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Strategies and Games : Theory and Practice
Dutta, Prajit K.
MIT Press
0262041693
9780262041690
9780585070223
English
Game theory, Equilibrium (Economics)
1999
HB144.D88 1999eb
330/.01/5193
Game theory, Equilibrium (Economics)
cover
Page III
Strategies and Games
Theory and Practice
Prajit K. Dutta
THE MIT PRESS CAMBRIDGE, MASSACHUSETTS LONDON, ENGLAND
page_iii
Page IV
© 1999 Massachusetts Institute of Technology
All rights reserved. No part of this book may be reproduced in any form by any electronic or mechanical
means (including photocopying, recording, or information storage and retrieval) without permission in
writing from the publisher.
This book was set in Melior and MetaPlus by Windfall Software using ZzTEX and was printed and bound in
the United States of America.
Library of Congress Cataloging-in-Publication Data
Dutta, Prajit K.
Strategies and games: theory and practice / Prajit K.
Dutta.
p. cm.
Includes bibliographical references and index.
ISBN 0-262-04169-3
1. Game theory. 2. Equilibrium (Economics). I. Title.
HB144.D88 1999
330'.01'15193dc21
98-42937
CIP
page_iv
Page V
MA AAR BABA KE
page_v
Page VII
BRIEF CONTENTS
Preface
A Reader's Guide
XXI
XXIX
Part One Introduction
1
Chapter 1 A First Look at the Applications
3
2 A First Look at the Theory
17
Two Strategic Form Games: Theory and Practice
33
3 Strategic Form Games and Dominant Strategies
35
4 Dominance Solvability
49
5 Nash Equilibrium
63
6 An Application" Cournot Duopoly
75
7 An Application: The Commons Problem
91
8 Mixed Strategies
103
9 Two Applications: Natural Monopoly and Bankruptcy Law
121
10 Zero-Sum Games
139
Three Extensive Form Games: Theory and Applications
155
11 Extensive Form Games and Backward Induction
157
12 An Application: Research and Development
179
13 Subgame Perfect Equilibrium
193
14 Finitely Repeated Games
209
15 Infinitely Repeated Games
227
16 An Application: Competition and Collusion in the NASDAQ Stock Market
243
17 An Application: OPEC
257
18 Dynamic Games with an Application to the Commmons Problem
275
Four Asymmetric Information Games: Theory and Applications
291
19 Moral Hazard and Incentives Theory
293
20 Games with Incomplete Information
309
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Page VIII
21 An Application: Incomplete Information in a Cournot Duopoly
331
22 Mechanism Design, the Revelation Principle, and Sales to an Unknown Buyer
349
23 An Application: Auctions
367
24 Signaling Games and the Lemons Problem
383
Five Foundations
401
25 Calculus and Optimization
403
26 Probability and Expectation
421
27 Utility and Expected Utility
433
28 Existence of Nash Equilibria
451
Index
465
page_viii
Page IX
CONTENTS
Preface
A Reader's Guide
XXI
XXIX
Part One Indroduction
1
Chapter 1 A First Look at the Applications
3
1.1 Gabes That We Play
3
1.2 Background
7
1.3 Examples
8
Summary
12
Exercises
12
Chapter 2 A First Look at the Theory
17
2.1 Rules of the Game: Background
17
2.2 Who, What, When: The Extensive Form
18
2.2.1 Information Sets and Strategies
20
2.3 Who What, When: The Normal (or Strategic) Form
21
2.4 How Much: Von Neumann-Morgenstern Utility Function
23
2.5 Representation of the Examples
25
Summary
27
Exercises
28
Part Two Strategic Form Games: Theory and Practice
33
Chapter 3 Strategic Form Games and Dominant Strategies
35
3.1 Strategic Form Games
35
3.1.1 Examples
36
3.1.2 Equivalence with the Extensive Form
39
3.2 Case Study The Strategic Form of Art Auctions
40
3.2.1 Art Auctions: A Description
40
3.2.2 Art Auctions: The Strategic Form
40
3.3 Dominant Strategy Solution
41
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Page X
3.4 Cae Study Again A Dominant Strategy at the Auction
43
Summary
44
Exercises
45
Chapter 4 Dominance Solvability
4.1 The Idea
49
49
4.1.1 Dominated and Undominated Strategies
49
4.1.2 Iterated Elimination of Dominated Strategies
51
4.1.3 More Examples
51
4.2 Case Study Electing the United Nations Secretary General
54
4.3 A More Formal Definition
55
4.4 A Discussion
57
Summary
59
Exercises
59
Chapter 5 Nash Equilibrium
5.1 The Concept
63
63
5.1.1 Intuition and Definition
63
5.1.2 Nash Parables
64
5.2 Examples
66
5.3 Case Study Nash Equilibrium in the Animal Kingdom
68
5.4 Relation Between the Solution Concepts
69
Summary
71
Exercises
71
Chapter 6 An Application: Cournot Duopoly
75
6.1 Background
75
6.2 The Basic Model
76
6.3 Cournot Nash Equilibrium
77
6.4 Cartel Solution
79
6.5 Case Study Today's OPEC
81
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Page XI
6.6 Variants on the Main Theme I: A Graphical Analysis
82
6.6.1 The IEDS Solution to the Cournot Model
84
6.7 Variants on the Main Theme II: Stackelberg Model
85
6.8 Variants on the Main Theme III: Generalization
86
Summary
87
Exercises
88
Chapter 7 An Application: The Commons Problem
91
7.1 Background: What is the Commons?
91
7.2 A Simple Model
93
7.3 Social Optimality
95
7.4 The Problem Worsens in a Large Population
96
7.5 Case Studies Buffalo, Global Warming, and the Internet
97
7.6 Averting a Tragedy
98
Summary
99
Exercises
100
Chapter 8 Mixed Strategies
8.1 Definition and Examples
103
103
8.1.1 What Is a Mixed Strategy?
103
8.1.2 Yet More Examples
106
8.2 An Implication
107
8.3 Mixed Strategies Can Dominate Some Pure Strategies
108
8.3.1 Implications for Dominant Strategy Solution and IEDS
8.4 Mixed Strategies are Good for Bluffing
109
110
8.5 Mixed Strategies and Nash Equilibrium
111
8.5.1 Mixed-Strategy Nash Equilibria in an Example
113
8.6 Case Study Random Drug Testing
114
Summary
115
Exercises
116
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Page XII
Chapter 9 Tow Applications: Naturla Monopoly and Bankruptcy Law
9.1 Chicken, Symmetric Games, and Symmetric Equilibria
121
121
9.1.1 Chicken
121
9.1.2 Symmetric Games and Symmetric Equilibria
122
9.2 Natural Monopoly
123
9.2.1 The Economic Background
123
9.2.2 A Simple Example
124
9.2.3 War of Attrition and a General Analysis
125
9.3 Bankruptcy Law
128
9.3.1 The Legal Background
128
9.3.2 A Numerical Example
128
9.3.3 A General Analysis
130
Summary
132
Exercises
133
Chapter 10 Zero-Sum Games
139
10.1 Definition and Examples
139
10.2 Playing Safe: Maxmin
141
10.2.1 The Concept
141
10.2.2 Examples
142
10.3 Playing Sound: Minmax
144
10.3.1 The Concept and Examples
144
10.3.2 Two Results
146
10.4 Playing Nash: Playing Both Safe and Sound
147
Summary
149
Exercises
149
Part Three Extensive Form Games: Theory and Applications
155
Chapter 11 Extensive Form Games and Backward Induction
11.1 The Extensive Form
157
157
11.1.1 A More Formal Treatment
158
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Page XIII
11.1.2 Strategies, Mixed Strategies, and Chance Nodes
160
11.2 Perfect Information Games: Definition and Examples
162
11.3 Backward Induction: Examples
165
11.3.1 The Power of Commitment
167
11.4 Backward Induction: A General Result
168
11.5 Connection With IEDS in the Strategic Form
170
11.6 Case Study Poison Pills and Other Takeover Deterrents
172
Summary
174
Exercises
175
Chapter 12 An Application: Research and Development
12.1 Background: R&D, Patents, and Ologopolies
12.1.1 A Patent Race in Progress: High-Definition Television
179
179
180
12.2 A Model of R&D
181
12.3 Backward Induction: Analysis of the Model
183
12.4 Some Remarks
188
Summary
189
Exercises
190
Chapter 13 Subgame Perfect Equilibrium
193
13.1 A Motivating Example
193
13.2 Subgames and Strategies Within Subgames
196
13.3 Subgame Perfect Equilibrium
197
13.4 Two More Examples
199
13.5 Some Remarks
202
13.6 Case Study Peace in the World War I Trenches
203
Summary
205
Exercises
205
Chapter 14 Finitely Repeated Games
14.1 Examples and Economic Applications
209
209
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Page XIV
14.1.1 Three Repeated Games and a Definition
209
14.1.2 Four Economic Applications
212
14.2 Finitely Repeated Games
214
14.2.1 Some General Conclusions
218
14.3 Case Study Treasury Bill Auctions
219
Summary
222
Exercises
222
Chapter 15 Infinitely Repeated Games
227
15.1 Detour Through Discounting
227
15.2 Analysis of Example 3: Trigger Strategies and Good Behavior
229
15.3 The Folk Theorem
232
15.4 Repeated Games With Imperfect Detection
234
Summary
237
Exercises
238
Chapter 16 An Application: Competition and Collusion in the NASDAQ Stock Market243
