Norms and Development:
Interdisciplinary Approach
Week 2
Explaining Social Norms I:
Evolutionary Perspective
A Story of Mr. RATIONAL
One day, he had his wallet ($50) stolen on
the street. As he is a very busy and
successful businessman, it’s just a waste
of time to try to get it back. He quickly
estimated the cost for recovering the wallet
is more than $200. He is so clever and
rational, he didn't chase the thief.
If the thief realizes that he’s a rational person,
he will be a sucker for the thief in future…
A Story of Mr. EMOTIONAL
He is a successful businessman but known
as very quick-tempered person. When he
had his wallet stolen, his temper flared up
and started to chase the thief. He is not
calculating the cost of chasing anymore.
He’s like an animal just driven by anger…
If the thief knows that he never gives up
chasing until he gets a wallet recovered,
he’ll never be a sucker for the thief.
Passions Within Reason
by Robert H. Frank (1989)

In the world, there are many situations in
which it is worse to try to be rational.

Emotion, that are considered to be a
representative of human irrationality, can
maximize self-interest that cannot be
achieved by rational individuals…
Huh?
Being irrational is rational?
Being rational is irrational?
The Limit of Rationality
t=1
Consequence
at time t
t=2
t=3
Action
t=4
t=5
t=100
Total consequences of an action
= the sum of consequence at each time
Forward vs. Backward Rationality

Rationality = maximization of self-interest

Forward Rationality = evaluation of the action
before the action is taken  with practical
and logical reasons, the range of
maximization is myopically limited.

Backward Rationality = evaluation of the
action long after the action was taken.
Range for backward
rationality
t=1
Range for forward
rationality
t=2
t=3
Action
t=4
t=5
t=100
Some Terminological Issues

The term “optimization” implicitly assumes
forward rationality.

Backward rationality seems to be optimizing.
But, remember that, when an agent with
backward rationality takes the action that was
optimum in the past, it may not be optimum
anymore in future (e.g., what if the
consequence of the action is evaluated at the
time t = 200?).
Mechanisms of Backward Rationality
1.
Genetic Evolution
Successful genotypes spread in a population via
Darwinian selection.
2.
Individual Learning
Trial and error increases the proportion of
successful behaviors within individuals.
3.
Social Learning
Successful behaviors/traits spread in a population
via imitation.

Evolutionary processes are the mechanisms
acquiring the behaviors that are backwardly
rational and emancipate us from the myopic
forward rationality.

This is exactly the reason why we pay
attention to the evolutionary processes as
initial candidates explaining human
behaviors that contradict with the principle of
forward-rationality.



We do NOT argue that all the human
behaviors are genetically encoded and
selected by genetic evolution.
We do NOT believe in that evolutionary
processes (in general) can explain everything
in the world.
We are just arguing that, when we find human
behaviors that look like conflicting with selfinterest, it is a simple and parsimonious
research strategy to check the possibility that
such behaviors are the product of backward
rationality.
Roadmap




Key issues in evolutionary processes
Report on Axelrod (1986)
What lacks in evolutionary perspective?
Discussion
Social Learning: Random Imitation



Let’s consider social learning.
Individuals in the population has a trait (A or
B). A is more beneficial than B.
An individual (social learner) randomly picks
another individual (target) and just imitate the
target’s trait.
Proportion of the trait A
1
0.8
0.6
0.4
0.2
0
1
18 35 52 69 86 103 120 137 154 171 188
Generation
Random social learning does not increase
an adaptive trait in a population.
Social Learning: Majority-Biased Imitation



Let’s consider social learning.
Individuals in the population has a trait (A or
B). A is more beneficial than B.
Imagine that an individual (social learner)
randomly picks three other individuals
(targets). If majority of the targets has the
trait X, the learner just imitates it. Otherwise,
don’t imitate.
Proportion of the trait A
1
0.8
0.6
0.4
0.2
0
1
18 35 52
69 86 103 120 137 154 171 188
Generation
Majority-biased social learning just increases
initially popular trait (but not an adaptive trait in
this case).
Social Learning: Payoff-Biased Imitation



Let’s consider social learning.
Individuals in the population has a trait (A or
B). A is more beneficial than B.
Imagine that an individual (social learner)
randomly picks another individual (target) and
imitate the target’s trait only when the benefit
of the target received in the past is larger
than the benefit the learners received.
Otherwise, he doesn't imitate the trait.
Proportion of the trait A
1
0.8
0.6
0.4
0.2
0
1
18 35 52 69 86 103 120 137 154 171 188
Generation
Payoff-biased social learning increases the
proportion of an adaptive trait.

Social learning is a catch-all word. Be careful
when you use this word. When we say
“evolution is an analogy of social learning”, it
means payoff-biased social learning.

Other types of social learning and their roles
are discussed next week…
Equilibrium and Path Dependency

As a result of evolutionary processes, a society
may reach at stable states in which no one can
get better outcome by changing the behaviors =
evolutionarily stable equilibrium.

There may be multiple equilibriums existing. In
such a case, history matters = path dependency.
An Example: Coordination Game
5
5
Individuals are
randomly matched
with another
individual and play
the game.

