)]
results in a loss of $0.5 million with 80
percent probability and a gain of $4.5
million with 80 percent probability.
Payoff (million $)
(b) Small factory
Probability (%)
80
20
–0.5 0
4.5
Payoff (million $)
ber00279_c11_365-401.indd 369
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Part II Economic Decision Making
Expected Payoff
The expected payoff of a
risky financial choice is a
weighted average of all the
possible payoffs, using the
probability of each payoff as
its weight.
To determine the average gain or loss from a risky financial choice, we can calculate its
expected payoff. The expected payoff is a weighted average of all the possible payoffs,
using the probability of each payoff as its weight.
To illustrate, suppose Ilya and Napoleon bet $5 on the flip of a coin. If the outcome is
heads (H), Napoleon pays Ilya $5; if tails (T), Ilya pays Napoleon $5. We compute Ilya’s
expected payoff as follows:
Ilya’s expected payoff (Probability of H) (Ilya’s payoff if H) (Probability of T) (Ilya’s payoff if T)
(1/2) (5) (1/2) (5) 0
Using the same logic, we see that Napoleon’s expected payoff is also zero.
More generally, suppose the payoff from a risky choice can take on one of N different values, P1, P2, . . . , PN, and that it turns out to be Pn with the probability n (the
Greek letter pi). We can calculate the expected payoff, abbreviated EP, using the following formula:
EP 1 P1 2 P2 ... N PN
(1)
The final column of Table 11.2 applies this formula to New Stuff ’s proposed factories. As
indicated, the two factories have exactly the same expected payoff, $500,000.
Notice that in these examples, the actual payoff never equals the expected payoff. In
general, the expected payoff from a risky choice is not necessarily the most likely outcome, and need not even be remotely likely. Rather, it’s the amount you would earn, on
average, if you were to make the same risky choice many times. To illustrate, take Ilya and
Napoleon’s coin flip. Were they to make the same bet, say, 10,000 times, the frequency of
heads and tails would be very close to 50 percent, so their average payoff would be very
close to zero.
Application 11.1
The Development of New Drugs
P
harmaceutical manufacturers have drawn widespread
criticism by charging high prices for life-saving drugs.
For example, the drugs used to treat HIV, the virus that
causes AIDS, can cost a patient tens of thousands of
dollars a year, placing treatment beyond the reach of many,
including millions in third world countries. Critics point out
that manufacturers could make these drugs available at
lower prices and still cover their costs of production.
The companies respond that drug prices must be high
enough to cover the costs not only of production, but of
development, including expensive clinical trials. Otherwise,
they would have no incentive to develop new drugs. In
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evaluating the cost of development, they emphasize, it’s
essential to account for the fact that success isn’t assured.
Only a minority of experimental drugs eventually win approval
by the Food and Drug Administration (FDA) and only after
clinical testing. Companies won’t develop new drugs unless,
on average, their profits cover their development costs.
How much does the typical approved drug need to
earn to cover a company’s development costs? Let’s say
that a company has to spend an average of $50 million on
development costs for each experimental drug, and that
the probability of approval by the FDA is 20 percent. If
each approved drug generates A dollars in profits (ignoring
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Chapter 11 Choices Involving Risk
development costs), then using formula (1), the expected
payoff from each experimental drug is:
Expected payoff (0.2 $A 0.8 $0) $50 million
Setting A 250 million, we find that the expected payoff
is exactly zero. So on average, a successful drug has to
generate a profit of at least $250 million after its approval to
justify the company’s development of experimental drugs.
Merely covering the $50 million in development costs for the
approved drug isn’t enough.
Experts disagree sharply about the costs of drug
development. According to one controversial study by
economists Joseph DiMasi, Ronald Hansen, and Henry
Grabowski, the average successful drug must generate
$802 million in profits after the FDA’s approval to cover a
company’s development costs.1 Other studies have arrived
at lower figures.2
Variability
Economists gauge financial risk by measuring the variability of gains and losses. Roughly
speaking, variability is low when the range of likely payoffs is narrow, and high when
the range is wide. With little variability, the actual payoff is almost always close to the
expected payoff. With substantial variability, the two amounts often differ significantly.
Histograms can help us to visualize variability. For example, look again at Figure
11.1, which summarizes the possible financial consequences of New Stuff ’s proposed
factories. Notice that the blue bars in Figure 11.1(b) are closer together—and closer to
the expected payoff of $0.5 million—than the red bars in Figure 11.1(a). That means the
payoffs from the small factory are less variable than the payoffs from the large one.
The difference between the actual payoff and the expected payoff is called a deviation. Greater variability is associated with larger deviations. Figures 11.2(a) and (b) illustrate the possible deviations from the expected payoff for New Stuff ’s proposed factories.
We’ve indicated the expected payoff for each of the proposed factories, $0.5 million, on
the horizontal axes. As Figure 11.2(a) shows, for the large factory there is an 80 percent
chance that the deviation will be $2 million (the horizontal distance between the tall
red bar and a vertical line drawn at the expected payoff), and a 20 percent chance that the
deviation will be $8 million (the horizontal distance between the short red bar and a
vertical line drawn at the expected payoff). In contrast, for the small factory there is an 80
percent chance that the deviation will be $1 million, and a 20 percent chance that it will
be $4 million.
Often, economists measure the variability of a risky financial payoff by calculating
either its variance or its standard deviation. The variance is the expected value of a squared
deviation. The calculation of variance involves three steps: first, compute the deviations
for all possible outcomes; second, square each deviation; third, find the weighted average
of all the possible squared deviations, using the probability of each squared deviation as
its weight. The standard deviation is the square root of the variance. Both the variance
and the standard deviation tell us something about the size of the typical deviation.
The variability of payoffs is
an indication of risk. With
little variability, the actual
payoff is almost always
close to the expected
payoff. With substantial
variability, the two amounts
often differ significantly.
A deviation is the difference
between the actual payoff
and the expected payoff.
The variance is the
expected value of a squared
deviation.
The standard deviation
is the square root of the
variance.
1Joseph
A. DiMasi, Ronald W. Hansen, and Henry G. Grabowski, “The Price of Innovation: New Estimates of Drug Development
Costs,” Journal of Health Economics 22, March 2003, pp. 151–185.
2For example, a study by the Global Alliance for TB Drug Development (The Economics of TB Drug Development, October 2001)
concludes that the average successful tuberculosis drug must generate between $115 and $250 million in profits to cover development
costs.
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Part II Economic Decision Making
Figure 11.2
(a) Large factory
Expected Payoff and Deviations
for New Stuff’s Investment in the
Thingamajig. The expected payoff
from each of New Stuff’s proposed
factories is $0.5 million. However,
the histograms show that the large
factory’s payoff is more variable—its
distribution is wider, and the deviations
are larger.
Deviation 2
80
Probability (%)
372
Deviation 8
20
–1.5
8.5
0 0.5
Payoff (million $)
(b) Small factory
Deviation 1
Probability (%)
80
Deviation 4
20
–0.5 0 0.5
4.5
Payoff (million $)
Expected
payoff
Table 11.3 calculates the variance and standard deviation of the payoffs for each of
New Stuff ’s proposed factories. For the large factory, the variance is 16 trillion dollars
squared, and the standard deviation is $4 million. For the small factory, the variance is 4
trillion dollars squared, and the standard deviation is $2 million. These numbers confirm
what we saw in Figure 11.2; the payoffs for the large factory are much more variable than
the payoffs for the small one.
11.2
RISK PREFERENCES
Fortunately, we don’t need to develop an entirely new theory to analyze decisions involving risk. We can simply apply the basic theory of consumer decision making that we
learned in Chapters 4 through 6.
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Chapter 11 Choices Involving Risk
373
Table 11.3
Variability of New Stuff’s Profits on the Thingamajig
Large Factory
Probability
Deviation
(million $)
Squared Dev.
(trillion $2)
Probability Squared Dev.
(trillion $2)
20%
80
8
2
64
4
12.8
3.2
16
4
Probability
Deviation
(million $)
Squared Dev.
(trillion $2)
Probability Squared Dev.
(trillion $2)
20%
80
4
1
16
1
3.2
0.8
4
2
Variance (trillion $2)
Standard dev. (million $)
Small Factory
Variance (trillion $2)
Standard dev. (million $)
Consumption Bundles
In Section 10.2 we learned how to apply that theory to decisions involving time. The key
was to think of a consumption bundle as a list of the quantities of each good consumed
at each point in time. (If you skipped Chapter 10, take a moment to read the short section titled the Timing of Consumption, which begins on page 338.) We can handle risk in
much the same way. That is, we can think of a consumption bundle as a list of the quantities of each good consumed in each possible state of nature.
To illustrate, let’s start with a simple problem. A consumer, whom we’ll call Maria, is
uncertain about the weather. There are only two possible states of nature, “sun” and “hurricane.” We’ll assume that Maria is familiar with the probability of each state from past
experience.
To keep the analysis simple, we’ll assume that Maria consumes only one good, called
“food,” which is measured in kilograms. Maria needn’t consume the same amount of food
when it’s sunny and when there’s a hurricane. Indeed, if a hurricane were to destroy some
of her property, forcing her to spend some of her money on repairs, she would most likely
consume less. Therefore, her consumption bundle must list both the amount of food she
eats when it’s sunny and the amount she eats when there’s a hurricane.
Figure 11.3 illustrates Maria’s potential consumption bundles. The horizontal axis
measures her consumption if it’s sunny, and the vertical axis measures her consumption
if there’s a hurricane. Each point on the graph corresponds to a distinct alternative. For
example, point B represents the consumption bundle consisting of 600 kg of food if it’s
sunny and 300 kg of food if there’s a hurricane.
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Part II Economic Decision Making
Figure 11.3
1,500
Food if hurricane (kg)
Consumption Bundles. Each point
on this graph corresponds to a different risky alternative. At point B, for
example, Maria eats 600 kg of food
if it’s sunny and 300 kg if there’s a
hurricane. For points like A on the 45degree line (also called the guaranteed
consumption line), she eats the same
amount, rain or shine. Maria’s expected
consumption is the same (500 kg) for
all points on the green line (also called
a constant expected consumption line).
As Maria moves away from point A in
either direction along the green line,
variability (risk) increases.
Constant expected
consumption line
(slope 2)
Guaranteed
consumption
line
Increasing
variability
(risk)
700
C
500
A
300
Increasing
variability
(risk)
B
400 500 600
750
Food if sunny (kg)
Guaranteed Consumption
For some bundles, the level of consumption is guaranteed in the sense that it does not depend on the state of nature. In Figure 11.3, those
bundles all lie along the 45-degree line through the origin, also known as the guaranteed consumption line. Notice, for example, that point A provides Maria with the same
amount of food, 500 kg, if it’s sunny and if there’s a hurricane. At all points below the
guaranteed consumption line, like point B, Maria eats more food if it’s sunny; at all points
above that line, like point C, she eats more food if there’s a hurricane.
The guaranteed
consumption line shows
the consumption bundles
for which the level of
consumption does not
depend on the state of
nature.
Expected Consumption and Variability
For bundles that don’t lie on the guaranteed consumption line in Figure 11.3, Maria’s payoff (the amount of food she consumes)
is uncertain. Given any particular bundle, we can compute her expected consumption
(abbreviated EC) by applying formula (1). Let’s use FS to stand for the amount of food she
consumes if it’s sunny, FH for the amount she consumes if there’s a hurricane, and for
the probability of sun (so that the probability of a hurricane is 1 ). Then
EC FS (1 ) FH
(2)
2/
For example, if the probability of sun is 3, FS is 600 kg, and FH is 300 kg, then expected
consumption is 500 kg.
Many of the bundles in Figure 11.3 share the same level of expected consumption.
Which ones? Let’s rearrange formula (2) as follows:3
FH 5
A constant expected
consumption line shows
all the risky consumption
bundles with the same level
of expected consumption.
(3)
Formula (3) implies that risky consumption bundles with the same level of expected
consumption (that is, the same value of EC) lie along a straight line with a slope of
/(1 ). We call this a constant expected consumption line.
3To
ber00279_c11_365-401.indd 374
EC
P
2 a
bF
12P
12P S
obtain formula (3) from formula (2), first subtract FS from both sides, and then divide through by 1 .
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Chapter 11 Choices Involving Risk
To illustrate, let’s suppose that 2/3. For all of the bundles on the green line in
Figure 11.3, expected consumption is 500 kg. Therefore, this is a constant expected consumption line. Notice that the slope of this line is /(1 ) 2. The line would be
steeper for a higher value of and flatter for a lower value.
Although every point on the green line in Figure 11.3 has the same level of expected
consumption, the variability of consumption differs. At point A, consumption doesn’t
depend on the weather, so there is no variability. As Maria moves away from point A in
either direction along the constant expected consumption line, her consumption becomes
increasingly variable. (Pick a point on this line and draw the histogram for the probability
distribution of Maria’s consumption. Notice that, as your chosen point moves farther away
from point A, the bars corresponding to sun and hurricane move farther apart.)
Preferences and Indifference Curves
If one consumption bundle guarantees more of every good than a second bundle, a consumer should prefer the first bundle to the second. This assumption reflects the More-IsBetter Principle, which we introduced in Chapter 4. Notice that a consumption bundle
does not have to guarantee a particular level of consumption to guarantee more consumption than a second bundle. For example, point D in Figure 11.4(a) guarantees a higher
level of food consumption than point B: with point D, consumption is higher in every state
of nature—both if it’s sunny (700 kg vs. 500 kg) and if there’s a hurricane (400 kg vs. 300
kg). Maria must therefore prefer point D to point B.
Assuming that more is better, we can illustrate Maria’s preferences for consumption
bundles by drawing indifference curves, exactly as we did in Chapter 4. For the reasons
Figure 11.4
Maria’s Preferences for Risky Consumption Bundles. The indifference curves show Maria’s preferences for risky consumption bundles. A high probability of a hurricane leads to relatively flat indifference curves, like the red ones shown in figure (a), and
a low probability leads to relatively steep indifference curves, like the blue ones shown in figure (b).
Guaranteed
consumption
line
C
A
500
400
300
D
B
500 700
Food if sunny (kg)
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(b) Low probability of hurricane
Food if hurricane (kg)
Food if hurricane (kg)
(a) High probability of hurricane
Guaranteed
consumption
line
500
400
A
D
500 700
Food if sunny (kg)
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Part II Economic Decision Making
discussed in Section 4.2, her indifference curves must be downward sloping lines, and
those that are farther from the origin must correspond to higher levels of well-being. For
example, the indifference curves in Figure 11.4(a) show that Maria prefers point C to
point A, and point A to point B. She is indifferent between points A and D. Given a choice
between these four alternatives, she’ll pick point C.
The slope of an indifference curve indicates the consumer’s willingness to shift
consumption from one state of nature to another. For example, the indifference curve that
runs through points A and D in Figure 11.4(a) implies that, starting from point D, Maria
is willing to give up 200 kg of food if it’s sunny in order to secure an additional 100 kg if
there’s a hurricane.
Usually, a consumer’s willingness to shift consumption from one state of nature to
another depends on the probabilities of those states. Therefore, a change in probabilities
changes the slopes of the consumer’s indifference curves. In Maria’s case, a high probability of a hurricane leads to relatively flat indifference curves, like the red ones shown in
Figure 11.4(a), and a low probability leads to relatively steep indifference curves, like the
blue ones shown in Figure 11.4(b). To understand why, suppose that Maria starts out with
point D. When the probability of a hurricane is high [Figure 11.4(a)], she is indifferent
between points A and D, and willing to swap 200 kg of food if it’s sunny for an additional
100 kg if there’s a hurricane. When the probability of hurricane falls, she becomes less
concerned about consumption if there’s a hurricane and more concerned about consumption if it’s sunny. Therefore, as shown in Figure 11.4(b), the same swap makes her worse
off; she prefers point D to point A. Since she now requires more than 100 kg of food if
there’s a hurricane to compensate for the loss of 200 kg of food if it’s sunny, her indifference curve is steeper. In other words, reducing the probability of a hurricane increases her
marginal rate of substitution for food if it’s sunny with food if there’s a hurricane.
The Concept of Risk Aversion
A person is risk averse if, in
comparing a riskless bundle
to a risky bundle with the
same level of expected
consumption, he prefers
the riskless bundle.
The certainty equivalent of
a risky bundle is the amount
of consumption which, if
provided with certainty,
would make the consumer
equally well off.
ber00279_c11_365-401.indd 376
Most people don’t like uncertainty. They pay money to eliminate or reduce risk—for example, by purchasing an insurance policy. They also avoid accepting a new risk unless they’re
adequately compensated. We say that a person is risk averse if, in comparing a riskless
bundle to a risky bundle with the same level of expected consumption, he prefers the riskless bundle. This definition captures the idea that, by itself, variability is a bad thing.
What does risk aversion imply about the shape of an indifference curve? Figure 11.5
reproduces the constant expected consumption line from Figure 11.3. This line crosses
the guaranteed consumption line at point A. Therefore, point A is the only riskless bundle
on the constant expected consumption line. If Maria is risk averse, she will prefer point A
to all other points on this line. This implies that one of her indifference curves lies tangent
to the constant expected consumption line at point A, and above all other points on that
line, as shown.
Risk-averse individuals do not avoid risk at all costs. Starting from a riskless position,
they are usually willing to accept some risk, provided that they receive adequate compensation in the form of higher expected consumption. For example, in Figure 11.5, Maria
will prefer bundle B to bundle A, even though bundle B involves more variability, and in
the event of a hurricane, lower consumption than bundle A.
Certainty Equivalents and Risk Premiums For risk-averse individuals, exposure
to risk reduces well-being. How do we measure that cost? To answer this question, we’ll
introduce the concept of a certainty equivalent. The certainty equivalent of a risky bun-
10/31/07 10:08:49 AM
Chapter 11 Choices Involving Risk
Figure 11.5
Food if hurricane (kg)
Constant expected
consumption line
Guaranteed
consumption
line
Indifference
curve
A
377
B
Risk Aversion. The constant expected
consumption line crosses the guaranteed consumption line at point A. Therefore, point A is the only riskless bundle
on the constant expected consumption
line. If Maria is risk averse, she prefers
point A to all other points on this line.
This implies that her indifference curve
lies tangent to the constant expected
consumption line at point A, and above
all other points on this line, as shown.
Food if sunny (kg)
Figure 11.6
Food if hurricane (kg)
Constant expected
consumption line
Risk
premium
for B
Guaranteed
consumption
line
Indifference
curves
A
C
B
Certainty
equivalent
of B
Certainty
equivalent
of B
Risk
premium
for B
The Certainty Equivalent and the
Risk Premium. The fixed level of food
consumption associated with point C
is the certainty equivalent of the risky
consumption bundle B. The risk premium
for bundle B is the difference between
the expected level of consumption at
B and B’s certainty equivalent. It’s also
the amount by which the consumer is
willing to reduce expected consumption
to eliminate all risks. Since the expected
level of consumption is the same at
points A and B, the risk premium for
bundle B equals the horizontal distance
(also the vertical distance) between
points C and A.
Food if sunny (kg)
dle is the amount of consumption which, if provided with certainty, would make the consumer equally well off. Figure 11.6 illustrates this concept. Let’s take any risky bundle,
like bundle B, and draw the consumer’s indifference curve through it. This curve intersects the 45-degree line at point C, which guarantees a fixed level of consumption. This
guaranteed level, which we can read from either the horizontal or vertical axis (as shown
in the figure), is the certainty equivalent of bundle B.
A risky bundle’s certainty equivalent tells us exactly what it’s worth to the consumer.
When faced with a choice between two risky bundles, the consumer always chooses the
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Part II Economic Decision Making
The risk premium of a risky
bundle is the difference
between its expected
consumption and the
consumer’s certainty
equivalent.
one with the higher certainty equivalent. So if one risky bundle is worth $5 for sure and
another is worth only $4 for sure, the first is plainly better than the second.
For a risk-averse individual, the certainty equivalent of a risky bundle is always less
than expected consumption. Why? Providing the same expected consumption with no risk
would clearly make the individual better off. In the figure, Maria prefers point A to point
B. To make Maria indifferent between bundle B and a riskless bundle, we have to reduce
her guaranteed consumption from the level associated with point A to the level associated
with point C.
The risk premium of a risky bundle is the difference between its expected consumption and the consumer’s certainty equivalent. In other words, it’s the amount by which the
consumer is willing to reduce expected consumption to eliminate all risk. In Figure 11.6,
the risk premium corresponds to the horizontal distance (equivalently, the vertical distance)
between point A (which has the same expected level of consumption as the risky bundle B)
and point C. The size of the risk premium reflects the psychological cost of risk.
WORKED-OUT PROBLEM
11.1
The Problem Suppose Maria’s indifference curves are given by the following
formula:
2
1
"FS 1 "FH 5 C
3
3
where C is a constant (the value of which differs from one indifference curve to
another). Maria receives 36 kg of food when it’s sunny and 81 kg when there’s a
hurricane. If the probability of sunshine, , equals 2/3, what is her expected food
consumption? What is the certainty equivalent of this risky bundle? What is the risk
premium?
The Solution Expected consumption is 2/3FS 1/3FH 2/3 36 1/3 81
51. To compute the certainty equivalent, we need to find a level of guaranteed
food consumption, F, that places Maria on the same indifference curve as the risky
bundle:
2
1
2
1
"36 1 "81 5 "F 1 "F
3
3
3
3
Simplifying, we get 4 3 !F, or F 49. So the certainty equivalent is 49 kg.
Since 51 49 2, the risk premium is 2 kg.
IN-TEXT EXERCISE 11.1
Repeat the calculation in worked-out problem 11.1
for each of the following risky consumption bundles.
(a) 81 kg when it’s sunny, and 36 kg when it rains.
(b) 225 kg when it’s sunny, and 144 kg when it rains.
(c) 9 kg when it’s sunny, and 900 kg when it rains.
Degrees of Risk Aversion
Some people need relatively little encouragement to take
a risk. Others prefer safe alternatives unless the cards are heavily stacked in their favor.
What do these differences imply about the shapes of their indifference curves?
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Chapter 11 Choices Involving Risk
Figure 11.7
Constant expected
consumption line
Food if hurricane (kg)
379
Guaranteed
consumption
line
A
C
D
B
Arnold
Maria
Food if sunny (kg)
Degrees of Risk Aversion. The red indifference curve belongs to Maria; the blue one
belongs to Arnold. Maria is less risk averse
than Arnold. Neither would trade the riskless
consumption bundle A for risky alternatives
that lie below both of their indifference curves,
like bundle B. Both would trade bundle A for
risky alternatives above both of their indifference curves, like bundle C. However, Maria
will trade bundle A for risky bundles between
the two indifference curves, like bundle D,
while Arnold will not.
Figure 11.7 reproduces Maria’s indifference curve (shown in red), along with an
indifference curve for another consumer, Arnold (shown in blue). Since Maria and Arnold
are both risk averse, both curves lie tangent to the constant expected consumption line
at point A. However, Arnold’s curve bends more sharply at the 45-degree line, and it lies
above Maria’s everywhere else. That means Arnold is more risk averse than Maria, starting from point A.
To understand why, notice that the two indifference curves divide the set of possible
alternatives into three parts. First, there are points below Maria’s indifference curve, like
point B. Neither Maria nor Arnold will swap bundle A for any risky bundle in this part
of the figure. Second, there are points above Arnold’s indifference curve, like point C.
Both Maria and Arnold will swap bundle A for any risky bundle in this part of the figure.
Finally, there are points between the two indifference curves, like point D. Maria will
exchange bundle A for a risky bundle in this part of the figure, but Arnold will not. Since
Maria is willing to take on a wider range of risk than Arnold, she’s less risk averse.
Greater risk aversion implies that for any risky bundle, the certainty equivalent is
lower and the risk premium higher than it is for less risk-averse consumers. In other
words, the psychological cost of risk exposure is greater for more risk-averse consumers.
As an example, take point D in Figure 11.7. Since Maria prefers point D to point A, her
certainty equivalent for point D must be larger than the level of consumption that point A
guarantees. Since Arnold prefers point A to point D, his certainty equivalent for point D
must be smaller than the level of consumption that point A guarantees. Maria’s certainty
equivalent is therefore larger than Arnold’s, and her risk premium is smaller.
Alternatives to Risk Aversion
Some people actually like to take risks, at least in some situations. We say that a person
is risk loving if, in comparing a riskless bundle to a risky bundle with the same level of
expected consumption, he prefers the risky bundle. This definition captures the idea that
variability is in itself a good thing. A risk-loving individual will voluntarily accept higher
variability even if it isn’t associated with a higher level of expected consumption.
What would Maria’s indifference curves look like if she were risk loving? Figure 11.8
reproduces the constant expected consumption line from Figure 11.3. Recall that point A
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A person is risk loving if, in
comparing a riskless bundle
to a risky bundle with the
same level of expected
consumption, he prefers
the risky bundle.
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Part II Economic Decision Making
Figure 11.8
Food if hurricane (kg)
A Risk Loving Consumer. As before,
point A is the only riskless bundle on
the constant expected consumption
line. If Maria is risk loving, she must
prefer all other points on this line to
point A. This implies that her indifference curve lies tangent to the constant
expected consumption line at point A
and below all other points on this line,
as shown.
Constant expected
consumption line
Guaranteed
consumption
line
A
Food if sunny (kg)
A person is risk neutral if
he is indifferent between all
bundles with the same level
of expected consumption.
is the only riskless bundle on this line. If Maria is risk loving, she will prefer every other
point on this line to point A. This implies that one of her indifference curves lies tangent
to the constant expected consumption line at point A, and below all other points on that
line, as shown.
Conceivably, some consumers may not care about risk one way or the other. As a
result, they are indifferent between all bundles with the same level of expected consumption. In comparing two risky bundles, they simply select the one with the highest level of
expected consumption. The indifference curves of these risk neutral consumers therefore coincide with the constant expected consumption lines.
Application 11.2
Betting on Sports
T
hough sports betting is illegal in 48 states, it is widespread.
According to some estimates, Americans may have
wagered as much as $380 billion on sporting events in 1999.4
This figure is expected to rise with the explosive growth of
Internet gambling.
Why is sports betting so popular? Evidence shows that
most people are risk averse: for example, they buy insurance.
Two risk-averse individuals will never bet against each other
if they agree on the probabilities of the outcomes. Being risk
averse, neither will bet unless he thinks the wager yields a
positive expected payoff. But since their payoffs always add
up to zero (one wins what the other loses), any wager that
has a positive expected payoff for one person has a negative
expected payoff for the other.
In contrast, if two risk-averse people disagree about the
probabilities of the outcomes, they may want to bet against
each other. Let’s say Germany is playing Brazil for the
world soccer championship. Helga and Luiz disagree about
Germany’s likelihood of winning; Helga thinks it’s 70 percent,
while Luiz thinks it’s 30 percent. What if they bet $100 on the
outcome, with Helga taking Germany and Luiz taking Brazil?
From Helga’s perspective, her expected payoff is $40 (since
4This
estimate appears in the 1999 Final Report of the National Gambling Impact Study Commission. Of course, since most gambling is unreported, the true figure is unknown.
Other estimates place total sports betting as low as $80 billion—still a hefty sum.
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Chapter 11 Choices Involving Risk
0.7 100 0.3 100 40). From Luiz’s perspective, his
expected payoff is also $40. Both think they’re getting a
positive expected payoff.
Is this a good explanation for sports betting? Not
necessarily. It begs the question of why Helga and Luiz
disagree about the likely outcome. Certainly, they could
come to different objective conclusions if they had different
information. However, two rational, risk-averse people
should never be willing to wager with each other for that
reason alone.
To see why, let’s change the example. Suppose Helga
and Luiz agree that Germany has a 30 percent chance of
beating Brazil. The day after the game, Helga picks up the
sports section of the newspaper while having coffee with
Luiz and reads that, despite the odds, Germany has won.
Assuming that Luiz hasn’t yet heard the result, they now have
different beliefs that reflect different information. Even so,
Luiz should refuse to take any gamble that Helga proposes.
Suppose she says, “I’ll bet you $100 that Germany beat Brazil
381
yesterday.” Her willingness to make this bet after reading
the paper should convince Luiz that Germany did indeed
win. By proposing the gamble, Helga effectively reveals her
information, thereby eliminating the difference in beliefs that
might have sustained a wager.
Even after considering what they might learn from the
other’s willingness to make the bet, Helga and Luiz could
both think they’re getting a better-than-fair deal if they start
out with different subjective beliefs. Maybe Helga has a gut
feeling that Germany will win, or Luiz is feeling lucky. In other
words, differences in feelings—unlike differences in hard
information—could explain sports betting.5
Of course, many people also gamble for entertainment.
For example, some think that placing a bet heightens the
excitement they feel when watching a game. But while that
is certainly part of the explanation, it isn’t the whole story.
People also place bets on the outcomes of games they don’t
watch, and many regular gamblers are convinced they can
consistently pick winners.
Expected Utility
In Chapter 4 we learned that it’s possible to represent a consumer’s indifference curves
with a utility function, which assigns a numerical index of well-being to each consumption bundle. In Maria’s case, a consumption bundle consists of the amount of food she
eats if it’s sunny, FS, and the amount of food she eats if there’s a hurricane, FH. Thus, we
can represent her indifference curves, like the ones in Figure 11.4(a) or (b), with a utility
function of the form U(FS, FH).
When we write a utility function like U(FS, FH), we are allowing for a very wide
range of possibilities. That flexibility may make it difficult to determine consumers’ preferences by observing their choices. To narrow down the range of possibilities, economists
usually make some additional assumptions.
Expected Utility Functions Let’s assume that the benefit Maria derives from food
depends only on the amount she eats, and not on the weather. The function W(F) will tell
us the size of that benefit. When it’s sunny, Maria eats FS kilograms and receives a benefit of W(FS). When there’s a hurricane, she eats FH kilograms and receives a benefit of
W(FH). It is then natural to assume that Maria’s utility is simply her expected benefit:
U(FS, FH) W(FS) (1 ) W(FH)
(4)
(Recall that is the probability of sun and 1 is the probability of a hurricane.)
Formula (4) is an example of an expected utility function.6 It assigns a benefit
level to each possible state of nature based only on what is consumed, and then takes the
5Another
possibility is that each thinks the other may not understand the implications of the information they possess.
6More
than 60 years ago, a mathematician named John von Neumann and an economist named Oskar Morgenstern proved an important theorem concerning expected utility: as long as someone’s choices satisfy a few simple assumptions (also known as the von
Neumann-Morgenstern axioms), they will behave as if they maximize an expected utility function when making choices involving
risk. Many economists think von Neumann and Morgenstern’s assumptions are reasonable; others disagree. We describe some alternatives in Section 13.4.
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An expected utility function
assigns a benefit level
to each possible state of
nature based only on what
is consumed, and then takes
the expected value of those
benefits.
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Part II Economic Decision Making
expected value of those benefits. In other words, it’s a weighted average of all the possible
benefit levels, using the probability of each benefit level as its weight.
What if Maria’s enjoyment of food depends on the weather? For example, what if she
tends to have a larger appetite when it’s sunny (perhaps because she gets more exercise)?
In that case, we may not be able to represent her indifference curves with an expected utility function like formula (4). Most (but certainly not all) economists nevertheless think
that the assumptions behind expected utility functions are reasonably accurate in most
circumstances.
WORKED-OUT PROBLEM
11.2
The Problem Suppose we can represent Maria’s preferences with an expected
utility function, and that W(F) !F. Suppose also that the probability of sun is 2/3,
and the probability of a hurricane is 1/3. Plot the indifference curve that runs through
the point (FS, FH) (400, 400). Plot the constant expected consumption line that runs
through this same point. Is Maria risk averse, risk loving, or risk neutral?
The Solution Maria’s expected utility function is
2
1
U 1 FS, FH 2 5 "FS 1 "FH
3
3
Using this formula, we see that U(400, 400) 20. So points on the indifference curve
through the point (FS, FH) (400, 400) satisfy the formula 23 !FS 13 !FH 20.
After multiplying through by 3, subtracting 2 !FS from both sides, and squaring, we
can rewrite this formula as:
FH (60 2"FS )2
We’ve plotted this indifference curve in Figure 11.9 by plugging in several numerical
values: for FS 300 we have FH 643; for FS 350 we have FH 510; for FS 400 we have FH 400; for FS 450 we have FH 309; and for FS 500 we have
FH 233.
Points on the constant expected consumption line that runs through the point
(FS, FH) (400, 400) satisfy the formula 23 FS 13 FH 400, which we can rewrite
as FH 1,200 2FS. We’ve also plotted that line in Figure 11.9. Notice that the
indifference curve lies tangent to the constant expected consumption line at the point
(FS, FH) (400, 400), and above all other points on that line. Maria is therefore risk
averse.
IN-TEXT EXERCISE 11.2
Repeat the calculation in worked-out problem 11.2,
but assume that W(F) F2.
Expected Utility and Risk Aversion Assuming that it’s possible to represent Maria’s
preferences with an expected utility function, we can determine her attitude toward risk
from the shape of her benefit function, W(F). If W(F) is concave (that is, if it flattens as F
increases), she’s risk averse; if it’s convex (that is, if it steepens as F increases), she’s risk
loving; and if it’s linear (that is, a straight line), she’s risk neutral.
Let’s examine the case where the benefit function W(F) is concave. Look at Figure 11.10, in which we’ve graphed the benefit function from worked-out problem 11.2,
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Chapter 11 Choices Involving Risk
Food if hurricane (kg)
Figure 11.9
Indifference
curve
700
600
500
400
Solution to Worked-Out Problem
11.2. Maria’s indifference curve is
shown in red. The constant expected
consumption line is shown in green.
Since her indifference curve lies tangent
to the constant expected consumption
line where it crosses the guaranteed
consumption line, and above the constant expected consumption line at all
other points, Maria is risk averse.
Guaranteed
consumption
line
Constant expected
consumption line
383
300
200
100
0
0
100
200
300
400
500
600
Food if sunny (kg)
Figure 11.10
25
W(FS )
W(EC)
20
Benefit
A
D
E
15
C
Expected
benefit
10
Risk
premium
W(FH )
5
B
FH
Expected
consumption
(EC)
Certainty
equivalent
0
0
100
200
300
400
Food (kg)
FS
500
600
Expected Utility for a Risk-Averse
Consumer. If Maria’s benefit function
is concave, she is risk averse. Consider
a consumption bundle consisting of FS
kg of food if it’s sunny, yielding a benefit
of W (FS), and FH kg of food if there’s a
hurricane, yielding a benefit of W (FH).
The coordinates of point C, which lies on
the straight line between points A and
B, correspond to expected consumption
and expected benefit. Since the concave
benefit function bows upward, a riskless
bundle with the same level of expected
consumption yields a larger benefit
(point D). Therefore, Maria prefers it to
the risky bundle.
W(F) !F. This is a typical concave function; as F increases, it becomes flatter. As we
have already seen in worked-out problem 11.2, this benefit function implies risk aversion.
Figure 11.10 shows that this is a consequence of the function’s concavity.
Let’s consider a consumption bundle consisting of FS kg of food if it’s sunny and FH
kg of food if there’s a hurricane. In the event of sun, Maria’s benefit is W(FS), shown as
point A in Figure 11.10. In the event of a hurricane, her benefit is W(FH), shown as point
B in the figure. Now consider point C. Its horizontal coordinate is the level of expected
consumption, and its vertical coordinate is the expected benefit. By definition, the coordinates of point C are weighted averages of the coordinates of points A and B, where the
probabilities of sun and hurricane are used as the weights. Therefore, point C must lie on
the straight line that connects points A and B, as shown.
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Part II Economic Decision Making
Now let’s consider a second consumption bundle with the same level of expected
consumption, but no risk. The benefit that Maria receives from this bundle is the vertical
coordinate of point D. Point D has the same horizontal coordinate as point C. However,
since the concave function W(F) bows upward between points A and B, point D’s vertical coordinate is larger than point C’s. Maria is therefore risk averse: she prefers the safe
bundle, which provides a benefit equal to point D’s vertical coordinate, to the risky one,
which provides an expected benefit equal to point C’s vertical coordinate.
Let’s check this conclusion numerically. In the figure, FS 441, FH 36, 2/3,
and W(F) !F. You can verify that W(FS) 21, W(FH) 6. Expected consumption
(EC) is 306, and Maria’s expected benefit is 16. For a riskless bundle providing 306 kg
of food in both states of nature, her benefit is approximately 17.5. Since 17.5 exceeds 16,
Maria prefers the riskless bundle.
Concavity of the benefit function is linked to the idea that scarcity makes a commodity more valuable. For instance, the marginal benefit of extra food in Figure 11.10
(equivalently, the slope of the benefit function) is greater at a low level of consumption
like FH than at a high level of consumption like FS. That is precisely why Maria is risk
averse. Starting with a risky consumption bundle, she is willing to give up some food in a
state of nature where food has low incremental value because it’s plentiful, for extra food
in a state of nature where food has high incremental value because it’s scarce.
Now let’s consider the case where the benefit function W(F) is convex. We can evaluate
a risky bundle using a diagram similar to Figure 11.10. With a convex benefit function, the
red curve would bow downward rather than upward. As a result, point D would be lower
than point C, rather than higher. (Check your understanding by drawing the figure.) This
configuration implies a preference for the risky bundle over a riskless one with the same
level of expected consumption. In other words, the consumer is risk loving. You have already
seen an example of this principle: the consumer from in-text exercise 11.2 is risk loving
because the benefit function W(F) F 2 is convex (check this by graphing the function).
What if the benefit function W(F) is linear? Once again, we can evaluate a risky bundle using a diagram similar to Figure 11.10. With a linear benefit function, the red curve
would be a straight line. As a result, point D would be the same as point C. (Check your
understanding by drawing the figure.) This configuration implies indifference between
the risky bundle and a riskless one with the same level of expected consumption. In other
words, the consumer is risk neutral.
Certainty Equivalents, Risk Premia, and Degrees of Risk Aversion
Figure
11.10 illustrates another way to find the certainty equivalent and risk premium for a risky
consumption bundle. The horizontal line between point C and the vertical axis intersects
the function W(F) at point E. The horizontal coordinate of point E is the certainty equivalent of the risky bundle. Why? If Maria consumes that amount of food with certainty,
her benefit will be the same as her expected benefit from the risky bundle. As shown in
the figure, Maria’s risk premium, which we defined previously as the difference between
expected consumption and the certainty equivalent, corresponds to the horizontal gap
between points C and E. Returning to our numerical example, we see that Maria’s certainty equivalent, CE, solves the formula W(CE) 16. This implies that CE 162 256
kg. Since expected consumption is 306 kg, Maria’s risk premium is 50 kg.
How does the shape of the benefit function W(F) relate to the degree of risk aversion?
Since concavity implies risk aversion, you won’t be surprised to learn that the greater the
concavity, the greater the risk aversion. To see why, look at Figure 11.11, which analyzes
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Chapter 11 Choices Involving Risk
Figure 11.11
Expected
benefit
25
A
Benefit
20
F
E
C
15
10
5
Arnold’s
risk
premium
Maria’s
risk
premium
B
FH
FS
0
0
100
Arnold’s
certainty
equivalent
200
300
400
500
Expected
consumption
(EC)
Maria’s
certainty
equivalent
600
Expected Utility and the Degree of
Risk Aversion. The red benefit function is for Maria, and the blue benefit
function is for Arnold. The blue curve
is more bowed than the red curve; it’s
more concave. Maria’s certainty equivalent is the horizontal coordinate of point
E and Arnold’s is the horizontal coordinate of point F. Maria’s risk premium
is the horizontal gap between points
C and E; Arnold’s is the horizontal gap
between points C and F. Since Arnold’s
certainty equivalent is smaller and his
risk premium larger than Maria’s, he is
more risk averse.
Food (kg)
the same risky bundle as Figure 11.10. We’ve reproduced a number of elements from
Figure 11.10, including Maria’s benefit function (in red), certainty equivalent, and risk
premium. The preferences of a second consumer, Arnold, correspond to the blue benefit
function. Notice that the blue curve is more bowed than the red curve; it’s more concave.
Using the same logic as before, we see that Arnold’s certainty equivalent is the horizontal
coordinate of point F, and his risk premium is the horizontal gap between points C and F.
Since Arnold’s certainty equivalent is smaller and his risk premium larger than Maria’s,
he is more risk averse.
11.3
INSURANCE
Much of life is inherently risky. While we can take some measures to improve the odds of
staying safe, secure, and healthy, many events are outside our control. To protect ourselves
against the financial consequences of these risks, we sometimes purchase insurance.
The Nature of Insurance
People address a wide range of risks by purchasing insurance policies. An insurance
policy is a contract that reduces the financial loss associated with some risky event, like a
burglary, an accident, an illness, or death. The simplest type of insurance policy specifies
a premium, which is the amount of money the policyholder pays for the policy, and a
benefit, which is the amount the policyholder receives if a specific loss occurs.
The purchaser of an insurance policy is essentially placing a bet. Let’s use the symbol
M to stand for the size of the premium, and the symbol B to stand for the size of the promised benefit. Having paid M, the policyholder receives B if a loss occurs, for a net gain of
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An insurance policy is a
contract that reduces the
financial loss associated
with some risky event, like
a burglary, an accident, an
illness or death.
An insurance premium is
the amount of money the
policyholder pays for the
insurance policy.
An insurance benefit is
the amount of money a
policyholder receives if a
specific loss occurs.
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Part II Economic Decision Making
An insurance policy is
actuarially fair if it’s
expected net payoff
is zero.
B M. If a loss doesn’t occur, he loses the premium, M. Since the policy pays off only
when a loss occurs, in a sense the consumer is betting that the loss will happen.
When the expected net payoff from an insurance policy is zero, we say that it’s actuarially fair. Sometimes we simply call it fair insurance. Purchasing an actuarially fair
insurance policy does not change the purchaser’s expected consumption; it simply raises
consumption in some states of nature and reduces it in others. Let’s use the Greek symbol
to stand for the probability of avoiding a loss, in which case the probability of sustaining a loss is 1 . Actuarial fairness requires that
( M) (1 ) (B M) 0
which implies that M B(1 ). That is, an actuarially fair insurance premium equals
the promised benefit times the probability of a loss; the price of insurance per dollar of
coverage is (1 ).
Insurance policies are usually less than actuarially fair. That is, the premium typically
exceeds the promised benefit times the probability of a loss (M B(1 )). This is
because insurance companies must come out ahead to cover their costs of operation. On
average, then, policyholders lose money; purchasing an actuarially unfair policy reduces
the purchaser’s expected consumption.
The Demand for Insurance
Considered in isolation, the typical insurance policy seems unattractive. Its payoff is
uncertain, and it reduces expected consumption. Risk-averse consumers are nevertheless
willing to make actuarially unfair bets by purchasing insurance because an insurance
policy cancels out other risks.
To illustrate, let’s suppose Maria earns $500 and would like to spend as much as possible on food, which costs $1 per kilogram. As before, the weather is uncertain; the probability of sun () is two-thirds, and the probability of a hurricane (1 ) is one-third.