16.1 The Background
243
16.2 The Analysis
245
16.2.1 A Model of the NASDAQ Market
245
16.2.2 Collusion
246
16.2.3 More on Collusion
248
16.3 The Broker-Dealer Relationship
249
16.3.1 Order Preferencing
249
16.3.2 Dealers Big and Small
250
16.4 The Epilogue
251
Summary
252
Exercises
252
Chapter 17 An Application: OPEC
257
17.1 Oil: A Historical Review
257
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Page XV
17.1.1 Production and Price History
258
17.2 A Simple Model of the Oil Market
259
17.3 Oil Prices and the Role of OPEC
260
17.4 Repteated Games With Demand Uncertainty
262
17.5 Unobserved Quota Violations
266
17.6 Some Further Comments
269
Summary
270
Exercises
271
Chapter 18 Dynamic Games With An Application to the Commons Problem
275
18.1 Dynamic Games: A Prologue
275
18.2 The Commons Problem: A Model
276
18.3 Sustainable Development and Social Optimum
278
18.3.1 A Computation of the Social Optimum
278
18.3.2 An Explanation of the Social Optimum
281
18.4 Achievable Development and Game Equilibrium
282
18.4.1 A Computation of the Game Equilibrium
282
18.4.2 An Explanation of the Equilibrium
284
18.4.3 A Comparison of the Socially Optimal and the Equilibrium Outcomes
285
18.5 Dynamic Games: An Epilogue
286
Summary
287
Exercises
288
Part Four Asymmetric Information Games: Theory and Applications
291
Chapter 19 Moral Hazard and Incentives Theory
293
19.1 Moral Hazard: Examples and a Definition
293
29.2 A Principal-Agent Model
295
19.2.1 Some Examples of Incentive Schemes
19.3 The Optimal Incentive Scheme
297
299
19.3.1 No Moral Hazard
299
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Page XVI
19.3.2 Moral Hazard
19.4 Some General Conclusions
19.4.1 Extensions and Generalizations
19.5 Case Study Compensating Primary Care Physicians in an HMO
299
301
303
304
Summary
305
Exercises
306
Chapter 20 Games with Incomplete Information
309
20.1 Some Examples
309
20.1.1 Some Analysis of the Examples
312
20.2 A Complete Analysis of Example 4
313
20.2.1 Bayes-Nash Equilibrium
313
20.2.2 Pure-Strategy Bayes-Nash Equilibria
315
20.2.3 Mixed-Strategy Bayes-Nash Equilibria
316
20.3 More General Considerations
318
20.3.1 A Modified Example
318
20.3.2 A General Framework
320
20.4 Dominance-Based Solution Concepts
321
20.5 Case Study Final Jeopardy
323
Summary
326
Exercises
326
Chapter 21 An Application: Incomplete Information in a Cournot Duopoly
21.1 A Model and its Equilibrium
331
331
21.1.1 The Basic Model
331
21.1.2 Bayes-Nash Equilibrium
332
21.2 The Complete Information Solution
336
21.3 Revealing Costs to a Rival
338
21.4 Two-Sided Incompleteness of Information
340
21.5 Generalizations and Extensions
341
21.5.1 Oligopoly
341
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Page XVII
21.5.2 Demand Uncertainty
342
Summary
343
Exercises
343
Chapter 22 Mechanism Design, The Revelation Priciple, and Sales to an Unknown
Buyer
349
22.1 Mechanism Design: The Economic Context
349
22.2 A Simple Example: Selling to a Buyer With an Unknown Valuation
351
22.2.1 Known Passion
351
22.2.2 Unknown Passion
352
22.3 Mechanism Design and the Revelation Principle
356
22.3.1 Single Player
356
22.3.2 Many Players
357
22.4 A More General Example: Selling Variable Amounts
358
22.4.1 Known Type
359
22.4.2 Unknown Type
359
Summary
362
Exercises
362
Chapter 23 An Application: Auctions
367
23.1 Background and Examples
367
23.1.1 Basic Model
369
23.2 Second-Price Auctions
369
23.3 First-Price Auctions
371
23.4 Optimal Auctions
373
23.4.1 How Well Do the First- and Second-Price Auctions Do?
375
23.5 Final Remarks
376
Summary
377
Exercises
378
Chapter 24 Signaling Games and the Lemons Problem
24.1 Motivation and Two Examples
383
383
24.1.1 A First Analysis of the Examples
385
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Page XVIII
24.2 A Definition, an Equilibrium Concept, and Examples
387
24.2.1 Definition
387
24.2.2 Perfect Bayesian Equilibrium
387
24.2.3 A Further Analysis of the Examples
389
24.3 Signaling Product Quality
391
24.3.1 The Bad Can Drive Out the Good
391
24.3.2 Good Can Signal Quality?
392
24.4 Case Study Used CarsA Market for Lemons?
394
24.5 Concluding Remarks
395
Summary
396
Exercises
396
Part Five Foundations
401
Chapter 25 Calculus and Optimization
403
25.1 A Calculus Primer
403
25.1.1 Functions
404
25.1.2 Slopes
405
25.1.3 Some Formulas
407
25.1.4 Concave Functions
408
25.2 An Optimization Theory Primer
409
25.2.1 Necessary Conditions
409
25.2.2 Sufficient Conditions
410
25.2.3 Feasibility Constraints
411
25.2.4 Quadratic and Log Functions
413
Summary
414
Exercises
415
Chapter 26 Probability and Expectation
421
26.1 Probability
421
26.1.1 Independence and Conditional Probability
26.2 Random Variables and Expectation
425
426
26.2.1 Conditional Expectation
427
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Page XIX
Summary
428
Exercises
428
Chapter 27 Utility and Expected Utility
433
27.1 Decision Making Under Certainty
433
27.2 Decision Making Under Uncertainty
436
27.2.1 The Expected Utility Theorem and the Expected Return Puzzle
437
27.2.2 Details on the Von Neumann-Morgenstern Theorem
439
27.2.3 Payoffs in a Game
441
27.3 Risk Aversion
441
Summary
444
Exercises
444
Chapter 28 Existence of Nash Equilibria
452
28.1 Definition and Examples
451
28.2 Mathematical Background: Fixed Points
453
28.3 Existence of Nash Equilibria: Results and Intuition
458
Summary
460
Exercises
461
Index
465
page_xix
Page XXI
PREFACE
This book evolved out of lecture notes for an undergraduate course in game theory that I have taught at
Columbia University for the past six years. On the first two occasions I took the straight road, teaching out
of available texts. But the road turned out to be somewhat bumpy; for a variety of reasons I was not satisfied
with the many texts that I considered. So the third time around I built myself a small bypass; I wrote a set of
sketchy lecture notes from which I taught while I assigned a more complete text to the students. Although
this compromise involved minimal costs to me, it turned out to be even worse for my students, since we were
now traveling on different roads. And then I (foolishly) decided to build my own highway; buoyed by a
number of favorable referee reports, I decided to turn my notes into a book. I say foolishly because I had no
idea how much hard work is involved in building a road. I only hope I built a smooth one.
The Book's Purpose And Its Intended Audience
The objective of this book is to provide a rigorous yet accessible introduction to game theory and its
applications, primarily in economics and business, but also in political science, the law, and everyday life.
The material is intended principally for two audiences: first, an undergraduate audience that would take this
course as an elective for an economics major. (My experience has been, however, that my classes are also
heavily attended by undergraduate majors in engineering and the sciences who take this course to fulfill their
economics requirement.) The many applications and case studies in the book should make it attractive to its
second audience, MBA students in business schools. In addition, I have tried to make the material useful to
graduate students in economics and related disciplinesPh.D. students in political science, Ph.D. students in
economics not specializing in economic theory, etc.who would like to have a source from which they can get
a self-contained, albeit basic, treatment of game theory.
Pedagogically I have had one overriding objective: to write a textbook that would take the middle road
between the anecdotal and the theorem-driven treatments of the subject. On the one hand is the approach
that teaches purely by examples and anecdotes. In my experience that leaves the students, especially the
brighter ones, hungering for more. On the other hand, there is the more advanced approach emphasizing a
rigorous treatment, but again, in my experience, if there are too few examples and applications it is difficult
to keep even the brighter students interested.
I have tried to combine the best elements of both approaches. Every result is precisely stated (albeit with
minimal notation), all assumptions are detailed, and at least a sketch of a proof is provided. The text also
contains nine chapter-length applications and twelve fairly detailed case studies.
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Page XXII
Distinctive Features Of The Book
I believe this book improves on available undergraduate texts in the following ways.
Content a full description of utility theory and a detailed analysis of dynamic game theory
The book provides a thorough discussion of the single-agent decision theory that forms the underpinning of
game theory. (That exercise takes up three chapters in Part Five.) More importantly perhaps, this is the first
text that provides a detailed analysis of dynamic strategic interaction (in Part Three). The theory of repeated
games is studied over two and a half chapters, including discussions of finitely and infinitely repeated games
as well as games with varying stage payoffs. I follow the theory with two chapter-length applications:
market-making on the NASDAQ financial market and the price history of OPEC. A discussion of dynamic
games (in which the game environment evolves according to players' previous choices) follows along with an
application to the dynamic commons problem. I believe many of the interesting applications of game theory
are dynamicstudent interest seems always to heighten when I get to this part of the courseand I have found
that every other text pays only cursory attention to many dynamic issues.
Style emphasis on a parallel development of theory and examples
Almost every chapter that introduces a new concept opens with numerical examples, some of which are well
known and many of which are not. Sometimes I have a leading example and at other times a set of (small)
examples. After explaining the exam-pies, I go to the concept and discuss it with reasonable rigor. At this
point I return to the examples and analyze the just introduced concept within the context of the examples. At
the end of a sectiona set of chapters on related ideasI devote a whole chapter, and sometimes two, to
economic applications of those ideas.
Length and Organization bite-sized chapters and a static to dynamic progression
I decided to organize the material within each chapter in such a fashion that the essential elements of a
whole chapter can be taught in one class (or a class and a half, depending on level). In my experience it has
been a lot easier to keep the students engaged with this structure than with texts that have individual
chapters that are, for example, over fifty pages long. The topics evolve in a natural sequence: static complete
information to dynamic complete information to static incomplete information. I decided to skip much of
dynamic incomplete information (other than signaling) because the questions in this part of the subject are a
lot easier than the answers (and my students seemed to have little stomach for equilibrium refinements, for
example). There are a few advanced topics as well; different instructors will have the freedom to decide
which subset of the advanced topics they would like to teach in their course. Sections that are more difficult
are marked with the symbol . Depending on level, some instructors will want to skip
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Page XXIII
these sections at first presentation, while others may wish to take extra time in discussing the material.
Exercises
At the end of each chapter there are about twenty-five to thirty problems (in the Exercises section). In
addition, within the text itself, each chapter has a number of questions (or concept checks) in which the
student is asked to complete a part of an argument, to compute a remaining case in an example, to check the
computation for an assertion, and so on. The point of these questions is to make sure that the reader is really
following the chapter's argument; I strongly encourage my students to answer these questions and often
include some of them in the problem sets.
Case Studies and Applications
At the end of virtually every theoretical chapter there is a case study drawn from real life to illustrate the
concept just discussed. For example, after the chapter on Nash equilibrium, there is a discussion of its usage
in understanding animal conflicts. After a chapter on backward induction (and the power of commitment),
there is a discussion of poison pills and other take-over deterrents. Similarly, at the end of each cluster of
similar topics there is a whole chapter-length application. These range from the tragedy of the commons to
bankruptcy law to incomplete information Cournot competition.
An Overview And Two Possible Syllabi
The book is divided into five parts. The two chapters of Part One constitute an Introduction. Part Two
(Chapters 3 through 10) covers Strategic Form Games: Theory and Practice, while Part Three (Chapters 11
through 18) concentrates on Extensive Form Games: Theory and Practice. In Part Four (Chapters 19
through 24) I discuss Asymmetric Information Games: Theory and Practice. Finally, Part Five (Chapters 25
through 28) consists of chapters on Foundations.