Evolutionary
processes increase
the proportion of
adaptive strategy.
0
0
0
0

4
4
Proportion of Apple
1
0.42
0.43
0.44
0.45
0.46
0.47
0.8
0.6
0.4
0.2
0
1
4
7
10 13 16 19 22 25 28
Generation
Very small initial difference results in totally different society.
Once a society is set on a different path, it cannot escape
from the self-reinforcing processes…
Implications of Multiple Equilibriums

Individuals with totally identical mentality or
psychology may create totally different societies.

Differences between societies cannot be
easily reduced into the differences in the
psychological mechanisms.

Consider the article in the Guardian. Why the
people, who are considered to be kind and nice,
are so messy in queuing?
When Socially Harmful Norms Persist or
Disappear: An Evolutionary Explanation
Equilibrium
A
Stable equilibrium
Equilibrium
B
1. Because a society is in stable
equilibrium, small perturbation
cannot easily move it to another
equilibrium.
When Socially Harmful Norms Persist or
Disappear: An Evolutionary Explanation
Equilibrium
A
Stable equilibrium
Equilibrium
B
1. Because a society is in stable
equilibrium, small perturbation
cannot easily move it to another
equilibrium.
2. If a norm that was socially beneficial in the past is
transformed into socially harmful norm, the equilibrium
may become less stable. This will enhance a society
moves to another stable equilibrium.
Equilibrium
A
Equilibrium
B
3. However, a society continues to stay in the same place
until when accumulated perturbation creates very large
shock. It is just because a harmful norm is still a stable
equilibrium.
Equilibrium
A
Equilibrium
B
Summary



Evolutionary processes have a potential for
explaining 1) seemingly irrational behaviors
and 2) cultural/societal differences with the
single principle of backward rationality.
Thus, evolutionary processes may save us
from the tyranny of infinite numbers of culturalspecific psychological concepts…
We focus on evolutionary processes not
because it can explain everything but because
it is a simple and parsimonious starting point.
Game Theory vs. Evolutionary Game Theory?
Q. Forward rationality assumed in game theory is
like Laplacean-demon. Does game theory make
the same prediction with evolutionary game
theory that assumes backward rationality?
A. Yes (but not always). Just remember that
evolutionary game theory investigates the
dynamic processes in which a society changes,
and game theory investigates where a society
reaches as a result of dynamic evolutionary
processes.
Let’s move to Axelrod (1986) that first
applied evolutionary perspective for
explaining WHY social norms exist.
What’s Wrong About Axelrod (1986)?
“The model makes the critical assumption that a
player’s vengefulness against nonpunishment
is the same as the player’s vengefulness
against an original defection. (p. 1102, left
column).”
 If this linkage is removed, social norms
cannot evolve.
Step 1
Step 2
Step 3


FF
PP
PF
cooperate
free ride
free ride
cooperate
punish
punish
cooperate
punish
free ride
PP is better than FF as FF is punished in both
steps 2 and 3.
However, PF is better off than PP at step 3. If PF
dominates PP, then FF can invade and this results
in the evolution of free riders at the step 1.
A Basic Model of Social Norm



Step 1. Norm-obedience
Step 2. Punishment of norm violators
Even if a social norm is beneficial to
everyone (=functional), it cannot solve the
free-rider problem.
What Have We Learned?
Some mechanism that makes punishment
(at least) less disadvantageous is necessary.
Why Free-Rider Problem Was Solved in
Kameda et al. (2003)?


The game structure is slightly different from a
standard game of social norms  the cost of
punishment is very small in communal sharing
norm.
Punishment results in the increase of immediate
benefit  non-acquirers can get a portion of
resource from an acquirer (i.e., looting). This
does not happen in standard game of social
norms.



Another solution is to stop interacting with
norm-violators (Aoki, 2001).
If people live in a small group and cannot
escape to the outside, like small villages in
mountains, punishment is cheap as it is
sufficient to break the relations (i.e., helping
in danger) with norm-violators.
It doesn't work, however, if mobility gets
higher or group size gets larger. It is difficult
to imagine other cases (except for the above
two) in which punishment is so cheap…
Summary and Conclusion
What Is Evolutionary Perspective?


This is a powerful and parsimonious
perspective that can explain several
(seemingly) conflicting phenomena with the
single principle.
As three mechanisms composing
evolutionary processes (individual/social
learning and evolution) are important forces,
evolutionary processes can be a kind of firstorder approximation of social dynamics. See
my talk at Ringberg.
What Lacks in Evolutionary Perspective?


However, it cannot explain the existence of
punishment of norm-violators in many
situations and thus insufficient for explaining
social norms.
Furthermore, this approach tends to ignore
psychological mechanisms underlying social
norms. For instance, where's a room for
(seemingly maladaptive) emotion, such as
guilt/shame, that seem to be crucial for
maintaining social norms?
Towards Cultural-Evolutionary Perspective


We tried to explain seemingly irrational
behaviors only with the single principle. In the
real world, however, several other forces are
working.
Cultural-evolutionary perspective is the
second-order approximation that provides
answers to the above questions by
incorporating other forces. Keep in mind that,
however, this perspective has the strong
basis on evolutionary-perspective.