If it’s sunny, Maria spends all $500 on food. If there’s a hurricane, she sustains a loss of
$300 due to property damage from flooding, and can spend only $200 on food. Thus, she
starts out with the risky consumption bundle labeled A in Figure 11.12. She can protect
herself from this serious risk by purchasing flood insurance (which pays off in the event
of a hurricane). Let’s see how the process works.
With full insurance, the
promised benefit equals
the potential loss.
The Demand for Fair Insurance In Figure 11.12, we’ve drawn the constant expected
consumption line through point A, and labeled it L. (From formula (3) on page 374, we
know that the slope of L is /(1 ) 2.) Since a fair insurance policy doesn’t change
Maria’s expected consumption, it moves her from point A to some other bundle on L.
We’ve marked the point where L crosses the guaranteed consumption line as point B.
To reach point B, Maria can purchase an insurance policy with a benefit level of $300 for
a premium of $100.7 In that case, the promised benefit equals the potential loss, giving
Maria full insurance. With full insurance, consumption is the same regardless of whether
a loss occurs. Here, Maria spends $400 on food—the amount she has left after paying her
insurance premium—regardless of the weather.
To reach any other point on the solid line connecting points A and B, Maria can buy
an insurance policy with a benefit that’s smaller than her potential loss. For example, she
might pay $50 for a policy that promises a $150 benefit, obtaining the midpoint between
7 We’ve seen that M B(1 ) with actuarially fair insurance. Here, with full insurance, B $300. Since 2/3, the actuarially
fair insurance premium is $300 (1 2/3) $100.
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Chapter 11 Choices Involving Risk
Figure 11.12
Food if hurricane (kg)
L (constant expected
consumption line)
Guaranteed
consumption
line
Benefit minus
insurance
premium
400
200
B
C
A
The Demand for Fair Insurance.
Maria starts out with a risky consumption bundle, at point A. By purchasing
actuarially fair insurance, she can
reach any point on the solid green line
segment connecting points A and B. If
she’s risk averse, she’ll choose point B,
representing full insurance.
Budget line
400 500
Insurance
premium
Food if sunny (kg)
A and B, marked as C in the figure. Because the promised benefit is less than the potential loss, this type of policy is called partial insurance. With partial insurance, Maria’s
consumption still depends on the weather, but it varies less than without insurance. At
point C, she consumes 450 kg of food if it’s sunny (spending her $500 income minus the
$50 insurance premium) and 300 kg when there’s a hurricane (spending her $500 income
minus the $50 premium, minus the $300 loss, plus the $150 benefit).
Maria cannot obtain any point on L to the left of point B or the right of point A. Points
to the left of point B would require an insurance contract that promises benefits in excess
of losses. Generally, insurance policies cover policyholders only up to the amount of the
actual loss; otherwise, policyholders would have incentives to create losses. Points to the
right of point A would require Maria to pay the insurance company in the event of a flood,
which is contrary to the definition of flood insurance.
Through fair insurance, Maria can therefore obtain any risky bundle on the solid
green line connecting points A and B. Consequently, that is her budget line.
If Maria is risk averse, she will purchase full insurance. To understand why, remember that for risk-averse consumers, points of tangency between indifference curves and
constant expected consumption lines lie on the guaranteed consumption line (as in Figure
11.5 on page 377). In Figure 11.12, we’ve drawn Maria’s indifference curve through point
B. Since point B is her most preferred point on L, it is certainly her best choice on the
solid portion of L between points A and B.
Notice that this conclusion doesn’t depend on the degree of Maria’s risk aversion.
Whether she’s slightly or severely risk averse, point B is her best choice on the constant
expected consumption line through point A, so she fully insures.
With partial insurance, the
promised benefit is less
than the potential loss.
The Demand for Less-than-Fair Insurance What if insurance is less than actuarially fair? Let’s suppose that the insurance company charges a premium of R dollars per
dollar of coverage (in other words, M BR), where R 1 . If Maria increases B by
one dollar, her consumption falls by R dollars if it’s sunny and rises by 1 R dollars if
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Part II Economic Decision Making
there’s a hurricane. Therefore, by purchasing insurance, Maria can obtain bundles on a
straight line through her starting point, A, with a slope of (1 R)/R.
To illustrate, let’s assume that Maria pays 50 cents for each dollar of promised benefit
(that is, R 0.5), rather than the actuarially fair price of 33 1/3 cents. Then (1 R)/R 1. In Figure 11.13, we’ve drawn a straight green line with a slope of 1 between points
A and D. That is Maria’s budget line. To reach point D, she must fully insure by purchasing a policy with a benefit level of $300. She pays a premium of $150, or $50 more than
her premium for fair insurance. As a result, her food consumption is 50 kg lower, regardless of the state of nature (350 kg instead of 400 kg). To reach any consumption level
between points A and D, she must partially insure. For example, she can achieve point E
by spending $100 to purchase a policy with a promised benefit of $200.
With less-than-fair insurance, Maria definitely won’t insure fully. Why not? The constant expected consumption line that runs through point D, shown as a broken green line
in the figure, has a slope of 2, so it’s steeper than the budget line. Because the indifference curve that runs through point D is tangent to this constant expected consumption
line, it passes below the budget line to the right of point D. So Maria’s best choice on that
line must lie to the right of point D. Point E is the best choice. Maria partially insures by
purchasing a policy with a benefit equal to two-thirds of her potential loss.
In contrast to fair insurance, the amount of less-than-fair insurance purchased does
depend on the policyholder’s degree of risk aversion. For example, a risk neutral consumer
will buy no insurance. Why? His indifference curves coincide with the constant expected
consumption lines in Figure 11.13. Point A is therefore his best choice. The same is true
if the consumer is slightly risk averse; his indifference curves will still be steeper than the
budget line (since they will bend only slightly as they move away from the guaranteed
consumption line), so point A will remain the best choice.
Figure 11.13
Food if hurricane (kg)
The Demand for Unfair
Insurance. Maria starts out with a
risky consumption bundle, at point A.
By purchasing actuarially unfair insurance, she can reach any point on the
solid green line connecting points A
and D. Her best choice, point E, lies to
the right of point D, which means she
partially insures.
Constant expected
consumption lines
Guaranteed
consumption
line
350
Benefit minus 300
insurance
premium 200
D
Budget line
E
A
350
400
500
Insurance
premium
Food if sunny (kg)
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Chapter 11 Choices Involving Risk
389
The Value of Insurance
Food if hurricane (kg)
Insurance makes people better off by protecting them from the consequences of a loss.
One way to measure the benefits it offers is to compare certainty equivalents.
Let’s start with fair insurance. Figure 11.14 reproduces points A and B from Figure
11.12, along with the budget line that runs between them. With fair insurance, Maria
chooses point B. Since this bundle involves no risk, its certainty equivalent is the same
as the amount consumed, 400 kg. Without insurance, Maria is stuck with point A. We’ve
drawn the indifference curve that runs through point A and marked its intersection
with the guaranteed consumption line as point G. The certainty equivalent of point A is
the horizontal (equivalently, vertical) coordinate of G, 300 kg. Fair insurance raises the
certainty equivalent of Maria’s best choice from 300 kg to 400 kg. The difference between
these two numbers, 100 kg, is the net value of fair insurance, considering both the benefits
and the premiums.
Now let’s turn to less-than-fair insurance. Figure 11.14 also reproduces points A, D,
and E from Figure 11.13, along with the budget line that runs between points A and D,
and the indifference curve that runs through point E. We’ve marked the point where this
indifference curve crosses the guaranteed consumption line as point H. Maria chooses the
risky bundle E by partially insuring. The certainty equivalent of point E is the horizontal
(equivalently, vertical) coordinate of point H, 375 kg. As before, without insurance, Maria
is stuck with point A, which has a certainty equivalent of 300 kg. Less-than-fair insurance
raises the certainty equivalent of her best choice from 300 kg to 375 kg. The difference
between these two numbers, 75 kg, is the net value of less-than-fair insurance. It isn’t as
great as the value of fair insurance, but it contributes significantly to Maria’s well-being.
Guaranteed
consumption
line
B
H
Budget lines
D
E
Value, unfair G
insurance
A
300 375 400
Value, fair
insurance
Food if sunny (kg)
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Constant expected
consumption line
Figure 11.14
The Value of Insurance. Maria starts
out with a risky consumption bundle at
point A. The certainty equivalent of this
risky bundle, given by the level of
consumption at point G, is 300 kg. With
actuarially fair insurance, Maria obtains
point B, which has a certainty equivalent
of 400 kg. The value of actuarially fair
insurance is therefore 400 kg – 300 kg
= 100 kg. With actuarially unfair insurance, Maria obtains point E. The certainty equivalent of this risky bundle,
given by the level of consumption at
point H, is 375 kg. The value of actuarially unfair insurance is therefore 375 kg
– 300 kg = 75 kg.
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Part II Economic Decision Making
Application 11.3
The Value of Life Annuities
N
one of us knows how long we’ll live. This uncertainty is
the source of significant financial risk. Most obviously,
the unexpected death of a family’s primary breadwinner can
have major financial consequences. That is why people buy
life insurance.
But uncertainty concerning the length of one’s life also
creates a second type of risk: the possibility that a person will
outlive his resources. Imagine for the moment that you’re 65
years old and newly retired. How well can you afford to live?
The answer depends on how long you’ll end up living. If you
expect to live a long time, you’ll have to watch your spending
to make sure you won’t run out of money. Even then you may
find yourself in trouble. For example, you might plan your
spending at age 65 on the assumption that you’ll probably die
by age 90. But what if you live to 95? Will you have enough
money to get by in those final years? Alternatively, you could
be extremely conservative and plan your spending at age 65
on the assumption that you’ll live forever. But you probably
won’t get to spend a large chunk of your wealth in that case.
To address the risk of living “too long,” people can
purchase an insurance policy called a life annuity. The
policyholder pays a premium (either in a lump sum or in
installments), and in return receives a constant income
for the rest of his life starting at some specified age. If he
dies quickly, his premium exceeds the present value of the
benefits received. But if he lives to a ripe old age, the present
value of the benefits received exceeds his premium. So in
effect, he is betting on living a long life. (With life insurance,
he is betting on dying quickly.)
Life annuities make planning easier. A new retiree can
lock in a living standard without worrying about outliving
his resources. And he can achieve a higher living standard
than if he conservatively planned his spending based on the
assumption that he’ll live forever.
How valuable is this form of insurance? According
to economists Olivia Mitchell, James Poterba, Mark
Warshawsky, and Jeffrey Brown, people who behave
according to the Life Cycle Hypothesis (discussed in Section
10.2) would give up 30 to 38 percent of their wealth at age
65 in exchange for the ability to purchase actuarially fair life
annuities!8
Most of us end up relying heavily on life annuities,
because Social Security and many private pension plans
provide benefits in that form. However, relatively few people
buy life annuities directly. In light of the enormous potential
value of these policies, their lack of popularity is something of
a puzzle. A number of explanations have been proposed. First,
annuities aren’t actuarially fair. Second, since Social Security
and pensions partially insure people against living too long,
additional insurance is less valuable than the figures in the
previous paragraph would suggest. Third, since benefits are
usually paid in nominal terms, the policyholder is exposed to
inflation risk. Fourth, people may be reluctant to lock up their
funds in annuities. Once money is invested in an annuity,
it becomes unavailable to heirs (since income is received
only while the investor lives), and the investor usually can’t
withdraw funds to cover unplanned expenses like uninsured
nursing home care. Consequently, the demand for annuities
may be low among those who want to leave bequests, or
who are concerned about large uninsured expenses.
Mitchell and her coauthors found that the first two
explanations are reasonably important, but fail to tell the
whole story. Because annuity premiums are actuarially unfair,
the typical person can expect to receive 15 to 20 percent
less in benefits than he pays in premiums. However, that
gap would have to be significantly larger (23 to 31 percent)
to offset the advantages of annuity insurance, even taking
Social Security and pensions into account. The study also
concludes that the third consideration—inflation risk—is of
little importance. Other evidence suggests that the fourth
explanation plays a more central role.9
8 Olivia S. Mitchell, James M. Poterba, Mark J. Washawsky, and Jeffrey R. Brown, “New Evidence on the Money’s Worth of Individual Annuities,” American Economic Review
89, December 1999, pp. 1299–1318.
9 See B. Douglas Bernheim, “How Strong Are Bequest Motives? Evidence Based on the Demand for Life Insurance and Annuities,” Journal of Political Economy 99, October
1991, pp. 899–927.
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Chapter 11 Choices Involving Risk
391
11.3
WORKED-OUT PROBLEM
The Problem The probability of sun is two-thirds, and the probability of a hurricane
is one-third. Maria’s indifference curves are given by the following formula:10
FHF S2 C
where FS is the amount of food she consumes if it’s sunny, FH is the amount of food
she consumes when there’s a hurricane, and C is a constant (the value of which differs
from one indifference curve to another). For indifference curves in this family, the
marginal rate of substitution for food if it’s sunny with food if there’s a hurricane is:
MRSSH 5 2
FH
FS
Without insurance, Maria can spend $400 if it’s sunny but only $75 if there’s a
hurricane (due to property losses from flooding). Food costs one dollar per kilogram.
She can purchase flood insurance for a premium of 40 cents for each dollar of
promised benefit. How much insurance will she purchase? What will she pay for it?
What is its value?
The Solution Maria’s available choices lie on a budget line like the one connecting
points A and D in Figure 11.14. In this case, R 0.4, so the slope of the budget line,
(1 R)/R, is 1.5. The formula for the budget line is therefore FH C 1.5FS,
where C is a constant. Since the budget line must pass through Maria’s starting point,
(FS, FH) (400, 75), we know that 75 C (1.5 400), so C 675.
Maria’s best choice is a point of tangency between the budget line and an
indifference curve, like point E in Figure 11.14. At any such point, the slope of the
budget line is the same as the slope of her indifference curve. Thus,
21.5 5 2MRSSH 5 22
FH
FS
From this formula, we can conclude that at any point of tangency, FH 0.75FS.
We know that Maria’s best choice satisfies both FH 675 1.5FS (it’s on
the budget line) and FH 0.75FS (it’s a point of tangency), so we can solve for it
algebraically. Substituting the second expression into the first, we get 0.75FS 675
1.5FS, so FS 675/2.25 300. In other words, she consumes 300 kg of food if
it’s sunny, at a cost of $300. Since Maria starts with $400 if it’s sunny, she must spend
$100 on insurance. That means her insurance benefit must be $250 (since 0.4 250
100). If there’s a hurricane, she consumes FH 75 250 100 225 kg of
food.
After purchasing insurance, Maria’s risky bundle is (FS, FH) (300, 225). Points
on the indifference curve that runs through this bundle satisfy the formula FH F 2S 225 3002 20,250,000. To find a riskless bundle on this indifference curve (one
for which FS and FH have the same value), we solve the formula F 3 20,250,000.
The solution is F 272.6. This is the certainty equivalent of Maria’s risky bundle
after purchasing insurance.
are Maria’s indifference curves when her preferences correspond to an expected utility function of the form U(FS, FH) log(FS) 1/3 log(FH) [that is, W(F) log(F)].
10 These
2/
3
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Part II Economic Decision Making
Maria’s initial bundle is (FS, FH) (400, 75). Points on the indifference curve that
runs through this bundle satisfy the formula FH F 2S 75 4002 12,000,000. To find
a riskless bundle on this indifference curve, we solve the formula F 3 12,000,000.
The solution is F 228.9. This is the certainty equivalent of Maria’s initial risky
bundle. Since 272.6 228.9 43.7, the value of insurance is $43.70.
IN-TEXT EXERCISE 11.3
Repeat worked-out problem 11.3 assuming that
flood insurance costs 50 cents for each dollar of promised benefit. Repeat it again
assuming that flood insurance costs $8/11 for each dollar of promised benefit.
What happens if the premium is higher than that amount?
11.4
The object of risk
management is to make
risky activities more
attractive by taking steps
to moderate the potential
losses while preserving
much of the potential gains.
OTHER METHODS OF MANAGING RISK
People often take risks voluntarily. For example, they invest in the stock market, start new
businesses, take jobs in start-up companies, and try out new products. Companies invest
in research and development, build new factories despite uncertain demand, and bring
new, untried products to market. In these cases, the expected reward is high enough to
justify risk-taking.
Often, we can make risky activities more attractive by taking steps to moderate potential losses while preserving much of our potential gains. Such steps are known as risk
management. As we’ve seen, one way to manage risks is to purchase insurance. When a
company builds a factory, for example, managers buy insurance to guard against losses
due to fire, theft, lawsuits, and other unforseen events. In this section, we’ll investigate
four other strategies for managing risk: risk sharing, hedging, diversification, and information acquisition. We’ll see how each of these strategies influences the demand for risky
assets. To learn more about the demand for risky assets, read Add-On 11A.
Risk Sharing
Risk sharing involves
dividing a risky prospect
among several people.
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One way to make a risky prospect more attractive is to divide it among several people, a
strategy known as risk sharing. A simple example will illustrate.
Suppose Maria earns $600 regardless of the weather, which she’s free to spend on
food. In other words, she starts out at point A in Figure 11.15 (we’ll continue to assume
that one kg of food costs $1). Suppose she’s offered the opportunity to acquire a sunscreen
concession at the local beach for an investment of $300. If it’s sunny, she makes $600 (net
of her investment), which means she can spend $1,200 on food (her earnings, $600, plus
her net profit, $600). If there’s a hurricane, she sells nothing and loses her investment.
In that case, she can spend only $300 on food (her earnings, $600, minus her investment, $300). So if Maria makes the investment, she obtains the risky consumption bundle
labeled B in the figure.
If the probability of sun is two-thirds, this investment has an expected payoff of $300
(Maria comes out ahead by $600 with a probability of two-thirds, and behind by $300
with a probability of one-third). Since this expected return is positive, point B lies above
the constant expected consumption line that runs through point A, shown as the broken
green line in Figure 11.15. So while the investment would expose Maria to risk, it would
also provide her with a higher level of expected consumption.
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Chapter 11 Choices Involving Risk
Figure 11.15
Food if hurricane (kg)
Constant expected
consumption line
600
480
450
Guaranteed
consumption
line
Budget line
A
D
C
300
B
600
393
840 900
Risk Sharing. Maria starts out with a
riskless consumption bundle at point A.
By making an investment, she can swap
bundle A for bundle B, which provides
higher expected consumption. Given
her preferences, however, she prefers
bundle A. If she can find partners to
split the investment and the profits, she
can reach points on the solid green line.
In that case, she would want to take a
share. Point D would be her best choice.
1,200
Food if sunny (kg)
Even so, Maria does not want to make the investment. As her indifference curves show, she prefers bundle A to bundle B. The higher
expected consumption offered at point B is simply not enough to offset
the risk of having little to eat if there’s a hurricane. In other words, the
investment looks attractive from the perspective of expected profit, but
it’s just too big a risk for Maria to take on alone.
Now let’s suppose Maria has a friend, Arnold, who has the same
earnings and the same preferences. What if the two of them make the
investment as equal partners? That is, each invests $150 and receives
$300 in net profit if it’s sunny. This strategy allows both Maria and
Arnold to reach point C in Figure 11.15 ($600 $300 $900 if it’s
sunny, and $600 $150 $450 if there’s a hurricane), which lies
above the indifference curve that runs through point A. Both Maria and
Arnold prefer point C to point A. Risk sharing allows both to benefit
© The New Yorker Collection 2004 Leo Cullum from
from an investment that neither would find attractive alone.
cartoonbank.com. All Rights Reserved.
By taking on more partners, Maria can create more opportunities
for risk sharing. With three equal partners, she would invest $100 and receive $200 in net
profit if it’s sunny; with four equal partners, she would invest $75 and receive $150 if it’s
sunny; and so forth. Maria could also take on unequal partners. For example, she could
arrange a 60-40 split of the investment and net profits with Arnold.
In practice, owners of companies often take on partners by issuing equity shares. An An equity share is a
equity share is a proportional claim on the ownership of a company. As an example, sup- proportional claim on the
ownership of a company.
pose the sunscreen concession has issued 1,000 shares, each of which sells for 30 cents
and gives its owner a claim on 1/1,000 of any profit earned. Maria could buy the entire
concession by acquiring all 1,000 shares for $300, which would place her at point B. Or
she could buy half of the concession by acquiring 500 shares for $150, which would put
her at point C. In fact, by purchasing the appropriate number of shares, Maria could come
very close to any point on the green line connecting points A and B in Figure 11.15. In
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Part II Economic Decision Making
effect, that line becomes Maria’s budget line. Her best choice is point D where she spends
$120 on 400 shares to acquire 40 percent of the concession (leaving her with $600 0.4
$600 $840 if it’s sunny and $600 $120 $480 if there’s a hurricane).
The logic of Figure 11.15 leads us to a remarkable conclusion. Even if Maria is
extremely risk averse and the expected profit from the investment is very small, she will
still want to invest something in it. Why? First, as long as Maria is risk averse, her indifference curve will lie tangent to the expected consumption line at point A. Increasing her risk
aversion will only cause her indifference curve to bend more sharply at that point. Second,
as long as the investment’s expected profit is positive, Maria’s budget line will extend
above the expected consumption line through point A. Combining these two observations,
we see that the budget line must always pass above the indifference curve, just as in the
figure. A small enough investment therefore necessarily makes Maria better off.
Application 11.4
Insurance Coverage for Satellites
E
very time someone reduces his exposure to financial
risk by purchasing an insurance policy, someone else
(usually an insurance company) accepts a new risk by selling
the policy. Why is the insurance company willing to take on
the policyholder’s risk? As long as the policy is less-thanactuarially fair, the company comes out ahead on average.11
If the risk is sufficiently small, the company may be willing
to absorb all of it at terms that are only slightly worse than
actuarially fair.
But what if the risk is extremely large? Take the case of
commercial satellites. The costs of building and launching
a single satellite regularly exceed $250 million, and satellite
owners have been known to seek more than $400 million in
coverage. Historically, roughly 1 out of every 10 satellites is
either destroyed on launch or fails within a year of reaching
orbit, so these ventures are extremely risky propositions for
insurers. For example, estimates place satellite insurers’
collective losses for 1998 at $1.9 billion, against only $860
million in premiums.
Not surprisingly, individual insurers are generally
unwilling (and often simply unable) to accept responsibility
for such catastrophic losses. Nevertheless, insurance
for satellites is readily available, and the premiums do not
appear to be wildly out of line with the expected benefits.12
Insurance companies manage to provide satellite coverage
at reasonably attractive rates by sharing risks. Even if a
company is unwilling to take on a 10 percent risk of losing
$250 million for a premium of $40 million, it might be perfectly
content with 1/25 of that policy—that is, a 10 percent risk
of losing $10 million for a premium of $1.6 million. If 25
companies will accept these terms, the satellite owner can
piece together the desired coverage.
In practice, insurance companies often save satellite
owners the trouble of making these arrangements. Sometimes
they form syndicates to insure large-scale projects. Sometimes one company takes on a large policy and then sells
interests in it to others through “reinsurance” agreements.
These types of arrangements allow insurers to spread the
risk more evenly through the industry, so that no single
insurer faces the prospect of a catastrophic loss.
11The seller may also care less about absorbing the risk than the policyholder, for two reasons. First, by insuring many unrelated risks, the seller can benefit from diversification,
discussed in the next section. Second, the seller may be less risk averse to begin with. Of course, sellers must also cover operating costs.
12 Historically,
7 percent of launches fail. Between 1995 and 2002, average premiums for launch insurance ranged from 7 to 15 percent of the covered amount.
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Chapter 11 Choices Involving Risk
Hedging and Diversification
Another way to make risky activities more attractive is to combine them appropriately
with other risky activities. We’ll discuss two versions of this strategy, hedging and diversification.
First, we need to introduce the statistical concept of correlation. Two variables are
positively correlated if they tend to move in the same direction. For example, the number
of minutes of sunshine per day is positively correlated with average temperature. Two
variables are negatively correlated if they tend to move in the opposite directions. The
number of minutes of sunshine per day is negatively correlated with rainfall. Two variables are uncorrelated if their movements tend to be unrelated. The number of minutes
of sunshine per day is uncorrelated with earthquake activity. Finally, two variables are
perfectly correlated if one is simply a multiple of the other. The number of inches of rain
and the number of millimeters of rain are perfectly correlated.
Hedging Hedging refers to the practice of taking on two risky activities with negatively correlated financial payoffs. We’ll illustrate this strategy by adding a new twist to
the example we discussed in the last section.
As before, we’ll assume Maria has the opportunity to buy the entire sunscreen concession, allowing her to reach point B in Figure 11.16. In addition, we’ll assume that
Maria can invest $300 in a portable generator distributorship. In the event of a hurricane,
the generator distributorship will turn a net profit of $600. In the event of sun, Maria will
lose her investment. Assuming as before that the probability of sun is two-thirds, the
expected payoff from this investment is zero [(1/3 $600) (2/3 $300) $0]. If Maria
invests in the generator distributorship but not the sunscreen concession, she obtains point
E in Figure 11.16.
Food when rainy (kg)
E
Guaranteed
consumption
line
900
G
F
A
600
300
B
300
600
900
Hedging is the practice of
taking on two risky activities
with negatively correlated
financial payoffs.
Figure 11.16
Constant expected
consumption line
1,200
Two variables are positively
correlated if they tend
to move in the same
direction. Two variables
are negatively correlated
if they tend to move in the
opposite direction. Two
variables are uncorrelated
if their movements tend
to be unrelated. Finally,
two variables are perfectly
correlated if one is simply a
multiple of the other.
1,200
Hedging a Risky Venture. Maria
starts out with a riskless consumption bundle at point A. By making an
investment, she can swap point A for
point B, which provides a higher level
of expected consumption, or point A
for point E, which provides the same
level of expected consumption. Neither
investment is attractive by itself. However, since each succeeds when the
other fails, Maria can hedge her bets by
making both investments. That strategy
allows her to reach point F, which is better than point A. With perfect information concerning the weather, she could
reach point G.
Food when sunny (kg)
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Part II Economic Decision Making
Taken individually, both these investments look unattractive to Maria (in Figure 11.16
points B and E lie on lower indifference curves than point A). Both investments are associated with substantial risk premiums. Indeed, no risk averse individual would ever consider the generator investment, which creates risk without offering her a higher expected
level of consumption.
Yet what if Maria bought both concessions? Doing so would require an investment of
$600, but would guarantee an overall net profit of $300 ($600 from the successful project
minus $300 from the unsuccessful one). Regardless of the weather, Maria would be able
to spend $900 on food (the $600 she earns plus $300 in net profit). In other words, Maria
would be able to reach point F in the figure, which is obviously better than point A. Taken
together, these investments are riskless: When Maria combines them, their risk premiums
disappear!
The critical feature of this example is that the profits from the two concessions are
perfectly negatively correlated. Maria hedges her bet on the sunscreen concession by
investing in the generator distributorship, and vice versa. That way, bad news on one
investment is always more than offset by good news on the other, so that she comes out
ahead.
Insurance is actually a form of hedging. That is, the benefit paid by a flood insurance
policy is perfectly negatively correlated with a loss from flooding. If Maria buys flood
insurance, she is hedging against the possibility of a flood.
Diversification is the
practice of undertaking
many risky activities, each
on a small scale, rather than
a few risky activities (or just
one) on a large scale.
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Diversification
Diversification refers to the practice of undertaking many risky
activities, each on a small scale, rather than a few risky activities (or just one) on a large
scale. The simplest argument for diversification is the old adage that it’s unwise to put all
your eggs in one basket. Dividing them among many baskets reduces risk.
To illustrate, let’s suppose that Maria wants to invest $300. She is thinking about two
start-up companies, Go for Broke, Inc., and Shoot the Moon, Inc. An investment in Go
For Broke, Inc., triples in value with 50 percent probability and becomes worthless with
50 percent probability. The same is true of an investment in Shoot the Moon, Inc. However, the payoffs for these investments are uncorrelated.
One alternative is for Maria to invest all of her money ($300) in a single company.
The histogram in Figure 11.17(a) shows the probability distribution of her payoff. Her
expected payoff is $450 (1/2 $900 1/2 $0 $450), which exceeds her investment
by $150. However, the risk is considerable—there’s a 50 percent chance that she’ll lose
everything. The standard deviation of her payoff is $450 (the square root of the expected
squared deviation, 1/2 4502 1/2 4502).
Alternatively, Maria could diversify, investing half of her money ($150) in each company. If both companies succeed, her payoff will be $900. Since each company succeeds
with 50 percent probability, the probability that both succeed at the same time is 1/2 1/ 1/ . If both companies fail, Maria’s payoff will be zero. Since each company fails
2
4
with 50 percent probability, the probability that both fail at the same time is 1/2 1/2 1/ . If one company succeeds and the other fails, Maria’s payoff will be $450, an outcome
4
that occurs with a probability of 1/2. (The probability that Go for Broke succeeds while
Shoot the Moon fails is 1/4, as is the probability that Shoot the Moon succeeds while Go
for Broke fails.) Maria’s overall expected payoff is still $450 (1/4 $900 1/2 $450 1/ $0 $450), but the standard deviation is lower, $318 (the square root of 1/ 4502
4
4
1/4 4502).
The histogram in Figure 11.17(b) shows this information graphically. Compared with
Figure 11.17(a), the payoff is less variable. Investing in two companies instead of one
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Chapter 11 Choices Involving Risk
Figure 11.17
Probability (%)
(a) No diversification
50
0
450
900
Payoff ($)
(b) Diversification
Probability (%)
397
Diversification of Risk. If Maria
invests $300 in Go for Broke, Inc., her
payoff will be $900 with 50 percent
probability, and $0 with 50 percent
probability, as shown in figure (a). If she
diversifies by investing $150 in Go for
Broke, Inc., and $150 in Shoot the Moon,
Inc. (the returns from which are uncorrelated with those from Go for Broke, Inc.),
she shifts probability to an intermediate payoff ($450), reducing variability
without changing her expected payoff,
as shown in figure (b).
50
25
0
450
900
Payoff ($)
moves half the bar at $0 to $450, and half the bar at $900 to $450. Extreme outcomes
become less likely, and a moderate outcome becomes more likely. So diversification
reduces risk without changing the expected payoff.
In this example, the payoffs from the two investments are uncorrelated. What if they
were perfectly positively correlated? In that case, Shoot the Moon would succeed when
Go for Broke succeeds and fail when Go for Broke fails. Investing $150 in each company
would then be equivalent to investing $300 in one of the companies. With perfect positive
correlation, there’s no benefit to diversification.
What if the payoffs from the two investments were perfectly negatively correlated? In
that case, Shoot the Moon would succeed when Go for Broke fails and fail when Go for
Broke succeeds. By investing in both companies, Maria would perfectly hedge her bets.
The risks would cancel out, delivering a payoff of $450 with certainty.
Comparing these three cases (no correlation, perfect positive correlation, and perfect
negative correlation), we see that as the correlation between the payoffs increases, the
risk-reducing effect of diversification decreases. Intuitively, diversification reduces risk
because a gain sometimes offsets a loss, leading to an intermediate outcome. When the
correlation is lower, offsetting outcomes are more likely, so diversification is more valuable. Hedging is simply a case in which diversification is particularly effective at reducing
risk because the payoffs from the hedged activities are negatively correlated, so that gains
usually offset losses.
This discussion underscores the point that it’s dangerous to evaluate risky activities
in isolation. The desirability of undertaking a risky project depends on other actual and
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Part II Economic Decision Making
potential risks, including opportunities for hedging and diversification. One of the most
important functions of the stock market, in fact, is that it allows people to diversify their
risky investments by purchasing small interests in many companies, instead of betting
everything on a single business. Application 11.5 describes one easy way to diversify.
Application 11.5
Diversification through Mutual Funds
I
n principle, diversification through the stock market seems
like a good idea. But in practice, many small investors find
it rather difficult. Picking the right companies requires a
good deal of research. Each investment has to be monitored
and adjusted as prices rise and fall. Brokerage fees can add
up, particularly if transactions involve only a few shares. For
someone who is putting, say, $10,000 in the stock market,
these costs are potentially prohibitive.
Mutual funds make diversifying investments easy. A
mutual fund raises money from investors by selling shares
in the fund and then invests the proceeds. Investors share
in the fund’s gains and losses until they either redeem their
shares from the fund or sell them to other investors.
There are several different types of mutual funds. For
example, equity funds invest only in common stocks, bond
funds invest only in bonds, and balanced funds invest in both.
Some funds specialize in certain types of stocks or bonds,
while others do not. Some funds are actively managed,
which means that the manager tries to choose investments
that will outperform some recognized index, like the S&P
500 (which is published by Standard and Poor’s and tracks
the performance of 500 prominent companies). Other funds,
known as index funds, try to match the performance of an
index, usually by holding the stocks that make up the index.
During the 1990s, total U.S. mutual fund investments
exploded, growing from $1.07 trillion to $6.85 trillion by the
end of the decade. In large part, that growth resulted from
the increased popularity of 401(k) pension plans and other
similar types of retirement savings accounts, which typically
require individual investors to allocate their savings among
particular mutual funds. As of year-end 2003, the mutual fund
industry served more than 260 million separate accounts,
with a total balance of $7.41 trillion. More than threequarters of that total was held in accounts owned directly
by households, and those accounts contained more than
18 percent of all U.S. household financial assets. Nearly 48
percent of U.S. households had at least one mutual fund
account. Overall, mutual funds held 22 percent of all U.S.
corporate equity.
While mutual fund companies tout a long list of
advantages over direct investment, some of their claims are
disputed. For example, there is little evidence that active
management improves investment outcomes. Actively
managed funds do outperform index funds some of the
time, but we would expect this to occur purely by chance,
and there is little evidence that anything more than chance
is involved. Nevertheless, mutual funds do offer individual
investors at least one indisputable advantage: convenient,
low-cost diversification.
WORKED-OUT PROBLEM
11.4
The Problem Suppose Maria starts out with $300, and that she seeks to maximize
her expected benefit, !X, where X measures her resources in dollars. She has three
options: keep the $300, invest it all in Go for Broke, Inc., or invest $150 each in Go
for Broke, Inc., and Shoot the Moon, Inc. Compute the certainty equivalent of each
option. Which is best? What is the benefit of diversification?
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Chapter 11 Choices Involving Risk
399
The Solution Keeping $300 guarantees a benefit of 17.3 (the square root of 300);
the certainty equivalent is clearly $300. Investing $300 in Go for Broke produces an
expected benefit of
1
1
"0 1 "900 5 15
2
2
Her certainty equivalent is the amount X that solves !X 15, so X 225. Investing
in both companies produces an expected benefit of
1
1
1
"0 1 "450 1 "900 5 18.1
4
2
4
Her certainty equivalent is the amount X that solves !X 18.1, so X 328.
Maria’s best choice is to invest in both companies. Her second best choice is
not to invest. The benefit of diversification is $103 ($328 $225) compared with
no diversification, and $28 ($328 $300) compared with the next best choice (no
investment).
IN-TEXT EXERCISE 11.4
Repeat worked-out problem 11.4 assuming that
Maria seeks to maximize the expected value of 2,000X X 2 (for values of X less
than 1,000).
Information Acquisition
People often try to reduce or eliminate risk by acquiring information. Because better
information about probable events leads to better decisions, it can reduce the likelihood
of undesirable outcomes.
To illustrate this point, let’s return to the example we used in our discussion of hedging. As before, Maria can invest in a sunscreen concession, a generator distributorship, or
both. She starts out at point A in Figure 11.16 (page 395); her investment options allow
her to reach points B, E, and F. As we’ve said, her best choice is to buy both concessions
(point F), yet that isn’t the ideal solution. Because the sunscreen concession does poorly
if there’s a hurricane and the generator distributorship does poorly if the sun shines, one
concession or the other will always do poorly, so Maria wastes the $300 she invests in it.
Can she do better?
If Maria could predict the weather perfectly, she would always make the right investment, investing in the sunscreen concession only when the sun is about to shine and the
generator distributorship only when a hurricane is about to hit. Rain or shine, she would
earn a net profit of $600, leaving her with $1,200 (including her earnings) to spend on
food. In other words, with perfect information, Maria would reach point G in Figure
11.16, which is significantly better than point F. Clearly, Maria has a strong incentive to
gather information about likely weather patterns.
Suppose Maria knows of a meteorological consultant who can predict the weather
perfectly. How much would she pay for his services? We know she ends up with $1,200
for sure if she can predict the weather, and only $900 for sure (through hedging) if she
can’t. That means she should be willing to pay up to $300 for perfect weather prediction.
In this example, the value of information is $300.
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C H A P T E R S U M M A RY
400
1.
2.
Part II Economic Decision Making
What is risk?
a. Sometimes we can assess the probability of a state of
nature by determining the frequency with which it has
occurred in the past. This concept is known as objective
probability. At other times, we may assess the probability
of a state of nature by using subjective judgment, a
concept known as subjective probability.
b. If we know the probability and payoff associated
with each state of nature, we can find the probability
distribution of the payoffs. We can also calculate the
expected payoff, as well as the standard deviation and the
variance, two measures of variability.
Risk preferences
a. To analyze decisions involving risk, we can apply the
theory developed in Chapters 4 through 6, thinking of a
consumption bundle as a list of the amount of each good
consumed in each state of nature. Indifference curves
represent the consumer’s preferences for consumption in
different states of nature.
b. With only two possible states of nature, a risk-averse
consumer’s preferred point on any constant expected
consumption line lies on the guaranteed consumption line.
c. With only two possible states of nature, we can find
the certainty equivalent of a risky consumption bundle
by identifying the indifference curve that runs through
the bundle and determining the level of consumption
that corresponds to the point where the curve crosses the
guaranteed consumption line.
d. The difference between a bundle’s expected level of
consumption and its certainty equivalent, known as the
risk premium, reflects the psychological cost of exposure
to risk.
e. At greater levels of risk aversion, indifference curves
bend more sharply where they cross the guaranteed
consumption line. The certainty equivalent of any risky
bundle is lower and the risk premium higher with greater
levels of risk aversion.
f. With only two possible states of nature, a risk-loving
consumer’s least preferred point on any constant expected
consumption line lies on the guaranteed consumption line.
For risk-neutral consumers, indifference curves coincide
with the constant expected consumption lines.
g. Under some conditions, we can use an expected utility
function to describe a consumer’s risk preferences.
3.
4.
h. For expected utility, a concave benefit function
implies risk aversion; a convex function implies riskloving preferences. A linear benefit function implies risk
neutrality.
Insurance
a. If insurance is actuarially fair, a risk-averse consumer
will purchase full insurance.
b. If insurance is less than actuarially fair, a risk-averse
consumer will purchase partial insurance or no insurance
at all. The amount of insurance purchased will depend on
the degree of risk aversion. Those who are not very risk
averse will purchase no insurance.
c. The value of insurance equals the difference between
the certainty equivalent of the consumer’s consumption
bundle after purchasing insurance and the certainty
equivalent of the bundle before purchasing insurance.
The greater the risk aversion, the higher the value of the
insurance.
Other methods of managing risks
a. One way to make a risky investment more attractive
is to share the risk by dividing it among several people.
Companies can expand the opportunities for risk sharing
by issuing equity shares. As long as an investment’s
expected payoff is positive, even an extremely risk-averse
person will benefit from taking a small share of it.
b. Hedging reduces risk, because when the payoffs from
two activities are negatively correlated, the gains offset
the losses.
c. Diversification reduces risk because it creates
opportunities for gains to offset losses, raising the
likelihood of intermediate outcomes. The risk-reducing
effects of diversification are smaller when the payoffs are
more positively correlated, making offsetting gains and
losses less likely. One of the most important functions
of the stock market is to allow people to diversify their
risky investments by purchasing small interests in
many companies, instead of betting everything on the
performance of a single business.
d. Better information about probable events leads to
better decisions, reducing the likelihood of a loss. The
value of information equals the difference between
the certainty equivalent of the risky outcome when an
individual is informed and the certainty equivalent of the
risky outcome when he is uninformed.
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A D D I T I O N A L E X E RC I S E S
401
Chapter 11 Choices Involving Risk
Exercise 11.1: List as many types of financial risk as possible
for each of the following activities: driving a car; going to
college; trying out a new brand of breakfast cereal; taking a
vacation in Europe; hiring a new employee.
Exercise 11.2: Compute the expected payoff and the standard
deviation for each of the following probability distributions:
a. A payoff of 200 with a probability of 0.4 and 500 with a
probability of 0.6.
b. A payoff is 110 with a probability of 0.2; 130 with a
probability of 0.3; 150 with a probability of 0.1; and 170
with a probability of 0.4.
c. Every possible payoff that’s a whole number between (and
including) 100 and 200, each with the same probability.
(Use a spreadsheet.)
Exercise 11.3: As in Figure 11.3 (page 374), draw the
constant expected consumption line through point A under
the assumption that the probability of a hurricane is:
a. 50 percent
b. 25 percent
c. 75 percent
Exercise 11.4: Suppose that consumption when it’s sunny
and consumption when there’s a hurricane are perfect
complements. The investor’s indifference curves are L-shaped,
and the corner of each L lies on the 45-degree line. Using
graphs, explain why these assumptions imply infinite risk
aversion.
Exercise 11.5: Repeat worked-out problem 11.1 (page 378),
assuming that the indifference curves are L-shaped, as in
exercise 11.4.
Exercise 11.6: The risk premium for a risky consumption
bundle is never larger than the difference between expected
consumption and the lowest payoff that occurs with a positive
probability. Explain why this statement is true. Assuming there
are only two possible outcomes, illustrate with a graph.
Exercise 11.7: Using a graph like the one in Figure 11.6
(page 377), show the risk premium and certainty equivalent
for a risky consumption bundle, assuming that the consumer is
risk-loving. Explain why the certainty equivalent exceeds the
expected level of consumption, and why the risk premium is
negative.
Exercise 11.8: Suppose that Maria seeks to maximize the
expected value of the benefit function W(F) 1,000F F 2
(for values of F below 100). Maria consumes 50 kg of food
when it’s sunny and 30 kg when there’s a hurricane. There’s
a 25 percent chance of a hurricane. Compute her expected
payoff, her expected utility, the certainty equivalent of her
risky consumption bundle, and the risk premium.
Exercise 11.9: Show graphically that a risk-loving consumer
would never purchase actuarially fair insurance.
Exercise 11.10: You’ve seen that the consumer’s degree of
risk aversion doesn’t affect the quantity of actuarially fair
insurance purchased (since all risk-averse consumers will
fully insure). Using graphs, show that the degree of risk
aversion does affect the value of insurance. Is the value of fair
insurance smaller or larger to a more risk-averse consumer?
Exercise 11.11: Initially, Maria consumes 400 kg of food
when it’s sunny and 75 kg of food when there’s a hurricane
(due to property losses from flooding). Her indifference curves
are L-shaped, as in exercise 11.4. Suppose that flood insurance
is available, and that the premium, M, for each dollar of
promised benefit is less than a dollar (but greater than zero).
Solve for Maria’s best choice as a function of M. How much
insurance will she buy, and how much food will she consume?
(Hint: Draw a graph. Does she partially insure or fully insure?)