I can suggest two possible syllabi for a one-semester course in game theory and applications. The first
stresses the applications end while the second covers all the theoretical topics. In terms of mathematical
requirements, the second is, naturally, more demanding and presumes that the students are at a higher level.
I have consequently included twenty chapters in the second syllabus and only eighteen in the first. (Note
that the numbers are chapter numbers.)
Syllabus 1 (Applications Emphasis)
1. A First Look at the Applications
3. Strategic Form Games and Dominant Strategies
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Page XXIV
4. Dominance Solvability
5. Nash Equilibrium
6. An Application: Cournot Duopoly
8. Mixed Strategies
9.Two Applications: Natural Monopoly and Bankruptcy Law
11. Extensive Form Games and Backward Induction
12. An Application: Research and Development
13. Subgame Perfect Equilibrium
15. Infinitely Repeated Games
16. An Application: Competition and Collusion in the NASDAQ Stock Market
17. An Application: OPEC
19. Moral Hazard and Incentives Theory
20. Games with Incomplete Information
22. Mechanism Design, the Revelation Principle, and Sales to an Unknown Buyer
23. An Application: Auctions
24.Signaling Games and the Lemons Problem
Syllabus 2 (Theory Emphasis)
2. A First Look at the Theory
27. Utility and Expected Utility
3. Strategic Form Games and Dominant Strategies
4. Dominance Solvability
5. Nash Equilibrium
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Page XXV
6. An Application: Cournot Duopoly
7. An Application: The Commons Problem
8. Mixed Strategies
10. Zero-Sum Games
28. Existence of Nash Equilibria
11. Extensive Form Games and Backward Induction
13. Subgame Perfect Equilibrium
14. Finitely Repeated Games
15. Infinitely Repeated Games
17. An Application: OPEC
18. Dynamic Games with an Application to the Commons Problem
20. Games with Incomplete Information
21. An Application: Incomplete Information in a Cournot Duopoly
22. Mechanism Design, the Revelation Principle, and Sales to an Unknown Buyer
23. An Application: Auctions
Prerequisites
I have tried to write the book in a manner such that very little is presumed of a reader's mathematics or
economics background. This is not to say that one semester each of calculus and statistics and a semester of
intermediate microeconomics will not help. However, students who do not already have this background but
are willing to put in extra work should be able to educate themselves sufficiently.
Toward that end, I have included a chapter on calculus and optimization, and one on probability and
expectation. Readers can afford not to read the two chapters if they already have the following knowledge.
In calculus, I presume knowledge of the slope of a function and a familiarity with slopes of the linear,
quadratic, log, and the square-root functions. In optimization theory, I use the first-order characterization of
an interior
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Page XXVI
optimum, that the slope of a maximand is zero at a maximum. As for probability, it helps to know how to
take an expectation. As for economic knowledge, I have attempted to explain all relevant terms and have
not presumed, for example, any knowledge of Pareto optimality, perfect competition, and monopoly.
Acknowledgments
This book has benefited from the comments and criticisms of many colleagues and friends. Tom Gresik at
Penn State, Giorgidi Giorgio at La Sapienza in Rome, Sanjeev Goyal at Erasmus, Matt Kahn at Columbia,
Amanda Bayer at Swarthmore, Rob Porter at Northwestern, and Charles Wilson at NYU were foolhardy
enough to have taught from preliminary versions of the text, and I thank them for their courage and
comments. In addition, the following reviewers provided very helpful comments:
Amanda Bayer, Swarthmore College
James Dearden, Lehigh University
Tom Gresik, Penn State
Ehud Kalai, Northwestern University
David Levine, UCLA
Michael Meurer, SUNY Buffalo
Yaw Nyarko, NYU
Robert Rosenthal, Boston University
Roberto Serrano, Brown University
Rangarajan Sundaram, NYU
A second group of ten referees provided extremely useful, but anonymous, comments.
My graduate students Satyajit Bose, Tack-Seung Jun, and Tsz-Cheong Lai very carefully read the entire
manuscript. Without their hawk-eyed intervention, the book would have many more errors. They are also
responsible for the Solutions Manual, which accompanies this text. My colleagues in the community, Venky
Bala, Terri Devine, Ananth
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Page XXVII
Madhavan, Mukul Majumdar, Alon Orlitsky, Roy Radner, John Rust, Paulo Siconolfi, and Raghu Sundaram,
provided support, sometimes simply by questioning my sanity in undertaking this project. My brother, Prajjal
Dutta, often provided a noneconomist's reality check. Finally, I cannot sufficiently thank my wife, Susan
Sobelewski, who provided critical intellectual and emotional support during the writing of this book.
page_xxvii
Page XXIX
A READER'S GUIDE
Game theory studies strategic situations. Suppose that you are a contestant on the quiz show "Jeopardy!" At
the end of the half hour contest (during Final Jeopardy) you have to make a wager on being able to answer
correctly a final question (that you have not yet been asked). If you answer correctly, your wager will be
added to your winnings up to that point; otherwise, the wager will be subtracted from your total. The two
other contestants also make wagers and their final totals are computed in an identical fashion. The catch is
that there will be only one winner: the contestant with the maximum amount at the very end will take home
his or her winnings while the other two will get (essentially) nothing.
Question: How much should you wager? The easy part of the answer is that the more confident you are in
your knowledge, the more you should bet. The difficult part is, how much is enough to beat out your rivals?
That clearly depends on how much they wager, that is, what their strategies are. It also depends on how
knowledgeable you think they are (after all, like you, they will bet more if they are more knowledgeable, and
they are also more likely to add to their total in that case). The right wager may also depend on how much
money you have already wonand how much they have won.
For instance, suppose you currently have $10,000 and they have $7,500 each. Then a $5,001 wagerand a
correct answerguarantees you victory. But that wager also guarantees you a lossif you answer
incorrectlyagainst an opponent who wagers only $2,500. You could have bet nothing and guaranteed victory
against the $2,500 opponent (since the rules of "Jeopardy!" allow all contestants to keep their winnings in
the event of a tie). Of course, the zero bet would have been out of luck against an opponent who bet
everything and answered correctly. And then there is a third possibility for you: betting everything . . .
As you can see the problem appears to be quite complicated. (And keep in mind that I did not even mention
additional relevant factors: estimates that you have about answering correctly or about the other contestants
answering correctly, that the others may have less than $5,000, that you may have more than $15,000, and
so forth.) However, game theory has the answer to this seemingly complicated problem! (And you will read
about it in Chapter 20.) The theory provides us with a systematic way to analyze questions such as: What are
the options available for each contestant? What are the consequences of various choices? How can we
model a contestant's estimate of the others' knowledge? What is a rational wager for a contestant?
In Chapter I you will encounter a variety of other examplesfrom real life, from economics, from politics,
from law, and from businesswhere game theory gives us the tools and the techniques to analyze the strategic
issues.
In terms of prerequisites for this book, I have attempted to write a self-contained text. If you have taken one
semester each of calculus, statistics, and intermediate microeconomics, you will find life easier. If you do not
have the mathematics background,
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it is essential that you acquire it. You should start with the two chapters in Part Five, one on calculus and
optimization, the other on probability and expectation. Read them carefully and do as many of the exercises
as possible. If the chapter on utility theory, also in Part Five, is not going to be covered in class, you should
read that carefully as well. As for economic knowledge, if you have not taken an intermediate
microeconomics class, it would help for you to pick up one of the many textbooks for that course and read
the chapters on perfect competition and monopoly.
I have tried to write each chapterand each part of the bookin a way that the level of difficulty rises as you
read through it. This approach facilitates jumping from topic to topic. If you are reading this book on your
ownand not as part of a classthen a good way to proceed is to read the foundational chapters (25 through 27)
first and then to read sequentially through each part. At a first reading you may wish to skip the last two
chapters within each part, which present more difficult material. Likewise you may wish to skip the last
conceptual section or so within each chapter (but don't skip the case studies!). Sections that are more
difficult are marked with the symbol ; you may wish to skip those sections as well at first reading (or to
read them at a more deliberate pace).
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PART ONE
INTRODUCTION
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Chapter 1
A First Look At The Applications
This chapter is organized in three sections. Section 1.1 will introduce you to some applications of game
theory while section 1.2 will provide a background to its history and principal subject matter. Finally, in
section 1.3, we will discuss in detail three specific games.
1.1 Games That We Play
If game theory were a company, its corporate slogan would be No man is an island. This is because the
focus of game theory is interdependence, situations in which an entire group of people is affected by the
choices made by every individual within that group. In such an interlinked situation, the interesting questions
include
What will each individual guess about the others' choices?
What action will each person take? (This question is especially intriguing when the best action depends on
what the others do.)
What is the outcome of these actions? Is this outcome good for the group as a whole? Does it make any
difference if the group interacts more than once?
How do the answers change if each individual is unsure about the characteristics of others in the group?
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The content of game theory is a study of these and related questions. A more formal definition of game
theory follows; but consider first some examples of interdependence drawn from economics, politics,
finance, law, and even our daily lives.
Art auctions (such as the ones at Christie's or Sotheby's where works of art from Braque to Veronese are
sold) and Treasury auctions (at which the United States Treasury Department sells U.S. government bonds
to finance federal budget expenditures): Chapters 3, 14, and 23, respectively
Voting at the United Nations (for instance, to select a new Secretary General for the organization): Chapter
4
Animal conflicts (over a prized breeding ground, scarce fertile females of the species, etc.): Chapter 5
Sustainable use of natural resources (the pattern of extraction of an exhaustible resource such as oil or a
renewable resource such as forestry): Chapters 7 and 18
Random drug testing at sports meets and the workplace (the practice of selecting a few athletes or workers
to take a test that identifies the use of banned substances): Chapter 8
Bankruptcy law (which specifies when and how much creditors can collect from a company that has gone
bankrupt): Chapter 9
Poison pill provisions (that give management certain latitude in fending off unwelcome suitors looking to
take over or merge with their company): Chapter 11
R&D expenditures (for example, by pharmaceutical firms): Chapter 12
Trench warfare in World War I (when armies faced each other for months on end, dug into rival
trench-lines on the borders between Germany and France): Chapter 13
OPEC (the oil cartel that controls half of the world's oil production and, hence, has an important say in
determining the price that you pay at the pump): Chapter 17
A group project (such as preparing a case study for your game theory class)
Game theory
A formal way to analyze interaction among a group of rational agents who
behave strategically.
