A fascinating,provocative,and important book. Douglas R.Hofstadter author of Godel,Escher.Bach THE Evolution OF Cooperation ROBERT AXELROD
CONTENTS PREFACE i PART I Introduction 1.The Problem ofCooperation 3 PART II The Emergence of Cooperation 2.The Success of TIT FOR TAT in Computer Tournaments 27 3.The Chronology ofCooperation 55 PART III Cooperation Without Friendship or Foresight 4.The Live-and-Let-Live System in Trench Warfare in World War I 73
CONTENTS PREFACE vi PART I Introduction 1. The Problem of Cooperation 3 PART II The Emergence of Cooperation 2. The Success of TIT FOR TAT in Computer Tournaments 27 3. The Chronology of Cooperation 55 PART III Cooperation Without Friendship or Foresight 4. The Live-and-Let-Live System in Trench Warfare in World War I 73 v
Contents 5.The Evolution ofCooperation in Biological Systems (with William D.Hamilton) 88 PART IV Advice for Participants and Reformers 6.How to Choose Effectively 109 7.How to Promote Cooperation 124 PART V Conclusions 8.The Social Structure ofCooperation 145 9. The Robustness ofReciprocity 169 APPENDIX A TOURNAMENT RESULTS 192 APPENDIX B PROOFS OF THE THEORETICAL PROPOSITIONS 206 NOTES 216 BIBLIOGRAPHY 223 INDEX 231
Contents 5. The Evolution of Cooperation in Biological Systems (with William D. Hamilton) 88 PART IV Advice for Participants and Reformers 6. How to Choose Effectively 109 7. How to Promote Cooperation 124 PART V Conclusions 8. The Social Structure of Cooperation 145 9. The Robustness of Reciprocity 169 APPENDIX A TOURNAMENT RESULTS 192 APPENDIX B PROOFS OF THE THEORETICAL PROPOSITIONS 206 NOTES 216 BIBLIOGRAPHY 223 INDEX 231
PREFACE THIS PROJECT began with a simple question:When should a person cooperate,and when should a person be selfish,in an ongoing interaction with another person? Should a friend keep providing favors to another friend who never reciprocates?Should a business provide prompt service to another business that is about to be bankrupt? How intensely should the United States try to punish the Soviet Union for a particular hostile act,and what pattern of behavior can the United States use to best elicit coopera- tive behavior from the Soviet Union? There is a simple way to represent the type of situation that gives rise to these problems.This is to use a particular kind of game called the iterated Prisoner's Dilemma.The game allows the players to achieve mutual gains from co- operation,but it also allows for the possibility that one player will exploit the other,or the possibility that neither will cooperate.As in most realistic situations,the players do not have strictly opposing interests.To find a good strategy to use in such situations,I invited experts in game theory to submit programs for a Computer Prisoner's Di- lemma Tournament-much like a computer chess tourna- ment.Each program would have available to it the history of the interaction so far and could use this history in mak- ing its choice of whether or not to cooperate on the current vii
PREFACE THIS PROJECT began with a simple question: When should a person cooperate, and when should a person be selfish, in an ongoing interaction with another person? Should a friend keep providing favors to another friend who never reciprocates? Should a business provide prompt service to another business that is about to be bankrupt? How intensely should the United States try to punish the Soviet Union for a particular hostile act, and what pattern of behavior can the United States use to best elicit cooperative behavior from the Soviet Union? There is a simple way to represent the type of situation that gives rise to these problems. This is to use a particular kind of game called the iterated Prisoner's Dilemma. The game allows the players to achieve mutual gains from cooperation, but it also allows for the possibility that one player will exploit the other, or the possibility that neither will cooperate. As in most realistic situations, the players do not have strictly opposing interests. To find a good strategy to use in such situations, I invited experts in game theory to submit programs for a Computer Prisoner's Dilemma Tournament—much like a computer chess tournament. Each program would have available to it the history of the interaction so far and could use this history in making its choice of whether or not to cooperate on the current vii
Preface move.Entries came from game theorists in economics,psy- chology,sociology,political science,and mathematics.I ran the fourteen entries and a random rule against each other in a round robin tournament.To my considerable surprise,the winner was the simplest of all the programs submitted,TIT FOR TAT.TIT FOR TAT is merely the strategy of starting with cooperation,and thereafter doing what the other player did on the previous move. I then circulated the results and solicited entries for a second round of the tournament.This time I received sixty-two entries from six countries.Most of the contes- tants were computer hobbyists,but there were also profes- sors of evolutionary biology,physics,and computer sci- ence,as well as the five disciplines represented in the first round.As in the first round,some very elaborate programs were submitted.There were also a number of attempts to improve on TIT FOR TAT itself.TIT FOR TAT was again sent in by the winner ofthe first round,Anatol Rapo- port of the University of Toronto.Again it won. Something very interesting was happening here.I sus- pected that the properties that made TIT FOR TAT so successful in the tournaments would work in a world where any strategy was possible.If so,then cooperation based solely on reciprocity seemed possible.But I wanted to know the exact conditions that would be needed to fos- ter cooperation on these terms.This led me to an evolu- tionary perspective:a consideration of how cooperation can emerge among egoists without central authority.The evo- lutionary perspective suggested three distinct questions. First,how can a potentially cooperative strategy get an ini- tial foothold in an environment which is predominantly noncooperative?Second,what type of strategy can thrive in a variegated environment composed of other individuals viⅷ
Preface move. Entries came from game theorists in economics, psychology, sociology, political science, and mathematics. I ran the fourteen entries and a random rule against each other in a round robin tournament. To my considerable surprise, the winner was the simplest of all the programs submitted, TIT FOR TAT. TIT FOR TAT is merely the strategy of starting with cooperation, and thereafter doing what the other player did on the previous move. I then circulated the results and solicited entries for a second round of the tournament. This time I received sixty-two entries from six countries. Most of the contestants were computer hobbyists, but there were also professors of evolutionary biology, physics, and computer science, as well as the five disciplines represented in the first round. As in the first round, some very elaborate programs were submitted. There were also a number of attempts to improve on TIT FOR TAT itself. TIT FOR TAT was again sent in by the winner of the first round, Anatol Rapoport of the University of Toronto. Again it won. Something very interesting was happening here. I suspected that the properties that made TIT FOR TAT so successful in the tournaments would work in a world where any strategy was possible. If so, then cooperation based solely on reciprocity seemed possible. But I wanted to know the exact conditions that would be needed to foster cooperation on these terms. This led me to an evolutionary perspective: a consideration of how cooperation can emerge among egoists without central authority. The evolutionary perspective suggested three distinct questions. First, how can a potentially cooperative strategy get an initial foothold in an environment which is predominantly noncooperative? Second, what type of strategy can thrive in a variegated environment composed of other individuals viii