Does your answer depend on the probability of a hurricane?
Explain.
Exercise 11.12: Why might it make sense for the same riskaverse person to both eliminate risk by purchasing insurance
and take on new risk by investing in the stock market?
Exercise 11.13: Recall the risk-sharing problem illustrated
in Figure 11.15 (page 393). Show graphically that if Maria
is less risk averse, she’ll want to buy a larger fraction of the
sunscreen concession (represented by a point to the right of
point D).
Exercise 11.14: Consider again the risk-sharing problem
illustrated in Figure 11.15 (page 393). If the sunscreen
concession becomes more profitable when the sun shines, what
happens to Maria’s best choice? Show your answer graphically.
Can you say whether she will want to own a larger or smaller
share of the concession?
Exercise 11.15: Suppose a consumer can buy equity shares
in any of several risky projects, all of which have the same
net expected payoff and the same variability. Suppose too that
the payoffs from the various projects are uncorrelated. If the
consumer is risk loving, should she diversify? What if she’s
risk neutral? Explain.
Exercise 11.16: Jeffrey, who is risk neutral, is thinking about
investing in one of two mutually exclusive projects. Project
A requires an investment of $200 up front. It pays $600 if it
rains, $800 if it snows, $400 if it hails, and $0 if it’s sunny.
Project B requires an investment of $300 up front. It pays $200
if it rains, $0 if it snows, $600 if it hails, and $700 if it’s sunny.
The probability of each outcome is 0.1 for rain, 0.3 for snow,
0.2 for hail, and 0.4 for sun.
a. What is the net expected payoff from each project? Which
is better for Jeffrey, and by how much?
b. Suppose that a meteorologist can forecast the weather
with perfect accuracy. How much will Jeffrey pay for the
information?
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MICROECONOMIC
THEORY
BASIC PRINCIPLES AND EXTENSIONS
TENTH EDITION
Walter Nicholson
Amherst College
Christopher Snyder
Dartmouth College
CHAPTER
9
Production Functions
The principal activity of any firm is to turn inputs into outputs. Because economists are interested in the
choices the firm makes in accomplishing this goal, but wish to avoid discussing many of the engineering
intricacies involved, they have chosen to construct an abstract model of production. In this model the
relationship between inputs and outputs is formalized by a production function of the form
q ¼ f ðk, l, m, …Þ,
(9.1)
where q represents the firm’s output of a particular good during a period,1 k represents the machine (that
is, capital) usage during the period, l represents hours of labor input, m represents raw materials used,2 and
the notation indicates the possibility of other variables affecting the production process. Equation 9.1 is
assumed to provide, for any conceivable set of inputs, the engineer’s solution to the problem of how best
to combine those inputs to get output.
MARGINAL PRODUCTIVITY
In this section we look at the change in output brought about by a change in one of the
productive inputs. For the purposes of this examination (and indeed for most of the purposes
of this book), it will be more convenient to use a simplified production function defined as
follows.
Production function. The firm’s production function for a particular good, q,
q ¼ f ðk, lÞ,
DEFINITION
(9.2)
shows the maximum amount of the good that can be produced using alternative combinations of capital ðkÞ and labor ðlÞ.
Of course, most of our analysis will hold for any two inputs to the production process we
might wish to examine. The terms capital and labor are used only for convenience. Similarly,
it would be a simple matter to generalize our discussion to cases involving more than two
inputs; occasionally, we will do so. For the most part, however, limiting the discussion to two
inputs will be quite helpful because we can show these inputs on two-dimensional graphs.
Marginal physical product
To study variation in a single input, we define marginal physical product as follows.
Here we use a lowercase q to represent one firm’s output. We reserve the uppercase Q to represent total output in a
market. Generally, we assume that a firm produces only one output. Issues that arise in multiproduct firms are discussed in a
few footnotes and problems.
1
2
In empirical work raw material inputs often are disregarded and output, q, is measured in terms of “value added.”
295
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Part 3 Production and Supply
DEFINITION
Marginal physical product. The marginal physical product of an input is the additional
output that can be produced by employing one more unit of that input while holding all
other inputs constant. Mathematically,
∂q
¼ fk ,
marginal physical product of capital ¼ MPk ¼
∂k
(9.3)
∂q
¼ fl .
marginal physical product of labor ¼ MPl ¼
∂l
Notice that the mathematical definitions of marginal product use partial derivatives, thereby
properly reflecting the fact that all other input usage is held constant while the input of
interest is being varied. For an example, consider a farmer hiring one more laborer to harvest
the crop but holding all other inputs constant. The extra output this laborer produces is that
farmhand’s marginal physical product, measured in physical quantities, such as bushels of
wheat, crates of oranges, or heads of lettuce. We might observe, for example, that 50 workers
on a farm are able to produce 100 bushels of wheat per year, whereas 51 workers, with the
same land and equipment, can produce 102 bushels. The marginal physical product of the
51st worker is then 2 bushels per year.
Diminishing marginal productivity
We might expect that the marginal physical product of an input depends on how much of
that input is used. Labor, for example, cannot be added indefinitely to a given field (while
keeping the amount of equipment, fertilizer, and so forth fixed) without eventually exhibiting
some deterioration in its productivity. Mathematically, the assumption of diminishing marginal physical productivity is an assumption about the second-order partial derivatives of the
production function:
∂MPk
∂2 f
¼ 2 ¼ fkk ¼ f11 < 0,
∂k
∂k
∂MPl
∂2 f
¼ 2 ¼ fll ¼ f22 < 0:
∂l
∂l
(9.4)
The assumption of diminishing marginal productivity was originally proposed by the
nineteenth-century economist Thomas Malthus, who worried that rapid increases in
population would result in lower labor productivity. His gloomy predictions for the future
of humanity led economics to be called the “dismal science.” But the mathematics of the
production function suggests that such gloom may be misplaced. Changes in the marginal
productivity of labor over time depend not only on how labor input is growing, but also on
changes in other inputs, such as capital. That is, we must also be concerned with ∂MPl =∂k ¼ flk .
In most cases, flk > 0, so declining labor productivity as both l and k increase is not a foregone
conclusion. Indeed, it appears that labor productivity has risen significantly since Malthus’
time, primarily because increases in capital inputs (along with technical improvements) have
offset the impact of diminishing marginal productivity alone.
Average physical productivity
In common usage, the term labor productivity often means average productivity. When it is
said that a certain industry has experienced productivity increases, this is taken to mean that
output per unit of labor input has increased. Although the concept of average productivity
is not nearly as important in theoretical economic discussions as marginal productivity is, it
receives a great deal of attention in empirical discussions. Because average productivity is
Chapter 9 Production Functions
easily measured (say, as so many bushels of wheat per labor-hour input), it is often used as a
measure of efficiency. We define the average product of labor (APl ) to be
output
q
f ðk, lÞ
¼
¼
.
(9.5)
APl ¼
labor input
l
l
Notice that APl also depends on the level of capital employed. This observation will prove to
be quite important when we examine the measurement of technical change at the end of this
chapter.
EXAMPLE 9.1 A Two-Input Production Function
Suppose the production function for flyswatters during a particular period can be represented by
(9.6)
q ¼ f ðk, lÞ ¼ 600k2 l 2 k3 l 3 .
To construct the marginal and average productivity functions of labor (l) for this function, we
must assume a particular value for the other input, capital (k). Suppose k ¼ 10. Then the
production function is given by
(9.7)
q ¼ 60,000l 2 1,000l 3 .
Marginal product. The marginal productivity function (when k ¼ 10) is given by
MPl ¼
∂q
¼ 120,000l 3,000l 2 ,
∂l
(9.8)
which diminishes as l increases, eventually becoming negative. This implies that q reaches a
maximum value. Setting MPl equal to 0,
120,000l 3,000l 2 ¼ 0
(9.9)
40l ¼ l 2
(9.10)
l ¼ 40
(9.11)
yields
or
as the point at which q reaches its maximum value. Labor input beyond 40 units per period
actually reduces total output. For example, when l ¼ 40, Equation 9.7 shows that q ¼ 32
million flyswatters, whereas when l ¼ 50, production of flyswatters amounts to only 25 million.
Average product. To find the average productivity of labor in flyswatter production, we
divide q by l, still holding k ¼ 10:
q
(9.12)
APl ¼ ¼ 60,000l 1,000l 2 .
l
Again, this is an inverted parabola that reaches its maximum value when
∂APl
¼ 60,000 2,000l ¼ 0,
∂l
(9.13)
which occurs when l ¼ 30. At this value for labor input, Equation 9.12 shows that
APl ¼ 900,000, and Equation 9.8 shows that MPl is also 900,000. When APl is at a
maximum, average and marginal productivities of labor are equal.3
(continued)
3
This result is quite general. Because
∂APl l ⋅ MPl q
¼
,
∂l
l2
at a maximum l MPl ¼ q or MPl ¼ APl .
297
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Part 3 Production and Supply
EXAMPLE 9.1 CONTINUED
Notice the relationship between total output and average productivity that is illustrated
by this example. Even though total production of flyswatters is greater with 40 workers
(32 million) than with 30 workers (27 million), output per worker is higher in the second
case. With 40 workers, each worker produces 800,000 flyswatters per period, whereas with
30 workers each worker produces 900,000. Because capital input (flyswatter presses) is held
constant in this definition of productivity, the diminishing marginal productivity of labor
eventually results in a declining level of output per worker.
QUERY: How would an increase in k from 10 to 11 affect the MPl and APl functions here?
Explain your results intuitively.
ISOQUANT MAPS AND THE RATE OF
TECHNICAL SUBSTITUTION
To illustrate possible substitution of one input for another in a production function, we use its
isoquant map. Again, we study a production function of the form q ¼ f ðk, lÞ, with the
understanding that “capital” and “labor” are simply convenient examples of any two inputs
that might happen to be of interest. An isoquant (from iso, meaning “equal”) records those
combinations of k and l that are able to produce a given quantity of output. For example, all
those combinations of k and l that fall on the curve labeled “q ¼ 10” in Figure 9.1 are capable
of producing 10 units of output per period. This isoquant then records the fact that there are
many alternative ways of producing 10 units of output. One way might be represented by
point A: We would use lA and kA to produce 10 units of output. Alternatively, we might prefer
FIGURE 9.1
An Isoquant Map
Isoquants record the alternative combinations of inputs that can be used to produce a given level of
output. The slope of these curves shows the rate at which l can be substituted for k while keeping
output constant. The negative of this slope is called the (marginal) rate of technical substitution
(RTS). In the figure, the RTS is positive and diminishing for increasing inputs of labor.
k per period
kA
A
q = 30
q = 20
kB
B
lA
lB
q = 10
l per period
Chapter 9 Production Functions
299
to use relatively less capital and more labor and therefore would choose a point such as B.
Hence, we may define an isoquant as follows.
Isoquant. An isoquant shows those combinations of k and l that can produce a given level of
DEFINITION
output (say, q0 ). Mathematically, an isoquant records the set of k and l that satisfies
f ðk, lÞ ¼ q0 .
(9.14)
As was the case for indifference curves, there are infinitely many isoquants in the k–l plane.
Each isoquant represents a different level of output. Isoquants record successively higher
levels of output as we move in a northeasterly direction. Presumably, using more of each of
the inputs will permit output to increase. Two other isoquants (for q ¼ 20 and q ¼ 30) are
shown in Figure 9.1. You will notice the similarity between an isoquant map and the
individual’s indifference curve map discussed in Part 2. They are indeed similar concepts,
because both represent “contour” maps of a particular function. For isoquants, however, the
labeling of the curves is measurable—an output of 10 units per period has a quantifiable
meaning. Economists are therefore more interested in studying the shape of production
functions than in examining the exact shape of utility functions.
The marginal rate of technical substitution (RTS)
The slope of an isoquant shows how one input can be traded for another while holding
output constant. Examining the slope provides information about the technical possibility of
substituting labor for capital. A formal definition follows.
Marginal rate of technical substitution. The marginal rate of technical substitution (RTS)
DEFINITION
shows the rate at which labor can be substituted for capital while holding output constant
along an isoquant. In mathematical terms,
dk .
(9.15)
RTS ðl for kÞ ¼
dl q¼q0
In this definition, the notation is intended as a reminder that output is to be held constant as l
is substituted for k. The particular value of this trade-off rate will depend not only on the level
of output but also on the quantities of capital and labor being used. Its value depends on the
point on the isoquant map at which the slope is to be measured.
RTS and marginal productivities
To examine the shape of production function isoquants, it is useful to prove the following
result: the RTS (of l for k) is equal to the ratio of the marginal physical productivity of labor
(MPl ) to the marginal physical productivity of capital (MPk ). We begin by setting up the total
differential of the production function:
dq ¼
∂f
∂f
⋅ dl þ
⋅ dk ¼ MPl ⋅ dl þ MPk ⋅ dk,
∂l
∂k
(9.16)
which records how small changes in l and k affect output. Along an isoquant, dq ¼ 0 (output
is constant), so
(9.17)
MPl ⋅ dl ¼ MPk ⋅ dk.
This says that along an isoquant, the gain in output from increasing l slightly is exactly
balanced by the loss in output from suitably decreasing k. Rearranging terms a bit gives
dk MPl
¼ RTS ðl for kÞ ¼
.
(9.18)
MPk
dl q¼q0
Hence the RTS is given by the ratio of the inputs’ marginal productivities.
300
Part 3 Production and Supply
Equation 9.18 shows that those isoquants that we actually observe must be negatively
sloped. Because both MPl and MPk will be nonnegative (no firm would choose to use a costly
input that reduced output), the RTS also will be positive (or perhaps zero). Because the slope
of an isoquant is the negative of the RTS, any firm we observe will not be operating on
the positively sloped portion of an isoquant. Although it is mathematically possible to devise
production functions whose isoquants have positive slopes at some points, it would not make
economic sense for a firm to opt for such input choices.
Reasons for a diminishing RTS
The isoquants in Figure 9.1 are drawn not only with a negative slope (as they should be) but
also as convex curves. Along any one of the curves, the RTS is diminishing. For high ratios of
k to l, the RTS is a large positive number, indicating that a great deal of capital can be given
up if one more unit of labor becomes available. On the other hand, when a lot of labor is
already being used, the RTS is low, signifying that only a small amount of capital can be
traded for an additional unit of labor if output is to be held constant. This assumption would
seem to have some relationship to the assumption of diminishing marginal productivity. A
hasty use of Equation 9.18 might lead one to conclude that a rise in l accompanied by a fall in
k would result in a fall in MPl , a rise in MPk , and, therefore, a fall in the RTS. The problem
with this quick “proof” is that the marginal productivity of an input depends on the level of
both inputs—changes in l affect MPk and vice versa. It is not possible to derive a diminishing
RTS from the assumption of diminishing marginal productivity alone.
To see why this is so mathematically, assume that q ¼ f ðk, lÞ and that fk and fl are positive
(that is, the marginal productivities are positive). Assume also that fkk < 0 and fll < 0 (that
the marginal productivities are diminishing). To show that isoquants are convex, we would
like to show that dðRTSÞ=dl < 0. Since RTS ¼ fl =fk , we have
dRTS dð fl =fk Þ
¼
.
dl
dl
(9.19)
Because fl and fk are functions of both k and l, we must be careful in taking the derivative of
this expression:
dRTS
f ð f þ flk ⋅ dk=dlÞ fl ð fkl þ fkk ⋅ dk=dlÞ
.
¼ k ll
dl
ð fk Þ2
(9.20)
Using the fact that dk=dl ¼ fl =fk along an isoquant and Young’s theorem (fkl ¼ flk ), we have
f 2 f 2fk fl fkl þ f 2l fkk
dRTS
¼ k ll
.
dl
ð f k Þ3
(9.21)
Because we have assumed fk > 0, the denominator of this function is positive. Hence the
whole fraction will be negative if the numerator is negative. Because fll and fkk are both
assumed to be negative, the numerator definitely will be negative if fkl is positive. If we can
assume this, we have shown that dRTS=dl < 0 (that the isoquants are convex)4.
Importance of cross-productivity effects
Intuitively, it seems reasonable that the cross-partial derivative fkl ¼ flk should be positive. If
workers had more capital, they would have higher marginal productivities. But, although this
is probably the most prevalent case, it does not necessarily have to be so. Some production
functions have fkl < 0, at least for a range of input values. When we assume a diminishing
4
As we pointed out in Chapter 2, functions for which the numerator in Equation 9.21 is negative are called (strictly) quasiconcave functions.
Chapter 9 Production Functions
RTS (as we will throughout most of our discussion), we are therefore making a stronger
assumption than simply diminishing marginal productivities for each input—specifically, we
are assuming that marginal productivities diminish “rapidly enough” to compensate for any
possible negative cross-productivity effects. Of course, as we shall see later, with three or
more inputs, things become even more complicated.
EXAMPLE 9.2 A Diminishing RTS
In Example 9.1, the production function for flyswatters was given by
q ¼ f ðk, lÞ ¼ 600k2 l 2 k3 l 3 .
(9.22)
General marginal productivity functions for this production function are
∂q
¼ 1,200k 2 l 3k 3 l 2 ,
∂l
∂q
MPk ¼ fk ¼
¼ 1,200kl 2 3k2 l 3 .
∂k
MPl ¼ fl ¼
(9.23)
Notice that each of these depends on the values of both inputs. Simple factoring shows that
these marginal productivities will be positive for values of k and l for which kl < 400.
Because
fll ¼ 1,200k2 6k 3 l
and
fkk ¼ 1,200l 2 6kl 3 ,
(9.24)
it is clear that this function exhibits diminishing marginal productivities for sufficiently large
values of k and l. Indeed, again by factoring each expression, it is easy to show that fll , fkk < 0 if
kl > 200. However, even within the range 200 < kl < 400 where the marginal productivity
relations for this function behave “normally,” this production function may not necessarily have
a diminishing RTS. Cross-differentiation of either of the marginal productivity functions
(Equation 9.23) yields
fkl ¼ flk ¼ 2,400kl 9k2 l 2 ,
(9.25)
which is positive only for kl < 266.
The numerator of Equation 9.21 will therefore definitely be negative for 200 < kl < 266,
but for larger-scale flyswatter factories the case is not so clear, because fkl is negative. When fkl
is negative, increases in labor input reduce the marginal productivity of capital. Hence, the
intuitive argument that the assumption of diminishing marginal productivities yields an
unambiguous prediction about what will happen to the RTS ð¼ fl =fk Þ as l increases and k
falls is incorrect. It all depends on the relative effects on marginal productivities of diminishing
marginal productivities (which tend to reduce fl and increase fk ) and the contrary effects of
cross-marginal productivities (which tend to increase fl and reduce fk ). Still, for this flyswatter
case, it is true that the RTS is diminishing throughout the range of k and l, where marginal
productivities are positive. For cases where 266 < kl < 400, the diminishing marginal productivities exhibited by the function are sufficient to overcome the influence of a negative
value for fkl on the convexity of isoquants.
QUERY: For cases where k ¼ l, what can be said about the marginal productivities of this
production function? How would this simplify the numerator for Equation 9.21? How does
this permit you to more easily evaluate this expression for some larger values of k and l?
301
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Part 3 Production and Supply
RETURNS TO SCALE
We now proceed to characterize production functions. A first question that might be asked
about them is how output responds to increases in all inputs together. For example, suppose
that all inputs were doubled: Would output double or would the relationship not be quite so
simple? This is a question of the returns to scale exhibited by the production function that has
been of interest to economists ever since Adam Smith intensively studied the production of
pins. Smith identified two forces that came into operation when the conceptual experiment of
doubling all inputs was performed. First, a doubling of scale permits a greater division of labor
and specialization of function. Hence, there is some presumption that efficiency might
increase—production might more than double. Second, doubling of the inputs also entails
some loss in efficiency because managerial overseeing may become more difficult given the
larger scale of the firm. Which of these two tendencies will have a greater effect is an
important empirical question.
Presenting a technical definition of these concepts is misleadingly simple.
DEFINITION
Returns to scale. If the production function is given by q ¼ f ðk, lÞ and if all inputs are
multiplied by the same positive constant t (where t > 1), then we classify the returns to scale
of the production function by
Effect on Output
Returns to Scale
I. f ðtk, tlÞ ¼ tf ðk, lÞ ¼ tq
Constant
II. f ðtk, tlÞ < tf ðk, lÞ ¼ tq
Decreasing
III. f ðtk, tlÞ > tf ðk, l Þ ¼ tq
Increasing
In intuitive terms, if a proportionate increase in inputs increases output by the same proportion, the production function exhibits constant returns to scale. If output increases less than
proportionately, the function exhibits diminishing returns to scale. And if output increases
more than proportionately, there are increasing returns to scale. As we shall see, it is theoretically possible for a function to exhibit constant returns to scale for some levels of input
usage and increasing or decreasing returns for other levels.5 Often, however, economists refer
to the degree of returns to scale of a production function with the implicit notion that only a
fairly narrow range of variation in input usage and the related level of output is being
considered.
Constant returns to scale
There are economic reasons why a firm’s production function might exhibit constant
returns to scale. If the firm operates many identical plants, it may increase or decrease
production simply by varying the number of them in current operation. That is, the firm
can double output by doubling the number of plants it operates, and that will require it to
employ precisely twice as many inputs. Alternatively, if one were modeling the behavior of
an entire industry composed of many firms, the constant returns-to-scale assumption might
5
A local measure of returns to scale is provided by the scale elasticity, defined as
eq, t ¼
∂f ðtk, tlÞ
t
,
⋅
∂t
f ðtk, tlÞ
where this expression is to be evaluated at t ¼ 1. This parameter can, in principle, take on different values depending on the
level of input usage. For some examples using this concept, see Problem 9.9.
Chapter 9 Production Functions
make sense because the industry can expand or contract by adding or dropping an arbitrary
number of identical firms (see Chapter 12). Finally, studies of the entire U.S. economy
have found that constant returns to scale is a reasonably good approximation to use for an
“aggregate” production function. For all of these reasons, then, the constant returns-toscale case seems worth examining in somewhat more detail.
When a production function exhibits constant returns to scale, it meets the definition of
“homogeneity” that we introduced in Chapter 2. That is, the production is homogeneous of
degree 1 in its inputs because
f ðtk, tlÞ ¼ t 1 f ðk, lÞ ¼ tq.
(9.26)
In Chapter 2 we showed that, if a function is homogeneous of degree k, its derivatives are
homogeneous of degree k 1. In this context this implies that the marginal productivity
functions derived from a constant returns-to-scale production function are homogeneous of
degree 0. That is,
∂f ðk, lÞ ∂f ðtk, tlÞ
¼
,
∂k
∂k
∂f ðk, lÞ ∂f ðtk, tlÞ
¼
MPl ¼
∂l
∂l
MPk ¼
(9.27)
for any t > 0. In particular, we can let t ¼ 1=l in Equations 9.27 and get
∂f ðk=l, 1Þ
,
∂k
∂f ðk=l, 1Þ
.
MPl ¼
∂l
MPk ¼
(9.28)
That is, the marginal productivity of any input depends only on the ratio of capital to labor
input, not on the absolute levels of these inputs. This fact is especially important, for
example, in explaining differences in productivity among industries or across countries.
Homothetic production functions
One consequence of Equations 9.28 is that the RTS ð¼ MPl =MPk Þ for any constant returnsto-scale production function will depend only on the ratio of the inputs, not on their
absolute levels. That is, such a function will be homothetic (see Chapter 2)—its isoquants
will be radial expansions of one another. This situation is shown in Figure 9.2. Along any ray
through the origin (where the ratio k=l does not change), the slopes of successively higher
isoquants are identical. This property of the isoquant map will be very useful to us on several
occasions.
A simple numerical example may provide some intuition about this result. Suppose a roof
can be installed in one day by three workers with one hammer each or by two workers with
two hammers each (these workers are ambidextrous). The RTS of hammers for workers is
therefore one for one—one extra hammer can be substituted for one worker. If this production process exhibits constant returns to scale, two roofs can be installed in one day either by
six workers with six hammers or by four workers with eight hammers. In the latter case, two
hammers are substituted for two workers, so again the RTS is one for one. In constant
returns-to-scale cases, expanding the level of production does not alter trade-offs among
inputs, so production functions are homothetic.
A production function can have a homothetic indifference curve map even if it does not
exhibit constant returns to scale. As we showed in Chapter 2, this property of homotheticity is
retained by any monotonic transformation of a homogeneous function. Hence, increasing or
decreasing returns to scale can be incorporated into a constant returns-to-scale function
303
304
Part 3 Production and Supply
FIGURE 9.2
Isoquant Map for a Constant Returns-to-Scale Production Function
For a constant returns-to-scale production function, the RTS depends only on the ratio of k to l, not
on the scale of production. Consequently, each isoquant will be a radial blowup of the unit isoquant.
Along any ray through the origin (a ray of constant k=l), the RTS will be the same on all isoquants.
k per period
q=3
q=2
q=1
l per period
through an appropriate transformation. Perhaps the most common such transformation is
exponential. So, if f ðk, lÞ is a constant returns-to-scale producton function, we can let
F ðk, lÞ ¼ ½ f ðk, lÞγ ,
(9.29)
where γ is any positive exponent. If γ > 1 then
F ðtk, tlÞ ¼ ½ f ðtk, tlÞγ ¼ ½tf ðk, lÞγ ¼ t γ ½ f ðk, lÞγ ¼ t γ F ðk, lÞ > tF ðk, lÞ
(9.30)
for any t > 1. Hence, this transformed production function exhibits increasing returns to
scale. An identical proof shows that the function F exhibits decreasing returns to scale for
γ < 1 . Because this function remains homothetic through all such transformations, we have
shown that there are important cases where the issue of returns to scale can be separated
from issues involving the shape of an isoquant. In the next section, we will look at how
shapes of isoquants can be described.
The n-input case
The definition of returns to scale can be easily generalized to a production function with n
inputs. If that production function is given by
q ¼ f ðx1 , x2 , …, xn Þ
(9.31)
and if all inputs are multiplied by t > 1, we have
f ðtx1 , tx2 , …, txn Þ ¼ t k f ðx1 , x2 , …, xn Þ ¼ t k q
(9.32)
for some constant k. If k ¼ 1, the production function exhibits constant returns to scale. Diminishing and increasing returns to scale correspond to the cases k < 1 and k > 1, respectively.
The crucial part of this mathematical definition is the requirement that all inputs be
increased by the same proportion, t . In many real-world production processes, this provision
may make little economic sense. For example, a firm may have only one “boss,” and that
Chapter 9 Production Functions
305
number would not necessarily be doubled even if all other inputs were. Or the output of a
farm may depend on the fertility of the soil. It may not be literally possible to double the acres
planted while maintaining fertility, because the new land may not be as good as that already
under cultivation. Hence, some inputs may have to be fixed (or at least imperfectly variable)
for most practical purposes. In such cases, some degree of diminishing productivity (a result
of increasing employment of variable inputs) seems likely, although this cannot properly be
called “diminishing returns to scale” because of the presence of inputs that are held fixed.
THE ELASTICITY OF SUBSTITUTION
Another important characteristic of the production function is how “easy” it is to substitute
one input for another. This is a question about the shape of a single isoquant rather than
about the whole isoquant map. Along one isoquant, the rate of technical substitution will
decrease as the capital-labor ratio decreases (that is, as k=l decreases); now we wish to define
some parameter that measures this degree of responsiveness. If the RTS does not change at all
for changes in k=l, we might say that substitution is easy because the ratio of the marginal
productivities of the two inputs does not change as the input mix changes. Alternatively, if the
RTS changes rapidly for small changes in k=l, we would say that substitution is difficult
because minor variations in the input mix will have a substantial effect on the inputs’ relative
productivities. A scale-free measure of this responsiveness is provided by the elasticity of
substitution, a concept we encountered in Part 2. Now we can provide a formal definition.
Elasticity of substitution. For the production function q ¼ f ðk, lÞ, the elasticity of substitution ðσÞ measures the proportionate change in k=l relative to the proportionate change in D E F I N I T I O N
the RTS along an isoquant. That is,
σ¼
percent ∆ðk=lÞ dðk=lÞ RTS
∂ ln k=l
∂ ln k=l
.
¼
¼
¼
⋅
percent ∆RTS
dRTS
k=l
∂ ln RTS ∂ ln fl =fk
(9.33)
Because along an isoquant, k=l and RTS move in the same direction, the value of σ is always
positive. Graphically, this concept is illustrated in Figure 9.3 as a movement from point A to
point B on an isoquant. In this movement, both the RTS and the ratio k=l will change; we are
interested in the relative magnitude of these changes. If σ is high, then the RTS will not change
much relative to k=l and the isoquant will be relatively flat. On the other hand, a low value of σ
implies a rather sharply curved isoquant; the RTS will change by a substantial amount as k=l
changes. In general, it is possible that the elasticity of substitution will vary as one moves along
an isoquant and as the scale of production changes. Often, however, it is convenient to assume
that σ is constant along an isoquant. If the production function is also homothetic, then—
because all the isoquants are merely radial blowups—σ will be the same along all isoquants. We
will encounter such functions later in this chapter and in many of its problems.6
The n-input case
Generalizing the elasticity of substitution to the many-input case raises several complications.
One approach is to adopt a definition analogous to Equation 9.33; that is, to define the
elasticity of substitution between two inputs to be the proportionate change in the ratio of
6
The elasticity of substitution can be phrased directly in terms of the production function and its derivatives in the constant
returns-to-scale case as
f ⋅f
σ¼ k l .
f ⋅ fk, l
But this form is quite cumbersome. Hence usually the logarithmic definition in Equation 9.33 is easiest to apply. For a
compact summary, see P. Berck and K. Sydsaeter, Economist’s Mathematical Manual (Berlin: Springer-Verlag, 1999),
chap. 5.
306
Part 3 Production and Supply
FIGURE 9.3
Graphic Description of the Elasticity of Substitution
In moving from point A to point B on the q ¼ q0 isoquant, both the capital-labor ratio (k=l) and the
RTS will change. The elasticity of substitution (σ) is defined to be the ratio of these proportional
changes; it is a measure of how curved the isoquant is.
k per
period
A
RTSA
RTSB
B
q = q0
(k /l ) A
(k /l ) B
l per period
the two inputs to the proportionate change in the RTS between them while holding output
constant.7 To make this definition complete, it is necessary to require that all inputs other
than the two being examined be held constant. However, this latter requirement (which is
not relevant when there are only two inputs) restricts the value of this potential definition. In
real-world production processes, it is likely that any change in the ratio of two inputs will also
be accompanied by changes in the levels of other inputs. Some of these other inputs may be
complementary with the ones being changed, whereas others may be substitutes, and to hold
them constant creates a rather artificial restriction. For this reason, an alternative definition of
the elasticity of substitution that permits such complementarity and substitutability in the
firm’s cost function is generally used in the n-good case. Because this concept is usually
measured using cost functions, we will describe it in the next chapter.
FOUR SIMPLE PRODUCTION FUNCTIONS
In this section we illustrate four simple production functions, each characterized by a
different elasticity of substitution. These are shown only for the case of two inputs, but
generalization to many inputs is easily accomplished (see the Extensions for this chapter).
7
That is, the elasticity of substitution between input i and input j might be defined as
σij ¼
∂ lnðxi =xj Þ
∂ lnð fj =fi Þ
for movements along f ðx1 , x2 , …, xn Þ ¼ c. Notice that the use of partial derivatives in this definition effectively requires that
all inputs other than i and j be held constant when considering movements along the c isoquant.
Chapter 9 Production Functions
Case 1: Linear (σ ¼ ∞)
Suppose that the production function is given by
q ¼ f ðk, lÞ ¼ ak þ bl.
(9.34)
It is easy to show that this production function exhibits constant returns to scale: For
any t > 1,
f ðtk, tlÞ ¼ atk þ btl ¼ t ðak þ blÞ ¼ tf ðk, lÞ.
(9.35)
All isoquants for this production function are parallel straight lines with slope b=a. Such an
isoquant map is pictured in panel (a) of Figure 9.4. Because the RTS is constant along any
straight-line isoquant, the denominator in the definition of σ (Equation 9.33) is equal to 0
and hence σ is infinite. Although this linear production function is a useful example, it is
FIGURE 9.4
Isoquant Maps for Simple Production Functions with Various Values for σ
Three possible values for the elasticity of substitution are illustrated in these figures. In (a), capital
and labor are perfect substitutes. In this case, the RTS will not change as the capital-labor ratio
changes. In (b), the fixed-proportions case, no substitution is possible. The capital-labor ratio is fixed
at b=a. A case of limited substitutability is illustrated in (c).
k per
period
k per
period
σ=∞
σ=0
–b
Slope = __
a
q
__3
a
q1
q2
q3
q2
q3
q1
l per period
(a)
(b)
q
3
__
b
k per
period
σ=1
q3
q2
q1
l per period
(c)
l per period
307
308
Part 3 Production and Supply
rarely encountered in practice because few production processes are characterized by such
ease of substitution. Indeed, in this case, capital and labor can be thought of as perfect
substitutes for each other. An industry characterized by such a production function could
use only capital or only labor, depending on these inputs’ prices. It is hard to envision such a
production process: Every machine needs someone to press its buttons, and every laborer
requires some capital equipment, however modest.
Case 2: Fixed proportions (σ ¼ 0)
The production function characterized by σ ¼ 0 is the important case of a fixed-proportions
production function. Capital and labor must always be used in a fixed ratio. The isoquants for
this production function are L-shaped and are pictured in panel (b) of Figure 9.4. A firm
characterized by this production function will always operate along the ray where the ratio k=l
is constant. To operate at some point other than at the vertex of the isoquants would be
inefficient, because the same output could be produced with fewer inputs by moving along
the isoquant toward the vertex. Because k=l is a constant, it is easy to see from the definition of
the elasticity of substitution that σ must equal 0.
The mathematical form of the fixed-proportions production function is given by
q ¼ minðak, blÞ,
a, b > 0,
(9.36)
where the operator “min” means that q is given by the smaller of the two values in parentheses.
For example, suppose that ak < bl; then q ¼ ak, and we would say that capital is the binding
constraint in this production process. The employment of more labor would not raise output,
and hence the marginal product of labor is zero; additional labor is superfluous in this case.
Similarly, if ak > bl, then labor is the binding constraint on output and additional capital is
superfluous. When ak ¼ bl, both inputs are fully utilized. When this happens, k=l ¼ b=a, and
production takes place at a vertex on the isoquant map. If both inputs are costly, this is the only
cost-minimizing place to operate. The locus of all such vertices is a straight line through the
origin with a slope given by b=a.
The fixed-proportions production function has a wide range of applications.8 Many
machines, for example, require a certain number of people to run them, but any excess
labor is superfluous. Consider combining capital (a lawn mower) and labor to mow a lawn. It
will always take one person to run the mower, and either input without the other is not able
to produce any output at all. It may be that many machines are of this type and require a fixed
complement of workers per machine.9
Case 3: Cobb-Douglas (σ ¼ 1)
The production function for which σ ¼ 1, called a Cobb-Douglas production function,10
provides a middle ground between the two polar cases previously discussed. Isoquants for
With the form reflected by Equation 9.35, the fixed-proportions production function exhibits constant returns to scale
because
8
f ðtk, tlÞ ¼ minðatk, btlÞ ¼ t minðak, blÞ ¼ tf ðk, lÞ
for any t > 1. As before, increasing or decreasing returns can be easily incorporated into the functions by using a nonlinear
transformation of this functional form—such as ½ f ðk, lÞγ , where γ may be greater than or less than 1.
9
The lawn mower example points up another possibility, however. Presumably there is some leeway in choosing what size
of lawn mower to buy. Hence, prior to the actual purchase, the capital-labor ratio in lawn mowing can be considered
variable: Any device, from a pair of clippers to a gang mower, might be chosen. Once the mower is purchased, however, the
capital-labor ratio becomes fixed.
10
Named after C. W. Cobb and P. H. Douglas. See P. H. Douglas, The Theory of Wages (New York: Macmillan Co.,
1934), pp. 132–f35.
Chapter 9 Production Functions
the Cobb-Douglas case have the “normal” convex shape and are shown in panel (c) of
Figure 9.4. The mathematical form of the Cobb-Douglas production function is given by
q ¼ f ðk, lÞ ¼ Ak a l b ,
(9.37)
where A, a, and b are all positive constants.
The Cobb-Douglas function can exhibit any degree of returns to scale, depending on the
values of a and b. Suppose all inputs were increased by a factor of t . Then
f ðtk, tlÞ ¼ AðtkÞa ðtlÞb ¼ At aþb ka l b
¼ t aþb f ðk, lÞ.
(9.38)
Hence, if a þ b ¼ 1, the Cobb-Douglas function exhibits constant returns to scale because
output also increases by a factor of t . If a þ b > 1 then the function exhibits increasing
returns to scale, whereas a þ b < 1 corresponds to the decreasing returns-to-scale case. It
is a simple matter to show that the elasticity of substitution is 1 for the Cobb-Douglas
function.11 This fact has led researchers to use the constant returns-to-scale version of
the function for a general description of aggregate production relationships in many
countries.
The Cobb-Douglas function has also proved to be quite useful in many applications
because it is linear in logarithms:
ln q ¼ ln A þ a ln k þ b ln l.
(9.39)
The constant a is then the elasticity of output with respect to capital input, and b is the
elasticity of output with respect to labor input.12 These constants can sometimes be
estimated from actual data, and such estimates may be used to measure returns to scale (by
examining the sum a þ b) and for other purposes.
Case 4: CES production function
A functional form that incorporates all of the three previous cases and allows σ to take on
other values as well is the constant elasticity of substitution (CES) production function first
introduced by Arrow et al. in 1961.13 This function is given by
q ¼ f ðk, lÞ ¼ ½k ρ þ l ρ γ=ρ
(9.40)
for ρ 1, ρ 6¼ 0, and γ > 0. This function closely resembles the CES utility function
discussed in Chapter 3, though now we have added the exponent γ=ρ to permit explicit
introduction of returns-to-scale factors. For γ > 1 the function exhibits increasing returns to
scale, whereas for γ < 1 it exhibits diminishing returns.
11
For the Cobb-Douglas function,
RTS ¼
fl bAka l b1 b k
¼
¼
fk
aAka1 l b a l
or
ln RTS ¼ lnðb=aÞ þ lnðk=lÞ.
Hence
σ¼
12
∂ ln k=l
¼ 1.
∂ ln RTS
See Problem 9.5.
K. J. Arrow, H. B. Chenery, B. S. Minhas, and R. M. Solow, “Capital-Labor Substitution and Economic Efficiency,”
Review of Economics and Statistics (August 1961): 225–50.
13
309
310
Part 3 Production and Supply
Direct application of the definition of σ to this function14 gives the important result that
σ¼
1
.
1ρ
(9.41)
Hence the linear, fixed-proportions, and Cobb-Douglas cases correspond to ρ ¼ 1,
ρ ¼ ∞, and ρ ¼ 0, respectively. Proof of this result for the fixed proportions and CobbDouglas cases requires a limit argument.
Often the CES function is used with a distributional weight, β ð0 β 1Þ, to indicate
the relative significance of the inputs:
q ¼ f ðk, lÞ ¼ ½βkρ þ ð1 βÞl ρ γ=ρ :
(9.42)
With constant returns to scale and ρ ¼ 0, this function converges to the Cobb-Douglas form
q ¼ f ðk, lÞ ¼ k β l 1β .
(9.43)
EXAMPLE 9.3 A Generalized Leontief Production Function
Suppose that the production function for a good is given by
pffiffiffiffiffiffiffiffi
q ¼ f ðk, lÞ ¼ k þ l þ 2 k ⋅ l .
(9.44)
This function is a special case of a class of functions named for the Russian-American
economist Wassily Leontief.15 The function clearly exhibits constant returns to scale because
pffiffiffiffiffi
f ðtk, tlÞ ¼ tk þ tl þ 2t kl ¼ tf ðk, lÞ.
(9.45)
Marginal productivities for the Leontief function are
fk ¼ 1 þ ðk=lÞ0:5 ,
(9.46)
fl ¼ 1 þ ðk=lÞ0:5 .
Hence, marginal productivities are positive and diminishing. As would be expected (because
this function exhibits constant returns to scale), the RTS here depends only on the ratio of
the two inputs
RTS ¼
fl
1 þ ðk=lÞ0:5
¼
.
fk
1 þ ðk=lÞ0:5
(9.47)
This RTS diminishes as k=l falls, so the isoquants have the usual convex shape.
14
For the CES function we have
RTS ¼
fl
ðγ=ρÞ ⋅ q ðγρÞ=γ ⋅ ρl ρ1
¼
¼
fk ðγ=ρÞ ⋅ q ðγρÞ=γ ⋅ ρk ρ1
l
k
ρ1
¼
k
l
1ρ
.
Applying the definition of the elasticity of substitution then yields
σ¼
∂ lnðk=lÞ
1
¼
.
∂ ln RTS 1 ρ
Notice in this computation that the factor ρ cancels out of the marginal productivity functions, thereby ensuring that these
marginal productivities are positive even when ρ is negative (as it is in many cases). This explains why ρ appears in two
different places in the definition of the CES function.
15
Lenotief was a pioneer in the development of input-output analysis. In input-output analysis, production is assumed to
take place with a fixed-proportions technology. The Leontief production function generalizes the fixed-proportions case.
For more details see the discussion of Leontief production functions in the Extensions to this chapter.
Chapter 9 Production Functions
There are two ways you might calculate the elasticity of substitution for this production
function. First, you might notice that in this special case the function can be factored as
pffiffiffiffiffi
pffiffiffi pffiffi
q ¼ k þ l þ 2 kl ¼ ð k þ l Þ2 ¼ ðk 0:5 þ l 0:5 Þ2 ,
(9.48)
which makes clear that this function has a CES form with ρ ¼ 0:5 and γ ¼ 1. Hence the
elasticity of substitution here is σ ¼ 1=ð1 ρÞ ¼ 2.
Of course, in most cases it is not possible to do such a simple factorization. A more exhaustive approach is to apply the definition of the elasticity of substitution given in footnote 6
of this chapter:
σ¼
¼
fk fl
½1 þ ðk=lÞ0:5 ½1 þ ðk=lÞ0:5 pffiffiffiffiffi
¼
f ⋅ fkl
q ⋅ ð0:5= kl Þ
2 þ ðk=lÞ0:5 þ ðk=lÞ0:5
1 þ 0:5ðk=lÞ0:5 þ 0:5ðk=lÞ0:5
¼ 2:
(9.49)
Notice that in this calculation the input ratio ðk=lÞ drops out, leaving a very simple result. In
other applications, one might doubt that such a fortuitous result would occur and hence
doubt that the elasticity of substitution is constant along an isoquant (see Problem 9.7). But
here the result that σ ¼ 2 is intuitively reasonable, because that value represents a
compromise between the elasticity of substitution for this production function’s linear part
ðq ¼ k þ l, σ ¼ ∞Þ and its Cobb-Douglas part ðq ¼ 2k0:5 l 0:5 , σ ¼ 1Þ.
QUERY: What can you learn about this production function by graphing the q ¼ 4 isoquant?