Game theory is a formal way to consider each of the following items:
group In any game there is more than one decision-maker; each decision-maker is referred to as a
"player."
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interaction What any one individual player does directly affects at least one other player in the
group.
strategic An individual player accounts for this interdependence in deciding what action to take.
rational While accounting for this interdependence, each player chooses her best action.
Let me now illustrate these four conceptsgroup, interaction, strategic, and rationalby discussing in detail
some of the examples given above.
Examples from Everyday Life
Working on a group project, a case study for the game theory class: The group comprises the students jointly
working on the case. Their interaction arises from the fact that a certain amount of work needs to get done
in order to write a paper; hence, if one student slacks off, somebody else has to put in extra hours the night
before the paper is due. Strategic play involves estimating the likelihood of freeloaders in the group, and
rational play requires a careful comparison of the benefits to a better grade against the costs of the extra
work.
Random drug testing (at the Olympics): The group is made up of competitive athletes and the International
Olympic Committee (IOC). The interaction is both between the athleteswho make decisions on training
regimens as well as on whether or not to use drugsand with the IOC, which needs to preserve the reputation
of the sport. Rational strategic play requires the athletes to make decisions based on their chances of
winning and, if they dope, their chances of getting caught. Similarly, it requires the IOC to determine drug
testing procedures and punishments on the basis of testing costs and the value of a clean-whistle reputation.
Examples from Economics and Finance
R&D efforts by pharmaceutical companies: Some estimates suggest that research and development (R&D)
expenditures constitute as much as 20% of annual sales of U.S. pharmaceutical companies and that, on
average, the development cost of a new drug is about $350 million dollars. Companies are naturally
concerned about issues such as which product lines to invest research dollars in, how high to price a new
drug, how to reduce the risk associated with a new drug's development, and the like. In this example, the
group is the set of drug companies. The interaction arises because the first developer of a drug makes the
most profits (thanks to the associated patent). R&D expenditures are strategic and rational if they are
chosen to maximize the profits from developing a new drug, given inferences about the competition's
commitment to this line of drugs.
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Treasury auctions: On a regular basis, the United States Treasury auctions off U.S. government securities.1
The principal bidders are investment banks such as Lehman Brothers or Merrill Lynch (who in turn sell the
securities off to their clients). The group is therefore the set of investment banks. (The bidders, in fact, rarely
change from auction to auction.) They interact because the other bids determine whether a bidder is
allocated any securities and possibly also the price that the bidder pays. Bidding is rational and strategic if
bids are based on the likely competition and achieve the right balance between paying too much and the risk
of not getting any securities.
Examples from Biology and Law
Animal behavior: One of the more fascinating applications of game theory in the last twenty-five years has
been to biology and, in particular, to the analyses of animal conflicts and competition. Animals in the wild
typically have to compete for scarce resources (such as fertile females or the carcasses of dead animals); it
pays, therefore, to discover such a resourceor to snatch it away from the discoverer. The problem is that
doing so can lead to a costly fight. Here the group of "players" is all the animals that have an eye on the
same prize(s). They interact because resources are limited. Their choices are strategic if they account for
the behavior of competitors, and are rational if they satisfy short-term goals such as satisfying hunger or
long-term goals such as the perpetuation of the species.
Bankruptcy law: In the United States once a company declares bankruptcy its assets can no longer be
attached by individual creditors but instead are held in safekeeping until such time as the company and its
creditors reach some understanding. However, creditors can move the courts to collect payments before the
bankruptcy declaration (although by doing so a creditor may force the company into bankruptcy). Here the
interaction among the group of creditors arises from the fact that any money that an individual creditor can
successfully seize is money that becomes unavailable to everyone else. Strategic play requires an estimation
of how patient other creditors are going to be and a rational choice involves a trade-off between collecting
early and forcing an unnecessary bankruptcy.
At this point, you may well ask what, then, is not a game? A situation can fail to be a game in either of two
casesthe one or the infinity case. By the one case, I mean contexts where your decisions affect no one but
yourself. Examples include your choice about whether or not to go jogging, how many movies to see this
week, and where to eat dinner. By the infinity case, I mean situations where your decisions do affect others,
but there are so many people involved that it is neither feasible nor sensible to keep track of what each one
does. For example, if you were to buy some stock in AT&T it is best to imagine that your purchase has left
the large body of shareholders in AT&T entirely unaffected. Likewise, if you are the owner of Columbia
Bagels in New York City, your decision on the price of onion bagels is unlikely to affect the citywidenot to
speak of the nationwideonion bagel price.
1These securities are Treasury Bonds and Bills, financial instruments that are held by the public
(or its representatives, such as mutual funds or pension funds). These securities promise to pay a
sum of money after a fixed period of time, say three months, a year, or five years. Additionally,
they may also promise to pay a fixed sum of money periodically over the lifetime of the security.
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Although many situations can be formalized as a game, this book will not provide you with a menu of
answers. It will introduce you to the methodology of games and illustrate that methodology with a variety of
examples. However, when faced with a particular strategic setting, you will have to incorporate its unique
(informational and other) features in order to come up with the right answer. What this book will teach you
is a systematic way to incorporate those features and it will give you a coherent way to analyze the
consequent game. Everyone of us acts strategically, whether we know it or not. This book is designed to help
you become a better strategist.
1.2 Background
The earliest predecessors of game theory are economic analyses of imperfectly competitive markets. The
pioneering analyses were those of the French economist Augustin Cournot (in the year 1838)2 and the
English economist Francis Edgeworth (1881)3 (with subsequent advances due to Bertrand and Stackelberg).
Cournot analyzed an oligopoly problemwhich now goes by the name of the Cournot modeland employed a
method of analysis which is a special case of the most widely used solution concept found in modern game
theory. We will study the Cournot model in some detail in Chapter 6.
An early breakthrough in more modern times was the study of the game of chess by E. Zermelo in 1913.
Zermelo showed that the game of chess always has a solution, in the sense that from any position on the
board one of the two players has a winning strategy.4 More importantly, he pioneered a technique for
solving a certain class of games that is today called backwards induction. We will study this procedure in
detail in Chapters 11 and 12.
The seminal works in modern times is a paper by John von Neumann that was published in 1928 and, more
importantly, the subsequent book by him and Oskar Morgenstern titled Theory of Games & Economic
Behavior (1944). Von Neumann was a multi-faceted man who made seminal contributions to a number of
subjects including computer science, statistics, abstract topology, and linear programming. His 1928 paper
resolved a long-standing puzzle in game theory.5 Von Neumann got interested in economic problems in part
because of the economist Oskar Morgenstern. Their collaboration dates to 1938 when Morgenstern came to
Princeton University, where Von Neumann had been a professor at the Institute of Advanced Study since
1933. Von Neumann and Morgenstern started by working on a paper about the connection between
economics and game theory and ended with the crown jewelthe Theory of Games & Economic Behavior.
In their book Yon Neumann and Morgenstern made three major contributions, in addition to formalizing the
concept of a game. First, they gave an axiom-based foundation to utility theory, a theory that explains just
what it is that players get from playing a game. (We will discuss this work in Chapter 27.) Second, they
thoroughly characterized the optimal solutions to what are called zero-sum games, two-player games in
which
2See Cournot's Researches Into the Mathematical Principles of the Theory of Wealth (especially
Chapter 7).
3See Mathematical Psychics: An Essay on the Application of Mathematics to the Moral Sciences.
4That, of course, is not the same thing as saying that the player can easily figure what this winning
strategy is!(It is also possible that neither player has a winning strategy but rather that the game will
end in a stalemate.)
5The puzzle was whether or not a class of games called zerosum gameswhich are defined in the next
paragraphalways have a solution. A famous French mathematician, Emile Borel, had conjectured in
1913 that they need not; Von Neumann proved that they must always have a solution.
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one player wins if and only if the other loses. Third, they introduced a version of game theory called
cooperative games. Although neither of these constructions are used very much in modem game theory,
they both played an important role in the development of game theory that followed the publication of their
book.6
The next great advance is due to John Nash who, in 1950, introduced the equilibrium (or solution) concept
which is the one most widely used in modern game theory. This solution conceptcalled, of course, Nash
equilibriumhas been extremely influential; in this book we will meet it for the first time in Chapter 5. Nash's
approach advanced game theory from zero-sum to nonzero-sum games (i.e., situations in which both players
could win or lose). As mentioned above, Nash's solution concept built on the earlier work of Cournot on
oligopolistic markets.7 For all this he was awarded the Nobel Prize for Economics in 1994.
Which brings us to John Harsanyi and Reinhard Selten who shared the Nobel Prize with John Nash. In two
papers dating back to 1965 and 1975, Reinhard Selten generalized the idea of Nash equilibrium to dynamic
games, settings where play unfolds sequentially through time.8 In such contexts it is extremely important to
consider the future consequences of one's present actions. Of course there can be many possible future
consequences and Selten offered a methodology to select among them a ''reasonable" forecast for future
play. We will study Selten's fundamental idea in Chapter 13 and its applications in Chapters 14 through 18.9
In 1967-1968, Harsanyi generalized Nash's ideas to settings in which players have incomplete information
about each others' choices or preferences. Since many economic problems are in fact characterized by such
incompleteness of information, Harsanyi's generalization was an important step to take. Incomplete
information games will be discussed in Chapter 20 and their applications can be found in Chapters 21
through 24.10
At this point you might be wondering why this subjectwhich promises to study such weighty matters as the
arms race, oligopoly markets, and natural resource usagegoes by the name of something quite as fun-loving
as game theory. Part of the reason for this is historical: Game theory is called game theory because parlor
gamespoker, bridge, chess, backgammon, and so onwere a convenient starting point to think about the
deeper conceptual issues regarding interaction, strategy, and rationality, which form the core of the subject.
Even as the terminology is not meant to suggest that the issues addressed are light or trivial in any way, it is
also hoped that the terminology will turn out to be somewhat appropriate and that you will have fun learning
the subject.11
1.3 Examples
To fix ideas, let us now work though three games in some detail.
1. Nim and Marienbad.
These are two parlor games that work as follows. There are two piles of matches and two players. The game
starts with player 1 and thereafter the
6In this book we will study zerosum games in some detail in Chapter 10. We will not, however, look
at cooperative game theory.
7John Nash wrote four papers on game theory, two on Nash equilibrium and two more on bargaining
theory (and he co-authored three others). Each of the four papers has greatly influenced the further
development of the discipline. (If you wish, perhaps at a later point in the course, to read the paper on
Nash equilibrium, look for "Equilibrium Points of N-person Games, 1950, Proceedings of the National
Academy of Sciences.) Unfortunately, health problems cut short what would have been a longer and
even more spectacular research career.
