經濟學原理（上）
Unit 16: Oligopoly
國立臺灣大學 經濟學系
林明仁 教授
【本著作除另有註明外，採取創用CC「姓名
標示－非商業性－相同方式分享」臺灣3.0版授
權釋出】
本課程指定教材為N. Gregory Mankiw: Principles of Economics
(2012), 6th edition.
參考教材為吳聰敏，「經濟學原理」，雙葉書廊。
本講義僅引用部分內容，請讀者自行準備。
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A newspaper contest, in which entrants are
asked to choose from a set of six
photographs of women that are the most
beautiful. Those who picked the most popular
face are then eligible for a prize.
Chapter 16: Oligopoly
Chapter 16: Oligopoly
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A naive strategy would be to choose the face that,
in the opinion of the entrant, is the most
beautiful.
A more sophisticated contest entrant, wishing to
maximize the chances of winning a prize, would
think about what the majority perception of
beauty is, and then make a selection based on
some inference from his knowledge of public
perceptions.
This can be carried one step further to take into
account the fact that other entrants would each
have their own opinion of what public
perceptions are.
Thus the strategy can be extended to the next
order and the next and so on, at each level
attempting to predict the eventual outcome of
the process based on the reasoning of
other rational agents.
Chapter 16: Oligopoly
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"It is not a case of choosing those [faces] that, to
the best of one's judgment, are really the
prettiest, nor even those that average opinion
genuinely thinks the prettiest. We have reached
the third degree where we devote our
intelligences to anticipating what average opinion
expects the average opinion to be. And there are
some, I believe, who practice the fourth, fifth and
higher degrees." (Keynes, General Theory of
Employment Interest and Money, 1936).
Keynes believed that similar behavior was at work
within the stock market.
Chapter 16: Oligopoly
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Level
Level
Level
Level
0=50
1=25
2=12.5
3=6.25
Chapter 16: Oligopoly
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Game theory is the study of how people behave in
strategic situations.
Strategic decisions are those in which each person, in
deciding what actions to take, must consider how
others might respond to that action.
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Chapter 16: Oligopoly
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The prisoners’ dilemma (囚犯的困境) is a very simple
game, but provides insight into the difficulty in
maintaining cooperation.
Often people (firms) fail to cooperate with one another
even when cooperation would make them better off.
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Chapter 16: Oligopoly
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The prisoners’ dilemma is a particular “game” between
two captured prisoners that illustrates why cooperation
is difficult to maintain even when it is mutually
beneficial.
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Chapter 16: Oligopoly
Chapter 16: Oligopoly
Bonnie’ s Decision
Confess
Bonnie gets 8 years
Remain Silent
Bonnie gets 20 years
Confess
Clyde gets 8 years
Clyde’s
Decision
Bonnie goes free
Clyde goes free
Bonnie gets 1 year
Remain
Silent
Clyde gets 20 years
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Clyde gets 1 year
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https://www.youtube.com/watch?v=p3Uos2f
zIJ0
Golden Ball
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Steal
Steal
Split
(0 , 0) (100,0)
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Split
(0,100) (50,50)
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Chapter 16: Oligopoly
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How to find out Equilibrium(s)?
1. The dominant strategy (優勢策略法)
DS is the best strategy for a player to follow regardless of
the strategies chosen by the other players
Or strictly dominated strategy(嚴格被支配策略)
2. Find out “no deviation point”, one by one
Notice:
◦ (1) sometimes there might not be any dominant strategy in the game
◦ (2) NE might be more than one, or zero
Ex: battle for the sexes.
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In PD, Cooperation is difficult to maintain, because
cooperation is not in the best interest of the individual player.
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Chapter 16: Oligopoly
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A Nash equilibrium is a situation in which economic
actors interacting with one another each choose their
best strategy given the strategies that all the others have
chosen.
(Confess, Confess) is a NE if, given that B confesses,
A’s best strategy is to confess, and vise versa.
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Chapter 16: Oligopoly
Samsung’s Decision
High Production: 40 Gal.
Low Production: 30 gal.
Samsung gets $1,500 profit
Samsung gets $1,600 profit
High
Production
40 gal.
Apple gets $1,600 profit
Apple’s
Decision
Apple gets $2,000 profit
Samsung gets $2,000 profit
Samsung gets $1,800 profit
Low
Production
30 gal.
Apple gets $1,500 profit
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Apple gets $1,800 profit
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Self-interest makes it difficult for the oligopoly to
maintain a cooperative outcome with low production,
high prices, and monopoly profits.
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Chapter 16: Oligopoly
Decision of the United States (U.S.)