Why does this function generalize the fixed proportions case?
TECHNICAL PROGRESS
Methods of production improve over time, and it is important to be able to capture these
improvements with the production function concept. A simplified view of such progress is
provided by Figure 9.5. Initially, isoquant q0 records those combinations of capital and labor
that can be used to produce an output level of q0 . Following the development of superior
production techniques, this isoquant shifts to q 00 . Now the same level of output can be
produced with fewer inputs. One way to measure this improvement is by noting that with
a level of capital input of, say, k1 , it previously took l2 units of labor to produce q0 , whereas
now it takes only l1 . Output per worker has risen from q0 =l2 to q0 =l1 . But one must be careful
in this type of calculation. An increase in capital input to k2 would also have permitted a
reduction in labor input to l1 along the original q0 isoquant. In this case, output per worker
would also rise, although there would have been no true technical progress. Use of the
production function concept can help to differentiate between these two concepts and
therefore allow economists to obtain an accurate estimate of the rate of technical change.
Measuring technical progress
The first observation to be made about technical progress is that historically the rate of
growth of output over time has exceeded the growth rate that can be attributed to the growth
in conventionally defined inputs. Suppose that we let
q ¼ Aðt Þf ðk, lÞ
(9.50)
be the production function for some good (or perhaps for society’s output as a whole). The
term AðtÞ in the function represents all the influences that go into determining q other than
k (machine-hours) and l (labor-hours). Changes in A over time represent technical progress.
311
312
Part 3 Production and Supply
FIGURE 9.5
Technical Progress
Technical progress shifts the q0 isoquant toward the origin. The new q0 isoquant, q 00 , shows that a
given level of output can now be produced with less input. For example, with k1 units of capital it
now only takes l1 units of labor to produce q0 , whereas before the technical advance it took l2 units of
labor.
k per
period
k2
k1
q0
q′0
l1
l2
l per period
For this reason, A is shown as a function of time. Presumably dA=dt > 0; particular levels of
input of labor and capital become more productive over time.
Differentiating Equation 9.50 with respect to time gives
dq
dA
df ðk, lÞ
¼
⋅ f ðk, l Þ þ A ⋅
dt
dt
dt
dA q
q
∂f dk
∂f dl
þ
þ
.
¼
⋅
⋅
⋅
dt A
f ðk, lÞ ∂k dt
∂l dt
(9.51)
Dividing by q gives
dq=dt
dA=dt
∂f =∂k dk
∂f =∂l dl
¼
þ
þ
⋅
⋅
q
A
f ðk, lÞ dt
f ðk, lÞ dt
(9.52)
dq=dt
dA=dt
∂f
k
dk=dt
∂f
l
dl=dt
¼
þ
þ
.
⋅
⋅
⋅
⋅
q
A
∂k f ðk, lÞ
k
∂l f ðk, lÞ
l
(9.53)
or
Now, for any variable x, (dx=dt )/x is the proportional rate of growth of x per unit of time.
We shall denote this by Gx .16 Hence, Equation 9.53 can be written in terms of growth rates as
Two useful features of this definition are: (1) Gx ⋅ y ¼ Gx þ Gy —that is, the growth rate of a product of two variables is
the sum of each one’s growth rate; and (2) Gx=y ¼ Gx Gy .
16
Chapter 9 Production Functions
Gq ¼ GA þ
∂f
k
∂f
l
⋅
⋅ Gk þ
⋅
⋅ Gl ,
∂k f ðk, lÞ
∂l f ðk, lÞ
(9.54)
but
∂f
k
∂q k
¼
¼ elasticity of output with respect to capital input
⋅
⋅
∂k f ðk, lÞ
∂k q
¼ eq, k
and
∂f
l
∂q l
¼
⋅
⋅ ¼ elasticity of output with respect to labor input
∂l f ðk, lÞ
∂l q
¼ eq;l .
Growth accounting
Therefore, our growth equation finally becomes
Gq ¼ GA þ eq, k Gk þ eq, l Gl .
(9.55)
This shows that the rate of growth in output can be broken down into the sum of two
components: growth attributed to changes in inputs (k and l) and other “residual” growth
(that is, changes in A) that represents technical progress.
Equation 9.55 provides a way of estimating the relative importance of technical progress
(GA ) in determining the growth of output. For example, in a pioneering study of the entire
U.S. economy between the years 1909 and 1949, R. M. Solow recorded the following values
for the terms in the equation:17
Gq ¼ 2:75 percent per year,
Gl ¼ 1:00 percent per year,
Gk ¼ 1:75 percent per year,
eq, l ¼ 0:65,
eq, k ¼ 0:35.
Consequently,
GA ¼ Gq eq, l Gl eq, k Gk
¼ 2:75 0:65ð1:00Þ 0:35ð1:75Þ
¼ 2:75 0:65 0:60
¼ 1:50.
(9.56)
The conclusion Solow reached, then, was that technology advanced at a rate of 1.5 percent
per year from 1909 to 1949. More than half of the growth in real output could be attributed
to technical change rather than to growth in the physical quantities of the factors of production. More recent evidence has tended to confirm Solow’s conclusions about the relative
importance of technical change. Considerable uncertainty remains, however, about the
precise causes of such change.
R. M. Solow, “Technical Progress and the Aggregate Production Function,” Review of Economics and Statistics 39
(August 1957): 312–f20.
17
313
314
Part 3 Production and Supply
EXAMPLE 9.4 Technical Progress in the Cobb-Douglas Production Function
The Cobb-Douglas production function provides an especially easy avenue for illustrating
technical progress. Assuming constant returns to scale, such a production function with
technical progress might be represented by
q ¼ Aðt Þf ðk, lÞ ¼ Aðt Þk α l 1α .
(9.57)
If we also assume that technical progress occurs at a constant exponential (θ), then we can
write Aðt Þ ¼ Ae θt and the production function becomes
q ¼ Ae θt kα l 1α .
(9.58)
A particularly easy way to study the properties of this type of function over time is to use
“logarithmic differentiation”:
∂ ln q
∂ ln q ∂q ∂q=∂t
∂½ln A þ θt þ α ln k þ ð1 αÞ ln l
¼
¼
¼ Gq ¼
⋅
∂t
∂q
∂t
q
∂t
∂ ln k
∂ ln l
(9.59)
þ ð1 αÞ ⋅
¼ θ þ αGk þ ð1 − αÞGl .
¼θþα⋅
∂t
∂t
So this derivation just repeats Equation 9.55 for the Cobb-Douglas case. Here the technical
change factor is explicitly modeled, and the output elasticities are given by the values of the
exponents in the Cobb-Douglas.
The importance of technical progress can be illustrated numerically with this function.
Suppose A ¼ 10, θ ¼ 0:03, α ¼ 0:5 and that a firm uses an input mix of k ¼ l ¼ 4. Then, at
t ¼ 0, output is 40ð¼ 10 ⋅ 40:5 ⋅ 40:5 Þ. After 20 years ðt ¼ 20Þ, the production function
becomes
q ¼ 10e 0:03⋅20 k0:5 l 0:5 ¼ 10 ⋅ ð1:82Þk 0:5 l 0:5 ¼ 18:2k0:5 l 0:5 .
(9.60)
In year 20 the original input mix now yields q ¼ 72:8. Of course, one could also have
produced q ¼ 72:8 in year 0, but it would have taken a lot more inputs. For example, with
k ¼ 13:25 and l ¼ 4, output is indeed 72.8 but much more capital is used. Output per unit of
labor input would rise from 10 (q=l ¼ 40=4) to 18:2 ð¼ 72:8=4) in either circumstance, but
only the first case would have been true technical progress.
Input-augmenting technical progress. It is tempting to attribute the increase in the
average productivity of labor in this example to, say, improved worker skills, but that
would be misleading in the Cobb-Douglas case. One might just as well have said that output
per unit of capital rose from 10 to 18.2 over the 20 years and attribute this rise to improved
machinery. A plausible approach to modeling improvements in labor and capital separately is
to assume that the production function is
q ¼ Aðe φt kÞα ðe εt lÞ1α ,
(9.61)
where φ represents the annual rate of improvement in capital input and ε represents the
annual rate of improvement in labor input. But, because of the exponential nature of the
Cobb-Douglas function, this would be indistinguishable from our original example:
q ¼ Ae ½αφþð1αÞεt kα l 1α ¼ Ae θt kα l 1α ,
(9.62)
where θ ¼ αφ þ ð1 αÞε. Hence, to study technical progress in individual inputs, it is
necessary either to adopt a more complex way of measuring inputs that allows for improving
quality or (what amounts to the same thing) to use a multi-input production function.
Chapter 9 Production Functions
315
QUERY: Actual studies of production using the Cobb-Douglas tend to find α 0.3. Use this
finding together with Equation 9.62 to discuss the relative importance of improving capital
and labor quality to the overall rate of technical progress.
SUMMARY
In this chapter we illustrated the ways in which economists
conceptualize the production process of turning inputs into
outputs. The fundamental tool is the production function,
which—in its simplest form—assumes that output per period
(q) is a simple function of capital and labor inputs during that
period, q ¼ f ðk, lÞ. Using this starting point, we developed
several basic results for the theory of production.
•
If all but one of the inputs are held constant, a relationship between the single-variable input and output can be
derived. From this relationship, one can derive the marginal physical productivity (MP) of the input as the
change in output resulting from a one-unit increase in
the use of the input. The marginal physical productivity
of an input is assumed to decline as use of the input
increases.
•
The entire production function can be illustrated by its
isoquant map. The (negative of the) slope of an isoquant
is termed the marginal rate of technical substitution
(RTS), because it shows how one input can be substituted for another while holding output constant. The
RTS is the ratio of the marginal physical productivities of
the two inputs.
•
Isoquants are usually assumed to be convex—they obey
the assumption of a diminishing RTS. This assumption
cannot be derived exclusively from the assumption of
diminishing marginal physical productivities. One must
also be concerned with the effect of changes in one input
on the marginal productivity of other inputs.
•
The returns to scale exhibited by a production function
record how output responds to proportionate increases
in all inputs. If output increases proportionately with
input use, there are constant returns to scale. If there
are greater than proportionate increases in output, there
are increasing returns to scale, whereas if there are less
than proportionate increases in output, there are decreasing returns to scale.
•
The elasticity of substitution ðσÞ provides a measure of
how easy it is to substitute one input for another in production. A high σ implies nearly linear isoquants, whereas
a low σ implies that isoquants are nearly L-shaped.
•
Technical progress shifts the entire production function
and its related isoquant map. Technical improvements
may arise from the use of improved, more-productive
inputs or from better methods of economic organization.
PROBLEMS
9.1
Power Goat Lawn Company uses two sizes of mowers to cut lawns. The smaller mowers have a 24-inch
blade and are used on lawns with many trees and obstacles. The larger mowers are exactly twice as big as
the smaller mowers and are used on open lawns where maneuverability is not so difficult. The two
production functions available to Power Goat are:
Output per Hour
(square feet)
Capital Input
(# of 2400 mowers)
Labor Input
Large mowers
8000
2
1
Small mowers
5000
1
1
a. Graph the q ¼ 40,000 square feet isoquant for the first production function. How much k and l
would be used if these factors were combined without waste?
316
Part 3 Production and Supply
b. Answer part (a) for the second function.
c. How much k and l would be used without waste if half of the 40,000-square-foot lawn were cut
by the method of the first production function and half by the method of the second? How
much k and l would be used if three fourths of the lawn were cut by the first method and one
fourth by the second? What does it mean to speak of fractions of k and l?
d. On the basis of your observations in part (c), draw a q ¼ 40,000 isoquant for the combined
production functions.
9.2
Suppose the production function for widgets is given by
q ¼ kl 0:8k 2 0:2l 2 ,
where q represents the annual quantity of widgets produced, k represents annual capital input, and l
represents annual labor input.
a. Suppose k ¼ 10; graph the total and average productivity of labor curves. At what level of labor
input does this average productivity reach a maximum? How many widgets are produced at
that point?
b. Again assuming that k ¼ 10, graph the MPl curve. At what level of labor input does MPl ¼ 0?
c. Suppose capital inputs were increased to k ¼ 20. How would your answers to parts (a) and (b)
change?
d. Does the widget production function exhibit constant, increasing, or decreasing returns to
scale?
9.3
Sam Malone is considering renovating the bar stools at Cheers. The production function for new bar
stools is given by
q ¼ 0:1k 0:2 l 0:8 ,
where q is the number of bar stools produced during the renovation week, k represents the number of
hours of bar stool lathes used during the week, and l represents the number of worker hours employed
during the period. Sam would like to provide 10 new bar stools, and he has allocated a budget of
$10,000 for the project.
a. Sam reasons that because bar stool lathes and skilled bar stool workers both cost the same
amount ($50 per hour), he might as well hire these two inputs in equal amounts. If Sam
proceeds in this way, how much of each input will he hire and how much will the renovation
project cost?
b. Norm (who knows something about bar stools) argues that once again Sam has forgotten his
microeconomics. He asserts that Sam should choose quantities of inputs so that their marginal
(not average) productivities are equal. If Sam opts for this plan instead, how much of each input
will he hire and how much will the renovation project cost?
c. Upon hearing that Norm’s plan will save money, Cliff argues that Sam should put the savings
into more bar stools in order to provide seating to more of his USPS colleagues. How many
more bar stools can Sam get for his budget if he follows Cliff’s plan?
d. Carla worries that Cliff’s suggestion will just mean more work for her in delivering food to bar
patrons. How might she convince Sam to stick to his original 10–bar stool plan?
Chapter 9 Production Functions
9.4
Suppose that the production of crayons ðqÞ is conducted at two locations and uses only labor as an input.
0.5
The production function in location 1 is given by q1 ¼ 10l 0.5
1 and in location 2 by q2 ¼ 50l 2 :
a. If a single firm produces crayons in both locations, then it will obviously want to get as large an
output as possible given the labor input it uses. How should it allocate labor between the
locations in order to do so? Explain precisely the relationship between l1 and l2 :
b. Assuming that the firm operates in the efficient manner described in part (a), how does total
output ðqÞ depend on the total amount of labor hired ðlÞ?
9.5
As we have seen in many places, the general Cobb-Douglas production function for two inputs is
given by
q ¼ f ðk, lÞ ¼ Akα l β ,
where 0 < α < 1 and 0 < β < 1: For this production function:
a. Show that fk > 0, fl > 0, fkk < 0, fll < 0, and fkl ¼ flk > 0.
b. Show that eq, k ¼ α and ee, l ¼ β:
c. In footnote 5, we defined the scale elasticity as
eq, t ¼
∂f ðtk, tlÞ
t
,
⋅
∂t
f ðtk, tlÞ
where the expression is to be evaluated at t ¼ 1: Show that, for this Cobb-Douglas function,
eq, t ¼ α þ β: Hence, in this case the scale elasticity and the returns to scale of the production
function agree (for more on this concept see Problem 9.9).
d. Show that this function is quasi-concave.
e. Show that the function is concave for α þ β 1 but not concave for α þ β > 1:
9.6
Suppose we are given the constant returns-to-scale CES production function
q ¼ ½k ρ þ l ρ 1=ρ .
a. Show that MPk ¼ ðq=kÞ1ρ and MPl ¼ ðq=lÞ1ρ :
b. Show that RTS ¼ ðl=kÞ1ρ ; use this to show that σ ¼ 1=ð1 ρÞ:
c. Determine the output elasticities for k and l, and show that their sum equals 1.
d. Prove that
q
¼
l
and hence that
ln
q
l
∂q
∂l
¼ σ ln
σ
∂q
.
∂l
Note: The latter equality is useful in empirical work, because we may approximate ∂q=∂l by
the competitively determined wage rate. Hence, σ can be estimated from a regression of
lnðq=lÞ on ln w:
317
318
Part 3 Production and Supply
9.7
Consider a generalization of the production function in Example 9.3:
pffiffiffiffiffi
q ¼ β0 þ β1 kl þ β2 k þ β3 l,
where
0 βi 1,
i ¼ 0,…,3.
a. If this function is to exhibit constant returns to scale, what restrictions should be placed on
the parameters β0 , . . . , β3 ?
b. Show that, in the constant returns-to-scale case, this function exhibits diminishing marginal
productivities and that the marginal productivity functions are homogeneous of degree 0.
c. Calculate σ in this case. Although σ is not in general constant, for what values of the β’s does
σ ¼ 0, 1, or ∞?
9.8
Show that Euler’s theorem implies that, for a constant returns-to-scale production function
½q ¼ f ðk, lÞ,
q ¼ fk ⋅ k þ fl ⋅ l:
Use this result to show that, for such a production function, if MPl > APl then MPk must be negative.
What does this imply about where production must take place? Can a firm ever produce at a point
where APl is increasing?
Analytical Problems
9.9 Local returns to scale
A local measure of the returns to scale incorporated in a production function is given by the scale
elasticity eq, t ¼ ∂f ðtk, tlÞ=∂t ⋅ t =q evaluated at t ¼ l:
a. Show that if the production function exhibits constant returns to scale then eq, t ¼ 1:
b. We can define the output elasticities of the inputs k and l as
eq, k
eq, l
∂f ðk, lÞ
⋅
∂k
∂f ðk, lÞ
¼
⋅
∂l
¼
k
,
q
l
.
q
Show that eq, t ¼ eq, k þ eq, l :
c. A function that exhibits variable scale elasticity is
q ¼ ð1 þ k1 l 1 Þ1 :
Show that, for this function, eq, t > 1 for q < 0.5 and that eq, t < 1 for q > 0.5:
d. Explain your results from part (c) intuitively. Hint: Does q have an upper bound for this
production function?
Chapter 9 Production Functions
319
9.10 Returns to scale and substitution
Although much of our discussion of measuring the elasticity of substitution for various production
functions has assumed constant returns to scale, often that assumption is not necessary. This problem
illustrates some of these cases.
a. In footnote 6 we showed that, in the constant returns-to-scale case, the elasticity of substitution
for a two-input production function is given by
σ¼
fk fl
.
f ⋅ fkl
Suppose now that we define the homothetic production function F as
F ðk, lÞ ¼ ½ f ðk, lÞγ ,
where f ðk, lÞ is a constant returns-to-scale production function and γ is a positive exponent.
Show that the elasticity of substitution for this production function is the same as the elasticity of
substitution for the function f :
b. Show how this result can be applied to both the Cobb-Douglas and CES production functions.
9.11 More on Euler’s theorem
Suppose that a production function f ðx1 , x2 , …, xn Þ is homogeneous of degree k: Euler’s theorem
X
shows that i xi fi ¼ k f , and this fact can be used to show that the partial derivatives of f are
homogeneous of degree k 1:
a. Prove that
Xn
i¼1
Xn
j¼1 xi xj fij
¼ kðk 1Þf :
b. In the case of n ¼ 2 and k ¼ 1, what kind of restrictions does the result of part (a) impose on
the second-order partial derivative f12 ? How do your conclusions change when k > 1 or k < 1?
c. How would the results of part (b) be generalized to a production function with any number of
inputs?
d. What are the implications of this problem for the parameters of the multivariable Cobbn
α
Douglas production function f ðx1 , x2 , …, xn Þ ¼ ∏i¼1 x i i for αi 0?
SUGGESTIONS FOR FURTHER READING
Clark, J. M. “Diminishing Returns.” In Encyclopaedia of the
Social Sciences, vol. 5. New York: Crowell-Collier and
Macmillan, 1931, pp. 144–f46.
Mas-Collell, A., M. D. Whinston, and J. R. Green. Microeconomic Theory. New York: Oxford University Press, 1995.
Lucid discussion of the historical development of the diminishing
returns concept.
Chapter 5 provides a sophisticated, if somewhat spare, review of
production theory. The use of the profit function (see Chapter 11) is
quite sophisticated and illuminating.
Douglas, P. H. “Are There Laws of Production?” American
Economic Review 38 (March 1948): 1–f41.
Shephard, R . W. Theory of Cost and Production Functions.
Princeton, NJ: Princeton University Press, 1978.
A nice methodological analysis of the uses and misuses of production
functions.
Extended analysis of the dual relationship between production and
cost functions.
Ferguson, C. E. The Neoclassical Theory of Production and
Distribution. New York: Cambridge University Press, 1969.
Silberberg, E., and W. Suen. The Structure of Economics: A
Mathematical Analysis, 3rd ed. Boston: Irwin/McGrawHill, 2001.
A thorough discussion of production function theory (as of 1970).
Good use of three-dimensional graphs.
Fuss, M., and McFadden, D. Production Economics: A Dual
Approach to Theory and Application. Amsterdam: NorthHolland, 1980.
An approach with a heavy emphasis on the use of duality.
Thorough analysis of the duality between production functions and
cost curves. Provides a proof that the elasticity of substitution can be
derived as shown in footnote 6 of this chapter.
Stigler, G. J. “The Division of Labor Is Limited by the
Extent of the Market.” Journal of Political Economy 59
(June 1951): 185–f93.
Careful tracing of the evolution of Smith’s ideas about economies of
scale.
320
Part 3 Production and Supply
EXTENSIONS
Many-Input Production Functions
Most of the production functions illustrated in Chapter 9 can be easily generalized to many-input cases.
Here we show this for the Cobb-Douglas and CES
cases and then examine two quite flexible forms that
such production functions might take. In all of these
examples, the β’s are nonnegative parameters and the
n inputs are represented by x1 , …, xn :
E9.1 Cobb-Douglas
The many-input Cobb-Douglas production function
is given by
n
Y
β
xi i .
(i)
q¼
function is generally not used in econometric
analyses of microeconomic data on firms. However, the function has a variety of general uses
in macroeconomics, as the next example illustrates.
The Solow growth model
The many-input Cobb-Douglas production function is
a primary feature of many models of economic growth.
For example, Solow’s (1956) pioneering model of
equilibrium growth can be most easily derived using a
two-input constant-returns-to-scale Cobb-Douglas
function of the form
Y ¼ AK α L 1α ,
i¼1
a.
This function exhibits constant returns to scale if
n
X
βi ¼ 1.
(ii)
where A is a technical change factor that can be represented by exponential growth of the form
A ¼ e at .
i¼1
b. In the constant-returns-to-scale Cobb-Douglas
function, βi is the elasticity of q with respect to
input xi : Because 0 β < 1; each input exhibits diminishing marginal productivity.
c. Any degree of increasing returns to scale can be
incorporated into this function, depending on
ε¼
n
X
βi .
(iii)
i¼1
d. The elasticity of substitution between any two
inputs in this production function is 1. This can
be shown by using the definition given in footnote 7 of this chapter:
σij ¼
∂ lnðxi =xj Þ
∂ lnð fj =fi Þ
.
Here
fj
fi
β 1
¼
βj x j j
β
∏i6¼j x i i
β
β 1
βi x i i ∏j 6¼i x j j
¼
βj
βi
⋅
xi
.
xj
Hence,
ln
fj
fi
¼ ln
βj
βi
x
þ ln i
xj
!
and σij ¼ 1: Because this parameter is so constrained in the Cobb-Douglas function, the
(iv)
(v)
Dividing both sides of Equation iv by L yields
y ¼ e at kα ,
(vi)
where
y ¼ Y =L and k ¼ K =L.
Solow shows that economies will evolve toward an
equilibrium value of k (the capital-labor ratio).
Hence cross-country differences in growth rates can
be accounted for only by differences in the technical
change factor, a:
Two features of Equation vi argue for including
more inputs in the Solow model. First, the equation as
it stands is incapable of explaining the large differences
in per capita output ðyÞ that are observed around the
world. Assuming α ¼ 0:3, say (a figure consistent with
many empirical studies), it would take cross-country
differences in K =L of as much as 4,000,000-to-1 to
explain the 100-to-1 differences in per capita income
observed—a clearly unreasonable magnitude. By introducing additional inputs, such as human capital,
these differences become more explainable.
A second shortcoming of the simple Cobb-Douglas
formulation of the Solow model is that it offers no
explanation of the technical change parameter, a—its
value is determined “exogenously.” By adding additional factors, it becomes easier to understand how
the parameter a may respond to economic incentives.
Chapter 9 Production Functions
This is the key insight of literature on “endogenous”
growth theory (for a summary, see Romer, 1996).
E9.2 CES
The many-input constant elasticity of substitution
(CES) production function is given by
hX
iε=ρ
, ρ 1.
(vii)
q¼
βi x ρi
By substituting mxi for each output, it is easy
to show that this function exhibits constant
returns to scale for ε ¼ 1: For ε > 1, the function exhibits increasing returns to scale.
b. The production function exhibits diminishing
marginal productivities for each input because
ρ 1:
c. As in the two-input case, the elasticity of substitution here is given by
a.
σ¼
1
,
1ρ
(viii)
and this elasticity applies to substitution
between any two of the inputs.
Checking the Cobb-Douglas in the Soviet Union
One way in which the multi-input CES function is used
is to determine whether the estimated substitution
parameter ðρÞ is consistent with the value implied by
the Cobb-Douglas ðρ ¼ 0, σ ¼ 1Þ: For example, in a
study of five major industries in the former Soviet
Union, E. Bairam (1991) finds that the Cobb-Douglas
provides a relatively good explanation of changes in
output in most major manufacturing sectors. Only for
food processing does a lower value for σ seem
appropriate.
The next three examples illustrate flexible-form
production functions that may approximate any general function of n inputs. In the Chapter 10 extensions, we examine the cost function analogues to some
of these functions, which are more widely used than
the production functions themselves.
E9.3 Nested production functions
In some applications, Cobb-Douglas and CES production functions are combined into a “nested” single
function. To accomplish this, the original n primary
inputs are categorized into, say, m general classes of
inputs. The specific inputs in each of these categories
are then aggregated into a single composite input, and
321
the final production function is a function of these m
composites. For example, assume there are three primary inputs, x1 , x2 , x3 : Suppose, however, that x1 and
x2 are relatively closely related in their use by firms (for
example, capital and energy) whereas the third input
(labor) is relatively distinct. Then one might want to
use a CES aggregator function to construct a composite input for capital services of the form
x4 ¼ ½γx ρ1 þ ð1 γÞx ρ2 1=ρ :
(ix)
Then the final production function might take a
Cobb-Douglas form:
q ¼ x α3 x β4 :
(x)
This structure allows the elasticity of substitution between x1 and x2 to take on any value ½σ ¼ 1=ð1 ρÞ
but constrains the elasticity of substitution between x3
and x4 to be one. A variety of other options are available depending on how precisely the embedded functions are specified.
The dynamics of capital/energy substitutability
Nested production functions have been widely used in
studies that seek to measure the precise nature of the
substitutability between capital and energy inputs. For
example, Atkeson and Kehoe (1999) use a model
rather close to the one specified in Equations ix and x
to try to reconcile two facts about the way in which
energy prices affect the economy: (1) Over time, use of
energy in production seems rather unresponsive to
price (at least in the short-run); and (2) across countries, energy prices seem to have a large influence over
how much energy is used. By using a capital service
equation of the form given in Equation ix with a low
degree of substitutability ðρ ¼ 2:3Þ—along with a
Cobb-Douglas production function that combines
labor with capital services—they are able to replicate
the facts about energy prices fairly well. They conclude,
however, that this model implies a much more negative
effect of higher energy prices on economic growth
than seems actually to have been the case. Hence they
ultimately opt for a more complex way of modeling
production that stresses differences in energy use
among capital investments made at different dates.
E9.4 Generalized Leontief
q¼
n X
n
X
i¼1 j ¼1
where βij ¼ βji .
βij
pffiffiffiffiffiffiffiffiffi
xi xj ,
322
Part 3 Production and Supply
a.
The function considered in Problem 9.7 is a
simple case of this function for the case n ¼ 2:
For n ¼ 3, the function would have linear
terms in the three inputs along with three radical terms representing all possible cross-products of the inputs.
b. The function exhibits constant returns to scale,
as can be shown by using mxi . Increasing
returns to scale can be incorporated into the
function by using the transformation
q0 ¼ qε ,
ε > 1.
c.
Because each input appears both linearly and
under the radical, the function exhibits diminishing marginal productivities to all inputs.
d. The restriction βij ¼ βji is used to ensure symmetry of the second-order partial derivatives.
E9.5 Translog
ln q ¼ β0 þ
n
X
βi ln xi þ 0:5
i¼1
n X
n
X
i¼1 j ¼1
βij ln xi ln xj ;
βij ¼ βji:
a.
Note that the Cobb-Douglas function is a special case of this function where β0 ¼ βij ¼ 0 for
all i, j :
b. As for the Cobb-Douglas, this function may
assume any degree of returns to scale. If
n
X
i¼1
βi ¼ 1
and
n
X
j ¼1
βij ¼ 0
for all i, then this function exhibits constant
returns to scale. The proof requires some care
in dealing with the double summation.
c.
Again, the condition βij ¼ βji is required to
ensure equality of the cross-partial derivatives.
Immigration
Because the translog production function incorporates
a large number of substitution possibilities among various inputs, it has been widely used to study the ways in
which newly arrived workers may substitute for existing workers. Of particular interest is the way in which
the skill level of immigrants may lead to differing reactions in the demand for skilled and unskilled workers in
the domestic economy. Studies of the United States
and many other countries (Canada, Germany, France,
and so forth) have suggested that the overall size of
such effects is modest, especially given relatively small
immigration flows. But there is some evidence that
unskilled immigrant workers may act as substitutes
for unskilled domestic workers but as complements to
skilled domestic workers. Hence increased immigration flows may exacerbate trends toward rising wage
differentials. For a summary, see Borjas (1994).
References
Atkeson, Andrew, and Patrick J. Kehoe. “Models of Energy
Use: Putty-Putty versus Putty-Clay.” American Economic
Review (September 1999): 1028–43.
Bairam, Erkin. “Elasticity of Substitution, Technical Progress and Returns to Scale in Branches of Soviet Industry:
A New CES Production Function Approach.” Journal of
Applied Economics (January–March 1991): 91–f96.
Borjas, G. J. “The Economics of Immigration.” Journal of
Economic Literature (December 1994): 1667–f1717.
Romer, David. Advanced Macroeconomics. New York:
McGraw-Hill, 1996.
Solow, R. M. “A Contribution to the Theory of Economic
Growth.” Quarterly Journal of Economics (February
1956): 65–f94.
CHAPTER
10
Cost Functions
In this chapter we illustrate the costs that a firm incurs when it produces output. In Chapter 11, we will pursue
this topic further by showing how firms make profit-maximizing input and output decisions.
DEFINITIONS OF COSTS
Before we can discuss the theory of costs, some difficulties about the proper definition of
“costs” must be cleared up. Specifically, we must differentiate between (1) accounting cost
and (2) economic cost. The accountant’s view of cost stresses out-of-pocket expenses, historical
costs, depreciation, and other bookkeeping entries. The economist’s definition of cost (which in
obvious ways draws on the fundamental opportunity-cost notion) is that the cost of any input is
given by the size of the payment necessary to keep the resource in its present employment.
Alternatively, the economic cost of using an input is what that input would be paid in its next
best use. One way to distinguish between these two views is to consider how the costs of various
inputs (labor, capital, and entrepreneurial services) are defined under each system.
Labor costs
Economists and accountants regard labor costs in much the same way. To accountants,
expenditures on labor are current expenses and hence costs of production. For economists,
labor is an explicit cost. Labor services (labor-hours) are contracted at some hourly wage rate
ðwÞ, and it is usually assumed that this is also what the labor services would earn in their best
alternative employment. The hourly wage, of course, includes costs of fringe benefits provided
to employees.
Capital costs
In the case of capital services (machine-hours), the two concepts of cost differ. In calculating
capital costs, accountants use the historical price of the particular machine under investigation
and apply some more-or-less arbitrary depreciation rule to determine how much of that
machine’s original price to charge to current costs. Economists regard the historical price of a
machine as a “sunk cost,” which is irrelevant to output decisions. They instead regard the
implicit cost of the machine to be what someone else would be willing to pay for its use. Thus
the cost of one machine-hour is the rental rate for that machine in its best alternative use. By
continuing to use the machine itself, the firm is implicitly forgoing what someone else would
be willing to pay to use it. This rental rate for one machine-hour will be denoted by v.1
1
Sometimes the symbol r is chosen to represent the rental rate on capital. Because this variable is often confused with the
related but distinct concept of the market interest rate, an alternative symbol was chosen here. The exact relationship
between v and the interest rate is examined in Chapter 17.
323
324
Part 3 Production and Supply
Costs of entrepreneurial services
The owner of a firm is a residual claimant who is entitled to whatever extra revenues or losses
are left after paying other input costs. To an accountant, these would be called profits (which
might be either positive or negative). Economists, however, ask whether owners (or entrepreneurs) also encounter opportunity costs by working at a particular firm or devoting some of
their funds to its operation. If so, these services should be considered an input, and some cost
should be imputed to them. For example, suppose a highly skilled computer programmer
starts a software firm with the idea of keeping any (accounting) profits that might be generated. The programmer’s time is clearly an input to the firm, and a cost should be inputted for it.
Perhaps the wage that the programmer might command if he or she worked for someone else
could be used for that purpose. Hence some part of the accounting profits generated by the
firm would be categorized as entrepreneurial costs by economists. Economic profits would be
smaller than accounting profits and might be negative if the programmer’s opportunity costs
exceeded the accounting profits being earned by the business. Similar arguments apply to the
capital that an entrepreneur provides to the firm.
Economic costs
In this book, not surprisingly, we use economists’ definition of cost.
DEFINITION
Economic cost. The economic cost of any input is the payment required to keep that input in
its present employment. Equivalently, the economic cost of an input is the remuneration the
input would receive in its best alternative employment.
Use of this definition is not meant to imply that accountants’ concepts are irrelevant to
economic behavior. Indeed, accounting procedures are integrally important to any manager’s
decision-making process because they can greatly affect the rate of taxation to be applied
against profits. Accounting data are also readily available, whereas data on economic costs
must often be developed separately. Economists’ definitions, however, do have the desirable
features of being broadly applicable to all firms and of forming a conceptually consistent
system. They therefore are best suited for a general theoretical analysis.
Two simplifying assumptions
As a start, we will make two simplifications about the inputs a firm uses. First, we assume that
there are only two inputs: homogeneous labor (l, measured in labor-hours) and homogeneous capital (k, measured in machine-hours). Entrepreneurial costs are included in capital
costs. That is, we assume that the primary opportunity costs faced by a firm’s owner are those
associated with the capital that the owner provides.
Second, we assume that inputs are hired in perfectly competitive markets. Firms can buy
(or sell) all the labor or capital services they want at the prevailing rental rates (w and v). In
graphic terms, the supply curve for these resources is horizontal at the prevailing factor prices.
Both w and v are treated as “parameters” in the firm’s decisions; there is nothing the firm can
do to affect them. These conditions will be relaxed in later chapters (notably Chapter 16), but
for the moment the price-taker assumption is a convenient and useful one to make.
Economic profits and cost minimization
Total costs for the firm during a period are therefore given by
total costs ¼ C ¼ wl þ vk,
(10.1)
where, as before, l and k represent input usage during the period. Assuming the firm
produces only one output, its total revenues are given by the price of its product ðpÞ times its
Chapter 10
Cost Functions
325
total output [q ¼ f ðk, lÞ, where f ðk, lÞ is the firm’s production function]. Economic profits
ðπÞ are then the difference between total revenues and total economic costs.
Economic profits. Economic profits ðπÞ are the difference between a firm’s total revenues and
DEFINITION
its total costs:
π ¼ total revenue total cost ¼ pq wl vk
¼ pf ðk, lÞ wl vk.
(10.2)
Equation 10.2 shows that the economic profits obtained by a firm are a function of the
amount of capital and labor employed. If, as we will assume in many places in this book, the
firm seeks maximum profits, then we might study its behavior by examining how k and l are
chosen so as to maximize Equation 10.2. This would, in turn, lead to a theory of supply and
to a theory of the “derived demand” for capital and labor inputs. In the next chapter we will
take up those subjects in detail. Here, however, we wish to develop a theory of costs that is
somewhat more general and might apply to firms that are not necessarily profit maximizers.
Hence, we begin the study of costs by finessing, for the moment, a discussion of output
choice. That is, we assume that for some reason the firm has decided to produce a particular
output level (say, q0 ). The firm’s revenues are therefore fixed at pq0 . Now we wish to examine
how the firm can produce q0 at minimal costs.
COST-MINIMIZING INPUT CHOICES
Mathematically, this is a constrained minimization problem. But before proceeding with a
rigorous solution, it is useful to state the result to be derived with an intuitive argument. To
minimize the cost of producing a given level of output, a firm should choose that point on the q0
isoquant at which the rate of technical substitution of l for k is equal to the ratio w=v: It should
equate the rate at which k can be traded for l in production to the rate at which they can be traded
in the marketplace. Suppose that this were not true. In particular, suppose that the firm were
producing output level q0 using k ¼ 10, l ¼ 10, and assume that the RTS were 2 at this
point. Assume also that w ¼ $1, v ¼ $1, and hence that w=v ¼ 1 (which is unequal to 2). At
this input combination, the cost of producing q0 is $20. It is easy to show this is not the minimal
input cost. For example, q0 can also be produced using k ¼ 8 and l ¼ 11; we can give up two
units of k and keep output constant at q0 by adding one unit of l. But at this input combination,
the cost of producing q0 is $19 and hence the initial input combination was not optimal. A
contradiction similar to this one can be demonstrated whenever the RTS and the ratio of the
input costs differ.
Mathematical analysis
Mathematically, we seek to minimize total costs given q ¼ f ðk, lÞ ¼ q0 . Setting up the
Lagrangian expression
(10.3)
ℒ ¼ wl þ vk þ λ½q0 f ðk, lÞ,
the first-order conditions for a constrained minimum are
∂ℒ
∂f
¼wλ
¼ 0,
∂l
∂l
∂ℒ
∂f
(10.4)
¼vλ
¼ 0,
∂k
∂k
∂ℒ
¼ q0 f ðk, lÞ ¼ 0,
∂λ
326
Part 3 Production and Supply
or, dividing the first two equations,
w
∂f =∂l
¼
¼ RTS ðl for kÞ.
(10.5)
v
∂f =∂k
This says that the cost-minimizing firm should equate the RTS for the two inputs to the
ratio of their prices.
Further interpretations
These first-order conditions for minimal costs can be manipulated in several different ways to
yield interesting results. For example, cross-multiplying Equation 10.5 gives
fk
f
¼ l.
(10.6)
v
w
That is: for costs to be minimized, the marginal productivity per dollar spent should be the
same for all inputs. If increasing one input promised to increase output by a greater amount
per dollar spent than did another input, costs would not be minimal—the firm should hire
more of the input that promises a bigger “bang per buck” and less of the more costly (in
terms of productivity) input. Any input that cannot meet the common benefit-cost ratio
defined in Equation 10.6 should not be hired at all.
Equation 10.6 can, of course, also be derived from Equation 10.4, but it is more
instructive to derive its inverse:
w
v
¼
¼ λ.
(10.7)
fl
fk
This equation reports the extra cost of obtaining an extra unit of output by hiring either added
labor or added capital input. Because of cost minimization, this marginal cost is the same no
matter which input is hired. This common marginal cost is also measured by the Lagrangian
multiplier from the cost-minimization problem. As is the case for all constrained optimization
problems, here the Lagrangian multiplier shows how much in extra costs would be incurred by
increasing the output constraint slightly. Because marginal cost plays an important role in a
firm’s supply decisions, we will return to this feature of cost minimization frequently.
Graphical analysis
Cost minimization is shown graphically in Figure 10.1. Given the output isoquant q0 , we wish
to find the least costly point on the isoquant. Lines showing equal cost are parallel straight lines
with slopes w=v. Three lines of equal total cost are shown in Figure 10.1; C1 < C2 < C3 . It is
clear from the figure that the minimum total cost for producing q0 is given by C1 , where the
total cost curve is just tangent to the isoquant. The cost-minimizing input combination is
l , k . This combination will be a true minimum if the isoquant is convex (if the RTS
diminishes for decreases in k=l). The mathematical and graphic analyses arrive at the same
conclusion, as follows.
OPTIMIZATION
PRINCIPLE
Cost minimization. In order to minimize the cost of any given level of input (q0 ), the firm
should produce at that point on the q0 isoquant for which the RTS (of l for k) is equal to the
ratio of the inputs’ rental prices ðw=vÞ.
Contingent demand for inputs
Figure 10.1 exhibits the formal similarity between the firm’s cost-minimization problem and
the individual’s expenditure-minimization problem studied in Chapter 4 (see Figure 4.6). In
both problems, the economic actor seeks to achieve his or her target (output or utility) at
minimal cost. In Chapter 5 we showed how this process is used to construct a theory of
compensated demand for a good. In the present case, cost minimization leads to a demand
for capital and labor input that is contingent on the level of output being produced. This is
Chapter 10
FIGURE 10.1
Cost Functions
Minimization of Costs Given q ¼ q0
A firm is assumed to choose k and l to minimize total costs. The condition for this minimization is
that the rate at which k and l can be traded technically (while keeping q ¼ q0 ) should be equal to the
rate at which these inputs can be traded in the market. In other words, the RTS (of l for k) should be
set equal to the price ratio w=v. This tangency is shown in the figure; costs are minimized at C1 by
choosing inputs k and l .
k per period
C1
C2
C3
k*
q0
l*
l per period
not, therefore, the complete story of a firm’s demand for the inputs it uses because it does not
address the issue of output choice. But studying the contingent demand for inputs provides
an important building block for analyzing the firm’s overall demand for inputs, and we will
take up this topic in more detail later in this chapter.
The firm’s expansion path
A firm can follow the cost-minimization process for each level of output: For each q, it finds
the input choice that minimizes the cost of producing it. If input costs (w and v) remain constant for all amounts the firm may demand, we can easily trace this locus of cost-minimizing
choices. This procedure is shown in Figure 10.2. The line 0E records the cost-minimizing
tangencies for successively higher levels of output. For example, the minimum cost for
producing output level q1 is given by C1 , and inputs k1 and l1 are used. Other tangencies in
the figure can be interpreted in a similar way. The locus of these tangencies is called the firm’s
expansion path, because it records how input expands as output expands while holding the
prices of the inputs constant.
As Figure 10.2 shows, the expansion path need not be a straight line. The use of some
inputs may increase faster than others as output expands. Which inputs expand more rapidly
will depend on the shape of the production isoquants. Because cost minimization requires that
the RTS always be set equal to the ratio w=v, and because the w=v ratio is assumed to be
constant, the shape of the expansion path will be determined by where a particular RTS occurs
on successively higher isoquants. If the production function exhibits constant returns to scale
(or, more generally, if it is homothetic), then the expansion path will be a straight line because
in that case the RTS depends only on the ratio of k to l. That ratio would be constant along
such a linear expansion path.