8The Selten papers are "Spieltheoretische Behandlung eines Oligopolmodells mit Nachfrage-tragheit"
(1965), Zietschrift für die gesamte Statswissenschaft, and Reexamination of the Perfectness Concept
for Equilibrium Points in Extensive Games (1975), International Journal of Game Theory.
9Many interesting applications of game theory have a sequential, or dynamic, character to them. Put
differently, there are few game situations where you are sure that you are never going to encounter
any of the other players ever again; as the good game theorist James Bond would say, "Never say
never again." We will discuss, in Chapters 15 and 16, games where you think (there is some chance)
that you will encounter the same players again, and in an identical context. In Chapters 17 and 18, we
will discuss games where you think you will encounter the same players again but possibly in a
differerent context.
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players take turns. When it is a player's turn, he can remove any number of matches from either pile. Each
player is required to remove some number of matches if either pile has matches remaining, and he can only
remove matches from one pile at a time.
In Nim, whichever player removes the last match wins the game. In Marienbad, the player who removes the
last match loses the game.
The interesting question for either of these games is whether or not there is a winning strategy, that is, is
there a strategy such that if you used it whenever it is your turn to move, you can guarantee that you will
win regardless of how play unfolds from that point on?
Analysis of Nim.
Call the two piles balanced if there is an equal number of matches in each pile; and call them unbalanced
otherwise. It turns out that if the piles are balanced, player 2 has a winning strategy. Conversely, if the piles
are unbalanced, player 1 has a winning strategy.
Let us consider the case where there is exactly one match in each pile; denote this (1,1). It is easy to see that
player 2 wins this game. It is not difficult either to see that player 2 also wins if we start with (2,2). For
example, if player 1 removes two matches from the first pile, thus moving the game to (0,2), then all player 2
has to do is remove the remaining two matches. On the other hand, if player I removes only one match and
moves the game, say, to (1,2), then player 2 can counter that by removing a match from the other pile. At
that point the game will be at (1,1) and now we know player 2 is going to win.
More generally, suppose that we start with n matches in each pile, n > 2. Notice that player I will never want
to remove the last match from either pile, that is, he would want to make sure that both piles have matches
in them.12 However, in that case, player 2 can ensure that after every one of his plays, there is an equal
number of matches in each pile. (How?)13 This means that sooner or later there will ultimately be one match
in each pile.
If we start with unbalanced piles, player I can balance the piles on his first play. Hence, by the above logic,
he has a winning strategy. The reason for that is clear: once the piles are balanced, it is as if we are starting
afresh with balanced piles but with player 2 going first. However, we know that the first to play loses when
the piles are balanced.
CONCEPT CHECK
Are there any other winning strategies in this game? What do you think might
happen if there are more than two piles? Do all such games, in which players
take turns making plays, have winning strategies? (Think of tic-tac-toe.)14
Similar logic can be applied to the analysis of Nim's cousin, Marienbad. Remember, though, in working
through the claims below that in Marienbad the last player to remove matches loses the game.
10The original I967-1968 Harsanyi papers are "Games with Incomplete Information Played by
Bayesian Players," Management Science. Do notas David Letterman would say after a Stupid
Human Tricks segmenttry them at home, just yet!
11There are several books that I hope you will graduate to once you are finished reading this one. Two
that I have found very useful for their theoretical treatments are Game Theory by Drew Fudenberg
and Jean Tirole (MIT Press) and An Introduction to Game Theory by Martin Osborne and Ariel
Rubinstein (MIT Press). If you want a more advanced treatment of any topic in this book, you could do
worse than pick up either of these two texts. A hook that is more applications oriented is Thinking
Strategically by Barry Nalebuff and Avinash Dixit (W. W. Norton).
12Else, player 2 can force a win by removing all the matches from the pile which has matches
remaining.
13Think of what happens if player 2 simply mimics everything that player 1 does, except with the other
pile.
14These three questions have been broken down into further bite-sized pieces in the Exercises section.
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CONCEPT CHECK
ANALYSIS OF MARIENBAD
We claim that: If the two piles are balanced with one match in each pile,
player 1 has a winning strategy. On the other hand, if the two piles are
balanced, with at least two matches in each pile, player 2 has a winning
strategy. Finally, if the two piles are unbalanced, player I has a winning
strategy. Try proving these claims.15
Note, incidentally, that in both of these games the first player to move (referred to in my discussion as player
1) has an advantage if the piles are unbalanced, but not otherwise.
2. Voting.
This example is an idealized version of committee voting. It is meant to illustrate the advantages of strategic
voting, in other words, a manner of voting in which a voter thinks through what the other voters are likely to
do rather than voting simply according to his preferences.16
Suppose that there are two competing bills, designated here as A and B, and three legislators, voters 1, 2 and
3, who vote on the passage of these bills. Either of two outcomes are possible: either A or B gets passed, or
the legislators choose to pass neither bill (and stay with the status quo law instead). The voting proceeds as
follows: first, bill A is pitted against bill B; the winner of that contest is then pitted against the status quo
which, for simplicity, we will call "neither"(or N). In each of the two rounds of voting, the bill that the
majority of voters cast their vote for, wins. The three legislators have the following preferences among the
available options.
voter 1:
voter 2:
voter 3:
(where
should be read as, "Bill A is preferred to bill B.")
Analysis.
Note that if the voters voted according to their preferences (i.e., truthfully) then A would win against B and
then, in round two, would also win against N. However, voter 3 would be very unhappy with this state of
affairs; she most prefers N and can in fact enforce that outcome by simply switching her first round vote to
B, which would then lose to N. Is that the outcome? Well, since we got started we might wish to then note
that, acknowledging this possibility, voter 2 can also switch her vote and get A elected (which is preferable
to N for this voter).
There is a way to proceed more systematically with the strategic analysis. To begin with, notice that in the
second round each voter might as well vote truthfully. This is because by voting for a less preferred option, a
legislator might get that passed. That would be clearly worse than blocking its passage. Therefore, if A wins
in the first round, the eventual outcome will be A, whereas if B wins, the eventual outcome will be N. Every
15Again you may prefer to work step by step through these questions in the Exercises section.
16This example may also be found in Fun and Games by Ken Binmore (D.C. Heath).
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rational legislator realizes this. So, in voting between A and B in the first round, they are actually voting
between A and N. Hence, voters I and 2 will vote for A in the first round and A will get elected.
CONCEPT CHECK
TRUTHFUL VOTING
In what way is the analysis of strategic voting different from that of truthful
voting? Is the conclusion different? Are the votes different?
3. Prisoners' Dilemma.
This is the granddaddy of simple games. It was first analyzed in 1953 at the Rand Corporationa fertile
ground for much of the early work in game theoryby Melvin Dresher and A1 Tucker.
The story underlying the Prisoners' Dilemma goes as follows. Two prisoners, Calvin and Klein, are hauled in
for a suspected crime. The DA speaks to each prisoner separately, and tells them that she more or less has
the evidence to convict them but they could make her work a little easier (and help themselves) if they
confess to the crime. She offers each of them the following deal: Confess to the crime, turn a witness for the
State, and implicate the other guyyou will do no time. Of course, your confession will be worth a lot less if
the other guy confesses as well. In that case, you both go in for five years. If you do not confess, however,
be aware that we will nail you with the other guy's confession, and then you will do fifteen years. In the
event that I cannot get a confession from either of you, I have enough evidence to put you both away for a
year."
Here is a representation of this situation:
Confess
Not Confess
Calvin \ Klein
Confess
5, 5
0, 15
Not Confess
15, 0
1, 1
Notice that the entries in the above table are the prison terms. Thus, the entry that corresponds to (Confess,
Not Confess)the entry in the first row, second columnis the length of sentence to Calvin (0) and Klein (15),
respectively, when Calvin confesses but Klein does not. Note that since these are prison terms, a smaller
number (of years) is preferred to a bigger number.
Analysis.
From the pair's point of view, the best outcome is (Not Confess, Not Confess). The problem is that if Calvin
thinks that Klein is not going to confess, he can walk free by ratting on Klein. Indeed, even if he thinks that
Klein is going to confessthe ratCalvin had better confess to save his skin. Surely the same logic runs through
Klein's mind. Consequently, they both end up confessing.
Two remarks on the Prisoners' Dilemma are worth making. First, this game is not zero-sum. There are
outcomes in which both players can gain, such as (Not Confess, Not
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Confess). Second, this game has been used in many applications. Here are two: (a) Two countries are in an
arms race. They would both rather spend little money on arms buildup (and more on education), but realize
that if they outspend the other country they will have a tactical superiority. If they spend the same (large)
amount, though, they will be deadlockedmuch the same way that they would be deadlocked if they both
spent the same, but smaller, amount. (b) Two parties to a dispute (a divorce, labor settlement, etc.) each
have the option of either bringing in a lawyer or not. If they settle (50-50) without lawyers, none of their
money goes to lawyers. If, however, only one party hires a lawyer, then that party gets better counsel and
can get more than 50% of the joint property (sufficiently more to compensate for the lawyer's fees). If they
both hire lawyers, they are back to equal shares, but now equal shares of a smaller estate.
Summary
1. Game theory is a study of interdependence. It studies interaction among a group of players who make
rational choices based on a strategic analysis of what others in the group might do.
2. Game theory can be used to study problems as widely varying as the use of natural resources, the election
of a United Nations Secretary General, animal behavior, and production strategies of OPEC.
3. The foundations of game theory go back 150 years. The main development of the subject is more recent,
however, spanning approximately the last fifty years, making game theory one of the youngest disciplines
within economics and mathematics.
4. Strategic analysis of games such as Nim and the Prisoners' Dilemma can expose the outcomes that will be
reached by rational players. These outcomes are not always desirable for the whole group of players.
Exercises
Section 1.1
1.1
Give three examples of game-like situations from your everyday life. Be sure in each
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case to identify the players, the nature of the interaction, the strategies available, and the objectives that
each player is trying to achieve.
1.2
Give three examples of economic problems that are not games. Explain why they are not.
1.3
Now give three examples of economic problems that are games. Explain why these situations qualify as
games.
1.4
Consider the purchase of a house. By carefully examining each of the four components of a game
situationgroup, interaction, rationality, and strategydiscuss whether this qualifies as a game.
1.5
Repeat the last question for a trial by jury. Be sure to outline carefully what each player's objectives might
be.
Consider the following scenario: The market for bagels in the Morningside Heights neighborhood of New
York City. In this example, the dramatis personae are the two bagel stores in the Columbia University
neighborhood, Columbia Bagels (CB) and University Food Market (UFM); and the interaction among them
arises from the fact that Columbia Bagels' sales depend on the price posted by University Food Market.