Arm
Disarm
U.S. at risk
U.S. at risk and weak
Arm
Decision
of the
Soviet Union
(USSR)
USSR at risk
USSR safe and powerful
U.S. safe and powerful
U.S. safe
Disarm
USSR at risk and weak
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USSR safe
Exxon ’
s Decision
Drill Two Wells
Drill Two
Wells
Chevron gets $4
million profit
Chevron’s
Decision
Exxon gets $6
million profit
Drill One
Well
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Exxon gets $4
million profit
Chevron gets $3
million profit
Drill One Well
Exxon gets $3
million profit
Chevron gets $6
million profit
Exxon gets $5
million profit
Chevron gets $5
million profit
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Say a guard cost $10, the benefit for store A
and B is $8 each.
Hire
Do Not Hire
Hire
(3, 3)
Do not Hire (8, -2)
(-2, 8)
(0,0)
Nash E= Both won’t hire, but it is not efficient! So we need some
kind of enforcement to ensure collective benefit, but it may not
needed to be government!
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葵丘之盟: 五命曰：無曲防，無遏糴 孟子:以鄰為壑
How to solve? 大一統！Internalize the problem!
A typical free rider problem: 平成三劍客
http://www.ettoday.net/dalemon/post/3130
Other example: 排隊與美國開車
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Firms that care about future profits will cooperate in
repeated games rather than cheating in a single game to
achieve a one-time gain.
Focal point (see note)
The players must be patient enough, and the period
must be long enough (infinite)
So the question becomes: what would happen if we
play PD for 500 times, or forever?
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Chapter 16: Oligopoly
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Goal: to minimize years in jail,
Strategy includes (but not limited to):
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(1) always hang on
(2) always confess
(3) strong retaliation (恨你一世人)
(4) tit-for-tat (an eye for an eye, 以直報怨？)
(5) confess even, not confess odd,….
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Chapter 16: Oligopoly
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Ultimatum Game(最後通牒賽局): give Anita (proposer)
$1, she then proposes a way to split the money, then
Zelda (receiver) can decide whether to accept or reject
the proposal.
If accept: as it is.
If reject: both get nothing.
What would a Homo Economicus do?
In lab experiment, basically all subjects are students….
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Chapter 16: Oligopoly
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Henrich, J., Boyd, R., Bowles, S., Camerer, C., Fehr, E., Gintis,
H., and McElreath, R. (2001). In search of Homo Economicus:
Behavioral Experiments in 15 Small-Scale Societies. AER
Papers and Proceedings. 91, 73–78.*
They went to 15 aboriginal societies.
Subjects played anonymously.
The stakes of most games were money, about 1 or 2 days of
wage income (though in some cases tobacco or other goods
were used).
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Chapter 16: Oligopoly
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Chapter 16: Oligopoly
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Not what game theory has predicted.(not surprise)
But the range is larger, compared to results form
industrial society (44%)
Culture matters: say, payoffs for cooperation:
Machi..&Tsimane: no cooperation, Lamelara: whale
hunting.
Does this mean altruism?(the answer is No)
Researchers had intervened the society!(the story of
pants)
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Chapter 16: Oligopoly
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What happen when you receive a nasty offer, and you
reject it?
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有一大半的提議都是50-50 對分，因為愈不公平的提議，
被回應者拒絕的可能性就愈大，80-20 或更低的方案則幾
乎全部遭到拒絕。
Sanfey et al.（ 2003） 一文則透過腦部正子斷層造影告
訴我們，當回應者看見不公平的方案、考慮是否拒絕時，
他們大腦中和情緒反應相關的前腦島 （anterior insula）
與權衡利害的背側前額葉皮質（dorsolateral prefrontal
cortex, DLPFC） 會有反應，而且當不公平的提案被拒絕
的時候，情緒反應的腦區活動特別劇烈。
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Chapter 16: Oligopoly
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囚犯兩難賽局計算以推理型思考主宰的「優勢可解賽局」
(如囚犯兩難賽局)，其推理過程雖然有所依循步驟，但也
相對緩慢。
事實上，有些賽局的選擇，相對而言並沒有足夠的推理依
據，但又需要更快速的決定時，則常依據直覺(如股票投
資)，但相關研究甚少。
Schelling(2005 Nobel Prize in Economics):在某些策
略互動中，決策者雙方必須在缺乏溝通的前提下做出相同
的選擇。這類仰賴直覺型思考的賽局，關鍵在於如何依循
直覺找出決策雙方關注的「焦點」(focal point)，即稱
作「協調賽局」(coordination game)(ex: 明天見個面
吧!)。這種思考過程通常相對快速，但較易受到情緒影響。
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Chapter 16: Oligopoly
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黃: lets put people in MRI, and let them play!
Results:在進行「優勢可解賽局」時，人類大腦皮
質的前額葉及顱葉(frontoparietal areas)活化程
度較高，而此一大腦神經網域與人類的推理思考有
相當的關聯。在「協調賽局」中，偏向直覺型思考
使得人類大腦中的腦島(insula)及前扣帶皮質區
(anterior cingulate cortex)相對地活躍。而最
近的研究中指出，此大腦區域與人類諸多社會行為
相關：如合作、信任、同理心，甚至是愛情。
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Chapter 16: Oligopoly