327
FIGURE 10.2
The Firm’s Expansion Path
The firm’s expansion path is the locus of cost-minimizing tangencies. Assuming fixed input prices,
the curve shows how inputs increase as output increases.
k per period
E
q3
k1
q2
C1
0
FIGURE 10.3
l1
C2
C3
q1
l per period
Input Inferiority
With this particular set of isoquants, labor is an inferior input, because less l is chosen as output
expands beyond q2 .
k per period
E
q4
q3
q2
q1
0
l per period
Chapter 10
Cost Functions
It would seem reasonable to assume that the expansion path will be positively sloped; that
is, successively higher output levels will require more of both inputs. This need not be the
case, however, as Figure 10.3 illustrates. Increases of output beyond q2 actually cause the
quantity of labor used to decrease. In this range, labor would be said to be an inferior input.
The occurrence of inferior inputs is then a theoretical possibility that may happen, even when
isoquants have their usual convex shape.
Much theoretical discussion has centered on the analysis of factor inferiority. Whether inferiority is likely to occur in real-world production functions is a difficult empirical question to
answer. It seems unlikely that such comprehensive magnitudes as “capital” and “labor” could be
inferior, but a finer classification of inputs may bring inferiority to light. For example, the use of
shovels may decline as production of building foundations (and the use of backhoes) increases.
In this book we shall not be particularly concerned with the analytical issues raised by this
possibility, although complications raised by inferior inputs will be mentioned in a few places.
EXAMPLE 10.1 Cost Minimization
The cost-minimization process can be readily illustrated with two of the production functions
we encountered in the last chapter.
1. Cobb-Douglas: q ¼ f ðk, lÞ ¼ kα l β . For this case the relevant Lagrangian expression for
minimizing the cost of producing, say, q0 is
ℒ ¼ vk þ wl þ λðq0 kα l β Þ,
and the first-order conditions for a minimum are
∂ℒ
¼ v λαkα1 l β ¼ 0,
∂k
∂ℒ
¼ w λβkα l β1 ¼ 0,
∂l
∂ℒ
¼ q0 kα l β ¼ 0.
∂λ
Dividing the second of these by the first yields
(10.8)
(10.9)
w
βkα l β1
β k
¼ ⋅ ,
¼
(10.10)
α1
β
αk l
v
α l
which again shows that costs are minimized when the ratio of the inputs’ prices is equal to
the RTS. Because the Cobb-Douglas function is homothetic, the RTS depends only on the
ratio of the two inputs. If the ratio of input costs does not change, the firms will use the same
input ratio no matter how much it produces—that is, the expansion path will be a straight
line through the origin.
As a numerical example, suppose α ¼ β ¼ 0.5, w ¼ 12, v ¼ 3, and that the firm wishes to
produce q0 ¼ 40. The first-order condition for a minimum requires that k ¼ 4l. Inserting that
into the production function (the final requirement in Equation 10.9), we have q0 ¼ 40 ¼
k0.5 l 0.5 ¼ 2l. So the cost-minimizing input combination is l ¼ 20 and k ¼ 80, and total costs
are given by vk þ wl ¼ 3ð80Þ þ 12ð20Þ ¼ 480. That this is a true cost minimum is suggested by
looking at a few other input combinations that also are capable of producing 40 units of output:
k ¼ 40, l ¼ 40, C ¼ 600,
k ¼ 10, l ¼ 160, C ¼ 2,220,
(10.11)
k ¼ 160, l ¼ 10, C ¼ 600.
Any other input combination able to produce 40 units of output will also cost more than 480.
Cost minimization is also suggested by considering marginal productivities. At the optimal point
(continued)
329
330
Part 3 Production and Supply
EXAMPLE 10.1 CONTINUED
MPk ¼ fk ¼ 0.5k 0.5 l 0.5 ¼ 0.5ð20=80Þ0.5 ¼ 0.25,
(10.12)
MPl ¼ fl ¼ 0.5k0.5 l 0.5 ¼ 0.5ð80=20Þ0.5 ¼ 1.0;
hence, at the margin, labor is four times as productive as capital, and this extra productivity
precisely compensates for the higher unit price of labor input.
2. CES: q ¼ f ðk, lÞ ¼ ðk ρ þ l ρ Þγ=ρ . Again we set up the Lagrangian expression
ℒ ¼ vk þ wl þ λ½q0 ðk ρ þ l ρ Þγ=ρ ,
(10.13)
and the first-order conditions for a minimum are
∂ℒ
¼ v λðγ=ρÞðk ρ þ l ρ ÞðγρÞ=ρ ðρÞk ρ1 ¼ 0,
∂k
∂ℒ
(10.14)
¼ w λðγ=ρÞðk ρ þ l ρ ÞðγρÞ=ρ ðρÞl ρ1 ¼ 0,
∂l
∂ℒ
¼ q0 ðk ρ þ l ρ Þðγ=ρÞ ¼ 0.
∂λ
Dividing the first two of these equations causes a lot of this mass of symbols to drop out,
leaving
w
l ρ1
k 1ρ
k 1=σ
k w σ
¼
¼
, or
:
(10.15)
¼
¼
v
k
l
l
l
v
Because the CES function is also homothetic, the cost-minimizing input ratio is independent
of the absolute level of production. The result in Equation 10.15 is a simple generalization of
the Cobb-Douglas result (when σ ¼ 1). With the Cobb-Douglas, the cost-minimizing capitallabor ratio changes directly in proportion to changes in the ratio of wages to capital rental rates.
In cases with greater substitutability ðσ > 1Þ, changes in the ratio of wages to rental rates cause
a greater than proportional increase in the cost-minimizing capital-labor ratio. With less
substitutability ðσ < 1Þ, the response is proportionally smaller.
QUERY: In the Cobb-Douglas numerical example with w=v ¼ 4, we found that the costminimizing input ratio for producing 40 units of output was k=l ¼ 80=20 ¼ 4. How would
this value change for σ ¼ 2 or σ ¼ 0.5? What actual input combinations would be used?
What would total costs be?
COST FUNCTIONS
We are now in a position to examine the firm’s overall cost structure. To do so, it will be
convenient to use the expansion path solutions to derive the total cost function.
DEFINITION
Total cost function. The total cost function shows that, for any set of input costs and for any
output level, the minimum total cost incurred by the firm is
C ¼ C ðv, w, qÞ.
(10.16)
Figure 10.2 makes clear that total costs increase as output, q, increases. We will begin by
analyzing this relationship between total cost and output while holding input prices fixed.
Then we will consider how a change in an input price shifts the expansion path and its related
cost functions.
Chapter 10
Cost Functions
331
Average and marginal cost functions
Although the total cost function provides complete information about the output-cost
relationship, it is often convenient to analyze costs on a per-unit-of-output basis because
that approach corresponds more closely to the analysis of demand, which focused on the price
per unit of a commodity. Two different unit cost measures are widely used in economics:
(1) average cost, which is the cost per unit of output; and (2) marginal cost, which is the cost
of one more unit of output. The relationship of these concepts to the total cost function is
described in the following definitions.
Average and marginal cost functions. The average cost function (AC) is found by comDEFINITION
puting total costs per unit of output:
C ðv, w, qÞ
.
(10.17)
average cost ¼ AC ðv, w, q Þ ¼
q
The marginal cost function (MC) is found by computing the change in total costs for a
change in output produced:
marginal cost ¼ MC ðv, w, qÞ ¼
∂C ðv, w, qÞ
:
∂q
(10.18)
Notice that in these definitions, average and marginal costs depend both on the level of output
being produced and on the prices of inputs. In many places throughout this book, we will
graph simple two-dimensional relationships between costs and output. As the definitions
make clear, all such graphs are drawn on the assumption that the prices of inputs remain
constant and that technology does not change. If input prices change or if technology advances, cost curves generally will shift to new positions. Later in this chapter, we will explore
the likely direction and size of such shifts when we study the entire cost function in detail.
Graphical analysis of total costs
Figures 10.4a and 10.5a illustrate two possible shapes for the relationship between total cost
and the level of the firm’s output. In Figure 10.4a, total cost is simply proportional to output.
Such a situation would arise if the underlying production function exhibits constant returns
to scale. In that case, suppose k1 units of capital input and l1 units of labor input are required
to produce one unit of output. Then
(10.19)
C ðq ¼ 1Þ ¼ vk1 þ wl1 .
To produce m units of output, then, requires mk1 units of capital and ml1 units of labor,
because of the constant returns-to-scale assumption.2 Hence
C ðq ¼ mÞ ¼ vmk1 þ wml1 ¼ mðvk1 þ wl1 Þ
¼ m ⋅ C ðq ¼ 1Þ,
(10.20)
and the proportionality between output and cost is established.
The situation in Figure 10.5a is more complicated. There it is assumed that initially the
total cost curve is concave; although initially costs rise rapidly for increases in output, that rate
of increase slows as output expands into the midrange of output. Beyond this middle range,
however, the total cost curve becomes convex, and costs begin to rise progressively more
rapidly. One possible reason for such a shape for the total cost curve is that there is some third
factor of production (say, the services of an entrepreneur) that is fixed as capital and labor
usage expands. In this case, the initial concave section of the curve might be explained by the
2
The input combination ml1 , mk1 minimizes the cost of producing m units of output because the ratio of the inputs is still
k1 =l1 and the RTS for a constant returns-to-scale production function depends only on that ratio.
332
Part 3 Production and Supply
FIGURE 10.4
Total, Average, and Marginal Cost Curves for the Constant Returns-to-Scale Case
In (a) total costs are proportional to output level. Average and marginal costs, as shown in (b), are
equal and constant for all output levels.
Total
costs
C
(a)
Output per
period
Average and
marginal costs
AC = MC
Output per
period
(b)
increasingly optimal usage of the entrepreneur’s services—he or she needs a moderate level of
production to utilize his or her skills fully. Beyond the point of inflection, however, the
entrepreneur becomes overworked in attempting to coordinate production, and diminishing
returns set in. Hence, total costs rise rapidly.
A variety of other explanations have been offered for the cubic-type total cost curve in
Figure 10.5a, but we will not examine them here. Ultimately, the shape of the total cost curve
is an empirical question that can be determined only by examining real-world data. In the
Extensions to this chapter, we illustrate some of the literature on cost functions.
Graphical analysis of average and marginal costs
Information from the total cost curves can be used to construct the average and marginal cost
curves shown in Figures 10.4b and 10.5b. For the constant returns-to-scale case (Figure 10.4),
this is quite simple. Because total costs are proportional to output, average and marginal costs
Chapter 10
FIGURE 10.5
Cost Functions
Total, Average, and Marginal Cost Curves for the Cubic Total Cost Curve Case
If the total cost curve has the cubic shape shown in (a), average and marginal cost curves will be
U-shaped. In (b) the marginal cost curve passes through the low point of the average cost curve at
output level q .
Total
costs
C
Output per
period
(a)
Average and
marginal costs
MC
AC
q*
Output per
period
(b)
are constant and equal for all levels of output.3 These costs are shown by the horizontal line
AC ¼ MC in Figure 10.4b.
For the cubic total cost curve case (Figure 10.5), computation of the average and marginal
cost curves requires some geometric intuition. As the definition in Equation 10.18 makes
clear, marginal cost is simply the slope of the total cost curve. Hence, because of the assumed
shape of the curve, the MC curve is U-shaped, with MC falling over the concave portion of
the total cost curve and rising beyond the point of inflection. Because the slope is always
positive, however, MC is always greater than 0. Average costs (AC) start out being equal to
3
Mathematically, because C ¼ aq (where a is the cost of one unit of output),
AC ¼
C
∂C
¼a¼
¼ MC .
q
∂q
333
334
Part 3 Production and Supply
marginal cost for the “first” unit of output.4 As output expands, however, AC exceeds MC,
because AC reflects both the marginal cost of the last unit produced and the higher marginal
costs of the previously produced units. So long as AC > MC, average costs must be falling.
Because the lower costs of the newly produced units are below average cost, they continue to
pull average costs downward. Marginal costs rise, however, and eventually (at q ) equal
average cost. Beyond this point, MC > AC, and average costs will be rising because they are
being pulled upward by increasingly higher marginal costs. Consequently, we have shown
that the AC curve also has a U-shape and that it reaches a low point at q , where AC and MC
intersect.5 In empirical studies of cost functions, there is considerable interest in this point of
minimum average cost. It reflects the “minimum efficient scale” (MES) for the particular
production process being examined. The point is also theoretically important because of the
role it plays in perfectly competitive price determination in the long run (see Chapter 12).
COST FUNCTIONS AND SHIFTS IN COST CURVES
The cost curves illustrated in Figures 10.4 and 10.5 show the relationship between costs and
quantity produced on the assumption that all other factors are held constant. Specifically,
construction of the curves assumes that input prices and the level of technology do not change.6
If these factors do change, the cost curves will shift. In this section, we delve further into the
mathematics of cost functions as a way of studying these shifts. We begin with some examples.
EXAMPLE 10.2 Some Illustrative Cost Functions
In this example we calculate the cost functions associated with three different production
functions. Later we will use these examples to illustrate some of the general properties of cost
functions.
1. Fixed Proportions: q ¼ f ðk, lÞ ¼ minðak, blÞ. The calculation of cost functions from their
underlying production functions is one of the more frustrating tasks for economics students.
4
Technically, AC ¼ MC at q ¼ 0. This can be shown by L’Hôpital’s rule, which states that if f ðaÞ ¼ gðaÞ ¼ 0 then
lim
x !a
f ðxÞ
f 0 ðxÞ
.
¼ lim
gðxÞ x !a g 0 ðxÞ
In this case, C ¼ 0 at q ¼ 0, and so
lim AC ¼ lim
q !0
q !0
C
∂C =∂q
¼ lim
¼ lim MC
q !0
q !0
q
1
or
AC ¼ MC at q ¼ 0,
which was to be shown.
5
Mathematically, we can find the minimum AC by setting its derivative equal to 0:
∂AC
∂ðC =qÞ q ⋅ ð∂C =∂qÞ C ⋅ 1 q ⋅ MC C
¼
¼ 0,
¼
¼
∂q
∂q
q2
q2
or
q ⋅ MC C ¼ 0
or
MC ¼ C =q ¼ AC .
For multiproduct firms, an additional complication must be considered. For such firms it is possible that the costs
associated with producing one output (say, q1 ) are also affected by the amount of some other output being produced ðq2 Þ.
In this case the firm is said to exhibit “economies of scope,” and the total cost function will be of the form Cðq1 , q2 , w, vÞ.
Hence, q2 must also be held constant in constructing the q1 cost curves. Presumably increases in q2 shift the q1 cost curves
downward.
6
Chapter 10
Cost Functions
So, let’s start with a simple example. What we wish to do is show how total costs depend on
input costs and on quantity produced. In the fixed-proportions case, we know that production will occur at a vertex of the L-shaped isoquants where q ¼ ak ¼ bl. Hence, total costs are
q q v
w
þ
total costs ¼ C ðv, w, q Þ ¼ vk þ wl ¼ v
þw
¼q
.
(10.21)
a
b
a
b
This is indeed the sort of function we want because it states total costs as a function of v, w,
and q only together with some parameters of the underlying production function. Because
of the constant returns-to-scale nature of this production function, it takes the special form
C ðv, w, qÞ ¼ qC ðv, w, 1Þ.
(10.22)
That is, total costs are given by output times the cost of producing one unit. Increases in
input prices clearly increase total costs with this function, and technical improvements that
take the form of increasing the parameters a and b reduce costs.
2. Cobb-Douglas: q ¼ f ðk, lÞ ¼ kα l β . This is our first example of burdensome computation,
but we can clarify the process by recognizing that the final goal is to use the results of cost
minimization to replace the inputs in the production function with costs. From Example 10.1
we know that cost minimization requires that
w
β k
α w
¼ ⋅
and so k ¼ ⋅ ⋅ l.
v
α l
β v
Substitution into the production function permits a solution for labor input in terms of q, v,
and w as
α w α αþβ
β
l
or l ¼ q 1=ðαþβÞ
⋅
β v
α
A similar set of manipulations gives
q ¼ kα l β ¼
α=ðαþβÞ
w α=ðαþβÞ v α=ðαþβÞ . (10.23)
α β=ðαþβÞ β=ðαþβÞ β=ðαþβÞ
w
v
.
β
Now we are ready to derive total costs as
k ¼ q 1=ðαþβÞ
C ðv, w, qÞ ¼ vk þ wl ¼ q 1=ðαþβÞ Bv α=ðαþβÞ w β=ðαþβÞ ,
(10.24)
(10.25)
where B ¼ ðα þ βÞαα=ðαþβÞ ββ=ðαþβÞ —a constant that involves only the parameters α and β.
Although this derivation was a bit messy, several interesting aspects of this Cobb-Douglas
cost function are readily apparent. First, whether the function is a convex, linear, or concave
function of output depends on whether there are decreasing returns to scale ðα þ β < 1Þ,
constant returns to scale ðα þ β ¼ 1Þ, or increasing returns to scale ðα þ β > 1Þ. Second, an
increase in any input price increases costs, with the extent of the increase being determined by
the relative importance of the input as reflected by the size of its exponent in the production
function. Finally, the cost function is homogeneous of degree 1 in the input prices—a general
feature of all cost functions, as we shall show shortly.
3. CES: q ¼ f ðk, lÞ ¼ ðkρ þ l ρ Þγ=ρ . For this case, your author will mercifully spare you the
algebra. To derive the total cost function, we use the cost-minimization condition specified in
Equation 10.15, solve for each input individually, and eventually get
C ðv, w, qÞ ¼ vk þ wl ¼ q 1=γ ðvρ=ðρ1Þ þ w ρ=ðρ1Þ Þðρ1Þ=ρ
¼ q 1=γ ðv 1σ þ w 1σ Þ1=ð1σÞ ,
(10.26)
where the elasticity of substitution is given by σ ¼ 1=ð1 ρÞ. Once again the shape of the
total cost is determined by the scale parameter ðγÞ for this production function, and the cost
function is increasing in both of the input prices. The function is also homogeneous of
degree 1 in those prices. One limiting feature of this form of the CES function is that the
(continued)
335
336
Part 3 Production and Supply
EXAMPLE 10.2 CONTINUED
inputs are given equal weights—hence their prices are equally important in the cost function. This feature of the CES is easily generalized, however (see Problem 10.7).
QUERY: How are the various substitution possibilities inherent in the CES function reflected
in the CES cost function in Equation 10.26?
Properties of cost functions
These examples illustrate some properties of total cost functions that are quite general.
1. Homogeneity. The total cost functions in Example 10.3 are all homogeneous of degree 1
in the input prices. That is, a doubling of input prices will precisely double the cost of
producing any given output level (you might check this out for yourself). This is a
property of all proper cost functions. When all input prices double (or are increased by
any uniform proportion), the ratio of any two input prices will not change. Because
cost minimization requires that the ratio of input prices be set equal to the RTS along a
given isoquant, the cost-minimizing input combination also will not change. Hence,
the firm will buy exactly the same set of inputs and pay precisely twice as much for them.
One implication of this result is that a pure, uniform inflation in all input costs will not
change a firm’s input decisions. Its cost curves will shift upward in precise correspondence to the rate of inflation.
2. Total cost functions are nondecreasing in q, v, and w. This property seems obvious,
but it is worth dwelling on it a bit. Because cost functions are derived from a costminimization process, any decline in costs from an increase in one of the function’s
arguments would lead to a contradiction. For example, if an increase in output from q1
to q2 caused total costs to decline, it must be the case that the firm was not minimizing
costs in the first place. It should have produced q2 and thrown away an output of
q2 q1 , thereby producing q1 at a lower cost. Similarly, if an increase in the price of an
input ever reduced total cost, the firm could not have been minimizing its costs in the
first place. To see this, suppose the firm was using the input combination k1 , l1 and that
w increases. Clearly that will increase the cost of the initial input combination. But if
changes in input choices actually caused total costs to decline, that must imply that there
was a lower-cost input mix than k1 , l1 initially. Hence we have a contradiction, and this
property of cost functions is established.7
3. Total cost functions are concave in input prices. It is probably easiest to illustrate this
property with a graph. Figure 10.6 shows total costs for various values of an input
price, say, w, holding q and v constant. Suppose that initially a wage rate of w1 prevails
7
A formal proof could also be based on the envelope theorem as applied to constrained minimization problems. Consider
the Lagrangian expression in Equation 10.3. As was pointed out in Chapter 2, we can calculate the change in the objective
in such an expression (here, total cost) with respect to a change in a variable by differentiating the Lagrangian expression.
Performing this differentiation yields
∂C ∂ℒ
¼
¼ λ ð¼ MC Þ
∂q
∂q
∂C
∂ℒ
¼ k 0,
¼
∂v
∂v
∂C ∂ℒ
¼ l 0.
¼
∂w
∂w
0,
Not only do these envelope results prove this property of cost functions, they also are quite useful in their own right, as we
will show later in this chapter.
Chapter 10
FIGURE 10.6
Cost Functions
Cost Functions Are Concave in Input Prices
With a wage rate of w1 , total costs of producing q1 are Cðv, w1 , q1 Þ. If the firm does not change its
input mix, costs of producing q1 would follow the straight line CPSEUDO . With input substitution,
actual costs Cðv, w, q1 Þ will fall below this line, and hence the cost function is concave in w.
Costs
C PSEUDO
C(v,w,q1)
C(v,w1,q1)
w1
w
and that the total costs associated with producing q1 are given by Cðv, w1 , q1 Þ. If the
firm did not change its input mix in response to changes_in wages, then
_ cost
_ _ its total
curve would be linear as reflected by the line CPSEUDO ð v, w, q1 Þ ¼ v k 1 þ wl 1 in the
figure. But a cost-minimizing firm probably would change the input mix it uses to
produce q1 when wages change, and these actual costs ½Cðv, w, q1 Þ would fall below
the “pseudo” costs. Hence, the total cost function must have the concave shape
shown in Figure 10.6. One implication of this finding is that costs will be lower
when a firm faces input prices that fluctuate around a given level than when they
remain constant at that level. With fluctuating input prices, the firm can adapt its input
mix to take advantage of such fluctuations by using a lot of, say, labor when its price is
low and economizing on that input when its price is high.
4. Average and marginal costs. Some, but not all, of these properties of total cost
functions carry over to their related average and marginal cost functions. Homogeneity
is one property that carries over directly. Because Cðtv, tw, qÞ ¼ tCðv, w, qÞ, we have
C ðtv, tw, qÞ tC ðv, w, qÞ
¼
¼ tAC ðv, w, qÞ
(10.27)
AC ðtv, tw, q Þ ¼
q
q
and8
MC ðtv, tw, q Þ ¼
8
∂C ðtv, tw, qÞ t ∂C ðv, w, qÞ
¼
¼ tMC ðv, w, qÞ.
∂q
∂q
(10.28)
This result does not violate the theorem that the derivative of a function that is homogeneous of degree k is homogeneous
of degree k − 1, because we are differentiating with respect to q and total costs are homogeneous with respect to input
prices only.
337
338
Part 3 Production and Supply
The effects of changes in q, v, and w on average and marginal costs are sometimes
ambiguous, however. We have already shown that average and marginal cost curves
may have negatively sloped segments, so neither AC nor MC is nondecreasing in q.
Because total costs must not decrease when an input price rises, it is clear that average
cost is increasing in w and v. But the case of marginal cost is more complex. The main
complication arises because of the possibility of input inferiority. In that (admittedly
rare) case, an increase in an inferior input’s price will actually cause marginal cost to
decline. Although the proof of this is relatively straightforward,9 an intuitive explanation for it is elusive. Still, in most cases, it seems clear that the increase in the price
of an input will increase marginal cost as well.
Input substitution
A change in the price of an input will cause the firm to alter its input mix. Hence, a full study of
how cost curves shift when input prices change must also include an examination of substitution
among inputs. To study this process, economists have developed a somewhat different measure
of the elasticity of substitution than the one we encountered in the theory of production.
Specifically, we wish to examine how the ratio of input usage (k=l) changes in response to a
change in w=v, while holding q constant. That is, we wish to examine the derivative
∂ðk=lÞ
∂ðw=vÞ
(10.29)
along an isoquant.
Putting this in proportional terms as
∂ðk=lÞ w=v
∂ ln k=l
¼
(10.30)
s¼
⋅
∂ðw=vÞ k=l
∂ ln w=v
gives an alternative and more intuitive definition of the elasticity of substitution.10 In the twoinput case, s must be nonnegative; an increase in w=v will be met by an increase in k=l (or, in the
limiting fixed-proportions case, k=l will stay constant). Large values of s indicate that firms
change their input proportions significantly in response to changes in relative input prices,
whereas low values indicate that changes in input prices have relatively little effect.
Substitution with many inputs
When there are only two inputs, the elasticity of substitution defined in Equation 10.30 is
identical to the concept we defined in Chapter 9 (see Equation 9.32). This can be shown by
remembering that cost minimization11 requires that the firm equate its RTS (of l for k) to the
input price ratio w=v. The major advantage of the definition of the elasticity of substitution in
Equation 10.30 is that it is easier to generalize to many inputs than is the definition based on
the production function. Specifically, suppose there are many inputs to the production
process ðx1 , x2 , …, xn Þ that can be hired at competitive rental rates ðw1 , w2 , …, wn Þ. Then
the elasticity of substitution between any two inputs ðsij Þ is defined as follows.
9
The proof follows the envelope theorem results presented in footnote 7. Because the MC function can be derived by
differentiation from the Lagrangian for cost minimization, we can use Young’s theorem to show
∂MC
∂ð∂ℒ=∂qÞ
∂2 ℒ
∂2 ℒ
∂k
¼
¼
¼
¼ .
∂v
∂v
∂v∂q
∂q∂v
∂q
Hence, if capital is a normal input, an increase in v will raise MC whereas, if capital is inferior, an increase in v will actually
reduce MC.
10
This definition is usually attributed to R. G. D. Allen, who developed it in an alternative form in his Mathematical
Analysis for Economists (New York: St. Martin’s Press, 1938), pp. 504–9.
11
In Example 10.1 we found that, for the CES production function, cost minimization requires that k=l ¼ ðw=vÞσ , so lnðk=lÞ ¼
σ lnðw=vÞ and therefore sk, l ¼ ∂ lnðk=lÞ=∂ lnðw=vÞ ¼ σ.
Chapter 10
Cost Functions
339
Elasticity of substitution. The elasticity of substitution12 between inputs xi and xj is
DEFINITION
given by
∂ðxi =xj Þ wj =wi
∂ lnðxi =xj Þ
,
(10.31)
¼
si, j ¼
⋅
∂ðwj =wi Þ xi =xj
∂ lnðwj =wi Þ
where output and all other input prices are held constant.
The major advantage of this definition in a multi-input context is that it provides the firm with
the flexibility to adjust inputs other than xi and xj (while holding output constant) when
input prices change. For example, a major topic in the theory of firms’ input choices is to
describe the relationship between capital and energy inputs. The definition in Equation 10.31
would permit a researcher to study how the ratio of energy to capital input changes when
relative energy prices rise while permitting the firm to make any adjustments to labor input
(whose price has not changed) that would be required for cost minimization. Hence this
would give a realistic picture of how firms actually behave with regard to whether energy and
capital are more like substitutes or complements. Later in this chapter we will look at this
definition in a bit more detail, because it is widely used in empirical studies of production.
Quantitative size of shifts in cost curves
We have already shown that increases in an input price will raise total, average, and (except in
the inferior input case) marginal costs. We are now in a position to judge the extent of such
increases. First, and most obviously, the increase in costs will be influenced importantly by the
relative significance of the input in the production process. If an input constitutes a large
fraction of total costs, an increase in its price will raise costs significantly. A rise in the wage
rate would sharply increase home-builders’ costs, because labor is a major input in construction. On the other hand, a price rise for a relatively minor input will have a small cost impact.
An increase in nail prices will not raise home costs very much.
A less obvious determinant of the extent of cost increases is input substitutability. If firms
can easily substitute another input for the one that has risen in price, there may be little
increase in costs. Increases in copper prices in the late 1960s, for example, had little impact on
electric utilities’ costs of distributing electricity, because they found they could easily substitute aluminum for copper cables. Alternatively, if the firm finds it difficult or impossible to
substitute for the input that has become more costly, then costs may rise rapidly. The cost of
gold jewelry, along with the price of gold, rose rapidly during the early 1970s, because there
was simply no substitute for the raw input.
It is possible to give a precise mathematical statement of the quantitative sizes of all of
these effects by using the elasticity of substitution. To do so, however, would risk further
cluttering the book with symbols.13 For our purposes, it is sufficient to rely on the previous
intuitive discussion. This should serve as a reminder that changes in the price of an input will
have the effect of shifting firms’ cost curves, with the size of the shift depending on the
relative importance of the input and on the substitution possibilities that are available.
Technical change
Technical improvements allow the firm to produce a given output with fewer inputs. Such
improvements obviously shift total costs downward (if input prices stay constant). Although
12
This definition is attributed to the Japanese economist M. Morishima, and these elasticities are sometimes referred to as
“Morishima elasticities.” In this version, the elasticity of substitution for substitute inputs is positive. Some authors reverse
the order of subscripts in the denominator of Equation 10.31, and in this usage the elasticity of substitution for substitute
inputs is negative.
13
For a complete statement see Ferguson, Neoclassical Theory of Production and Distribution (Cambridge: Cambridge
University Press, 1969), pp. 154–60.
340
Part 3 Production and Supply
the actual way in which technical change affects the mathematical form of the total cost curve
can be complex, there are cases where one may draw simple conclusions. Suppose, for
example, that the production function exhibits constant returns to scale and that technical
change enters that function as described in Chapter 9 (that is, q ¼ Aðt Þf ðk, lÞ where
Að0Þ ¼ 1Þ. In this case, total costs in the initial period are given by
(10.32)
C0 ¼ C0 ðv, w, qÞ ¼ qC0 ðv, w, 1Þ.
Because the same inputs that produced one unit of output in period 0 will produce Aðt Þ
units of output in period t , we know that
Ct ðv, w, Aðt ÞÞ ¼ Aðt ÞCt ðv, w, 1Þ ¼ C0 ðv, w, 1Þ;
therefore, we can compute the total cost function in period t as
(10.33)
qC0 ðv, w, 1Þ C0 ðv, w, qÞ
¼
.
(10.34)
Aðt Þ
Aðt Þ
Hence, total costs fall over time at the rate of technical change. Note that in this case technical
change is “neutral” in that it does not affect the firm’s input choices (so long as input prices stay
constant). This neutrality result might not hold in cases where technical progress takes a more
complex form or where there are variable returns to scale. Even in these more complex cases,
however, technical improvements will cause total costs to fall.
Ct ðv, w, q Þ ¼ qCt ðv, w, 1Þ ¼
EXAMPLE 10.3 Shifting the Cobb-Douglas Cost Function
In Example 10.2 we computed the Cobb-Douglas cost function as
(10.35)
C ðv, w, qÞ ¼ q 1=ðαþβÞ Bv α=ðαþβÞ w β=ðαþβÞ ,
α=ðαþβÞ β=ðαþβÞ
where B ¼ ðα þ βÞα
β
. As in the numerical illustration in Example 10.1, let’s
assume that α ¼ β ¼ 0.5, in which case the total cost function is greatly simplified:
(10.36)
C ðv, w, qÞ ¼ 2qv 0.5 w 0.5 .
This function will yield a total cost curve relating total costs and output if we specify particular
values for the input prices. If, as before, we assume v ¼ 3 and w ¼ 12, then the relationship is
pffiffiffiffiffiffi
C ð3, 12, qÞ ¼ 2q 36 ¼ 12q,
(10.37)
and, as in Example 10.1, it costs 480 to produce 40 units of output. Here average and
marginal costs are easily computed as
C
¼ 12,
AC ¼
q
(10.38)
∂C
MC ¼
¼ 12.
∂q
As expected, average and marginal costs are constant and equal to each other for this constant
returns-to-scale production function.
Changes in input prices. If either input price were to change, all of these costs would change
also. For example, if wages were to increase to 27 (an easy number with which to work), costs
would become
pffiffiffiffiffiffi
C ð3, 27, qÞ ¼ 2q 81 ¼ 18q,
AC ¼ 18,
(10.39)
MC ¼ 18.
Notice that an increase in wages of 125 percent raised costs by only 50 percent here, both
because labor represents only 50 percent of all costs and because the change in input prices
encouraged the firm to substitute capital for labor. The total cost function, because it is
Chapter 10
Cost Functions
derived from the cost-minimization assumption, accomplishes this substitution “behind the
scenes”—reporting only the final impact on total costs.
Technical progress. Let’s look now at the impact that technical progress can have on costs.
Specifically, assume that the Cobb-Douglas production function is
(10.40)
q ¼ Aðt Þk 0.5 l 0.5 ¼ e .03t k0.5 l 0.5 .
That is, we assume that technical change takes an exponential form and that the rate of technical change is 3 percent per year. Using the results of the previous section (Equation 10.34)
yields
C0 ðv, w, qÞ
(10.41)
¼ 2qv 0.5 w 0.5 e . 03t :
Aðt Þ
So, if input prices remain the same then total costs fall at the rate of technical improvement—
that is, at 3 percent per year. After, say, 20 years, costs will be (with v ¼ 3, w ¼ 12)
pffiffiffiffiffiffi
C20 ð3, 12, qÞ ¼ 2q 36 ⋅ e . 60 ¼ 12q ⋅ ð0.55Þ ¼ 6.6q,
(10.42)
AC20 ¼ 6.6,
MC20 ¼ 6.6.
Consequently, costs will have fallen by nearly 50 percent as a result of the technical change.
This would, for example, more than have offset the wage rise illustrated previously.
Ct ðv, w, q Þ ¼
QUERY: In this example, what are the elasticities of total costs with respect to changes in
input costs? Is the size of these elasticities affected by technical change?
Contingent demand for inputs and Shephard’s lemma
As we described earlier, the process of cost minimization creates an implicit demand for inputs.
Because that process holds quantity produced constant, this demand for inputs will also be
“contingent” on the quantity being produced. This relationship is fully reflected in the firm’s
total cost function and, perhaps surprisingly, contingent demand functions for all of the firm’s
inputs can be easily derived from that function. The process involves what has come to be
called Shephard’s lemma,14 which states that the contingent demand function for any input is
given by the partial derivative of the total cost function with respect to that input’s price.
Because Shephard’s lemma is widely used in many areas of economic research, we will provide
a relatively detailed examination of it.
The intuition behind Shephard’s lemma is straightforward. Suppose that the price of labor
(w) were to increase slightly. How would this affect total costs? If nothing else changed, it seems
that costs would rise by approximately the amount of labor ðlÞ that the firm was currently hiring.
Roughly speaking, then, ∂C=∂w ¼ l, and that is what Shephard’s lemma claims. Figure 10.6
makes roughly the same point graphically. Along the “pseudo” cost function all inputs are held
constant, so an increase in the wage increases costs in direct proportion to the amount of
labor used. Because the true cost function is tangent to the pseudo-function at the current
wage, its slope (that is, its partial derivative) also will show the current amount of labor
input demanded.
Technically, Shephard’s lemma is one result of the envelope theorem that was first
discussed in Chapter 2. There we showed that the change in the optimal value in a constrained
optimization problem with respect to one of the parameters of the problem can be found by
14
Named for R. W. Shephard, who highlighted the important relationship between cost functions and input demand
functions in his Cost and Production Functions (Princeton, NJ: Princeton University Press, 1970).
341
342
Part 3 Production and Supply
differentiating the Lagrangian expression for that optimization problem with respect to this
changing parameter. In the cost-minimization case, the Lagrangian expression is
_
ℒ ¼ vk þ wl þ λ½ q f ðk, lÞ
(10.43)
and the envelope theorem applied to either input is
∂C ðv, w, qÞ
∂ℒðv, w, q, λÞ
¼
¼ kc ðv, w, qÞ,
∂v
∂v
∂C ðv, w, qÞ
∂ℒðv, w, q, λÞ
¼
¼ l c ðv, w, qÞ,
∂w
∂w
(10.44)
where the notation is intended to make clear that the resulting demand functions for capital
and labor input depend on v, w, and q. Because quantity produced enters these functions,
input demand is indeed contingent on that variable. This feature of the demand functions is
also reflected by the “c” in the notation.15 Hence, the demand relations in Equation 10.44
do not represent a complete picture of input demand because they still depend on a variable
that is under the firm’s control. In the next chapter, we will complete the study of input
demand by showing how the assumption of profit maximization allows us to effectively
replace q in the input demand relationships with the market price of the firm’s output, p.
EXAMPLE 10.4 Contingent Input Demand Functions
In this example, we will show how the total cost functions derived in Example 10.2 can be
used to derive contingent demand functions for the inputs capital and labor.
1. Fixed Proportions: Cðv, w, qÞ ¼ qðv=a þ w=bÞ. For this cost function, contingent demand functions are quite simple:
∂C ðv, w, qÞ
q
¼ ,
k c ðv, w, qÞ ¼
∂v
a
(10.45)
∂C ðv, w, qÞ q
c
l ðv, w, qÞ ¼
¼ .
∂w
b
In order to produce any particular output with a fixed proportions production function at
minimal cost, the firm must produce at the vertex of its isoquants no matter what the inputs’
prices are. Hence, the demand for inputs depends only on the level of output, and v and w do
not enter the contingent input demand functions. Input prices may, however, affect total input
demands in the fixed proportions case because they may affect how much the firm can sell.
2. Cobb-Douglas: Cðv, w, qÞ ¼ q 1=ðαþβÞ Bv α=ðαþβÞ wβ=ðαþβÞ . In this case, the derivation is
messier but also more instructive:
∂C
α
¼
kc ðv, w, qÞ ¼
⋅ q 1=ðαþβÞ Bv β=ðαþβÞ w β=ðαþβÞ
∂v
αþβ
w β=ðαþβÞ
α
¼
,
⋅ q 1=ðαþβÞ B
αþβ
v
(10.46)
∂C
β
c
1=ðαþβÞ
α=ðαþβÞ α=ðαþβÞ
¼
Bv
w
l ðv, w, qÞ ¼
⋅q
∂w
αþβ
w α=ðαþβÞ
β
¼
:
⋅ q 1=ðαþβÞ B
αþβ
v
15
The notation mirrors that used for compensated demand curves in Chapter 5 (which were derived from the expenditure
function). In that case, such demand functions were contingent on the utility target assumed.
Chapter 10
Cost Functions
Consequently, the contingent demands for inputs depend on both inputs’ prices. If we
assume α ¼ β ¼ 0.5 (so B ¼ 2), these reduce to
w 0.5
w 0.5
¼q
,
k c ðv, w, q Þ ¼ 0.5 ⋅ q ⋅ 2 ⋅
v
(10.47)
w
vw 0.5
0.5
l c ðv, w, q Þ ¼ 0.5 ⋅ q ⋅ 2 ⋅
¼q
.
v
v
With v ¼ 3, w ¼ 12, and q ¼ 40, Equations 10.47 yield the result we obtained previously: that
the firm should choose the input combination k ¼ 80, l ¼ 20 to minimize the cost of
producing 40 units of output. If the wage were to rise to, say, 27, the firm would choose the
input combination k ¼ 120, l ¼ 40=3 to produce 40 units of output. Total costs would rise
from 480 to 520, but the ability of the firm to substitute capital for the now more expensive
labor does save considerably. For example, the initial input combination would now cost 780.
3. CES: Cðv, w, qÞ ¼ q 1=γ ðv 1σ þ w1σ Þ1=ð1σÞ . The importance of input substitution is
shown even more clearly with the contingent demand functions derived from the CES
function. For that function,
∂C
1
σ=ð1σÞ
¼
ð1 σÞv σ
kc ðv, w, qÞ ¼
⋅ q 1=γ ðv1σ þ w 1σ Þ
∂v
1σ
¼ q 1=γ ðv1σ þ w 1σ Þσ=ð1σÞ vσ ,
∂C
1
σ=ð1σÞ
ð1 σÞw σ
l c ðv, w, qÞ ¼
¼
⋅ q 1=γ ðv1σ þ w 1σ Þ
∂w
1σ
(10.48)
¼ q 1=γ ðv1σ þ w 1σ Þσ=ð1σÞ w σ .
These functions collapse when σ ¼ 1 (the Cobb-Douglas case), but we can study examples
with either more ðσ ¼ 2Þ or less ðσ ¼ 0.5Þ substitutability and use Cobb-Douglas as the
middle ground. If we assume constant returns to scale ðγ ¼ 1Þ and v ¼ 3, w ¼ 12, and q ¼
40, then contingent demands for the inputs when σ ¼ 2 are
k c ð3, 12, 40Þ ¼ 40ð31 þ 121 Þ2 ⋅ 32 ¼ 25:6,
(10.49)
l c ð3, 12, 40Þ ¼ 40ð31 þ 121 Þ2 ⋅ 122 ¼ 1:6:
That is, the level of capital input is 16 times the amount of labor input. With less substitutability ðσ ¼ 0.5Þ, contingent input demands are
kc ð3, 12, 40Þ ¼ 40ð30:5 þ 120:5 Þ1 ⋅ 30:5 ¼ 120,
(10.50)
l c ð3, 12, 40Þ ¼ 40ð30:5 þ 120:5 Þ1 ⋅ 120:5 ¼ 60.
So, in this case, capital input is only twice as large as labor input. Although these various cases
cannot be compared directly because different values for σ scale output differently, we can, as an
example, look at the consequence of a rise in w to 27 in the low-substitutability case. With
w ¼ 27, the firm will choose k ¼ 160, l ¼ 53.3. In this case, the cost savings from substitution
can be calculated by comparing total costs when using the initial input combination
(¼ ð3Þ120 þ 27ð60Þ ¼ 1980) to total costs with the optimal combination (¼ ð3Þ160 þ
27ð53:3Þ ¼ 1919). Hence, moving to the optimal input combination reduces total costs by
only about 3 percent. In the Cobb-Douglas case, cost savings are over 20 percent.
QUERY: How would total costs change if w increased from 12 to 27 and the production
function took the simple linear form q ¼ k þ 4l? What light does this result shed on the other
cases in this example?
343
344
Part 3 Production and Supply
SHEPHARD’S LEMMA AND THE ELASTICITY
OF SUBSTITUTION
One especially nice feature of Shephard’s lemma is that it can be used to show how to derive
information about input substitution directly from the total cost function through differentiation. Using the definition in Equation 10.31 yields
si, j ¼
∂ lnðxi =xj Þ
∂ lnðwj =wi Þ
¼
∂ lnðCi =Cj Þ
∂ lnðwj =wi Þ
,
(10.51)
where Ci and Cj are the partial derivatives of the total cost function with respect to the input
prices. Once the total cost function is known (perhaps through econometric estimation),
information about substitutability among inputs can thus be readily obtained from it. In the
Extensions to this chapter, we describe some of the results that have been obtained in this
way. Problems 10.11 and 10.12 provide some additional details about ways in which substitutability among inputs can be measured.
SHORT-RUN, LONG-RUN DISTINCTION
It is traditional in economics to make a distinction between the “short run” and the “long
run.” Although no very precise temporal definition can be provided for these terms, the
general purpose of the distinction is to differentiate between a short period during which
economic actors have only limited flexibility in their actions and a longer period that provides
greater freedom. One area of study in which this distinction is quite important is in the theory
of the firm and its costs, because economists are interested in examining supply reactions over
differing time intervals. In the remainder of this chapter, we will examine the implications of
such differential response.