1.6
By considering a few sample prices, say, 40, 45, and 50 centsand likely bagel sales at these pricescan you
quantify how CB's sales revenue might depend on UFM's price? And vice versa?
1.7
For your numbers what would be a rational strategic price for CB if, say, UFM's bagels were priced at 45
cents? What if UFM raised its price to 50 cents?
Consider yet another scenario: Presidential primaries. The principal group of players are the candidates
themselves. Only one of them is going to win his party's nomination; hence, the interaction among them.
1.8
What are the strategic choices available to a candidate? (Hint: Think of political issues that a candidate can
highlight, how much time he can spend in any given state, etc.)
1.9
What is the objective against which we can measure the rationality of a candidate's choice? Should the
objective only be the likelihood of winning?17
17Bear in mind the hope once articulated by a young politician from Massachusetts, John E
Kennedy, that his margin of victory would be narrow; Kennedy explained that his father "hated to
overspend!
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Section 1.3
1.10
Show in detail that player 2 has a winning strategy in Nim if the two piles of matches are balanced. [Your
answer should follow the formalism introduced in the text; in particular, every configuration of matches
should be written as (m, n) and removing matches should be represented as a reduction in either m or n.]
1.11
Show that player 2 has exactly one winning strategy. In other words, show that if the winning strategy of
question 1.10 is not followed, then player I can at some point in the game turn the tables on player 2.
1.12
Verbally analyze the game of tic-tac-toe. Show that there is not a winning strategy in this game.
The next four questions have to do with a three pile version of Nim. The rules of the game are identical to
the case when there are two piles. In particular, each player can only choose from a single pile at a time and
can remove any number of the matches remaining in a pile. The last player to remove matches wins.
1.13
Show that if the piles have an equal number of matches, then player 1 has a winning strategy. [You may
wish to try out the configurations (1, 1,1) and (2, 2, 2) to get a feeling for this argument.]
1.14
Show that the same result is true if two of the piles have an equal number of matches; that is, show that
player I has a winning strategy in this case. [This time you might first try out the configurations (1, 1, p) and
(2, 2, p) where p is a number different from 1 and 2, respectively.]
1.15
Show that if the initial configuration of matches is (3, 2, 1)or any permutation of that configurationthen
player 2 has a winning strategy. As in the previous questions, carefully demonstrate what this winning
strategy is.
1.16
Use your answer in the previous questions to show that if the initial configuration is (3, 2, p)or (3,1, p) or (1,
2, p)where p is any number greater than 3, then player I has a winning strategy.
The next three questions have to do with the game of Marienbad played by two players.
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1.17
Show that if the configuration is (1, 1) then player 1 has a winning strategy.
1.18
On the other hand, if the two piles are balanced, with at least two matches in each pile, player 2 has a
winning strategy. Prove in detail that this must be the case.
1.19
Finally, show that if the two piles are unbalanced, player I has a winning strategy.
1.20
Consider the voting model of the second example of section 1.3 (pg. 10). Prove that in the second round,
each voter can do no better than vote truthfully according to her preferences.
1.21
Suppose voter 3's preferences were
(instead of
as in the text). What would be the
outcome of truthful voting in this case? What about strategic voting?
1.22
Write down a payoff matrix that corresponds to the legal scenario discussed at the end of the chapter (p. 12).
Give two alternative specifications of payoffs, the first in which this does correspond to a Prisoners'
Dilemma and the second in which it does not.
Suppose the Prisoners' Dilemma were modified by allowing a third choice for each playerPartly Confess.
Suppose further that the prison sentences (in years) in this modified game are as follows.
Calvin \ Klein
Confess
Not
Partly
Confess
2, 2
0, 5
1, 3
Not
5, 0
,
4,
Partly
3,1
,4
1,1
(As always, keep in mind that shorter prison terms are preferred by each player to longer prison terms.)
1.23
Is it true that Calvin is better off confessing to the crime no matter what Klein does? Explain.
1.24
Is there any other outcome in this gameother than both players confessingwhich is sensible? Your answer
should informally explain why you find any other outcome sensible (if you do).
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Chapter 2
A First Look At The Theory
This chapter will provide an introduction to game theory's toolkit; the formal structures within which we can
study strategic interdependence. Section 2.1 gives some necessary background. Sections 2.2 and 2.3 detail
the two principal ways in which a game can be written, the Extensive Form and the Strategic Form of a
game. Section 2.4 contains a discussion of utilityor payofffunctions, and Section 2.5 concludes with a revisit
to some of the examples discussed in the previous chapter.
2.1 Rules Of The Game: Background
Every game is played by a set of rules which have to specify four things.
1. who is playingthe group of players that strategically interacts
2. what they are playing withthe alternative actions or choices, the strategies, that each player has available
3. when each player gets to play (in what order)
4. how much they stand to gain (or lose) from the choices made in the game
In each of the examples discussed in Chapter 1, these four components were described verbally. A verbal
description can be very imprecise and tedious and so it is desirable to find a more compact description of the
rules. The two principal representations of (the rules off a game are called, respectively, the normal (or
strategic) form of a game and the extensive form; these terms will be discussed later in this chapter.
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Common knowledge about the rules
Every player knows the rules of a game and that fact is commonly known.
There is, however, a preliminary question to ask before we get to the rules: what is the rule about knowing
the rules? Put differently, how much are the players in a game supposed to know about the rules? In game
theory it is standard to assume common knowledge about the rules.
That everybody has knowledge about the rules means that if you asked any two players in the game a
question about who, what, when, or how much, they would give you the same answer. This does not mean
that all players are equally well informed or equally influential; it simply means that they know the same
rules. To understand this better, think of the rules of the game as being like a constitution (of a country or a
clubor, for that matter, a country club.) The constitution spells out the rules for admitting new members,
electing a President, acquiring new property, and so forth. Every member of this club is supposed to have a
copy of the constitution; in that sense they all have knowledge of the rules. This does not mean that they all
get to make the same choices or that they all have the same information when they make their choices. For
instance, perhaps it is only the Executive Committee members who decide whether the club should build a
new tennis court. In making this decision, they may furthermore have access to reports about the financial
health of the club that are not made available to all members. The point is that both of these rulesthe
Executive Committee's decision-making power and access to confidential reportsare in the club's
constitution and hence are known to everyone.
This established, the next question is: does everyone know that everyone knows? Common knowledge of the
rules goes even a few steps further: first, it says yes, everybody knows that the constitution is available to all.
Second, it says that everybody knows that everybody knows that the constitution is widely available. And
third, that everybody knows that everybody knows that everybody knows, ad infinitum.1 In a two-player
game, common knowledge of the rules says not only that player I knows the rules, but that she also knows
that player 2 knows the rules, knows that 2 knows that I knows the rules, knows that 2 knows that I knows
that 2 knows the rules, and so on.
In the next two sections we will discuss the two alternative representations of the three rules who, what, and
when. The final rule, how much, will be discussed in section 2.4.
2.2 Who, What, When: The Extensive FOrm
Extensive form
A pictorial representation of the rules. The main pictorial form is called the
game tree, which is made up of a root and branches arranged in order.
The extensive form is a pictorial representation of the rules. Its main pictorial form is called the game tree.
Much like an ordinary tree, a game tree starts from a root; at this starting point, or root, one of the players
has to make a choice. The various choices available to this player are represented as branches emanating
from the root. For example, in the game tree given by Figure 2.1, below, the root is denoted a; there are
three branches emerging from the root which correspond to the three choices b(us), c(ab), and s(ubway).2
At the end of each one of the branches that emerge from the root, either of two things can happen. The tree
might itself end with that branch; this signifies an end to the
1It may seem completely mysterious to you why we cannot simply stop with the assertion
"everybody knows the rules." The reason is that, knowing the rules, there might be certain
behaviors that a player will normally not undertake. However, if a player is unsure about whether
or not the others know that he knows the rules, he will consequently be unsure about whether the
others realize that he will not undertake those behaviors. This sort of doubt in players' minds can
have a dramaticand unreasonableimpact on what they end up doing, hence the need to assume
every level of knowledge.
2This tree could represent, for example, transportation choices in New York City; a player can either
take the bus, a cab, or the subway to his destination. Note that driving one's own car is not one of the
optionsthese are choices in New York City after all!
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FIGURE 2.1
FIGURE 2.2
game. Alternatively, it might split into further branches. In Figure 2.1, for instance, the tree ends after each
of the three branches b, c, and s. On the other hand, in Figure 2.2 each branch further divides in two. The
branch splits into E(xpress) and L(ocal); the implication is that after the initial choice s is made, the player
gets to choose again between the two options E and L (whether to stay on the Local train or to switch to an
Express). The end of branch s, where the subsequent decision between E and L is made, is called a decision
node of the tree. Figure 2.2 is therefore a two-stage decision problem with a single player.
Of greater interest is a situation where a different player gets to make the second choice. For instance
suppose that two players are on their way to see a Broadway musical that is in great demand, such as Rent.
The demand is so great that there is exactly one ticket left; whoever arrives first will get that ticket. Hence,
we have a game. The first player (player 1) leaves home a little earlier than player 2; in that sense he makes
his choice at the root of the game tree and subsequently the other player makes her transportation choice.
The extensive form of this game is represented in Figure 2.3.
From these building blocks we can draw more complicated game trees, trees that allow more than two
players to interact, allow many choices at each decision node, and allow each player to choose any number
of times. The extensive form answers the question whoany individual who has a decision node in the game
tree is a player in the game. It also answers the question what; the branches that come out of a decision node
represent the different choices available at that point. Finally, it answers the question
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FIGURE 2.3
when; for example, a node that is four branches removed from the root is reached only after these first four
choices have been made.
2.2.1 Information Sets and Strategies
The extensive forms discussed above permit only one player to move at a time; the next question is how to
represent simultaneous moves within the extensive form. The key idea here is that a player will act in the
same way in either of two circumstances; first, if he literally chooses at the same time as his opponent and
second, if he actually chooses after his opponent but is unaware of his opponent's choice. Consequently, a
simultaneous move in the Prisoners' Dilemma can be represented by Figure 2.4. In this figure, the ''first"
choice is player 1's while the "second" choice is that of player 2. Notice that there is an oval that encircles
the two (second-stage) decision nodes of player 2. By collecting the two decision nodes into one oval we are
signifying that player 2 is unable to distinguish between the two nodes, that is, he cannot tell whether the
first decision of player I was c or n.
Information set
A collection of decision nodes that a player cannot distinguish between.
The oval here is called an information set.3
Strategy
A strategy for a player specifies what to do at every information set at which
the player has to make a choice.
Finally, every player needs a strategy to play a game! A strategy is a blueprint for action; for every decision
node it tells the player how to choose. More precisely, since a player cannot distinguish between the nodes
within any one information set, a strategy specifies what to do at each set.
For example, in the theater game above (Figure 2.3), player I has a single decision node, the root. Thus, he
has three possible strategies to choose from: b, c, or s. Player 2 has three decision nodes; what to do if player
I took the bus, what to do if he took a cab instead, and, finally, what choice to make if player I hopped on
the subway. Hence every strategy of player 2 must have three components, one for each of her decision
nodes. A possible strategy for player 2 is (s, s, b); the first entry specifies her choice if player 1 takes the bus
(and this choice is s); the second component specifies player 2's choice if 1 takes a cab (and this choice is
also s); and the third entry is player 2's choice conditional
3Information sets will play an important role in a class of games called games of asymmetric
information that we will study in Chapters 19 through 24. At that point we will discuss, in greater
detail, properties that information sets must satisfy.
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FIGURE 2.4
on having seen player 1 take the subway (and in this strategy that conditional choice is b).
CONCEPT CHECK
HOW MANY STRATEGIES ARE THERE?
Can you show that player 2 has 33, that is, 27 strategies? Can you enumerate
some of
them?
A pair of strategies, one for player 1 and the other for player 2, determines the way in which the game
actually gets played. For example, suppose that player 1 chooses the strategy c while player 2 chooses (s, s,
b). Since the strategy for player 2, conditional on player 1 taking a cab, is to pick s, the pair of strategies
yields as outcome: 1 takes a cab and 2 follows by subway. In any game, a collection of strategies, one for
each player, will determine which branch of the game tree will actually get played out.
2.3 Who, What, When: The Normal (Or Strategic) Form
Strategic form
A complete list of who the players are, what strategies are available to each of
them, and how much each gets.
An alternative way to represent the rules of a game is called the normal or strategic form.
For example, the strategic form of the theater game can be represented in a table in which the three rows
correspond to the strategies of player 1 and the 33 columns correspond to the strategies of player 2. In each
cell of the table we write the how much rule, in other words, the payoffs associated with that pair of
strategies. Since we have yet
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TABLE 2.1
Player 1 \ Player 2
b
c
s
sss
N,T
T,N
T,N
ssb
N,T
T,N
T,N
ssc
N,T
T,N
N,T
bbs
T,N
T,N
T,N
...
ccb
N,T
T,N
T,N
ccs
N,T
T,N
T,N
ccc
N,T
T,N
N,T
to discuss payoff functions, for now we will simply write the outcomerather than the payoffsin each cell.
Suppose that player 1the first person to start for the theatergets the ticket regardless of player 2's mode of
transport as long as he takes a cab. He also gets the ticket if he travels by subway provided that 2 has not
taken a cab, and likewise gets the ticket after catching the bus if 2 catches a bus as well. Writing T for Ticket
and N for Nuttin' the outcomes are presented in Table 2.1. (Note that in each cell the outcome for player 1 is
the first of the listed pair of outcomes.)
Whenever we have a two-player game, we can represent the strategic form as a table. The rows will stand
for the strategies of player 1, the columns for the strategies of player 2, and the entry in a cell for the payoffs
of the two players from the associated pair of strategies.
You might be wondering about the when question: in a strategic form, who moves when? The simplest
context for the strategic form is a one-time simultaneous move game such as the Prisoners' Dilemma. In this
case, each player makes only one choice and, hence, every strategy has a single element. But we can also
study sequential move games in strategic form; strategies then are more complicated, and they answer the
question of who moves when. A useful interpretation of the strategic form in such cases is that the players
choose their strategies simultaneously although the game itself is played sequentially.
For instance, in the theater game, suppose player 1 chooses s while simultaneously player 2 chooses (c, s, c)c
if player 1 picks b, s if he picks c, and c if he travels by s. These strategies are chosen simultaneously in that
neither player knows the opponent's strategy at the time of their choice. However, the actual play of the
game is sequential. By the choice of strategies player 1 leaves first by subway; player 2 observes that and
then follows by cab.
In summary, the extensive and strategic forms are two ways to represent a game.4 For the purpose of clarity,
this text uses the strategic form to study games that are played simultaneously. This is the content of Part II
(Chapters 3 through 10). Conversely, we will employ the extensive form to study sequential game situations;
these will be studied in Part III (Chapters 11 through 18). At the beginning of each part there will be a more
detailed description of the two game forms; Chapter 3 does this for the strategic form while Chapter 11
details the extensive form. Part IV, Chapters 19 through 24, will use both representations.
4Later in this book you will see that the two representations are interchangeable; every extensive
form game can be written in strategic form and, likewise, every game in strategic form can be
represented in extensive form.
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2.4 How Much: Von Neumann-Morgenstern Utility Function
The last rule specifies how much: how much does each player stand to gain or lose by playing the game (in
the way that she does)? Put differently, what is the payoffor utilityfunction of a player, a function that
would specify the payoff to a player for every possible strategy combination that sheand the othersmight
pick? When the outcome of a game is monetary, each player pays out or receives money; the amount of the
winnings is a candidate for the payoff. But what of games in which the outcome is not monetarygames such
as the theater game, the Prisoners' Dilemma, Nim, or the voting game?
To start with, note that a player will typically have opinions about which strategy combinations are
preferable. For instance, in the Prisoners' Dilemma each prisoner is able to rank the four possible strategy
outcomes: Most preferred is the lenient sentence of a canary (who implicates the nonconfessing partner).
Next in preference is the outcome in which neither confesses. Further down is the outcome of both
confessing, and the worst outcome is to be done in by the other guy. This suggests that we could simply
attach numbers that correspond to the rankssay, 4, 3, 2, and 1and call those numbers the payoffs. A higher
payoff would signify a preferred alternative.
This argument can be made more generally. The various outcomes in a game can be thought of as different
options from among which a player has to choose. If the player's preferences between these options satisfy
certain consistency requirements, then she can systematically rank the various outcomes. Any numbering
that corresponds to the rankinga higher number for a higher rankcan then be viewed as a payoff or utility
function.5
In the extensive form these utility numbers would get written at each one of the nodes where the game
terminates. For instance, in the theater game there are two possible outcomes: either player 1 gets the ticket
or player 2 does. Presumably, each player would rather have the ticket than not; hence, any pair of numbers,
p(T) and p(N), with p(T) > p(N), would serve as a payoff function in this game for player 1. (Likewise for
player 2, any two numbers f(T) and f(N), with f(T) > f(N), would serve as a payoff function.) Filling in the
payoffs, the extensive form of this game is depicted in Figure 2.5.
In the strategic form, the payoff numbers would get written in the cells of the strategic matrix. The theater
game's strategic form would therefore look like Table 2.2. (Only some of the cells have been filled in; by
referring to Table 2.1 you should fill in some of the remaining ones.)
Matters are a little more complicated if the game's outcome is not known for sure. This can happen for a
variety of reasons. A player may choose her strategy in a probabilistic fashion by, for example, letting a coin
toss determine which of two possible strategies she will go with. It is also possible that there may be some
inherent uncertainty in the play of the game; for instance, if several firms are competing for the market share
of a new product, then nobody knows for sure how the market will view that product.
5A more detailed discussion of this subject can be found in Chapter 27. You should especially read
the section titled Decision-Making Under Certainty.
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FIGURE 2.5
TABLE 2.2
1\2
sss
b
c
s
ssb
p(N),f(T)
p(T), f(N)
ssc
...
bbs
ccb
ccs
ccc
p(T), f(N)
When there is uncertainty a simple ranking of the outcomes will no longer suffice. In their book Von
Neumann and Morgenstern asked the following question: Under what conditions can we treat the payoff to
an uncertain outcome as the average of the payoffs to the underlying certain outcomes? More concretely,
suppose that player 2 picks the strategy sss (she always travels by subway) while player 1 tosses a cointaking
a bus if the coin comes up heads or a cab if it comes up tails. In this case, there is a 50% chance that player 1
will get the remaining ticket and a 50% chance that he will not. Under what properties of player 1's
preferences is this uncertain outcome worth a payoff halfway between the certain outcomes, p1(T) and
p1(N)? In other words, under what conditions is it worth the payoff
?
Expected utility
Preferences satisfy expected utility when the payoff to an uncertain outcome
is precisely the average payoff of the underlying certain outcomes.
You canand should!read more about Von Neumann and Morgenstern's answer in Chapter 27; they offer
conditions under which preferences satisfy the expected utility hypothesis. In this book, we will presume
that each player's preferences do satisfy these required conditions. When there is no uncertainty in the
underlying game, or in the way players choose to play the game, you may continue to think of the payoffs as
simply a ranking.
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2.5 Representation Of The Examples
In this section we will examine the extensive and strategic forms of the three examples that were discussed
in detail at the end of Chapter 1.
Example 1: Nim
Suppose, to begin with, there are two matches in one pile and a single match in the other pile. Let us write
this configuration as (2,1). Winning is preferred to losing and, hence, the payoff number associated with
winning must be higher than the one that corresponds to losing; suppose that these numbers are,
respectively, 1 and -1. Figure 2.6 represents the extensive form of this game.6
The strategic form representation is as follows:
1\2
1L
lR
rL
rR
u
1,-1
1,-1
1,-1
1,-1
m
-1,1
-1,1
-1,1
-1,1
d
1,-1
-1,1
1,-1
-1,1
If there are more matches in either pile at the beginning of the game, then the game tree would simply be
bigger. For instance, if the configuration is (2,2), then branches that come out of the root would lead to any
one of the following configurations: (2,1), (1,2), (2,0), and (0,2). From (2,1) onwards, the game tree would
look exactly like the tree in Figure 2.6; similarly, from (1,2) onwards, except in this case everything would
be switched around since it is the first pile, rather than the second, that has the single match. From (2,0) and
(0,2) onwards the tree would look like the part of the tree in Figure 2.6 starting from those configurations.
The full extensive form of this scenario is depicted in Figure 2.7.
FIGURE 2.6
6For compactness, I have written u, m, and d to be the three actions of player I that take the game
to (1,1), (0,1), and (2,0), respectively. Similarly, 1 and r correspond to player 2 taking the game
from (1,1) to (1,0) and (0,1), respectively, while L and R have him take the game from (2,0) to (1,0)
and (0,0).
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FIGURE 2.7
FIGURE 2.8
Example 2: Voting Game
Suppose that a voter gets a utility payoff of 1 if her favorite bill is passed, 0 if her second choice is passed,
and -1 if her least favorite choice is passed. The extensive form representation of this game with two
representative payoffs is shown in Figure 2.8.