To illustrate why short-run and long-run reactions might differ, assume that capital input
is held fixed at a level of k1 and that (in the short run) the firm is free to vary only its labor
input.16 Implicitly, we are assuming that alterations in the level of capital input are infinitely
costly in the short run. As a result of this assumption, the short-run production function is
q ¼ f ðk1 , lÞ,
(10.52)
where this notation explicitly shows that capital inputs may not vary. Of course, the level of
output still may be changed if the firm alters its use of labor.
Short-run total costs
Total cost for the firm continues to be defined as
C ¼ vk þ wl
(10.53)
for our short-run analysis, but now capital input is fixed at k1 . To denote this fact, we will
write
(10.54)
SC ¼ vk1 þ wl,
where the S indicates that we are analyzing short-run costs with the level of capital input
fixed. Throughout our analysis, we will use this method to indicate short-run costs, whereas
long-run costs will be denoted by C, AC, and MC. Usually we will not denote the level of
capital input explicitly, but it is understood that this input is fixed.
16
Of course, this approach is for illustrative purposes only. In many actual situations, labor input may be less flexible in the
short run than is capital input.
Chapter 10
Cost Functions
345
Fixed and variable costs
The two types of input costs in Equation 8.53 are given special names. The term vk1 is
referred to as (short-run) fixed costs; because k1 is constant, these costs will not change in the
short run. The term wl is referred to as (short-run) variable costs—labor input can indeed be
varied in the short run. Hence we have the following definitions.
Short-run fixed and variable costs. Short-run fixed costs are costs associated with inputs
DEFINITION
that cannot be varied in the short run. Short-run variable costs are costs of those inputs that
can be varied so as to change the firm’s output level.
The importance of this distinction is to differentiate between variable costs that the firm can
avoid by producing nothing in the short run and costs that are fixed and must be paid
regardless of the output level chosen (even zero).
Nonoptimality of short-run costs
It is important to understand that total short-run costs are not the minimal costs for producing
the various output levels. Because we are holding capital fixed in the short run, the firm does not
have the flexibility of input choice that we assumed when we discussed cost minimization earlier
in this chapter. Rather, to vary its output level in the short run, the firm will be forced to use
“nonoptimal” input combinations: The RTS will not be equal to the ratio of the input prices.
This is shown in Figure 10.7. In the short run, the firm is constrained to use k1 units of capital.
To produce output level q0 , it therefore will use l0 units of labor. Similarly, it will use l1 units of
labor to produce q1 and l2 units to produce q2 . The total costs of these input combinations are
given by SC0 , SC1 , and SC2 , respectively. Only for the input combination k1 , l1 is output being
produced at minimal cost. Only at that point is the RTS equal to the ratio of the input prices.
From Figure 10.7, it is clear that q0 is being produced with “too much” capital in this short-run
situation. Cost minimization should suggest a southeasterly movement along the q0 isoquant,
indicating a substitution of labor for capital in production. Similarly, q2 is being produced with
“too little” capital, and costs could be reduced by substituting capital for labor. Neither of these
substitutions is possible in the short run. Over a longer period, however, the firm will be able to
change its level of capital input and will adjust its input usage to the cost-minimizing combinations. We have already discussed this flexible case earlier in this chapter and shall return to it to
illustrate the connection between long-run and short-run cost curves.
Short-run marginal and average costs
Frequently, it is more useful to analyze short-run costs on a per-unit-of-output basis rather
than on a total basis. The two most important per-unit concepts that can be derived from the
short-run total cost function are the short-run average total cost function (SAC) and the shortrun marginal cost function (SMC). These concepts are defined as
total costs
SC
¼
,
SAC ¼
total output
q
(10.55)
change in total costs
∂SC
SMC ¼
¼
,
change in output
∂q
where again these are defined for a specified level of capital input. These definitions for
average and marginal costs are identical to those developed previously for the long-run, fully
flexible case, and the derivation of cost curves from the total cost function proceeds in
exactly the same way. Because the short-run total cost curve has the same general type of
cubic shape as did the total cost curve in Figure 10.5, these short-run average and marginal
cost curves will also be U-shaped.
346
Part 3 Production and Supply
FIGURE 10.7
“Nonoptimal” Input Choices Must Be Made in the Short Run
Because capital input is fixed at k, in the short run the firm cannot bring its RTS into equality with the
ratio of input prices. Given the input prices, q0 should be produced with more labor and less capital
than it will be in the short run, whereas q2 should be produced with more capital and less labor than it
will be.
k per
period
SC2
SC 0
SC1 = C
k1
q2
q1
q0
l0
l1
l2
l per period
Relationship between short-run and long-run cost curves
It is easy to demonstrate the relationship between the short-run costs and the fully flexible
long-run costs that were derived previously in this chapter. Figure 10.8 shows this relationship for both the constant returns-to-scale and cubic total cost curve cases. Short-run total
costs for three levels of capital input are shown, although of course it would be possible to
show many more such short-run curves. The figures show that long-run total costs ðCÞ are
always less than short-run total costs, except at that output level for which the assumed fixed
capital input is appropriate to long-run cost minimization. For example, as in Figure 10.7,
with capital input of k1 the firm can obtain full cost minimization when q1 is produced.
Hence, short-run and long-run total costs are equal at this point. For output levels other than
q1 , however, SC > C, as was the case in Figure 10.7.
Technically, the long-run total cost curves in Figure 10.8 are said to be an “envelope” of
their respective short-run curves. These short-run total cost curves can be represented parametrically by
short-run total cost ¼ SC ðv, w, q, kÞ,
(10.56)
and the family of short-run total cost curves is generated by allowing k to vary while holding
v and w constant. The long-run total cost curve C must obey the short-run relationship in
Equation 10.56 and the further condition that k be cost minimizing for any level of output.
A first-order condition for this minimization is that
FIGURE 10.8
Two Possible Shapes for Long-Run Total Cost Curves
By considering all possible levels of capital input, the long-run total cost curve (C) can be traced.
In (a), the underlying production function exhibits constant returns to scale: in the long run, though
not in the short run, total costs are proportional to output. In (b), the long-run total cost curve has a
cubic shape, as do the short-run curves. Diminishing returns set in more sharply for the short-run
curves, however, because of the assumed fixed level of capital input.
Total
costs
SC (k2)
SC (k1)
SC (k0)
q0
C
q2 Output per period
q1
(a) Constant returns to scale
Total
costs
SC (k2)
C
SC (k1)
SC (k0)
q0
q1
q2
Output
per period
(b) Cubic total cost curve case
∂SC ðv, w, q, kÞ
¼ 0.
(10.57)
∂k
Solving Equations 10.56 and 10.57 simultaneously then generates the long-run total
cost function. Although this is a different approach to deriving the total cost function, it
should give precisely the same results derived earlier in this chapter—as the next example
illustrates.
348
Part 3 Production and Supply
EXAMPLE 10.5 Envelope Relations and Cobb-Douglas Cost Functions
Again we start with the Cobb-Douglas production function q ¼ kα l β , but now we hold
capital input constant at k1 . So, in the short run,
q ¼ k α1 l β
or
α=β
l ¼ q 1=β k 1
,
(10.58)
and total costs are given by
α=β
SC ðv, w, q, k1 Þ ¼ vk1 þ wl ¼ vk1 þ wq 1=β k1 .
(10.59)
Notice that the fixed level of capital enters into this short-run total cost function in two
ways: (1) k1 determines fixed costs; and (2) k1 also in part determines variable costs because
it determines how much of the variable input (labor) is required to produce various levels of
output. To derive long-run costs, we require that k be chosen to minimize total costs:
∂SC ðv, w, q, kÞ
α
1=β ðαþβÞ=β
¼ 0.
(10.60)
¼vþ
⋅ wq k
∂k
β
Although the algebra is messy, this equation can be solved for k and substituted into
Equation 10.59 to return us to the Cobb-Douglas cost function:
C ðv, w, qÞ ¼ Bq 1=ðαþβÞ vα=ðαþβÞ w β=ðαþβÞ .
(10.61)
Numerical example. If we again let α ¼ β ¼ 0.5, v ¼ 3, and w ¼ 12, then the short-run
cost function is
(10.62)
SC ð3, 12, q, kÞ ¼ 3k1 þ 12q 2 k1
1 .
In Example 10.1 we found that the cost-minimizing level of capital input for q ¼ 40 was
k ¼ 80. Equation 10.62 shows that short-run total costs for producing 40 units of output
with k ¼ 80 is
1
3q 2
¼ 240 þ
20
80
(10.63)
¼ 240 þ 240 ¼ 480,
which is just what we found before. We can also use Equation 10.62 to show how costs differ in
the short and long run. Table 10.1 shows that, for output levels other than q ¼ 40, short-run
costs are larger than long-run costs and that this difference is proportionally larger the farther
one gets from the output level for which k ¼ 80 is optimal.
SC ð3, 12, q, 80Þ ¼ 3.80 þ 12 ⋅ q 2 ⋅
TABLE 10.1
Difference between Short-Run and Long-Run Total Cost, k ¼ 80
q
C ¼ 12q
SC ¼ 240 þ 3q 2 =20
10
120
255
20
240
300
30
360
375
40
480
480
50
600
615
60
720
780
70
840
975
80
960
1200
Chapter 10
TABLE 10.2
Cost Functions
Unit Costs in the Long Run and the Short Run, k ¼ 80
q
AC
MC
SAC
SMC
10
12
12
25.5
3
20
12
12
15.0
6
30
12
12
12.5
9
40
12
12
12.0
12
50
12
12
12.3
15
60
12
12
13.0
18
70
12
12
13.9
21
80
12
12
15.0
24
It is also instructive to study differences between the long-run and short-run per-unit costs
in this situation. Here AC ¼ MC ¼ 12. We can compute the short-run equivalents (when
k ¼ 80) as
SC
240
3q
¼
þ
,
SAC ¼
q
q
20
(10.64)
∂SC
6q
¼
.
SMC ¼
∂q
20
Both of these short-run unit costs are equal to 12 when q ¼ 40. However, as Table 10.2 shows,
short-run unit costs can differ significantly from this figure, depending on the output level that
the firm produces. Notice in particular that short-run marginal cost increases rapidly as output
expands beyond q ¼ 40 because of diminishing returns to the variable input (labor). This
conclusion plays an important role in the theory of short-run price determination.
QUERY: Explain why an increase in w will increase both short-run average cost and short-run
marginal cost in this illustration, but an increase in v affects only short-run average cost.
Graphs of per-unit cost curves
The envelope total cost curve relationships exhibited in Figure 10.8 can be used to show
geometric connections between short-run and long-run average and marginal cost curves.
These are presented in Figure 10.9 for the cubic total cost curve case. In the figure, short-run
and long-run average costs are equal at that output for which the (fixed) capital input is
appropriate. At q1 , for example, SACðk1 Þ ¼ AC because k1 is used in producing q1 at
minimal costs. For movements away from q1 , short-run average costs exceed long-run average costs, thus reflecting the cost-minimizing nature of the long-run total cost curve.
Because the minimum point of the long-run average cost curve (AC) plays a major role in
the theory of long-run price determination, it is important to note the various curves that
pass through this point in Figure 10.9. First, as is always true for average and marginal cost
curves, the MC curve passes through the low point of the AC curve. At q1 , long-run average
and marginal costs are equal. Associated with q1 is a certain level of capital input (say, k1 ); the
short-run average cost curve for this level of capital input is tangent to the AC curve at its
minimum point. The SAC curve also reaches its minimum at output level q1 . For movements
away from q1 , the AC curve is much flatter than the SAC curve, and this reflects the greater
flexibility open to firms in the long run. Short-run costs rise rapidly because capital inputs are
fixed. In the long run, such inputs are not fixed, and diminishing marginal productivities do
349
350
Part 3 Production and Supply
FIGURE 10.9
Average and Marginal Cost Curves for the Cubic Cost Curve Case
This set of curves is derived from the total cost curves shown in Figure 10.8. The AC and MC curves
have the usual U-shapes, as do the short-run curves. At q1 , long-run average costs are minimized.
The configuration of curves at this minimum point is quite important.
Costs
MC
SMC (k2)
SAC (k2)
AC
SAC (k1)
SAC (k0)
SMC (k1)
SMC (k0)
q0
q1
q2
Output per
period
not occur so abruptly. Finally, because the SAC curve reaches its minimum at q1 , the shortrun marginal cost curve (SMC) also passes through this point. The minimum point of the AC
curve therefore brings together the four most important per-unit costs: at this point,
AC ¼ MC ¼ SAC ¼ SMC .
(10.65)
For this reason, as we shall show in Chapter 12, the output level q1 is an important equilibrium point for a competitive firm in the long run.
SUMMARY
In this chapter we examined the relationship between the
level of output a firm produces and the input costs associated
with that level of production. The resulting cost curves
should generally be familiar to you because they are widely
used in most courses in introductory economics. Here we
have shown how such curves reflect the firm’s underlying
production function and the firm’s desire to minimize costs.
By developing cost curves from these basic foundations, we
were able to illustrate a number of important findings.
•
A firm that wishes to minimize the economic costs of producing a particular level of output should choose that
input combination for which the rate of technical substitution (RTS) is equal to the ratio of the inputs’ rental
prices.
•
Repeated application of this minimization procedure
yields the firm’s expansion path. Because the expansion
path shows how input usage expands with the level of
output, it also shows the relationship between output
level and total cost. That relationship is summarized by
the total cost function, Cðq, v, wÞ, which shows production costs as a function of output levels and input prices.
•
The firm’s average cost ðAC ¼ C=qÞ and marginal cost
ðMC ¼ ∂C=∂qÞ functions can be derived directly from
the total cost function. If the total cost curve has a general
cubic shape then the AC and MC curves will be U-shaped.
•
All cost curves are drawn on the assumption that the
input prices are held constant. When input prices change,
Chapter 10
cost curves will shift to new positions. The extent of the
shifts will be determined by the overall importance of the
input whose price has changed and by the ease with
which the firm may substitute one input for another.
Technical progress will also shift cost curves.
•
Input demand functions can be derived from the firm’s
total cost function through partial differentiation. These
input demand functions will depend on the quantity of
Cost Functions
351
output that the firm chooses to produce and are therefore called “contingent” demand functions.
•
In the short run, the firm may not be able to vary some
inputs. It can then alter its level of production only by
changing its employment of variable inputs. In so doing,
it may have to use nonoptimal, higher-cost input combinations than it would choose if it were possible to vary
all inputs.
PROBLEMS
10.1
In a famous article [J. Viner, “Cost Curves and Supply Curves,” Zeitschrift fur Nationalokonomie 3
(September 1931): 23–46], Viner criticized his draftsman who could not draw a family of SAC curves
whose points of tangency with the U-shaped AC curve were also the minimum points on each SAC
curve. The draftsman protested that such a drawing was impossible to construct. Whom would you
support in this debate?
10.2
Suppose that a firm produces two different outputs, the quantities of which are represented by q1 and q2 .
In general, the firm’s total costs can be represented by Cðq1 , q2 Þ. This function exhibits economies of
scope if Cðq1 , 0Þ þ Cð0, q2 Þ > Cðq1 , q2 Þ for all output levels of either good.
a. Explain in words why this mathematical formulation implies that costs will be lower in this
multiproduct firm than in two single-product firms producing each good separately.
b. If the two outputs are actually the same good, we can define total output as q ¼ q1 þ q2 .
Suppose that in this case average cost ð¼ C=qÞ falls as q increases. Show that this firm also enjoys
economies of scope under the definition provided here.
10.3
Professor Smith and Professor Jones are going to produce a new introductory textbook. As true
scientists, they have laid out the production function for the book as
q ¼ S 1=2 J 1=2 ,
where q ¼ the number of pages in the finished book, S ¼ the number of working hours spent by
Smith, and J ¼ the number of hours spent working by Jones.
Smith values his labor as $3 per working hour. He has spent 900 hours preparing the first draft.
Jones, whose labor is valued at $12 per working hour, will revise Smith’s draft to complete the book.
a. How many hours will Jones have to spend to produce a finished book of 150 pages? Of 300
pages? Of 450 pages?
b. What is the marginal cost of the 150th page of the finished book? Of the 300th page? Of the
450th page?
10.4
Suppose that a firm’s fixed proportion production function is given by
q ¼ minð5k, 10lÞ.
a. Calculate the firm’s long-run total, average, and marginal cost functions.
b. Suppose that k is fixed at 10 in the short run. Calculate the firm’s short-run total, average, and
marginal cost functions.
352
Part 3 Production and Supply
c. Suppose v ¼ 1 and w ¼ 3. Calculate this firm’s long-run and short-run average and marginal
cost curves.
10.5
A firm producing hockey sticks has a production function given by
pffiffiffiffiffiffiffiffi
q ¼ 2 k ⋅ l.
In the short run, the firm’s amount of capital equipment is fixed at k ¼ 100. The rental rate for k is
v ¼ $1, and the wage rate for l is w ¼ $4.
a. Calculate the firm’s short-run total cost curve. Calculate the short-run average cost curve.
b. What is the firm’s short-run marginal cost function? What are the SC, SAC, and SMC for the
firm if it produces 25 hockey sticks? Fifty hockey sticks? One hundred hockey sticks? Two
hundred hockey sticks?
c. Graph the SAC and the SMC curves for the firm. Indicate the points found in part (b).
d. Where does the SMC curve intersect the SAC curve? Explain why the SMC curve will always
intersect the SAC curve at its lowest point.
_
Suppose now that capital used for producing hockey sticks is fixed at k in the short run.
_
e. Calculate the firm’s total costs as a function of q, w, v, and k.
f. Given q, w, and v, how should the capital stock be chosen to minimize total cost?
g. Use your results from part (f ) to calculate the long-run total cost of hockey stick production.
h. For w ¼ $4, v ¼ $1, graph the long-run total cost curve for hockey stick production. Show that
_
this is an envelope for the short-run curves computed in part (a) by examining values of k of
100, 200, and 400.
10.6
An enterprising entrepreneur purchases two firms to produce widgets. Each firm produces identical
products, and each has a production function given by
pffiffiffiffiffiffiffiffi
q ¼ ki li , i ¼ 1, 2.
The firms differ, however, in the amount of capital equipment each has. In particular, firm 1 has
k1 ¼ 25 whereas firm 2 has k2 ¼ 100. Rental rates for k and l are given by w ¼ v ¼ $1.
a. If the entrepreneur wishes to minimize short-run total costs of widget production, how should
output be allocated between the two firms?
b. Given that output is optimally allocated between the two firms, calculate the short-run total,
average, and marginal cost curves. What is the marginal cost of the 100th widget? The 125th
widget? The 200th widget?
c. How should the entrepreneur allocate widget production between the two firms in the long
run? Calculate the long-run total, average, and marginal cost curves for widget production.
d. How would your answer to part (c) change if both firms exhibited diminishing returns to
scale?
10.7
Suppose the total-cost function for a firm is given by
C ¼ qw 2=3 v 1=3 .
a. Use Shephard’s lemma to compute the constant output demand functions for inputs l and k.
b. Use your results from part (a) to calculate the underlying production function for q.
Chapter 10
Cost Functions
10.8
Suppose the total-cost function for a firm is given by
pffiffiffiffiffiffi
C ¼ qðv þ 2 vw þ wÞ.
a. Use Shephard’s lemma to compute the constant output demand function for each input,
k and l.
b. Use the results from part (a) to compute the underlying production function for q.
c. You can check the result by using results from Example 10.2 to show that the CES cost function
with σ ¼ 0:5, ρ ¼ 1 generates this total-cost function.
Analytical Problems
10.9 Generalizing the CES cost function
The CES production function can be generalized to permit weighting of the inputs. In the two-input
case, this function is
q ¼ f ðk, lÞ ¼ ½ðakÞρ þ ðblÞρ γ=ρ .
a. What is the total-cost function for a firm with this production function? Hint: You can, of
course, work this out from scratch; easier perhaps is to use the results from Example 10.2 and
reason that the price for a unit of capital input in this production function is v=a and for a unit of
labor input is w=b.
b. If γ ¼ 1 and a þ b ¼ 1, it can be shown that this production function converges to the CobbDouglas form q ¼ ka l b as ρ ! 0. What is the total cost function for this particular version of the
CES function?
c. The relative labor cost share for a two-input production function is given by wl=vk. Show that
this share is constant for the Cobb-Douglas function in part (b). How is the relative labor share
affected by the parameters a and b?
d. Calculate the relative labor cost share for the general CES function introduced above. How is
that share affected by changes in w=v? How is the direction of this effect determined by the
elasticity of substitution, σ? How is it affected by the sizes of the parameters a and b?
10.10 Input demand elasticities
The own-price elasticities of contingent input demand for labor and capital are defined as
∂l c w
∂k c v
el c , w ¼
⋅ c , ekc , v ¼
⋅ .
∂w l
∂v k c
a. Calculate el c , w and ekc , v for each of the cost functions shown in Example 10.2.
b. Show that, in general, el c , w þ el c , v ¼ 0.
c. Show that the cross-price derivatives of contingent demand functions are equal—that is, show
that ∂l c =∂v ¼ ∂kc =∂w. Use this fact to show that sl el c , v ¼ sk ekc , w where sl , sk are, respectively, the
share of labor in total cost ðwl=CÞ and of capital in total cost ðvk=CÞ.
d. Use the results from parts (b) and (c) to show that sl el c, w þ sk ekc, w ¼ 0.
e. Interpret these various elasticity relationships in words and discuss their overall relevance to a
general theory of input demand.
10.11 The elasticity of substitution and input demand elasticities
The definition of the (Morishima) elasticity of substitution (Equation 10.51) can also be described in
terms of input demand elasticities. This illustrates the basic asymmetry in the definition.
353
354
Part 3 Production and Supply
a. Show that if only wj changes, si, j ¼ ex c , wj ex c , wj .
i
j
b. Show that if only wi changes, sj , i ¼ ex c , wi ex c , wi .
j
i
P
c. Show that if the production function takes the general CES form q ¼ ½ n x ρi 1=ρ for ρ 6¼ 0, then
all of the Morishima elasticities are the same: si, j ¼ 1=ð1 ρÞ ¼ σ. This is the only case in which
the Morishima definition is symmetric.
10.12 The Allen elasticity of substitution
Many empirical studies of costs report an alternative definition of the elasticity of substitution between
inputs. This alternative definition was first proposed by R. G. D. Allen in the 1930s and further clarified
by H. Uzawa in the 1960s. This definition builds directly on the production function–based elasticity of
substitution defined in footnote 6 of Chapter 9: Ai, j ¼ Cij C=Ci Cj , where the subscripts indicate partial
differentiation with respect to various input prices. Clearly, the Allen definition is symmetric.
a. Show that Ai, j ¼ ex c , wj =sj , where sj is the share of input j in total cost.
i
b. Show that the elasticity of si with respect to the price of input j is related to the Allen elasticity by
esi , pj ¼ sj ðAi, j 1Þ.
c. Show that, with only two inputs, Ak, l ¼ 1 for the Cobb-Douglas case and Ak, l ¼ σ for the
CES case.
d. Read Blackorby and Russell (1989: “Will the Real Elasticity of Substitution Please Stand Up?”)
to see why the Morishima definition is preferred for most purposes.
SUGGESTIONS FOR FURTHER READING
Allen, R. G. D. Mathematical Analysis for Economists. New
York: St. Martin’s Press, 1938, various pages—see index.
Complete (though dated) mathematical analysis of substitution possibilities and cost functions. Notation somewhat difficult.
Blackorby, C., and R. R. Russell. “Will the Real Elasticity of
Substitution Please Stand Up? (A Comparison of the Allen/
Uzawa and Morishima Elasticities).” American Economic
Review (September 1989): 882–88.
A nice clarification of the proper way to measure substitutability
among many inputs in production. Argues that the Allen/Uzawa
definition is largely useless and that the Morishima definition is by
far the best.
Ferguson, C. E. The Neoclassical Theory of Production and
Distribution. Cambridge: Cambridge University Press,
1969, Chap. 6.
Nice development of cost curves; especially strong on graphic analysis.
Fuss, M., and D. McFadden. Production Economics: A Dual
Approach to Theory and Applications. Amsterdam: NorthHolland, 1978.
Difficult and quite complete treatment of the dual relationship between production and cost functions. Some discussion of empirical
issues.
Knight, H. H. “Cost of Production and Price over Long
and Short Periods.” Journal of Political Economics 29 (April
1921): 304–35.
Classic treatment of the short-run, long-run distinction.
Silberberg, E., and W. Suen. The Structure of Economics: A
Mathematical Analysis, 3rd ed. Boston: Irwin/McGrawHill, 2001.
Chapters 7–9 have a great deal of material on cost functions. Especially recommended are the authors’ discussions of “reciprocity effects”
and their treatment of the short-run–long-run distinction as an
application of the Le Chatelier principle from physics.
Sydsaeter, K., A. Strom, and P. Berck. Economists’ Mathematical Manual, 3rd ed. Berlin: Springer-Verlag, 2000.
Chapter 25 provides a succinct summary of the mathematical concepts in this chapter. A nice summary of many input cost functions,
but beware of typos.
Chapter 10
Cost Functions
355
EXTENSIONS
The Translog Cost Function
The two cost functions studied in Chapter 10 (the
Cobb-Douglas and the CES) are very restrictive in
the substitution possibilities they permit. The CobbDouglas implicitly assumes that σ ¼ 1 between any
two inputs. The CES permits σ to take any value, but
it requires that the elasticity of substitution be the same
between any two inputs. Because empirical economists
would prefer to let the data show what the actual
substitution possibilities among inputs are, they have
tried to find more flexible functional forms. One especially popular such form is the translog cost function,
first made popular by Fuss and McFadden (1978). In
this extension we will look at this function.
∂ ln C
¼ β1 þ 2β3 ln v þ β5 ln w,
∂ ln v
(iii)
∂ ln C
sl ¼
¼ β2 þ 2β4 ln w þ β5 ln v.
∂ ln w
In the Cobb-Douglas case ðβ3 ¼ β4 ¼ β5 ¼ 0Þ
these shares are constant, but with the general
translog function they are not.
sk ¼
•
E10.1 The translog with two inputs
In Example 10.2, we calculated the Cobb-Douglas
cost function in the two-input case as Cðq, v, wÞ ¼
Bq 1=ðαþβÞ v α=ðαþβÞ wβ=ðαþβÞ . If we take the natural logarithm of this we have
ln C ðq, v, wÞ ¼ ln B þ ½1=ðα þ βÞ ln q
þ ½α=ðα þ βÞ ln v
þ ½β=ðα þ βÞ ln w.
(i)
That is, the log of total costs is linear in the logs of
output and the input prices. The translog function
generalizes this by permitting second-order terms in
input prices:
¼ sl 0 þ
For the function to be homogeneous of degree 1 in
input prices, it must be the case that β1 þ β2 ¼ 1
and β3 þ β4 þ β5 ¼ 0.
•
This function includes the Cobb-Douglas as the
special case β3 ¼ β4 ¼ β5 ¼ 0. Hence, the function can be used to test statistically whether the
Cobb-Douglas is appropriate.
•
Input shares for the translog function are especially easy to compute using the result that si ¼
ð∂ ln CÞ=ð∂ ln wi Þ. In the two-input case, this yields
∂ ln sk ∂2 ln C
β
¼ sl þ 5 .
⋅
∂sk
sk
∂v∂w
(iv)
Observe that, in the Cobb-Douglas case
ðβ5 ¼ 0Þ, the contingent price elasticity of demand for k with respect to the wage has a simple
form: ekc , w ¼ sl . A similar set of manipulations
yields el c , w ¼ sk þ 2β4 =sl and, in the CobbDouglas case, el c , w ¼ sk . Bringing these two
elasticities together yields
ln C ðq, v, wÞ¼ ln q þ β0 þ β1 ln v þ β2 ln w
þ β3 ðln vÞ2 þ β4 ðln wÞ2
(ii)
þ β5 ln v ln w,
where this function implicitly assumes constant returns
to scale (because the coefficient of ln q is 1.0)—
although that need not be the case.
Some of the properties of this function are:
•
Calculating the elasticity of substitution in the
translog case proceeds by using the result given
in Problem 10.11 that sk, l ¼ ekc , w el c , w . Making this calculation is straightforward (provided
one keeps track of how to use logarithms):
C
∂ ln Cv ∂ ln Cv ⋅ ∂∂ ln
ln v
¼
ekc , w ¼
∂ ln w
∂ ln w
C ∂ ln C ln v þ ln ∂∂ ln
ln v
¼
∂ ln w
sk, l ¼ ekc , w el c , w
β5
2β4
sk
sl
s β 2sk β4
(v)
¼1þ l 5
.
sk sl
Again, in the Cobb-Douglas case we have
¼ 1, as should have been expected.
¼ sl þ sk þ
sk, l
•
The Allen elasticity of substitution (see Problem
10.12) for the translog function is Ak, l ¼
1 þ β5 =sk sl . This function can also be used to
calculate that the (contingent) cross-price elasticity of demand is ekc , w ¼ sl Ak, l ¼ sl þ β5 =sk , as
was shown previously. Here again, Ak, l ¼ 1 in the
Cobb-Douglas case. In general, however, the
Allen and Morishima definitions will differ even
with just two inputs.
356
Part 3 Production and Supply
E10.2 The many-input translog
cost function
Most empirical studies include more than two inputs.
The translog cost function is especially easy to generalize to these situations. If we assume there are n inputs,
each with a price of wi ði ¼ 1, nÞ, then this function is
n
X
C ðq, w1 , …, wn Þ ¼ ln q þ β0 þ
βi ln wi
i¼1
þ 0:5
n X
n
X
i¼1 j ¼1
βij ln wi ln wj ,
(vi)
where we have once again assumed constant returns
to scale. This function requires βij ¼ βji , so each term
for which i 6¼ j appears twice in the final double sum
(which explains the presence of the 0.5 in the expression). For this function to be homogeneous of
degree 1 in the input prices, it must be the case that
Xn
Xn
i¼1 βi ¼ 1 and
i¼1 βij ¼ 0. Two useful properties
of this function are:
•
Input shares take the linear form
n
X
si ¼ βi þ
βij ln wj .
(vii)
j ¼1
Again, this shows why the translog is usually
estimated in a share form. Sometimes a term in
ln q is also added to the share equations to allow
for scale effects on the shares (see Sydsæter,
Strøm, and Berck, 2000).
•
The elasticity of substitution between any two
inputs in the translog function is given by
sj βij si βjj
.
(viii)
si, j ¼ 1 þ
si sj
Hence, substitutability can again be judged directly from the parameters estimated for the
translog function.
E10.3 Some applications
The translog cost function has become the main choice
for empirical studies of production. Two factors account for this popularity. First, the function allows a
fairly complete characterization of substitution patterns
among inputs—it does not require that the data fit any
prespecified pattern. Second, the function’s format
incorporates input prices in a flexible way so that one
can be reasonably sure that he or she has controlled for
such prices in regression analysis. When such control is
assured, measures of other aspects of the cost function
(such as its returns to scale) will be more reliable.
One example of using the translog function to
study input substitution is the study by Westbrook
and Buckley (1990) of the responses that shippers
made to changing relative prices of moving goods
that resulted from deregulation of the railroad and
trucking industries in the United States. The authors
look specifically at the shipping of fruits and vegetables
from the western states to Chicago and New York.
They find relatively high substitution elasticities
among shipping options and so conclude that deregulation had significant welfare benefits. Doucouliagos
and Hone (2000) provide a similar analysis of deregulation of dairy prices in Australia. They show that
changes in the price of raw milk caused dairy processing
firms to undertake significant changes in input usage.
They also show that the industry adopted significant
new technologies in response to the price change.
An interesting study that uses the translog primarily
to judge returns to scale is Latzko’s (1999) analysis of
the U.S. mutual fund industry. He finds that the elasticity of total costs with respect to the total assets managed
by the fund is less than 1 for all but the largest funds
(those with more than $4 billion in assets). Hence, the
author concludes that money management exhibits
substantial returns to scale. A number of other studies
that use the translog to estimate economies of scale focus
on municipal services. For example, Garcia and Thomas
(2001) look at water supply systems in local French
communities. They conclude that there are significant
operating economies of scale in such systems and that
some merging of systems would make sense. Yatchew
(2000) reaches a similar conclusion about electricity
distribution in small communities in Ontario, Canada.
He finds that there are economies of scale for electricity
distribution systems serving up to about 20,000 customers. Again, some efficiencies might be obtained from
merging systems that are much smaller than this size.
References
Doucouliagos, H., and P. Hone. “Deregulation and Subequilibrium in the Australian Dairy Processing Industry.” Economic Record (June 2000): 152–62.
Fuss, M., and D. McFadden, Eds. Production Economics: A
Dual Approach to Theory and Applications. Amsterdam:
North Holland, 1978.
Garcia, S., and A. Thomas. “The Structure of Municipal
Water Supply Costs: Application to a Panel of French
Chapter 10
Local Communities.” Journal of Productivity Analysis
(July 2001): 5–29.
Latzko, D. “Economies of Scale in Mutual Fund Administration.” Journal of Financial Research (Fall 1999): 331–39.
Sydsæter, K., A. Strøm, and P. Berck. Economists’ Mathematical Manual, 3rd ed. Berlin: Springer-Verlag, 2000.
Westbrook, M. D., and P. A. Buckley. “Flexible Functional
Forms and Regularity: Assessing the Competitive
Cost Functions
357
Relationship between Truck and Rail Transportation.”
Review of Economics and Statistics (November 1990):
623–30.
Yatchew, A. “Scale Economies in Electricity Distribution: A
Semiparametric Analysis.” Journal of Applied Econometrics (March/April 2000): 187–210.
CHAPTER
11
Profit Maximization
In Chapter 10 we examined the way in which firms minimize costs for any level of output they choose. In this
chapter we focus on how the level of output is chosen by profit-maximizing firms. Before investigating that
decision, however, it is appropriate to discuss briefly the nature of firms and the ways in which their choices
should be analyzed.
THE NATURE AND BEHAVIOR OF FIRMS
As we pointed out at the beginning of our analysis of production, a firm is an association of
individuals who have organized themselves for the purpose of turning inputs into outputs.
Different individuals will provide different types of inputs, such as workers’ skills and varieties
of capital equipment, with the expectation of receiving some sort of reward for doing so.
Contractual relationships within firms
The nature of the contractual relationship between the providers of inputs to a firm may be
quite complicated. Each provider agrees to devote his or her input to production activities
under a set of understandings about how it is to be used and what benefit is to be expected
from that use. In some cases these contracts are explicit. Workers often negotiate contracts
that specify in considerable detail what hours are to be worked, what rules of work are to be
followed, and what rate of pay is to be expected. Similarly, capital owners invest in a firm
under a set of explicit legal principles about the ways in which that capital may be used, the
compensation the owner can expect to receive, and whether the owner retains any profits or
losses after all economic costs have been paid. Despite these formal arrangements, it is clear
that many of the understandings between the providers of inputs to a firm are implicit ;
relationships between managers and workers follow certain procedures about who has the
authority to do what in making production decisions. Among workers, numerous implicit
understandings exist about how work tasks are to be shared; and capital owners may delegate
much of their authority to managers and workers to make decisions on their behalf (General
Motors’ shareholders, for example, are never involved in how assembly-line equipment will
be used, though technically they own it). All of these explicit and implicit relationships
change in response to experiences and events external to the firm. Much as a basketball
team will try out new plays and defensive strategies, so too firms will alter the nature of their
internal organizations to achieve better long-term results.1
The initial development of the theory of the firm from the notion of the contractual relationships involved can be found in
R. H. Coase, “The Nature of the Firm,” Economica (November 1937): 386–405.
1
358
Chapter 11
Profit Maximization
359
Modeling firms’ behavior
Although some economists have adopted a “behavioral” approach to studying firms’ decisions, most have found that approach too cumbersome for general purposes. Rather, they
have adopted a “holistic” approach that treats the firm as a single decision-making unit and
sweeps away all the complicated behavioral issues about relationships among input providers.
Under this approach, it is often convenient to assume that a firm’s decisions are made by a
single dictatorial manager who rationally pursues some goal, usually profit maximization.
That is the approach we take here. In Chapter 18 we look at some of the informational issues
that arise in intrafirm contracts.
PROFIT MAXIMIZATION
Most models of supply assume that the firm and its manager pursue the goal of achieving the
largest economic profits possible. Hence we will use the following definition.
Profit-maximizing firm. A profit-maximizing firm chooses both its inputs and its outputs
DEFINITION
with the sole goal of achieving maximum economic profits. That is, the firm seeks to make the
difference between its total revenues and its total economic costs as large as possible.
This assumption—that firms seek maximum economic profits—has a long history in economic literature. It has much to recommend it. It is plausible because firm owners may
indeed seek to make their asset as valuable as possible and because competitive markets may
punish firms that do not maximize profits. The assumption also yields interesting theoretical
results that can explain actual firms’ decisions.
Profit maximization and marginalism
If firms are strict profit maximizers, they will make decisions in a “marginal” way. The
entrepreneur will perform the conceptual experiment of adjusting those variables that can
be controlled until it is impossible to increase profits further. This involves, say, looking at the
incremental, or “marginal,” profit obtainable from producing one more unit of output, or at
the additional profit available from hiring one more laborer. As long as this incremental profit
is positive, the extra output will be produced or the extra laborer will be hired. When the
incremental profit of an activity becomes zero, the entrepreneur has pushed that activity far
enough, and it would not be profitable to go further. In this chapter, we will explore the
consequences of this assumption by using increasingly sophisticated mathematics.
Output choice
First we examine a topic that should be very familiar: what output level a firm will produce in
order to obtain maximum profits. A firm sells some level of output, q, at a market price of p
per unit. Total revenues ðRÞ are given by
RðqÞ ¼ pðqÞ ⋅ q,
(11.1)
where we have allowed for the possibility that the selling price the firm receives might be
affected by how much it sells. In the production of q, certain economic costs are incurred
and, as in Chapter 10, we will denote these by CðqÞ.
The difference between revenues and costs is called economic profits ðπÞ. Because both
revenues and costs depend on the quantity produced, economic profits will also. That is,
πðqÞ ¼ pðqÞ ⋅ q C ðqÞ ¼ RðqÞ C ðqÞ.
(11.2)
360
Part 3 Production and Supply
The necessary condition for choosing the value of q that maximizes profits is found by
setting the derivative of Equation 11.2 with respect to q equal to 0:2
dπ
dR
dC
¼ π0 ðqÞ ¼
¼ 0,
dq
dq
dq
(11.3)
so the first-order condition for a maximum is that
dR
dC
¼
.
dq
dq
(11.4)
This is a mathematical statement of the “marginal revenue equals marginal cost” rule usually
studied in introductory economics courses. Hence we have the following.
OPTIMIZATION
PRINCIPLE
Profit maximization. To maximize economic profits, the firm should choose that output
for which marginal revenue is equal to marginal cost. That is,
MR ¼
dR
dC
¼
¼ MC .
dq
dq
(11.5)
Second-order conditions
Equation 11.4 or 11.5 is only a necessary condition for a profit maximum. For sufficiency, it is
also required that
d 2 π dπ0 ðqÞ ¼
< 0,
(11.6)
dq 2 q¼q dq q¼q
or that “marginal” profit must be decreasing at the optimal level of q. For q less than q (the
optimal level of output), profit must be increasing ½π0 ðqÞ > 0; and for q greater than q ,
profit must be decreasing ½π0 ðqÞ < 0. Only if this condition holds has a true maximum been
achieved. Clearly the condition holds if marginal revenue is decreasing (or constant) in q and
marginal cost is increasing in q.
Graphical analysis
These relationships are illustrated in Figure 11.1, where the top panel depicts typical cost and
revenue functions. For low levels of output, costs exceed revenues and so economic profits are
negative. In the middle ranges of output, revenues exceed costs; this means that profits are
positive. Finally, at high levels of output, costs rise sharply and again exceed revenues. The
vertical distance between the revenue and cost curves (that is, profits) is shown in Figure 11.1b.
Here profits reach a maximum at q . At this level of output it is also true that the slope of
the revenue curve (marginal revenue) is equal to the slope of the cost curve (marginal cost).
It is clear from the figure that the sufficient conditions for a maximum are also satisfied at
this point, because profits are increasing to the left of q and decreasing to the right of q .
Output level q is therefore a true profit maximum. This is not so for output level q .
Although marginal revenue is equal to marginal cost at this output, profits are in fact at a
minimum there.
2
Notice that this is an unconstrained maximization problem; the constraints in the problem are implicit in the revenue and
cost functions. Specifically, the demand curve facing the firm determines the revenue function, and the firm’s production
function (together with input prices) determines its costs.
Chapter 11
FIGURE 11.1
Profit Maximization
Marginal Revenue Must Equal Marginal Cost for Profit Maximization
Because profits are defined to be revenues ðRÞ minus costs ðCÞ, it is clear that profits reach a maximum
when the slope of the revenue function (marginal revenue) is equal to the slope of the cost function
(marginal cost). This equality is only a necessary condition for a maximum, as may be seen by
comparing points q (a true maximum) and q (a true minimum), points at which marginal revenue
equals marginal cost.
Revenues,
costs
C
R
q**
q*
Output per period
q*
Output per period
(a)
Profits
0
Losses
(b)
MARGINAL REVENUE
It is the revenue obtained from selling one more unit of output that is relevant to the profitmaximizing firm’s output decision. If the firm can sell all it wishes without having any effect
on market price, the market price will indeed be the extra revenue obtained from selling one
more unit. Phrased in another way: if a firm’s output decisions will not affect market price,
then marginal revenue is equal to the price at which a unit sells.
361
362
Part 3 Production and Supply
A firm may not always be able to sell all it wants at the prevailing market price, however. If
it faces a downward-sloping demand curve for its product, then more output can be sold only
by reducing the good’s price. In this case the revenue obtained from selling one more unit
will be less than the price of that unit because, in order to get consumers to take the extra
unit, the price of all other units must be lowered. This result can be easily demonstrated. As
before, total revenue ðRÞ is the product of the quantity sold ðqÞ times the price at which it is
sold ð pÞ, which may also depend on q. Marginal revenue ðMRÞ is then defined to be the
change in R resulting from a change in q.
DEFINITION
Marginal revenue. We define
marginal revenue ¼ MRðqÞ ¼
dR
d½pðqÞ ⋅ q
dp
¼
¼ p þq⋅
:
dq
dq
dq
(11.7)
Notice that the marginal revenue is a function of output. In general, MR will be different for
different levels of q. From Equation 11.7 it is easy to see that, if price does not change as
quantity increases ðdp=dq ¼ 0Þ, marginal revenue will be equal to price. In this case we say
that the firm is a price taker because its output decisions do not influence the price it receives.
On the other hand, if price falls as quantity increases ðdp=dq < 0Þ, marginal revenue will be
less than price. A profit-maximizing manager must know how increases in output will affect
the price received before making an optimal output decision. If increases in q cause market
price to fall, this must be taken into account.