The strategic form of the voting game is somewhat complicated to represent and so we will suspend that
discussion until the next chapter.
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FIGURE 2.9
Example 3: Prisoners' Dilemma
Suppose we write a prison term of 5 years as a utility payoff of -5, and so on. The extensive form of this
game is shown in Figure 2.9.
(Note that simultaneous moves have been represented using an information set.) The strategic form is as
follows:
c
n
l\2
c
-5,-5
0,-15
n
-15,0
-1,-1
Summary
1. The rules of a game have to specify who the players are, what choices are available to each player, and
how much each player gets from a set of choices made by the group of players.
2. There are two principal representations of the rules of a game, the extensive form and the strategic form.
3. The extensive form is a pictorial representation of the game. It specifies the order in which players make
choices, how many times each player gets to choose (and what choices are available to her each time), and
the eventual payoffs to each player for any sequence of choices.
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4. The strategic form is a representation in which the each player's choices (strategies) and the payoffs for a
set of choices are specified. You can think of this as the right game form when players make once for all
choices.
5. The payoffs in a game should be thought of as Von Neumann-Morgenstern utilities. For an uncertain
situation, payoffs should be computed by taking an expectation over the possible resolutions of the
uncertainty.
Exercises
Section 2.2
2.1
Consider the following decision situation. You have a choice to make about which two courses to take and
you have available four courses, A, B, C and D. Depict this problem in a tree form.
2.2
Suppose that after deciding which two courses to take you have a further decision to make: which course
you will concentrate your efforts on. To keep matters simple, suppose thatif you take courses B and C, for
instanceyou can either choose to Work Hard for B or Work Hard for C. Depict this full decision problem.
2.3
Draw the game tree for Nim with initial configuration (3, 2). Assume that the payoff for winning is 1 while
that for losing is 0.
2.4
Do the same for Marienbad with initial configuration (3, 3).
2.5
Consider the following game of "divide the dollar." There is a dollar to be split between two players. Player
1 can make any offer to player 2 in increments of 25 cents; that is, player 1 can make offers of 0 cents, 25
cents, 50 cents, 75 cents, and $1. An offer is the amount of the original dollar that player 1 would like player
2 to have. After player 2 gets an offer, she has the option of either accepting or rejecting the offer. If she
accepts, she gets the offered amount and player 1 keeps the remainder. If she rejects, neither player gets
anything. Draw the game tree.
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2.6
`Write down the modified version of the "divide the dollar" game in which player 2 can make a counteroffer
if she does not accept player l's offer. After player 2 makes her counterofferif she doesplayer 1 can accept or
reject the counteroffer. As before, if there is no agreement after the two rounds of offers, neither player gets
anything. If there is an agreement in either round then each player gets the amount agreed to.
2.7
Consider the following variant of the "divide the dollar" game. Players 1 and 2 move simultaneously; 1
makes an offer to 2 and 2 specifies what would be an acceptable offer. For instance, player 1 might make an
offer of 50 cents and player 2 might simultaneously set 25 cents as an acceptable offer. If player 1's offer is
at least as large as what is acceptable to player 2, then we will say that there is an agreement and player 1
will pay player 2 the amount of his offer. Alternatively, if player l's offer is smaller than what player 2
specifies as acceptable, there is no agreement, in which case neither player gets anything. Draw the game
tree for this game.
Section 2.3
2.8
Write down the strategies of player 1 in the "divide the dollar" game of question 2.5. Then do the same for
player 2.
2.9
Use your answer to the previous question to write down the strategic form of the "divide the dollar" game.
(You do not have to list every strategy for player 2.)
2.10
Write down the strategic form of the simultaneous move "divide the dollar" game of question 2.7.
2.11
Write down the strategic form of Nim when the initial configuration is (2,1). (You do not have to fill in the
payoffs of all the cells, but do fill in some.)
2.12
Consider the Morningside Heights Bagel Market example that is described in the previous chapter. Assume
that prices are simultaneously chosen by University Food Market and Columbia Bagels and that they can be
40, 45, or 50 cents. Assume that the cost of production is 25 cents a bagel. Assume further that the market is
of fixed size; 1000 bagels sell every day in this neighborhood and whichever store has the cheaper price gets
all of the business. If the prices are the same, then the market is shared equally. Write down the strategic
form of this game, with payoffs being each store's profits.
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2.13
Redo the previous question with the total number of bagels sold being, respectively, 1500, 1000, and 500
bagels at the three possible prices of 40, 45, and 50 cents. (Assume that all other factors remain unchanged.)
2.14
Redo question 2.12 such that the store with the cheaper price gets 75% of the business. (Assume that
everything else remains unchanged.)
2.15
Redo question 2.12 yet again, presuming that Columbia Bagels has, inherently, the tastier bagel and,
therefore, when the prices are the same Columbia Bagels gets 75% of the business. (Assume that everything
else remains unchanged.)
Section 2.4
2.16
Let us return to the course-work problem (question 2.2). Suppose that working hard produces a grade of A
while not working hard produces a grade of B. Fill in the payoffs to that decision problem.
2.17
Redo the extensive form of the theater game from this chapter to allow for the possibility that the first
person to get to the theater has a further choice to make between a good seat costing $60 and a not-so-good
(but, nevertheless, expensive) seat costing $40. (The later arrival then gets the remaining ticket.)
2.18
Discuss briefly how you might redo the original extensive form of the theater game if there is a 50% chance
that the show's star might be replaced by an understudy for that evening's performance.
2.19
How would you compute the payoffs to the game of question 2.18?
Consider the following group project example. Three studentsAndrew, Dice, and Claysimultaneously work
together on a problem set for their game theory class. The instructor has asked them, in fact, to submit a joint
problem set. Each student can choose to work hard (H) or goof off (G). If all three students work hard, their
assignment will get an A; if at least two students work hard, the assignment will get a B; if only one student
works hard, the assignment will get a C; and, finally, if nobody works hard the assignment will get an F.
Denote the payoff function p; this payoff depends on the grade and the amount of work. For example, the
payoff to H and a grade of B is denoted p(H, B).7
7A natural assumption is that a better grade is preferred to a worse grade, but goofing off is
preferred to working hard. For instance, p(G, B) > p(H, B) > p(H, C).
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2.20
Write out the extensive form.
2.21
The strategic form is easiest to read if it is written in two parts. First, consider the case where Clay is
expected to be a hard worker. Andrew and Dice can choose either H or G. Write down the strategic form.
2.22
There is also a second possibility, namely that Clay chooses to goof off. Show that in this case, the strategic
matrix becomes
Andrew \ Dice
H
G
H
p(H, B), p(H, B)p(G, B)
p(H, C), p(G, C), p(G, C)
G
p(G, C), p(H, C), p(G, C)
p(G, F), p(G, F), p(G, F)
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PART TWO
STRATEGIC FORM GAMES: THEORY AND PRACTICE
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Chapter 3
Strategic Form Games and Dominant Strategies
In this chapter we will discuss two concepts: in section 3.1, we will examine in greater detail the strategic
form representation of a game. Then, in section 3.3, we will look at the first of several solution concepts that
are applied to strategic form games, the dominant strategy solution. Sections 3.2 and 3.4 will serve as
practical illustrations for the two concepts. While section 3.2 will discuss the strategic form of an art auction,
section 3.4 will hunt for the dominant strategy solution in such an auction.
3.1 Strategic Form Games
The strategic form of a game is specified by three objects.
1. the list of players in the game
2. the set of strategies available to each player
3. the payoffs associated with any strategy combination (one strategy per player)
The payoffs should be thought of as Von Neumann-Morgenstern utilities. The simplest kind of game is one in
which there are two playerslabel them player 1 and player 2and each player has exactly two strategies. As
an illustration, consider a game in which player 1's two strategies are labelled High and Low and player 2's
strategies are called North and South. The four possible strategy combinations in this game are (High,
North), (High, South), (Low, North), and (Low, South). The payoffs are specified for each player for every
one of the four strategy combinations. A more compact representation of this strategic form is by way of a 2
× 2 matrix.
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Player 1 \ Player 2
High
Low
North
p1(H, N), p2(H, N)
p1(L, N), p2(L, N)
South
p1(H, S), p2(H, S)
p1(L, S), p2(L, S)
Here, for example, pl(H, N), p2(H, N) are the payoffs to the two players if the strategy combination (High,
North) is played.
When there are more than two players, and each player has more than two strategies, it helps to have a
symbolic representation because the matrix representation can become very cumbersome very quickly.
Throughout the book, we will use the following symbols for the three components of the strategic form:
players will be labelled 1, 2, . . . N. A representative player will be denoted the i-th player, that is, the index i
will run from 1 through N. Player i's strategies will be denoted in general as si and sometimes a specific
strategy will be marked or and so on. A strategy choice of all players other than player i will be
denoted s-i. Finally, pi will denote player i's payoff (or Van Neumann-Morgenstern utility) function. For a
combination of strategies, , , . . . , one strategy for each player, player i's payoff will be denoted
.
3.1.1 Examples
Let us develop intuition for the strategic form through a series of examples. We start with two player-two
strategy games.
Example 1: Prisoners' Dilemma (c = confess, nc = not confess)
This is the first example that we met in Chapter 1the tale of Calvin and Klein.1
Calvin \ Klein
c
nc
c
0,0
7,-2
nc
-2,7 5,5
Example 2: Battle of the Sexes (F = football, 0 = opera)
The (somewhat sexist) story for the Battle of the Sexes game goes as follows. A husband and wife are trying
to determine whether to go to the opera or to a football game. They each, respectively, prefer the football
game and the opera. At the same time, each of them would rather go with the spouse than go alone.
Husband \ Wife
F
O
F
3,1 0,0
O
0,0 1,3
1The entries in each cell are now in utility units, unlike in Chapter 1 where they represented
lengths of prison terms. Hence, a bigger number here is better than a smaller one.
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Example 3: Matching Pennies (h = heads, t = tails)
Two players write down either heads or tails on a piece of paper. If they have written down the same thing,
player 2 gives player 1 a dollaror, strictly speaking, 1 utility unit. If they have written down different things
then player 1 pays 2 instead.
Player 1 \ Player 2
h
t
h
1,-1 -1,1
t
-1,1 1,-1
Example 4: Hawk-Dove (or Chicken) (t = tough, c = concede)
Two (young) players are engaged in a conflict situation. For instance, they may be racing their cars towards
each other on Main Street, while being egged on by their many friends. If player 1 hangs tough and stays in
the center of the road while the other player concedeschickens outby moving out of the way, then all glory is
his and the other player eats humble pie. If they both hang tough they end up with broken bones, while if
they both concede they have their bodiesbut not their prideintact.
Player 1 \ Pl...