EXAMPLE 11.1 Marginal Revenue from a Linear Demand Function
Suppose a shop selling sub sandwichs (also called grinders, torpedoes, or, in Philadelphia,
hoagies) faces a linear demand curve for its daily output over period ðqÞ of the form
q ¼ 100 10p.
Solving for the price the shop receives, we have
q
þ 10,
p¼
10
and total revenues (as a function of q) are given by
R ¼ pq ¼
q 2
þ 10q.
10
(11.8)
(11.9)
(11.10)
The sub firm’s marginal revenue function is
dR
q
¼
þ 10,
(11.11)
MR ¼
dq
5
and in this case MR < p for all values of q. If, for example, the firm produces 40 subs per
day, Equation 11.9 shows that it will receive a price of $6 per sandwich. But at this level of
output Equation 11.11 shows that MR is only $2. If the firm produces 40 subs per day then
total revenue will be $240 ð¼ $6 40Þ, whereas if it produced 39 subs then total revenue
would be $238 ð¼ $6.1 39Þ because price will rise slightly when less is produced. Hence
the marginal revenue from the 40th sub sold is considerably less than its price. Indeed, for
q ¼ 50, marginal revenue is zero (total revenues are a maximum at $250 ¼ $5 50), and
any further expansion in daily sub output will actually result in a reduction in total revenue
to the firm.
To determine the profit-maximizing level of sub output, we must know the firm’s marginal costs. If subs can be produced at a constant average and marginal cost of $4, then
Equation 11.11 shows that MR ¼ MC at a daily output of 30 subs. With this level of output,
Chapter 11
Profit Maximization
each sub will sell for $7 and profits are $90 ½¼ ð$7 $4Þ ⋅ 30. Although price exceeds average and marginal cost here by a substantial margin, it would not be in the firm’s interest to
expand output. With q ¼ 35, for example, price will fall to $6.50 and profits will fall to
$87.50 ½¼ ð$6.50 $4.00Þ ⋅ 35. Marginal revenue, not price, is the primary determinant of
profit-maximizing behavior.
QUERY: How would an increase in the marginal cost of sub production to $5 affect the
output decision of this firm? How would it affect the firm’s profits?
Marginal revenue and elasticity
The concept of marginal revenue is directly related to the elasticity of the demand curve
facing the firm. Remember that the elasticity of demand (eq, p ) is defined as the percentage
change in quantity demanded that results from a 1 percent change in price:
dq=q
dq p
¼
⋅ .
dp=p
dp q
Now, this definition can be combined with Equation 11.7 to give
eq, p ¼
qdp
q dp
¼p 1þ ⋅
MR ¼ p þ
dq
p dq
¼ p 1þ
1
eq, p
!
.
(11.12)
If the demand curve facing the firm is negatively sloped, then eq, p < 0 and marginal revenue
will be less than price, as we have already shown. If demand is elastic ðeq, p < 1Þ, then marginal revenue will be positive. If demand is elastic, the sale of one more unit will not affect
price “very much” and hence more revenue will be yielded by the sale. In fact, if demand
facing the firm is infinitely elastic ðeq, p ¼ ∞Þ, marginal revenue will equal price. The firm is,
in this case, a price taker. However, if demand is inelastic ðeq, p > 1Þ, marginal revenue will be
negative. Increases in q can be obtained only through “large” declines in market price, and
these declines will actually cause total revenue to decrease.
The relationship between marginal revenue and elasticity is summarized by Table 11.1.
TABLE 11.1
Relationship between Elasticity and Marginal Revenue
eq, p < 1
MR > 0
eq, p ¼ 1
MR ¼ 0
eq, p > 1
MR < 0
Price–marginal cost markup
If we assume the firm wishes to maximize profits, this analysis can be extended to illustrate the
connection between price and marginal cost. Setting MR ¼ MC yields
!
1
MC ¼ p 1 þ
eq, p
or
eq, p
p
.
¼
1 þ eq, p
MC
(11.13)
That is, the “markup” of price over marginal cost depends in a very specific way on the elasticity of demand facing the firm. First, notice that this demand must be elastic ðeq, p < 1Þ.
363
364
Part 3 Production and Supply
If demand were inelastic, the ratio in Equation 11.13 would be negative and the equation would
be nonsensical. This simply reflects that, when demand is inelastic, marginal revenue is negative
and cannot be equated to a positive marginal cost. It is important to stress that it is the demand
facing the firm that must be elastic. This may be consistent with an overall inelastic demand for
the product in question if the firm faces competition from other firms producing the same good.
Equation 11.13 implies that the markup over marginal cost will be higher the closer eq, p is
to 1. If the demand facing the firm is infinitely elastic (perhaps because there are many other
firms producing the same good), then eq, p ¼ ∞ and there is no markup ðp=MC ¼ 1Þ. On
the other hand, with an elasticity of demand of (say) eq, p ¼ 2, the markup over marginal
cost will be 100 percent (i.e., p=MC ¼ 2).
Marginal revenue curve
Any demand curve has a marginal revenue curve associated with it. If, as we sometimes
assume, the firm must sell all its output at one price, it is convenient to think of the demand
curve facing the firm as an average revenue curve. That is, the demand curve shows the revenue
per unit (in other words, the price) yielded by alternative output choices. The marginal
revenue curve, on the other hand, shows the extra revenue provided by the last unit sold. In
the usual case of a downward-sloping demand curve, the marginal revenue curve will lie below
the demand curve because, according to Equation 11.7, MR < p. In Figure 11.2 we have
drawn such a curve together with the demand curve from which it was derived. Notice that for
output levels greater than q1 , marginal revenue is negative. As output increases from 0 to q1 ,
total revenues ðp ⋅ qÞ increase. However, at q1 total revenues ð p1 ⋅ q1 Þ are as large as possible;
beyond this output level, price falls proportionately faster than output rises.
FIGURE 11.2
Market Demand Curve and Associated Marginal Revenue Curve
Because the demand curve is negatively sloped, the marginal revenue curve will fall below the demand
(“average revenue”) curve. For output levels beyond q1 , MR is negative. At q1 , total revenues
ð p1 q1 Þ are a maximum; beyond this point additional increases in q actually cause total revenues
to fall because of the concomitant declines in price.
Price
D (average revenue)
p1
0
q1
Quantity per period
MR
Chapter 11
Profit Maximization
In Part 2 we talked in detail about the possibility of a demand curve’s shifting because of
changes in income, prices of other goods, or preferences. Whenever a demand curve does
shift, its associated marginal revenue curve shifts with it. This should be obvious, because a
marginal revenue curve cannot be calculated without referring to a specific demand curve.
EXAMPLE 11.2 The Constant Elasticity Case
In Chapter 5 we showed that a demand function of the form
(11.14)
q ¼ ap b
has a constant price elasticity of demand, and that this elasticity is given by the parameter b.
To compute the marginal revenue function for this function, first solve for p:
p¼
1
a
1=b
q 1=b ¼ kq 1=b ,
(11.15)
where k ¼ ð1=aÞ1=b . Hence
R ¼ pq ¼ kq ð1þbÞ=b
and
MR ¼ dR=dq ¼
1 þ b 1=b
1þb
kq
p.
¼
b
b
(11.16)
For this particular function, then, MR is proportional to price. If, for example, eq, p ¼ b ¼ 2,
then MR ¼ 0.5p. For a more elastic case, suppose b ¼ 10; then MR ¼ 0.9p. The MR curve
approaches the demand curve as demand becomes more elastic. Again, if b ¼ ∞; then
MR ¼ p; that is, in the case of infinitely elastic demand, the firm is a price taker. For inelastic
demand, on the other hand, MR is negative (and profit maximization would be impossible).
QUERY: Suppose demand depended on other factors in addition to p. How would this
change the analysis of this example? How would a change in one of these other factors shift
the demand curve and its marginal revenue curve?
SHORT-RUN SUPPLY BY A PRICE-TAKING FIRM
We are now ready to study the supply decision of a profit-maximizing firm. In this chapter we
will examine only the case in which the firm is a price taker. In Part 5 we will be looking at
other cases in considerably more detail. Also, we will focus only on supply decisions in the
short run here. Long-run questions concern entry and exit by firms and are the primary focus
of the next chapter. The firm’s set of short-run cost curves is therefore the appropriate model
for our analysis.
Profit-maximizing decision
Figure 11.3 shows the firm’s short-run decision. The market price3 is given by P . The
demand curve facing the firm is therefore a horizontal line through P . This line is labeled
P ¼ MR as a reminder that an extra unit can always be sold by this price-taking firm without
affecting the price it receives. Output level q provides maximum profits, because at q price
is equal to short-run marginal cost. The fact that profits are positive can be seen by noting that
3
We will usually use an uppercase italic P to denote market price for a single good here and in later chapters. When notation
is complex, however, we will sometimes revert to using a lowercase p.
365
366
Part 3 Production and Supply
FIGURE 11.3
Short-Run Supply Curve for a Price-Taking Firm
In the short run, a price-taking firm will produce the level of output for which SMC ¼ P . At P , for
example, the firm will produce q . The SMC curve also shows what will be produced at other prices.
For prices below SAVC, however, the firm will choose to produce no output. The heavy lines in the
figure represent the firm’s short-run supply curve.
Market
price
SMC
P **
SAC
P * = MR
SAVC
P ***
P1
q*** q* q**
0
Quantity
per period
price at q exceeds average costs. The firm earns a profit on each unit sold. If price were below
average cost (as is the case for P ), the firm would have a loss on each unit sold. If price and
average cost were equal, profits would be zero. Notice that at q the marginal cost curve has a
positive slope. This is required if profits are to be a true maximum. If P ¼ MC on a negatively
sloped section of the marginal cost curve then this would not be a point of maximum profits,
because increasing output would yield more in revenues (price times the amount produced)
than this production would cost (marginal cost would decline if the MC curve has a negative
slope). Consequently, profit maximization requires both that P ¼ MC and that marginal cost
be increasing at this point.4
The firm’s short-run supply curve
The positively sloped portion of the short-run marginal cost curve is the short-run supply
curve for this price-taking firm. That curve shows how much the firm will produce for every
possible market price. For example, as Figure 11.3 shows, at a higher price of P the firm
will produce q because it is in its interest to incur the higher marginal costs entailed by q .
With a price of P , on the other hand, the firm opts to produce less (q ) because only a
4
Mathematically: because
πðqÞ ¼ Pq C ðqÞ,
profit maximization requires (the first-order condition)
π0 ðqÞ ¼ P MC ðqÞ ¼ 0
and (the second-order condition)
π00 ðqÞ MC 0 ðqÞ < 0.
0
Hence it is required that MC ðqÞ > 0; marginal cost must be increasing.
Chapter 11
Profit Maximization
367
lower output level will result in lower marginal costs to meet this lower price. By considering
all possible prices the firm might face, we can see by the marginal cost curve how much
output the firm should supply at each price.
The shutdown decision. For very low prices we must be careful about this conclusion.
Should market price fall below P1 , the profit-maximizing decision would be to produce
nothing. As Figure 11.3 shows, prices less than P1 do not cover average variable costs. There
will be a loss on each unit produced in addition to the loss of all fixed costs. By shutting down
production, the firm must still pay fixed costs but avoids the losses incurred on each unit
produced. Because, in the short run, the firm cannot close down and avoid all costs, its best
decision is to produce no output. On the other hand, a price only slightly above P1 means the
firm should produce some output. Although profits may be negative (which they will be if
price falls below short-run average total costs, the case at P ), the profit-maximizing
decision is to continue production as long as variable costs are covered. Fixed costs must
be paid in any case, and any price that covers variable costs will provide revenue as an offset to
the fixed costs.5 Hence we have a complete description of this firm’s supply decisions in
response to alternative prices for its output. These are summarized in the following definition.
Short-run supply curve. The firm’s short-run supply curve shows how much it will produce
DEFINITION
at various possible output prices. For a profit-maximizing firm that takes the price of its
output as given, this curve consists of the positively sloped segment of the firm’s short-run
marginal cost above the point of minimum average variable cost. For prices below this level,
the firm’s profit-maximizing decision is to shut down and produce no output.
Of course, any factor that shifts the firm’s short-run marginal cost curve (such as changes in
input prices or changes in the level of fixed inputs employed) will also shift the short-run
supply curve. In Chapter 12 we will make extensive use of this type of analysis to study the
operations of perfectly competitive markets.
EXAMPLE 11.3 Short-Run Supply
In Example 10.5 we calculated the short-run total-cost function for the Cobb-Douglas
production function as
α=β
SC ðv, w, q, kÞ ¼ vk1 þ wq 1=β k1
,
(11.17)
(continued)
5
Some algebra may clarify matters. We know that total costs equal the sum of fixed and variable costs,
SC ¼ SFC þ SVC ,
and that profits are given by
π ¼ R SC ¼ P ⋅ q SFC SVC .
If q ¼ 0, then variable costs and revenues are 0 and so
π ¼ SFC .
The firm will produce something only if π > SFC. But that means that
P ⋅ q > SVC
or
P > SVC =q.
368
Part 3 Production and Supply
EXAMPLE 11.3 CONTINUED
where k1 is the level of capital input that is held constant in the short run.6 Short-run
marginal cost is easily computed as
∂SC
w
α=β
(11.18)
¼ q ð1βÞ=β k 1 .
SMC ðv, w, q, k1 Þ ¼
∂q
β
Notice that short-run marginal cost is increasing in output for all values of q. Short-run
profit maximization for a price-taking firm requires that output be chosen so that market
price ðP Þ is equal to short-run marginal cost:
w
α=β
¼ P,
(11.19)
SMC ¼ q ð1βÞ=β k1
β
and we can solve for quantity supplied as
w β=ð1βÞ α=ð1βÞ β=ð1βÞ
k1
P
.
(11.20)
β
This supply function provides a number of insights that should be familiar from earlier
economics courses: (1) the supply curve is positively sloped—increases in P cause the firm to
produce more because it is willing to incur a higher marginal cost;7 (2) the supply curve is
shifted to the left by increases in the wage rate, w—that is, for any given output price, less is
supplied with a higher wage; (3) the supply curve is shifted outward by increases in capital
input, k—with more capital in the short run, the firm incurs a given level of short-run
marginal cost at a higher output level; and (4) the rental rate of capital, v, is irrelevant to
short-run supply decisions because it is only a component of fixed costs.
q¼
Numerical example. We can pursue once more the numerical example from Example 10.5,
where α ¼ β ¼ 0.5, v ¼ 3, w ¼ 12, and k1 ¼ 80. For these specific parameters, the supply
function is
w 1
P
40P
10P
1
1
¼
¼
.
(11.21)
q¼
⋅ ðk1 Þ ⋅ P ¼ 40 ⋅
0.5
w
12
3
That this computation is correct can be checked by comparing the quantity supplied at
various prices with the computation of short-run marginal cost in Table 10.2. For example,
if P ¼ 12 then the supply function predicts that q ¼ 40 will be supplied, and Table 10.2
shows that this will agree with the P ¼ SMC rule. If price were to double to P ¼ 24, an
output level of 80 would be supplied and, again, Table 10.2 shows that when q ¼ 80,
SMC ¼ 24. A lower price (say P ¼ 6) would cause less to be produced ðq ¼ 20Þ.
Before adopting Equation 11.21 as the supply curve in this situation, we should also check
the firm’s shutdown decision. Is there a price where it would be more profitable to produce
q ¼ 0 than to follow the P ¼ SMC rule? From Equation 11.17 we know that short-run
variable costs are given by
α=β
SVC ¼ wq 1=β k1
(11.22)
and so
SVC
α=β
¼ wq ð1βÞ=β k 1 .
q
6
(11.23)
Because capital input is held constant, the short-run cost function exhibits increasing marginal cost and will therefore yield
a unique profit-maximizing output level. If we had used a constant returns-to-scale production function in the long run,
there would have been no such unique output level. We discuss this point later in this chapter and in Chapter 12.
7
In fact, the short-run elasticity of supply can be read directly from Equation 11.20 as β=ð1 βÞ.
Chapter 11
Profit Maximization
369
A comparison of Equation 11.23 with Equation 11.18 shows that SVC=q < SMC for all
values of q provided that β < 1. So in this problem there is no price low enough such that,
by following the P ¼ SMC rule, the firm would lose more than if it produced nothing.
In our numerical example, consider the case P ¼ 3. With such a low price the firm would
opt for q ¼ 10. Total revenue would be R ¼ 30, and total short-run costs would be
SC ¼ 255 (see Table 10.1). Hence, profits would be π ¼ R SC ¼ 225. Although the
situation is dismal for the firm, it is better than opting for q ¼ 0. If it produces nothing it
avoids all variable (labor) costs but still loses 240 in fixed costs of capital. By producing
10 units of output, its revenues cover variable costs ðR SVC ¼ 30 15 ¼ 15Þ and contribute 15 to offset slightly the loss of fixed costs.
QUERY: How would you graph the short-run supply curve in Equation 11.21? How would
the curve be shifted if w rose to 15? How would it be shifted if capital input increased to
k1 ¼ 100? How would the short-run supply curve be shifted if v fell to 2? Would any of these
changes alter the firm’s determination to avoid shutting down in the short run?
PROFIT FUNCTIONS
Additional insights into the profit-maximization process for a price-taking firm8 can be obtained by looking at the profit function. This function shows the firm’s (maximized) profits as
depending only on the prices that the firm faces. To understand the logic of its construction,
remember that economic profits are defined as
π ¼ Pq C ¼ Pf ðk, lÞ vk wl.
(11.24)
Only the variables k and l [and also q ¼ f ðk, lÞ] are under the firm’s control in this
expression. The firm chooses levels of these inputs in order to maximize profits, treating the
three prices P , v, and w as fixed parameters in its decision. Looked at in this way, the firm’s
maximum profits ultimately depend only on these three exogenous prices (together with the
form of the production function). We summarize this dependence by the profit function.
Profit function. The firm’s profit function shows its maximal profits as a function of the
DEFINITION
prices that the firm faces:
ΠðP , v, wÞ max πðk, lÞ ¼ max½Pf ðk, lÞ vk wl.
k, l
k, l
(11.25)
In this definition we use an upper case to indicate that the value given by the function is the
maximum profits obtainable given the prices. This function implicitly incorporates the form
of the firm’s production function—a process we will illustrate in Example 11.4. The profit
function can refer to either long-run or short-run profit maximization, but in the latter case
we would need also to specify the levels of any inputs that are fixed in the short run.
Properties of the profit function
As for the other optimized functions we have already looked at, the profit function has a
number of properties that are useful for economic analysis.
8
Much of the analysis here would also apply to a firm that had some market power over the price it received for its product,
but we will delay a discussion of that possibility until Part 5.
370
Part 3 Production and Supply
1. Homogeneity. A doubling of all of the prices in the profit function will precisely double
profits—that is, the profit function is homogeneous of degree 1 in all prices. We have
already shown that marginal costs are homogeneous of degree 1 in input prices, hence
a doubling of input prices and a doubling of the market price of a firm’s output will
not change the profit-maximizing quantity it decides to produce. But, because both
revenues and costs have doubled, profits will double. This shows that with pure
inflation (where all prices rise together) firms will not change their production plans
and the levels of their profits will just keep up with that inflation.
2. Profit functions are nondecreasing in output price, P. This result seems obvious—a firm
could always respond to a rise in the price of its output by not changing its input or
output plans. Given the definition of profits, they must rise. Hence, if the firm changes
its plans, it must be doing so in order to make even more profits. If profits were to
decline, the firm would not be maximizing profits.
3. Profit functions are nonincreasing in input prices, v, and w. Again, this feature of the
profit function seems obvious. A proof is similar to that used above in our discussion of
output prices.
4. Profit functions are convex in output prices. This important feature of profit functions
says that the profits obtainable by averaging those available from two different output
prices will be at least as large as those obtainable from the average9 of the two prices.
Mathematically,
ΠðP1 , v, wÞ þ ΠðP2 , v, wÞ
2
Π
P1 þ P2
, v, w .
2
(11.26)
The intuitive reason for this is that, when firms can freely adapt their decisions to two different prices, better results are possible than when they can make only one set of choices in
response to the single average price. More formally, let P3 ¼ ðP1 þ P2 Þ=2 and let qi , ki , li
represent the profit-maximizing output and input choices for these various prices. Because of
the profit-maximization assumption implicit in the function , we can write
P1 q3 vk3 wl3
P q vk3 wl3
þ 2 3
2
2
P q vk1 wl1
P q vk2 wl2
þ 2 2
1 1
2
2
ΠðP1 , v, wÞ þ ΠðP2 , v, wÞ
(11.27)
,
≡
2
ΠðP3 , v, wÞ ≡ P3 q3 vk3 wl3 ¼
which proves Equation 11.26. The convexity of the profit function has many applications to
topics such as price stabilization. Some of these are discussed in the Extensions to this
chapter.
Envelope results
Because the profit function reflects an underlying process of unconstrained maximization, we
may also apply the envelope theorem to see how profits respond to changes in output and
input prices. This application of the theorem yields a variety of useful results. Specifically,
9
Although we only discuss a simple averaging_of prices here, it is clear that with convexity a condition similar to Equation
11.26 holds for any weighted average price P ¼ tP1 þ ð1 t ÞP2 , where 0 t 1.
Chapter 11
Profit Maximization
using the definition of profits shows that
∂ΠðP , v, wÞ
¼ qðP , v, wÞ,
∂P
∂ΠðP , v, wÞ
(11.28)
¼ kðP , v, wÞ,
∂v
∂ΠðP , v, wÞ
¼ lðP , v, wÞ.
∂w
Again, these equations make intuitive sense: a small change in output price will increase
profits in proportion to how much the firm is producing, whereas a small increase in the
price of an input will reduce profits in proportion to the amount of that input being
employed. The first of these equations says that the firm’s supply function can be calculated
from its profit function by partial differentiation with respect to the output price.10 The
second and third equations show that input demand functions11 can also be derived from
the profit functions. Because the profit function itself is homogeneous of degree 1, all of the
functions described in Equations 11.28 are homogeneous of degree 0. That is, a doubling of
both output and input prices will not change the input levels that the firm chooses; nor will
this change the firm’s profit-maximizing output level. All these findings also have short-run
analogues, as will be shown later with a specific example.
Producer surplus in the short run
In Chapter 5 we discussed the concept of “consumer surplus” and showed how areas below
the demand curve can be used to measure the welfare costs to consumers of price changes.
We also showed how such changes in welfare could be captured in the individual’s expenditure function. The process of measuring the welfare effects of price changes for firms is similar
in short-run analysis, and this is the topic we pursue here. However, as we show in the next
chapter, measuring the welfare impact of price changes for producers in the long run requires
a very different approach because most such long-term effects are felt not by firms themselves
but rather by their input suppliers. In general it is this long-run approach that will prove more
useful for our subsequent study of the welfare impacts of price changes.
Because the profit function is nondecreasing in output prices, we know that if P2 > P1
then
ΠðP2 , …Þ ΠðP1 , …Þ,
and it would be natural to measure the welfare gain to the firm from the price change as
(11.29)
welfare gain ¼ ΠðP2 , …Þ ΠðP1 , …Þ.
Figure 11.4 shows how this value can be measured graphically as the area bounded by the
two prices and above the short-run supply curve. Intuitively, the supply curve shows the
minimum price that the firm will accept for producing its output. Hence, when market price
rises from P1 to P2 , the firm is able to sell its prior output level ðq1 Þ at a higher price and also
opts to sell additional output ðq2 q1 Þ for which, at the margin, it likewise earns added
profits on all but the final unit. Hence, the total gain in the firm’s profits is given by area
P2 ABP1 . Mathematically, we can make use of the envelope results from the previous section
to derive
10
This relationship is sometimes referred to as “Hotelling’s lemma”—after the economist Harold Hotelling, who
discovered it in the 1930s.
11
Unlike the input demand functions derived in Chapter 10, these input demand functions are not conditional on output
levels. Rather, the firm’s profit-maximizing output decision has already been taken into account in the functions. This
demand concept is therefore more general than the one we introduced in Chapter 10, and we will have much more to say
about it in the next section.
371
372
Part 3 Production and Supply
FIGURE 11.4
Changes in Short-Run Producer Surplus Measure Firm Profits
If price rises from P1 to P2 then the increase in the firm’s profits is given by area P2 ABP1 . At a price of
P1 , the firm earns short-run producer surplus given by area P0 BP1 . This measures the increase in shortrun profits for the firm when it produces q1 rather than shutting down when price is P0 or below.
Market
price
SMC
A
P2
P1
B
P0
q1
P2
welfare gain ¼
q2
q
P2
∫ qðP Þ dP ¼ ∫ ∂P∂Π dP ¼ ΠðP , …Þ ΠðP , …Þ.
2
P1
1
(11.30)
P1
Thus, the geometric and mathematical measures of the welfare change agree.
Using this approach, we can also measure how much the firm values the right to produce
at the prevailing market price relative to a situation where it would produce no output. If we
denote the short-run shutdown price as P0 (which may or may not be a price of zero), then
the extra profits available from facing a price of P1 are defined to be producer surplus:
P1
producer surplus ¼ ΠðP1 , …Þ ΠðP0 , …Þ ¼
∫ qðP Þ dP .
(11.31)
P0
This is shown as area P1 BP0 in Figure 11.4. Hence we have the following formal definition.
DEFINITION
Producer surplus. Producer surplus is the extra return that producers earn by making
transactions at the market price over and above what they would earn if nothing were
produced. It is illustrated by the size of the area below the market price and above the supply
curve.
In this definition we have made no distinction between the short run and the long run,
though our development so far has involved only short-run analysis. In the next chapter we
will see that the same definition can serve dual duty by describing producer surplus in the long
Chapter 11
Profit Maximization
run, so using this generic definition works for both concepts. Of course, as we will show, the
meaning of long-run producer surplus is quite different from what we have studied here.
One more aspect of short-run producer surplus should be pointed out. Because the firm
produces no output at its shutdown price, we know that ðP0 , …Þ ¼ vk1 ; that is, profits at
the shutdown price are solely made up of losses of all fixed costs. Therefore,
producer surplus ¼ ΠðP1 , …Þ ΠðP0 , …Þ
¼ ΠðP1 , …Þ ðvk1 Þ ¼ ΠðP1 , …Þ þ vk1 .
(11.32)
That is, producer surplus is given by current profits being earned plus short-run fixed costs.
Further manipulation shows that magnitude can also be expressed as
producer surplus ¼ ΠðP1 , …Þ ΠðP0 , …Þ
(11.33)
¼ P1 q1 vk1 wl1 þ vk1 ¼ P1 q1 wl1 .
In words, a firm’s short-run producer surplus is given by the extent to which its revenues
exceed its variable costs—this is, indeed, what the firm gains by producing in the short run
rather than shutting down and producing nothing.
EXAMPLE 11.4 A Short-Run Profit Function
These various uses of the profit function can be illustrated with the Cobb-Douglas production function we have been using. Since q ¼ kα l β and since we treat capital as fixed at k1 in the
short run, it follows that profits are
π ¼ Pk α1 l β vk1 wl.
(11.34)
To find the profit function we use the first-order conditions for a maximum to eliminate l
from this expression:
∂π
w 1=ðβ1Þ
¼ βPk α1 l β1 w ¼ 0 so l ¼
.
(11.35)
∂l
βPk α1
We can simplify the process of substituting this back into the profit equation by letting
A ¼ ðw=βPkα1 Þ. Making use of this shortcut, we have
ΠðP , v, w, k1 Þ ¼ Pk α1 A β=ðβ1Þ vk1 wA 1=ðβ1Þ
¼ wA 1=ðβ1Þ Pk α1
A
1 vk1
w
1 β β=ðβ1Þ 1=ð1βÞ α=ð1βÞ
(11.36)
w
P
k1
vk1 .
ββ=ðβ1Þ
Though admittedly messy, this solution is what was promised—the firm’s maximal profits
are expressed as a function of only the prices it faces and its technology. Notice that the
firm’s fixed costs ðvk1 Þ enter this expression in a simple linear way. The prices the firm faces
determine the extent to which revenues exceed variable costs; then fixed costs are subtracted
to obtain the final profit number.
Because it is always wise to check that one’s algebra is correct, let’s try out the numerical
example we have been using. With α ¼ β ¼ 0.5, v ¼ 3, w ¼ 12, and k1 ¼ 80, we know that
at a price of P ¼ 12 the firm will produce 40 units of output and use labor input of l ¼ 20.
Hence profits will be π ¼ R C ¼ 12 ⋅ 40 3 ⋅ 80 12 ⋅ 20 ¼ 0. The firm will just break
even at a price of P ¼ 12. Using the profit function yields
¼
(11.37)
ΠðP , v, w, k1 Þ ¼ Πð12, 3, 12, 80Þ ¼ 0:25 ⋅ 121 ⋅ 122 ⋅ 80 3 ⋅ 80 ¼ 0.
Thus, at a price of 12, the firm earns 240 in profits on its variable costs, and these are
precisely offset by fixed costs in arriving at the final total. With a higher price for its output,
(continued)
373
374
Part 3 Production and Supply
EXAMPLE 11.4 CONTINUED
the firm earns positive profits. If the price falls below 12, however, the firm incurs short-run
losses.12
Hotelling’s lemma. We can use the profit function in Equation 11.36 together with the
envelope theorem to derive this firm’s short-run supply function:
∂Π
w β=ðβ1Þ α=ð1βÞ β=ð1βÞ
¼
k1
P
,
(11.38)
∂P
β
which is precisely the short-run supply function that we calculated in Example 11.3 (see
Equation 11.20).
qðP , v, w, k1 Þ ¼
Producer surplus. We can also use the supply function to calculate the firm’s short-run
producer surplus. To do so, we again return to our numerical example: α ¼ β ¼ 0.5, v ¼ 3,
w ¼ 12, and k1 ¼ 80. With these parameters, the short-run supply relationship is q ¼ 10P =3
and the shutdown price is zero. Hence, at a price of P ¼ 12, producer surplus is
12
12
10P
10P 2 dP ¼
(11.39)
producer surplus ¼
¼ 240.
6 3
∫
0
0
This precisely equals short-run profits at a price of 12 ðπ ¼ 0Þ plus short-run fixed costs
ð¼ vk1 ¼ 3 ⋅ 80 ¼ 240Þ. If price were to rise to (say) 15 then producer surplus would
increase to 375, which would still consist of 240 in fixed costs plus total profits at the higher
price ð ¼ 135Þ.
QUERY: How is the amount of short-run producer surplus here affected by changes in the
rental rate for capital, v? How is it affected by changes in the wage, w?
PROFIT MAXIMIZATION AND INPUT DEMAND
Thus far, we have treated the firm’s decision problem as one of choosing a profit-maximizing
level of output. But our discussion throughout has made clear that the firm’s output is, in
fact, determined by the inputs it chooses to employ, a relationship that is summarized by the
production function q ¼ f ðk, lÞ. Consequently, the firm’s economic profits can also be
expressed as a function of only the inputs it employs:
πðk, lÞ ¼ Pq C ðqÞ ¼ Pf ðk, lÞ ðvk þ wlÞ.
(11.40)
Viewed in this way, the profit-maximizing firm’s decision problem becomes one of choosing the appropriate levels of capital and labor input.13 The first-order conditions for a
maximum are
∂π
∂f
¼P
v ¼ 0,
∂k
∂k
(11.41)
∂π
∂f
¼P
w ¼ 0.
∂l
∂l
In Table 10.2 we showed that if q ¼ 40 then SAC ¼ 12. Hence zero profits are also indicated by P ¼ 12 ¼ SAC.
12
Throughout our discussion in this section, we assume that the firm is a price taker so the prices of its output and its inputs
can be treated as fixed parameters. Results can be generalized fairly easily in the case where prices depend on quantity.
13
Chapter 11
Profit Maximization
375
These conditions make the intuitively appealing point that a profit-maximizing firm should
hire any input up to the point at which the input’s marginal contribution to revenue is equal
to the marginal cost of hiring the input. Because the firm is assumed to be a price taker in its
hiring, the marginal cost of hiring any input is equal to its market price. The input’s marginal
contribution to revenue is given by the extra output it produces (the marginal product)
times that good’s market price. This demand concept is given a special name as follows.
Marginal revenue product. The marginal revenue product is the extra revenue a firm
DEFINITION
receives when it employs one more unit of an input. In the price-taking14 case, MRPl ¼
Pfl and MRPk ¼ Pfk .
Hence, profit maximization requires that the firm hire each input up to the point at which its
marginal revenue product is equal to its market price. Notice also that the profit-maximizing
Equations 11.41 also imply cost minimization because RTS ¼ fl =fk ¼ w=v.
Second-order conditions
Because the profit function in Equation 11.40 depends on two variables, k and l, the secondorder conditions for a profit maximum are somewhat more complex than in the singlevariable case we examined earlier. In Chapter 2 we showed that, to ensure a true maximum,
the profit function must be concave. That is,
πkk ¼ fkk < 0,
πll ¼ fll < 0,
(11.42)
and
πkk πll π2kl ¼ fkk fll fkl2 > 0.
Therefore, concavity of the profit relationship amounts to requiring that the production
function itself be concave. Notice that diminishing marginal productivity for each input is not
sufficient to ensure increasing marginal costs. Expanding output usually requires the firm to
use more capital and more labor. Thus we must also ensure that increases in capital input do
not raise the marginal productivity of labor (and thereby reduce marginal cost) by a large
enough amount to reverse the effect of diminishing marginal productivity of labor itself. The
second part of Equation 11.42 therefore requires that such cross-productivity effects be
relatively small—that they be dominated by diminishing marginal productivities of the inputs.
If these conditions are satisfied, then marginal costs will be increasing at the profit-maximizing
choices for k and l, and the first-order conditions will represent a local maximum.
Input demand functions
In principle, the first-order conditions for hiring inputs in a profit-maximizing way can be
manipulated to yield input demand functions that show how hiring depends on the prices
that the firm faces. We will denote these demand functions by
capital demand ¼ kðP , v, wÞ,
(11.43)
labor demand ¼ lðP , v, wÞ.
Notice that, contrary to the input demand concepts discussed in Chapter 10, these demand
functions are “unconditional”—that is, they implicitly permit the firm to adjust its output to
changing prices. Hence, these demand functions provide a more complete picture of how
prices affect input demand than did the contingent demand functions introduced in
Chapter 10. We have already shown that these input demand functions can also be derived
from the profit function through differentiation; in Example 11.5, we show that process
14
If the firm is not a price taker in the output market, then this definition is generalized by using marginal revenue in place
of price. That is, MRPl ¼ ∂R=∂l ¼ ∂R=∂q ⋅ ∂q=∂l ¼ MR ⋅ MPl . A similiar derivation holds for capital input.
376
Part 3 Production and Supply
explicitly. First, however, we will explore how changes in the price of an input might be
expected to affect the demand for it. To simplify matters we look only at labor demand, but
the analysis of the demand for any other input would be the same. In general, we conclude
that the direction of this effect is unambiguous in all cases—that is, ∂l=∂w 0 no matter
how many inputs there are. To develop some intuition for this result, we begin with some
simple cases.
Single-input case
One reason for expecting ∂l=∂w to be negative is based on the presumption that the marginal
physical product of labor declines as the quantity of labor employed increases. A decrease in w
means that more labor must be hired to bring about the equality w ¼ P ⋅ MPl : A fall in w
must be met by a fall in MPl (because P is fixed as required by the ceteris paribus assumption),
and this can be brought about by increasing l. That this argument is strictly correct for the
case of one input can be shown as follows. Write the total differential of the profit-maximizing
Equation 11.41 as
∂f
∂l
dw ¼ P ⋅ l ⋅
⋅ dw
∂l ∂w
or
∂l
1
0,
(11.44)
¼
∂w
P ⋅ fll
where the final inequality holds because the marginal productivity of labor is assumed to be
diminishing ðfll 0Þ. Hence we have shown that, at least in the single-input case, a ceteris
paribus increase in the wage will cause less labor to be hired.
Two-input case
For the case of two (or more) inputs, the story is more complex. The assumption of a
diminishing marginal physical product of labor can be misleading here. If w falls, there will
not only be a change in l but also a change in k as a new cost-minimizing combination of
inputs is chosen. When k changes, the entire fl function changes (labor now has a different
amount of capital to work with), and the simple argument used previously cannot be made.
First we will use a graphic approach to suggest why, even in the two-input case, ∂l=∂w must
be negative. A more precise, mathematical analysis is presented in the next section.
Substitution effect
In some ways, analyzing the two-input case is similar to the analysis of the individual’s
response to a change in the price of a good that was presented in Chapter 5. When w falls,
we can decompose the total effect on the quantity of l hired into two components. The first
of these components is called the substitution effect. If q is held constant at q1 , then there
will be a tendency to substitute l for k in the production process. This effect is illustrated in
Figure 11.5a. Because the condition for minimizing the cost of producing q1 requires that
RTS ¼ w=v, a fall in w will necessitate a movement from input combination A to combination B. And because the isoquants exhibit a diminishing RTS, it is clear from the diagram that
this substitution effect must be negative. A decrease in w will cause an increase in labor hired if
output is held constant.
Output effect
It is not correct, however, to hold output constant. It is when we consider a change in q (the
output effect) that the analogy to the individual’s utility-maximization problem breaks down.
Chapter 11
FIGURE 11.5
Profit Maximization
The Substitution and Output Effects of a Decrease in the Price of a Factor
When the price of labor falls, two analytically different effects come into play. One of these, the
substitution effect, would cause more labor to be purchased if output were held constant. This is
shown as a movement from point A to point B in (a). At point B, the cost-minimizing condition
ðRTS ¼ w=vÞ is satisfied for the new, lower w. This change in w=v will also shift the firm’s expansion
path and its marginal cost curve. A normal situation might be for the MC curve to shift downward in
response to a decrease in w as shown in (b). With this new curve ðMC 0 Þ a higher level of output ðq2 Þ
will be chosen. Consequently, the hiring of labor will increase (to l2 ), also from this output effect.
Price
k per period
MC
MC′
k1
A
C
k2
q2
B
P
q1
l1
l2
l per period
(a) The isoquant map
q1 q2
Output
per period
(b) The output decision
Consumers have budget constraints, but firms do not. Firms produce as much as the available
demand allows. To investigate what happens to the quantity of output produced, we must
investigate the firm’s profit-maximizing output decision. A change in w, because it changes
relative input costs, will shift the firm’s expansion path. Consequently, all the firm’s cost curves
will be shifted, and probably some output level other than q1 will be chosen. Figure 11.5b
shows what might be considered the “normal” case. There the fall in w causes MC to shift
downward to MC 0 . Consequently, the profit-maximizing level of output rises from q1 to q2 .
The profit-maximizing condition (P ¼ MC) is now satisfied at a higher level of output.
Returning to Figure 11.5a, this increase in output will cause even more l to be demanded as
long as l is not an inferior input (see below). The result of both the substitution and output
effects will be to move the input choice to point C on the firm’s isoquant map. Both effects
work to increase the quantity of labor hired in response to a decrease in the real wage.
The analysis provided in Figure 11.5 assumed that the market price (or marginal revenue,
if this does not equal price) of the good being produced remained constant. This would be an
appropriate assumption if only one firm in an industry experienced a fall in unit labor costs.
However, if the decline were industrywide then a slightly different analysis would be required. In that case all firms’ marginal cost curves would shift outward, and hence the
industry supply curve would shift also. Assuming that output demand is downward sloping,
this will lead to a decline in product price. Output for the industry and for the typical firm will
still increase and (as before) more labor will be hired, but the precise cause of the output effect
is different (see Problem 11.11).
Cross-price effects
We have shown that, at least in simple cases, ∂l=∂w is unambiguously negative; substitution
and output effects cause more labor to be hired when the wage rate falls. From Figure 11.5 it
should be clear that no definite statement can be made about how capital usage responds to
377
378
Part 3 Production and Supply
the wage change. That is, the sign of ∂k=∂w is indeterminate. In the simple two-input case,
a fall in the wage will cause a substitution away from capital; that is, less capital will be used
to produce a given output level. However, the output effect will cause more capital to be
demanded as part of the firm’s increased production plan. Thus substitution and output
effects in this case work in opposite directions, and no definite conclusion about the sign of
∂k=∂w is possible.
A summary of substitution and output effects
The results of this discussion can be summarized by the following principle.
OPTIMIZATION
PRINCIPLE
Substitution and output effects in input demand. When the price of an input falls, two
effects cause the quantity demanded of that input to rise:
1. the substitution effect causes any given output level to be produced using more of the
input; and
2. the fall in costs causes more of the good to be sold, thereby creating an additional
output effect that increases demand for the input.
For a rise in input price, both substitution and output effects cause the quantity of an input
demanded to decline.
We now provide a more precise development of these concepts using a mathematical
approach to the analysis.
A mathematical development
Our mathematical development of the substitution and output effects that arise from the
change in an input price follows the method we used to study the effect of price changes in
consumer theory. The final result is a Slutsky-style equation that resembles the one we
derived in Chapter 5. However, the ambiguity stemming from Giffen’s paradox in the theory
of consumption demand does not occur here.
We start with a reminder that we have two concepts of demand for any input (say, labor):
(1) the conditional demand for labor, denoted by l c ðv, w, qÞ; and (2) the unconditional
demand for labor, which is denoted by lðP , v, wÞ. At the profit-maximizing choice for labor
input, these two concepts agree about the amount of labor hired. The two concepts also
agree on the level of output produced (which is a function of all the prices):
lðP , v, wÞ ¼ l c ðv, w, qÞ ¼ l c ðv, w, qðP , v, wÞÞ.
(11.45)
Differentiation of this expression with respect to the wage (and holding the other prices
constant) yields
∂lðP , v, wÞ
∂l c ðv, w, qÞ
∂l c ðv, w, qÞ ∂qðP , v, wÞ
¼
þ
.
(11.46)
⋅
∂w
∂w
∂q
∂w
So, the effect of a change in the wage on the demand for labor is the sum of two components: a
substitution effect in which output is held constant; and an output effect in which the wage
change has its effect through changing the quantity of output that the firm opts to produce.
The first of these effects is clearly negative—because the production function is quasi-concave
(i.e., it has convex isoquants), the output-contingent demand for labor must be negatively
sloped. Figure 11.5b provides an intuitive illustration of why the output effect in Equation 11.46 is negative, but it can hardly be called a proof. The particular complicating factor is
the possibility that the input under consideration (here, labor) may be inferior. Perhaps oddly,
inferior inputs also have negative output effects, but for rather arcane reasons that are best
Chapter 11
Profit Maximization
relegated to a footnote.15 The bottom line, however, is that Giffen’s paradox cannot occur
in the theory of the firm’s demand for inputs: input demand functions are unambiguously
downward sloping. In this case the theory of profit maximization imposes more restrictions on
what might happen than does the theory of utility maximization. In Example 11.5 we show
how decomposing input demand into its substitution and output components can yield useful
insights into how changes in input prices actually affect firms.
EXAMPLE 11.5 Decomposing Input Demand into Substitution and Output Components
To study input demand we need to start with a production function that has two features:
(1) the function must permit capital-labor substitution (because substitution is an important
part of the story); and (2) the production function must exhibit increasing marginal costs (so
that the second-order conditions for profit maximization are satisfied). One function that
satisfies these conditions is a three-input Cobb-Douglas function when one of the inputs is
held fixed. So, let q ¼ f ðk, l, gÞ ¼ k0.25 l 0.25 g 0.5 , where k and l are the familiar capital and
labor inputs and g is a third input (size of the factory) that is held fixed at g ¼ 16 (square
meters?) for all of our analysis. The short-run production function is therefore q ¼ 4k0.25 l 0.25 .
We assume that the factory can be rented at a cost of r per square meter per period. To study
the demand for (say) labor input, we need both the total cost function and the profit function
implied by this production function. Mercifully, your author has computed these functions
for you as
C ðv, w, r, q Þ ¼
q 2 v 0.5 w 0.5
þ 16r
8
(11.47)
and
ΠðP , v, w, rÞ ¼ 2P 2 v0.5 w 0.5 16r.
(11.48)
As expected, the costs of the fixed input ð gÞ enter as a constant in these equations, and these
costs will play very little role in our analysis.
Envelope Results
Labor-demand relationships can be derived from both of these functions through
differentiation:
l c ðv, w, r, qÞ ¼
∂C
q 2 v 0.5 w 0.5
¼
16
∂w
(11.49)
and
∂Π
¼ P 2 v0.5 w 1.5 .
(11.50)
∂w
These functions already suggest that a change in the wage has a larger effect on total labor
demand than it does on contingent labor demand because the exponent of w is more
negative in the total demand equation. That is, the output effect must also be playing a role
here. To see that directly, we turn to some numbers.
lðP , v, w, rÞ ¼ (continued)
15
In words, an increase in the price of an inferior reduces marginal cost and thereby increases output. But when output
increases, less of the inferior input is hired. Hence the end result is a decrease in quantity demanded in response to an
increase in price. A formal proof makes extensive use of envelope relationships:
output effect ¼
∂l c ∂q
∂l c ∂l
∂l c
¼
¼
⋅
⋅
∂q ∂w
∂q ∂P
∂q
2
⋅
∂q
.
∂P
Because the second-order conditions for profit maximization require that ∂q=∂P > 0, the output effect is clearly negative.
379
380
Part 3 Production and Supply
EXAMPLE 11.5 CONTINUED
Numerical example. Let’s start again with the assumed values that we have been using in
several previous examples: v ¼ 3, w ¼ 12, and P ¼ 60. Let’s first calculate what output the
firm will choose in this situation. To do so, we need its supply function:
∂Π
qðP , v, w, rÞ ¼
(11.51)
¼ 4Pv 0.5 w 0.5 .
∂P
With this function and the prices we have chosen, the firm’s profit-maximizing output level
is (surprise) q ¼ 40. With these prices and an output level of 40, both of the demand
functions predict that the firm will hire l ¼ 50. Because the RTS here is given by k=l, we also
know that k=l ¼ w=v, so at these prices k ¼ 200.
Suppose now that the wage rate rises to w ¼ 27 but that the other prices remain unchanged. The firm’s supply function (Equation 11.51) shows that it will now produce
q ¼ 26.67. The rise in the wage shifts the firm’s marginal cost curve upward and, with a
constant output price, this causes the firm to produce less. To produce this output, either of
the labor-demand functions can be used to show that the firm will hire l ¼ 14.8. Hiring of
capital will also fall to k ¼ 133.3 because of the large reduction in output.
We can decompose the fall in labor hiring from l ¼ 50 to l ¼ 14.8 into substitution and
output effects by using the contingent demand function. If the firm had continued to
produce q ¼ 40 even though the wage rose, Equation 11.49 shows that it would have
used l ¼ 33.33. Capital input would have increased to k ¼ 300. Because we are holding
output constant at its initial level of q ¼ 40, these changes represent the firm’s substitution
effects in response to the higher wage.
The decline in output needed to restore profit maximization causes the firm to cut back
on its output. In doing so it substantially reduces its use of both inputs. Notice in particular
that, in this example, the rise in the wage not only caused labor usage to decline sharply but
also caused capital usage to fall because of the large output effect.
QUERY: How would the calculations in this problem be affected if all firms had experienced
the rise in wages? Would the decline in labor (and capital) demand be greater or smaller than
found here?
SUMMARY
In this chapter we studied the supply decision of a profitmaximizing firm. Our general goal was to show how such a
firm responds to price signals from the marketplace. In addressing that question, we developed a number of analytical
results.
•
•
In order to maximize profits, the firm should choose to
produce that output level for which marginal revenue
(the revenue from selling one more unit) is equal to
marginal cost (the cost of producing one more unit).
If a firm is a price taker then its output decisions do not
affect the price of its output, so marginal revenue is given
by this price. If the firm faces a downward-sloping demand for its output, however, then it can sell more only
at a lower price. In this case marginal revenue will be less
than price and may even be negative.
•
Marginal revenue and the price elasticity of demand are
related by the formula
!
1
MR ¼ P 1 þ
,
eq, p
where P is the market price of the firm’s output and eq, p
is the price elasticity of demand for its product.
•
The supply curve for a price-taking, profit-maximizing
firm is given by the positively sloped portion of its marginal cost curve above the point of minimum average
variable cost (AVC). If price falls below minimum AVC,
the firm’s profit-maximizing choice is to shut down and
produce nothing.
•
The firm’s reactions to changes in the various prices it
faces can be studied through use of its profit function,
Chapter 11
ðP , v, wÞ. That function shows the maximum profits
that the firm can achieve given the price for its output,
the prices of its input, and its production technology.
The profit function yields particularly useful envelope
results. Differentiation with respect to market price
yields the supply function while differentiation with respect to any input price yields (the negative of ) the
demand function for that input.
•
Profit Maximization
The profit function can also be used to calculate changes
in producer surplus.
•
Profit maximization provides a theory of the firm’s derived demand for inputs. The firm will hire any input up
to the point at which its marginal revenue product is just
equal to its per-unit market price. Increases in the price
of an input will induce substitution and output effects
that cause the firm to reduce hiring of that input.
Short-run changes in market price result in changes to
the firm’s short-run profitability. These can be measured
graphically by changes in the size of producer surplus.
PROBLEMS
11.1
John’s Lawn Moving Service is a small business that acts as a price taker (i.e., MR ¼ P ). The prevailing
market price of lawn mowing is $20 per acre. John’s costs are given by
total cost ¼ 0.1q 2 þ 10q þ 50,
where q ¼ the number of acres John chooses to cut a day.
a. How many acres should John choose to cut in order to maximize profit?
b. Calculate John’s maximum daily profit.
c. Graph these results and label John’s supply curve.
11.2
Would a lump-sum profits tax affect the profit-maximizing quantity of output? How about a proportional tax on profits? How about a tax assessed on each unit of output? How about a tax on labor input?
11.3
This problem concerns the relationship between demand and marginal revenue curves for a few
functional forms.
a. Show that, for a linear demand curve, the marginal revenue curve bisects the distance between
the vertical axis and the demand curve for any price.
b. Show that, for any linear demand curve, the vertical distance between the demand and marginal
revenue curves is 1=b ⋅ q, where b ð< 0Þ is the slope of the demand curve.
c. Show that, for a constant elasticity demand curve of the form q ¼ aP b , the vertical distance
between the demand and marginal revenue curves is a constant ratio of the height of the
demand curve, with this constant depending on the price elasticity of demand.
d. Show that, for any downward-sloping demand curve, the vertical distance between the demand
and marginal revenue curves at any point can be found by using a linear approximation to the
demand curve at that point and applying the procedure described in part (b).
e. Graph the results of parts (a)–(d) of this problem.
381
382
Part 3 Production and Supply
11.4
Universal Widget produces high-quality widgets at its plant in Gulch, Nevada, for sale throughout the
world. The cost function for total widget production ðqÞ is given by
total cost ¼ 0.25q 2 .
Widgets are demanded only in Australia (where the demand curve is given by q ¼ 100 2P ) and
Lapland (where the demand curve is given by q ¼ 100 4P ). If Universal Widget can control the
quantities supplied to each market, how many should it sell in each location in order to maximize total
profits? What price will be charged in each location?
11.5
The production function for a firm in the business of calculator assembly is given by
pffiffi
q ¼ 2 l,
where q denotes finished calculator output and l denotes hours of labor input. The firm is a price taker
both for calculators (which sell for P ) and for workers (which can be hired at a wage rate of w
per hour).
a. What is the total cost function for this firm?
b. What is the profit function for this firm?
c. What is the supply function for assembled calculators ½qðP , wÞ?
d. What is this firm’s demand for labor function ½lðP , wÞ?
e. Describe intuitively why these functions have the form they do.
11.6
The market for high-quality caviar is dependent on the weather. If the weather is good, there are many
fancy parties and caviar sells for $30 per pound. In bad weather it sells for only $20 per pound. Caviar
produced one week will not keep until the next week. A small caviar producer has a cost function
given by
C ¼ 0:5q 2 þ 5q þ 100,
where q is weekly caviar production. Production decisions must be made before the weather (and the
price of caviar) is known, but it is known that good weather and bad weather each occur with a
probability of 0.5.
a. How much caviar should this firm produce if it wishes to maximize the expected value of its
profits?
b. Suppose the owner of this firm has a utility function of the form
utility ¼
pffiffiffiffi
π,
where π is weekly profits. What is the expected utility associated with the output strategy
defined in part (a)?
c. Can this firm owner obtain a higher utility of profits by producing some output other than that
specified in parts (a) and (b)? Explain.
d. Suppose this firm could predict next week’s price but could not influence that price. What
strategy would maximize expected profits in this case? What would expected profits be?
Chapter 11
Profit Maximization
11.7
The Acme Heavy Equipment School teaches students how to drive construction machinery. The
number of students that the school can educate per week is given by q ¼ 10 minðk, lÞr , where k is the
number of backhoes the firm rents per week, l is the number of instructors hired each week, and γ is a
parameter indicating the returns to scale in this production function.
a. Explain why development of a profit-maximizing model here requires 0 < γ < 1.
b. Suppposing γ ¼ 0.5, calculate the firm’s total cost function and profit function.
c. If v ¼ 1000, w ¼ 500, and P ¼ 600, how many students will Acme serve and what are its
profits?
d. If the price students are willing to pay rises to P ¼ 900, how much will profits change?
e. Graph Acme’s supply curve for student slots, and show that the increase in profits calculated in
part (d) can be plotted on that graph.
11.8
How would you expect an increase in output price, P , to affect the demand for capital and labor inputs?
a. Explain graphically why, if neither input is inferior, it seems clear that a rise in P must not reduce
the demand for either factor.
b. Show that the graphical presumption from part (a) is demonstrated by the input demand
functions that can be derived in the Cobb-Douglas case.
c. Use the profit function to show how the presence of inferior inputs would lead to ambiguity in
the effect of P on input demand.
Analytical Problems
11.9 A CES profit function
With a CES production function of the form q ¼ ðkρ þ l ρ Þγ=ρ a whole lot of algebra is needed to compute
the profit function as ð P , v, wÞ ¼ KP 1=ð1γÞ ð v1σ þ w1σ Þγ=ð1σÞðγ1Þ , where σ ¼ 1=ð1 ρÞ and K is a
constant.
a. If you are a glutton for punishment (or if your instructor is), prove that the profit function
takes this form. Perhaps the easiest way to do so is to start from the CES cost function in
Example 10.2.
b. Explain why this profit function provides a reasonable representation of a firm’s behavior only
for 0 < γ < 1.
c. Explain the role of the elasticity of substitution ðσÞ in this profit function.
d. What is the supply function in this case? How does σ determine the extent to which that
function shifts when input prices change?
e. Derive the input demand functions in this case. How are these functions affected by the size of σ?
11.10 Some envelope results
Young’s theorem can be used in combination with the envelope results in this chapter to derive some
useful results.
a. Show that ∂lðP , v, wÞ=∂v ¼ ∂kðP , v, wÞ=∂w. Interpret this result using subtitution and output
effects.
b. Use the result from part (a) to show how a unit tax on labor would be expected to affect capital
input.
c. Show that ∂q=∂w ¼ ∂l=∂P . Interpret this result.
d. Use the result from part (c) to discuss how a unit tax on labor input would affect quantity
supplied.
383
384
Part 3 Production and Supply
11.11 More on the derived demand with two inputs
The demand for any input depends ultimately on the demand for the goods that input produces. This
can be shown most explicitly by deriving an entire industry’s demand for inputs. To do so, we assume
that an industry produces a homogeneous good, Q , under constant returns to scale using only capital
and labor. The demand function for Q is given by Q ¼ DðP Þ, where P is the market price of the good
being produced. Because of the constant returns-to-scale assumption, P ¼ MC ¼ AC. Throughout
this problem let Cðv, w, 1Þ be the firm’s unit cost function.
a. Explain why the total industry demands for capital and labor are given by K ¼ QCv and
L ¼ QCw .
b. Show that
∂K
¼ QCvv þ D 0 C 2v
∂v
and
∂L
¼ QCww þ D 0 C 2w .
∂w
w
C
v vw
and
Cww ¼
c. Prove that
Cvv ¼
v
C .
w vw
d. Use the results from parts (b) and (c) together with the elasticity of substitution defined as
σ ¼ CCvw =Cv Cw to show that
∂K
wL σK
D 0K 2
¼
þ
⋅
Q2
∂v
Q vC
and
∂L
vK σL
D 0L2
.
¼
þ
⋅
Q2
∂w
Q wC
e. Convert the derivatives in part (d) into elasticities to show that
eK , v ¼ sL σ þ sK eQ , P and eL, w ¼ sK σ þ sL eQ , P ,
where eQ , P is the price elasticity of demand for the product being produced.
f. Discuss the importance of the results in part (e) using the notions of substitution and output
effects from Chapter 11.
Note: The notion that the elasticity of the derived demand for an input depends on the price elasticity of
demand for the output being produced was first suggested by Alfred Marshall. The proof given here
follows that in D. Hamermesh, Labor Demand (Princeton, NJ: Princeton University Press, 1993).
11.12 Cross-price effects in input demand
With two inputs, cross-price effects on input demand can be easily calculated using the procedure
outlined in Problem 11.11.
a. Use steps (b), (d), and (e) from Problem 11.11 to show that
eK , w ¼ sL ðσ þ eQ , P Þ
and
eL, v ¼ sK ðσ þ eQ , P Þ.
b. Describe intuitively why input shares appear somewhat differently in the demand elasticities in
part (e) of Problem 11.11 than they do in part (a) of this problem.
c. The expression computed in part (a) can be easily generalized to the many-input case as
exi , wj ¼ sj ðAi, j þ eQ , P Þ, where Ai, j is the Allen elasticity of substitution defined in Problem
10.12. For reasons described in Problems 10.11 and 10.12, this approach to input demand
in the multi-input case is generally inferior to using Morishima elasticities. One oddity
might be mentioned, however. For the case i ¼ j this expression seems to say that eL, w ¼
sL ðAL, L þ eQ , P Þ, and if we jumped to the conclusion that AL, L ¼ σ in the two-input case
then this would contradict the result from Problem 11.11. You can resolve this paradox
by using the definitions from Problem 10.12 to show that, with two inputs, AL, L ¼
ðsK =sL Þ ⋅ AK , L ¼ ðsK =sL Þ ⋅ σ and so there is no disagreement.
Chapter 11
Profit Maximization
385
SUGGESTIONS FOR FURTHER READING
Ferguson, C. E. The Neoclassical Theory of Production and
Distribution. Cambridge, UK: Cambridge University Press,
1969.
Provides a complete analysis of the output effect in factor demand.
Also shows how the degree of substitutability affects many of the
results in this chapter.
Hicks, J. R. Value and Capital, 2nd ed. Oxford: Oxford
University Press, 1947.
Samuelson, P. A. Foundations of Economic Analysis. Cambridge, MA: Harvard University Press, 1947.
Early development of the profit function idea together with a nice
discussion of the consequences of constant returns to scale for market
equilibrium.
Sydsaeter, K., A. Strom, and P. Berck. Economists’ Mathematical Manual, 3rd ed. Berlin: Springer-Verlag, 2000.
The Appendix looks in detail at the notion of factor complementarity.
Chapter 25 offers formulas for a number of profit and factor demand
functions.
Mas-Colell, A., M. D. Whinston, and J. R. Green. Microeconomic Theory. New York: Oxford University Press, 1995.
Varian, H. R. Microeconomic Analysis, 3rd ed. New York:
W. W. Norton, 1992.
Provides an elegant introduction to the theory of production using
vector and matrix notation. This allows for an arbitrary number of
inputs and outputs.
Includes an entire chapter on the profit function. Varian offers a novel
approach for comparing short- and long-run responses using the Le
Chatelier principle.
386
Part 3 Production and Supply
EXTENSIONS
Applications of the Profit Function
In Chapter 11 we introduced the profit function. That
function summarizes the firm’s “bottom line” as it
depends on the prices it faces for its outputs and
inputs. In these extensions we show how some of the
properties of the profit function have been used to
assess important empirical and theoretical questions.
E11.1 Convexity and price stabilization
Convexity of the profit function implies that a firm will
generally prefer a fluctuating output price to one that is
stabilized (say, through government intervention) at
its mean value. The result runs contrary to the direction
of economic policy in many less developed countries,
which tends to stress the desirability of stabilization of
commodity prices. Several factors may account for this
seeming paradox. First, many plans to “stabilize” commodity prices are in reality plans to raise the average
level of these prices. Cartels of producers often have
this as their primary goal, for example. Second, the
convexity result applies for a single price-taking firm.
From the perspective of the entire market, total revenues from stabilized or fluctuating prices will depend on
the nature of the demand for the product.1 A third
complication that must be addressed in assessing price
stabilization schemes is firms’ expectations of future
prices. When commodities can be stored, optimal production decisions in the presence of price stabilization
schemes can be quite complex. Finally, the purpose of
price stabilization schemes may in some situations be
focused more on reducing risks for the consumers of
basic commodities (such as food) than on the welfare
of producers. Still, this fundamental property of the
profit function suggests caution in devising price stabilization schemes that have desirable long-run effects on
producers. For an extended theoretical analysis of these
issues, see Newbury and Stiglitz (1981).
E11.2 Producer surplus and the
short-run costs of disease
Disease episodes can severely disrupt markets, leading
to short-run losses in producer and consumer surplus.
For firms, these losses can be computed as the shortrun losses of profits from temporarily lower prices for
their output or from the temporarily higher input
prices they must pay. A particular extensive set of
such calculations is provided by Harrington, Krupnick,
and Spofford (1991) in their detailed study of a giardiasis outbreak in Pennsylvania in 1983. Although consumers suffered most of the losses associated with this
outbreak, the authors also calculate substantial losses
for restaurants and bars in the immediate area. Such
losses arose both from reduced business for these firms
and from the temporary need to use bottled water and
other high-cost inputs in their operations. Quantitative
calculations of these losses are based on profit functions
described by the authors.
E11.3 Profit functions and productivity
measurement
In Chapter 9 we showed that total factor productivity
growth is usually measured as
GA ¼ Gq sk Gk sl Gl ,
where
dx=dt
d ln x
¼
x
dt
and where sk and sl are the shares of capital and labor in
total costs, respectively. One difficulty with making this
calculation is that it requires measuring changes in
input usage over time—a measurement that can be
especially difficult for capital. The profit function provides an alternative way of measuring the same phenomenon without estimating input usage directly. To
understand the logic of this approach, consider the
production function we wish to examine, q ¼ f ðk, l, t Þ.
We want to know how output would change over time
if input levels were held constant. That is, we wish to
measure ∂ðln qÞ=∂t ¼ ft =f . Notice the use of partial
differentiation in this expression—in words, we want to
know the proportionate change in f over time when
other inputs are held constant. If the production function exhibits constant returns to scale and if the firm is a
price taker for both inputs and its output, it is fairly easy2
to show that this partial derivative is the measure of
Gx ¼
1
Specifically, for a constant elasticity demand function, total revenue will
be a concave function of price if demand is inelastic but convex if demand
is elastic. Hence, in the elastic case, producers will obtain higher total
revenues from a fluctuating price than from a price stabilized at its mean
value.
2
The proof proceeds by differentiating the production function logarithmically with respect to time as Gq ¼ dðln qÞ=dt ¼ eq, k Gk þ eq, l Gl þ ft =f
and then recognizing that, with constant returns to scale and price-taking
behavior, eq, k ¼ sk and eq, l ¼ sl .
Chapter 11
changing total factor productivity we want—that
is, GA ¼ ft =f . Now consider the profit function,
ðP , v, w, t Þ. By definition, profits are given by
π ¼ Pq vk wl ¼ Pf vk wl,
so
∂ ln Π Pft
¼
Π
∂t
and thus
ft
Π ∂ ln Π
Π ∂ ln Π
¼
¼
.
(i)
⋅
⋅
f
Pf
∂t
Pq
∂t
So, in this special case, changes in total factor productivity can be inferred from the share of profits in
total revenue and the time derivative of the log of the
profit function. But this conclusion can be readily
generalized to cases of nonconstant returns to scale
and even to firms that produce multiple outputs (see
e.g. Kumbhakar, 2002). Hence, for situations where
input and output prices are more readily available
than input quantities, using the profit function is an
attractive way to proceed.
Three examples of this use for the profit function
might be mentioned. Karagiannis and Mergos (2000)
reassess the major increases in total factor productivity
that have been experienced by U.S. agriculture during
the past 50 years using the profit function approach.
They find results that are broadly consistent with those
using more conventional measures. Huang (2000)
adopts the same approach in a study of Taiwanese
GA ¼
Profit Maximization
387
banking and finds significant increases in productivity
that could not be detected using other methods. Finally, Coelli and Perelman (2000) use a modified
profit function approach to measure the relative efficiency of European railroads. Perhaps not surprisingly,
they find that Dutch railroads are the most efficient in
Europe whereas those in Italy are the least efficient.
References
Coelli, T., and S. Perelman. “Technical Efficiency of
European Railways: A Distance Function Approach.”
Applied Economics (December 2000): 1967–76.
Harrington, W. A., J. Krupnick, and W. O. Spofford.
Economics and Episodic Disease: The Benefits of Preventing a Giardiasis Outbreak. Baltimore: Johns Hopkins
University Press, 1991.
Huang, T. “Estimating X-Efficiency in Taiwanese Banking
Using a Translog Shadow Profit Function.” Journal of
Productivity Analysis (November 2000): 225–45.
Karagiannis, G., and G. J. Mergos. “Total Factor Productivity Growth and Technical Change in a Profit Function
Framework.” Journal of Productivity Analysis (July
2000): 31–51.
Kumbhakar, S. “Productivity Measurement: A Profit
Function Approach.” Applied Economics Letters (April
2002): 331–34.
Newbury, D. M. G., and J. E. Stiglitz. The Theory of
Commodity Price Stabilization. Oxford: Oxford University Press, 1981.
Brief Answers to Queries
705
outcome is an equilibrium because the firm’s best
response to NE would be NJ , inducing both types
of worker to pool on E.
sharply convex. It seems possible that an L-shaped isoquant might be approximated for particular coefficients of the linear and radical terms.
8.12
9.4
In equilibrium, type H obtains an expected payoff of
j w cH ¼ cL cH . This exceeds the payoff of 0 from
deviating to NE. Type L pools with type H on E with
probability e . But de =d PrðH Þ ¼ ðπ wÞ=π. Since
this expression is positive, type L must increase its
probability of playing E to offset an increase in PrðH Þ
and still keep player 2 indifferent between J and NJ .
Because the composite technical change factor is θ ¼
αφ þ ð1 αÞε, a value of α ¼ 0:3 implies that technical improvements in labor will be weighted more
highly in determining the overall result.
CHAPTER 10
10.1
8.13
Players earn more in more informative equilibria. Suppose 0 < d < 1=3. In a babbling equilibrium, player 1
earns expected payoff ð1 dÞ=2 and player 2 earns
ð1 þ dÞ=2. In the most informative equilibrium, player 1
earns 1 and player 2 earns 1 d, lower payoffs than in
the babbling equilibrium if d < 1=3. In theory, there is
no difference between announcing “A or C” and announcing the agreed-upon synonym “purple.” In practice, it might be difficult for players to coordinate on the
meaning of a nonsense word. Languages are more efficient the more precise they are and the more widespread agreement there is about meanings, but there
may be trade-offs between these two features.
If σ ¼ 2, ρ ¼ 0:5, k=l ¼ 16, l ¼ 8=5, k ¼ 128=5,
C ¼ 96.
If σ ¼ 0:5, ρ ¼ 1, k=l ¼ 2, l ¼ 60, k ¼ 120, C ¼
1080.
Notice that changes in σ also change the scale of the
production function, so the total cost figures cannot
be compared directly.
10.2
The expression for unit costs is ðv 1σ þ w1σ Þ1=ð1σÞ .
If σ ¼ 0 then this function is linear in w þ v. For σ > 0
the function is increasingly convex, showing that large
increases in w can be offset by small decreases in v.
CHAPTER 9
10.3
9.1
The elasticities are given by the exponents in the cost
functions and are unaffected by technical change as
modeled here.
Now, with k ¼ 11:
q ¼ 72,600l 2 1,331l 3 ,
MPl ¼ 145,200l 3,993l 2 ,
APl ¼ 72,600l 1,331l 2 :
In this case, APl reaches its maximal value at l ¼ 27:3
rather than at l ¼ 30.
9.2
Since k and l enter f symmetrically, if k ¼ l then fk ¼ fl
and fkk ¼ fll . Hence, the numerator of Equation 9.21
will be negative if fkl > fll . Combining Equations 9.24
and 9.25 (and remembering k ¼ l) shows this holds
for k ¼ l < 20.
9.3
The q ¼ 4 isoquant contains the points k ¼ 4, l ¼ 0;
k ¼ 1, l ¼ 1; and k ¼ 0, l ¼ 4. It is therefore fairly
10.4
In this case σ ¼ ∞. With w ¼ 4v, cost minimization
could use the inputs in any combination (for q constant) without changing costs. A rise in w would cause
the firm to switch to using only capital and would not
affect total costs. This shows that the impact on costs
of an increase in the price of a single input depends
importantly on the degree of substitution.
10.5
Because capital costs are fixed in the short run, they do
not affect short-run marginal costs (in mathematical
terms, the derivative of a constant is zero). Capital
costs do, however, affect short-run average costs. In
Figure 10.9 an increase in v would shift MC, AC, and
all of the SATC curves upward, but would leave the
SMC curves unaffected.
706
Brief Answers to Queries
CHAPTER 11
percent) predicts a price rise of 4.5 percent, very close
to the number in the example.
11.1
If MC ¼ 5, profit maximization requires q ¼ 25. Now
P ¼ 7:50, R ¼ 187:50, C ¼ 125, and π ¼ 62:50.
11.2
Factors other than p can be incorporated into the
constant term a. These would shift D and MR but
would not affect the elasticity calculations.
11.3
When w rises to 15, supply shifts inward to q ¼ 8P =5.
When k increases to 100, supply shifts outward to
q ¼ 25P =6. A change in v would not affect short-run
marginal cost or the shutdown decision.
11.4
A change in v has no effect on SMC but it does affect
fixed costs. A change in w would affect SMC and
short-run supply.
11.5
12.4
The short-run supply curve is given by Q s ¼
0:5P þ 750, and the short-term equilibrium price is
$643. Each firm earns approximately $2,960 in profits
in the short run.
12.5
Total and average costs for Equation 12.55 exceed
those for Equation 12.42 for q > 15:9. Marginal costs
for Equation 12.55 always exceed those for Equation 12.42. Optimal output is lower with Equation
12.55 than with Equation 12.42 because marginal
costs increase more than average costs.
12.6
Losses from a given restriction in quantity will be
greater when supply and/or demand is less elastic.
The actor with the least elastic response will bear the
greater share of the loss.
A rise in wages for all firms would shift the market
supply curve upward, raising the product price. Because total output must fall given a negatively sloped
demand curve, each firm must produce less. Again,
both substitution and output effects would then be
negative.
An increase in t unambiguously increases deadweight
loss. Because increases in t reduce quantity, however,
total tax revenues are subject to countervailing effects.
Indeed, if t =ðP þ t Þ 1=eQ , P then dtQ =dt < 0.
CHAPTER 12
12.8
12.1
The ability to sum incomes in this linear case would
require that each person have the same coefficient for
income. Because each person faces the same price,
aggregation requires only adding the price coefficients.
12.2
A value for β other than 0.5 would mean that the
exponent of price would not be 1.0. The higher is β
the more price elastic is short-run supply.
12.3
Following steps similar to those used to derive Equation 10.36 yields
eQ , β
eP , β ¼
eS, P eQ , P
Here eQ , β ¼ eQ , w ¼ 0:5, so eP , β ¼ ð0:5Þ=2:2 ¼
0:227. Multiplication by 0.20 (since wages rose 20
12.7
Total transfer to domestic producers is (in billions)
0:5 ð11:7Þ þ 0:5ð0:5Þð0:7Þ ¼ 6:03. This would be
gained as rents to those inputs that give the auto
supply curve its positive slope. With a quota, domestic
producers may also be able to gain some portion of
what would have been tariff revenue.
CHAPTER 13
13.1
An increase in labor input will shift the first frontier out
uniformly. In the second case, such an increase will
shift the y-intercept out farther than the x-intercept
because good y uses labor intensively.
13.2
In all three scenarios the total value of output is 200w,
composed half of wages and half of profits. With the
shift in supply, consumers still devote 100w to each
714
c.
d.
e.
f.
Solutions to Odd-Numbered Problems
rðW Þ ! 1=μ.
Let A ¼ 1=μ.
U ðW Þ ¼ θðW 2 2μW þ μ2 Þ.
For example, function may be unbounded and so
St. Petersburg paradox can be regenerated.
7.11
a. Risk aversion is an unwillingness to substitute between
states.
b. R ¼ 1 implies perfect substitution, R ¼ ∞ implies
zero substitution.
c. Depends on whether goods are gross substitutes or
gross complements.
d. i. R 3.
ii. A 2 percent premium roughly compensates for
a 10 percent gamble with R ¼ 3.
brunette, then one would prefer to deviate to blond
for payoff a rather than b.
b. Playing brunette provides a certain payoff of b
and blond provides a payoff of a with probability
ð1 pÞn1 (the probability no other player approaches
the blond). Equating the two payoffs yields p ¼
1 ðb=aÞ1=ðn1Þ .
c. The probability the blond is approached by at least
one male equals 1 minus the probability no males
approach her: 1 ð1 p Þn ¼ 1 ðb=aÞn=ðn1Þ .
This expression is decreasing in n because n=ðn 1Þ
is decreasing in n and b=a is a fraction.
8.7
a. If utility of wealth is homothetic, then uniform tax will
not affect allocation.
b. Increases incentives to hold risky assets, especially for
those less risk averse.
c. Tax on asset returns will increase allocation to risky
assets—see graph in detailed solutions.
a. The best-response function is lLC ¼ 3:5 þ l2 =4 for the
low-cost type of player 1, lHC_¼ 2:5 þ l2 =4 for the _
high-cost type, and l2 ¼ 3 þ l 1 =4 for player 2, where l 1
is the average for player 1. Solving these equations
¼ 4:5, l ¼ 3:5, and l ¼ 4.
yields lLC
HC
2
c. The low-cost type of player 1 earns 20.25 in the
Bayesian-Nash equilibrium and 20.55 in the
full-information game, so it would prefer to signal
its type if it could. Similar calculations show that
the high-cost player would like to hide its type.
CHAPTER 8
8.9
7.13
8.1
a. ðC, F Þ
b. Each player randomizes over the two actions with
equal probability.
c. Players each earn 4 in the pure-strategy equilibrium.
Players 1 and 2 earn 6 and 7, respectively, in the
mixed-strategy equilibrium.
d. The extensive form is similar to Figures 18.1 and 18.2
but has three branches from each node rather than two.
8.3
a. The extensive form is similar to Figures 8.1 and 8.2.
b. (Don’t veer, veer) and (veer, don’t veer)
c. Players randomize with equal probabilities over the
two actions.
d. Teen 2 has four contingent strategies: always veer,
never veer, do the same as Teen 1, and do the
opposite of Teen 1.
e. The first is (don’t veer, always veer), the second is
(don’t veer, do the opposite), and the third is (veer,
never veer).
f. (Don’t veer, do the opposite) is a subgame-perfect
equilibrium.
8.5
a. If all play blond, then one would prefer to deviate
to brunette to obtain a positive payoff. If all play
For any strategy profile besides the dominant-strategy
equilibrium, each player would have an incentive to
deviate to its dominant strategy, ruling out the profile
as a Nash equilibrium.
8.11
a. The condition for cooperation to be sustainable with
one period of punishment is δ 1, so one period of
punishment is not enough. Two periods of punishment are enough as long as δ2 þ δ 1 0, or
δ 0:62.
b. The required condition is that the present discounted
value of the payoffs from cooperating, 2=ð1 δÞ,
exceed that from deviating, 3þ
δð1 δ10 Þ=ð1 δÞ þ 2δ11 =ð1 δÞ. Simplifying,
2δ δ11 1 0. Using numerical or graphical
methods, this condition can be shown to be δ 0:50025,
not much stricter than the condition for cooperation
with infinitely many periods of punishment ðδ 1=2Þ.
CHAPTER 9
9.1
a. k ¼ 10 and l ¼ 5.
b. k ¼ 8 and l ¼ 8.
c. k ¼ 9, l ¼ 6:5, k ¼ 9:5, and l ¼ 5:75 (fractions of
hours).
d. The isoquant is linear between solutions (a) and (b).
Solutions to Odd-Numbered Problems
9.3
a.
b.
c.
d.
q ¼ 10, k ¼ 100, l ¼ 100, C ¼ 10,000.
q ¼ 10, k ¼ 33, l ¼ 132, C ¼ 8,250.
q ¼ 12:13, k ¼ 40, l ¼ 160, C ¼ 10,000.
Carla’s ability to influence the decision depends on
whether she can impose any costs on the bar if she is
unhappy serving the additional tables. Such ability
depends on whether Carla is a draw for Cheers’
customers.
9.5
Let A ¼ 1 for simplicity.
a. fk ¼ αkα1 l β > 0, fl ¼ βkα l β1 > 0,
fkk ¼ αðα 1Þkα2 l β < 0,
fll ¼ βðβ 1Þkα l β2 < 0,
fkl ¼ flk ¼ αβkα1 l β1 > 0:
b. eq, k ¼ fk ⋅ k=q ¼ α, eq, l ¼ fl ⋅ l=q ¼ β.
c.
f ðtk, tlÞ ¼ t αþβ f ðk, lÞ;
∂f ðtk, tlÞ=∂t ⋅ t =f ðk, lÞ ¼ ðα þ βÞt αþβ .
At t ¼ 1 this is just α þ β.
d., e. Apply the definitions using the derivatives from
part (a).
9.7
a. β0 ¼ 0.
pffiffiffiffiffiffiffi
pffiffiffiffiffiffiffi
b. MPk ¼ β2 þ 12 β1 l=k; MPL ¼ β3 þ 12 β1 k=l .
c. In general, σ is not constant. If β2 ¼ β3 ¼ 0, σ ¼ 1.
If β1 ¼ 0, σ ¼ ∞.
715
10.3
a., b. q ¼ 150, J ¼ 25, MC ¼ 4;
q ¼ 300, J ¼ 100, MC ¼ 8;
q ¼ 450, J ¼ 225, MC ¼ 12.
10.5
a.
pffiffiffiffi
pffiffiffiffiffiffiffiffi
q ¼ 2 k ⋅ l ; k ¼ 100, q ¼ 20 l, l ¼ q 2 =400.
SC ¼ vk þ wl ¼ 100 þ q 2 =100,
SAC ¼ SC=q ¼ 100=q þ q=100.
b. SMC ¼ q=50.
q
SC
25
106:25
SAC
4:25
SMC
0:50
50
125
2:5
1
100
200
2
2
200
500
2:5
4
c., d. As long as the marginal cost of producing one more
unit is below the average-cost curve, average costs
will be falling. Similarly, if the marginal cost of
producing one more unit is higher than the
average cost, then average costs will be rising.
Therefore, the SMC curve must intersect the
SAC
_ curve at _its lowest point.
e. C_ ¼ vk þ wq 2 =4k.
f. k ¼ ðq=2Þw1=2 v 1=2 .
g. C ¼ qw1=2 v1=2 .
h. Yields an envelope relationship.
10.7
9.9
a. If f ðtk, tlÞ ¼ tf ðk, lÞ then eq, t ¼ ∂f ðtk, tlÞ=∂t ⋅ t =f ðtk, tlÞ.
If t ! 1 then f ðk, lÞ=f ðk, lÞ ¼ 1.
b. Apply Euler’s theorem and use part (a): f ðk, lÞ ¼
fk k þ fl l.
c. eq, t ¼ 2ð1 qÞ. Hence q < 0:5 implies eq, t > 1 and
q > 0:5 implies eq, t < 1.
d. The production function has an upper bound of q ¼ 1.
9.11
a. Apply Euler’s theorem to each fi .
b. With n ¼ 2, k2 fkk þ 2klfkl þ l 2 fll ¼ kðk 1Þf ðk, lÞ. If
k ¼ 1, this implies fkl > 0. If k > 1, it is even clearer
that fkl must be positive. For k < 1, the case is not so clear.
c. Implies
thatPfij > 0 is more common for k ¼ 1.
P
d. ð αi Þ2 αi ¼ kðk 1Þ.
a.
l ¼ ∂C=∂w ¼ 23 qðv=wÞ1=3 .
k ¼ 13 qðw=vÞ2=3 .
b. q ¼ Bl 2=3 k1=3 where B is a constant.
10.9
a.
b.
c.
d.
C ¼ q 1=γ ½ðv=aÞ1σ þ ðw=bÞ1σ 1=ð1σÞ .
C ¼ qa a b b va wb .
wl=vk ¼ b=a.
l=k ¼ ½ðv=aÞ=ðw=bÞσ so wl=vk ¼ ðv=wÞσ1 ðb=aÞσ .
Labor’s relative share is an increasing function of b=a.
If σ > 1, labor’s share moves in the same direction
as v=w. If σ < 1, labor’s relative share moves in
the opposite direction to v=w. This accords with
intuition on how substitutability should affect shares.
10.11
CHAPTER 10
10.1
a. The draftsman is right because the minimum of
SAC curves occurs where the slope is zero. In the
constant-returns-to-scale case, both are correct.
a. si, j ¼ ∂ ln Ci =∂ ln wj ∂ ln Cj =∂ ln wj ¼ ex c , wj ex c , wj .
i
j
b. si, j ¼ ∂ ln Cj =∂ ln wi ∂ ln Ci =∂ ln wi ¼ ex c , wi ex c , wi .
j
i
c. See detailed solutions.
716
Solutions to Odd-Numbered Problems
CHAPTER 11
CHAPTER 12
11.1
12.1
11.3
12.3
a. q ¼ 50.
b. π ¼ 200.
c. q ¼ 5P 50.
a., b. q ¼ a þ bP , P ¼ q=b a=b, R ¼ Pq ¼
ðq 2 aqÞ=b, mr ¼ 2q=b a=b, and the mr curve
has double the slope of the demand curve, so
d mr ¼ q=b.
c. mr ¼ P ð1 þ 1=eÞ ¼ P ð1 þ 1=bÞ.
d. It follows since e ¼ ∂q=∂P ⋅ P =q.
pffiffiffiffi
a. q ¼ 10 P p20.
ffiffiffiffi
b. Q ¼ 1,000 P 2,000.
c. P ¼ 25; Q ¼ 3,000.
a.
b.
c.
d.
11.5
a.
b.
c.
d.
C ¼ wq 2 =4.
πðP , wÞ ¼ P 2 =w.
q ¼ 2P =w.
lðP , wÞ ¼ P 2 =w2 .
11.7
a. Diminishing returns is needed to ensure that a
profit-maximizing output choice exists.
b. Cðq, v, wÞ ¼ ðw þ vÞq 2 =100, ΠðP , v, wÞ ¼
25P 2 =ðw þ vÞ.
c. q ¼ ∂Π=∂P ¼ 50P =ðw þ vÞ ¼ 20, Π ¼ 6,000.
d. q ¼ 30, Π ¼ 13,500.
P ¼ 6.
q ¼ 60,100 10,000P .
P ¼ 6:01, P ¼ 5:99.
eq, p ¼ 600.
a0 P ¼ 6.
b0 Q ¼ 359,800 59,950P .
c0 P ¼ 6:002; P ¼ 5:998.
d0 eq, p ¼ 0:6; eq, p ¼ 3,597.
12.5
a. n ¼ 50, Q ¼ 1,000, q ¼ 20, P ¼ 10, and w ¼ 200.
b. n ¼ 72, Q ¼ 1,728, q ¼ 24, P ¼ 14, and w ¼ 288.
c. The increase for the makers ¼ $5,368. The linear
approximation for the supply curve yields approximately
the same result.
12.7
a.
b.
c.
d.
P ¼ 11, Q ¼ 500, and r ¼ 1.
P ¼ 12, Q ¼ 1,000, and r ¼ 2.
∆PS ¼ 750.
∆ rents ¼ 750.
11.9
b. Diminishing returns is needed to ensure increasing
marginal cost.
c. σ determines how firms adapt to disparate input prices.
d. q ¼ ∂Π=∂P
:
¼ 1=ð1 γÞKP γ=ðγ1Þ ðv 1σ þ w1σ Þγ=ð1σÞðγ1Þ
The size of σ does not affect the supply elasticity,
but greater substitutability implies that increases in
one input price will shift the supply curve less.
e. See detailed solutions.
11.11
a. Shephard’s lemma shows Cv , Cw are the demands for
inputs when Q ¼ 1. The result follows from the assumption of constant returns to scale.
b. Differentiate results from part (a).
c. Follows because C is homogeneous of degree 1 in the
input prices.
d. Substitution
e. Substitution of elasticity definitions.
f. The substitution effect is similar to that for a single firm.
The output effect is derived from moving along the
demand curve for the product.
12.9
a. Use exponential demand and supply: Q D ¼ aP b ,
Q S ¼ cP d . If P is supplier price, then demand is
Q D ¼ að1 þ t Þb P b and equilibrium requires
að1 þ t Þb =c ¼ P db . Taking logs of this expression,
using the approximation that lnð1 þ t Þ t , and
differentiating with respect to t yields d ln P =dt ¼
b=ðd bÞ. A similar expression holds for demand
price.
b. DW 0:5∆P ∆Q 0:5tP0 ∆ ln Q ⋅ Q 0 ¼ 0:5td ⋅
∆ ln P ⋅ P0 Q 0 ¼ 0:5t 2 ½db=ðd bÞP0 Q 0 .
c. These results are almost identical to those in the chapter,
and may often be easier to use.
12.11
a. Foreign supply curve augments domestic one (see
graph in detailed solutions).
b. Tariff shifts foreign portion of supply curve (see graph).
c. Loss of consumer surplus is similar to perfectly elastic
case. Tariff also causes a loss of some foreign producer
surplus in this case.