The Evolution of Cooperation: Revised Edition by Robert Axelrod


Why do people cooperate? More specifically, why do people on different sides of a conflict cooperate in environments that ask them to do the opposite (ex: war)? This book explores how specific environments and actions can promote cooperation and what causes cooperation to brake down.

The main example used in this book is the prisoner's dilemma. It goes like this:

"Two members of a criminal gang are arrested and imprisoned. Each prisoner is in solitary confinement with no means of communicating with the other. The prosecutors lack sufficient evidence to convict the pair on the principal charge, but they have enough to convict both on a lesser charge. Simultaneously, the prosecutors offer each prisoner a bargain. Each prisoner is given the opportunity either to betray the other by testifying that the other committed the crime, or to cooperate with the other by remaining silent. The possible outcomes are:"

  • If A and B each betray the other, each of them serves two years in prison.
  • If A betrays B but B remains silent, A will be set free and B will serve three years in prison.
  • If A remains silent but B betrays A, A will serve three years in prison and B will be set free.
  • If A and B both remain silent, both of them will serve only one year in prison (on the lesser charge).
A vs BB Stays SilentB betrays A
A Stays Silent-1/-1-3/0
A betrays B0/-3-2/-2

What strategy works best in the long run? Do you betray the other person to save yourself? Do you cooperate and hope the other person doesn't betray you?

The prisoners dilemma is a simple, one-and-done example of how the dynamic would work. In real life, however, there are dilemmas like this that are continuously going on. Trade agreements between countries, for example. Do you screw over the other country for your own gain or do you cooperate and hope they don't betray you?

The strategy Axelrod proposes is called "tit for tat" and it's very simple: You cooperate on the first move, and then you keep copying your opponents last move. If they betray you, you betray them on the next move. if they cooperate, you cooperate on the next move.

There are a couple of principles which foster cooperation:

  • Avoidance of unnecessary conflict. (i.e.: not betraying the other person first.)
  • Being able to be provoked (i.e.: retaliating when you are betrayed)
  • Being able to forgive a provocation
  • Clear, predictable behavior

Axelrod then goes on and gives examples of situations where these principles have led to cooperation.


  • "On the next-to-last move neither player will have an incentive to cooperate since they can both anticipate a defection by the other player on the very last move. Such a line of reasoning implies that the game will unravel all the way back to mutual defection on the first move."
  • "What makes it possible for cooperation to emerge is the fact that the players might meet again. This possibility means that the choices made today not only determine the outcome of this move, but can also influence the later choices of the players. The future can therefore cast a shadow back upon the present and thereby affect the current strategic situation."
  • "TIT FOR TAT is the policy of cooperating on the first move and then doing whatever the other player did on the previous move."
  • "The analysis of the data from these tournaments reveals four properties which tend to make a decision rule successful: avoidance of unnecessary conflict by cooperating as long as the other player does, provocability in the face of an uncalled for defection by the other, forgiveness after responding to a provocation, and clarity of behavior so that the other player can adapt to your pattern of action."
  • "Surprisingly, there is a single property which distinguishes the relatively high-scoring entries from the relatively low-scoring entries. This is the property of being nice, which is to say never being the first to defect."
  • "A major lesson of this tournament is the importance of minimizing echo effects in an environment of mutual power. When a single defection can set off a long string of recriminations and counterrecriminations, both sides suffer. A sophisticated analysis of choice must go at least three levels deep to take account of these echo effects."
  • "[Other algorithms] were too competitive for their own good. In the first place, many of them defected early in the game without provocation, a characteristic which was very costly in the long run."
  • "once a manufacturer begins to go under, even his best customers begin refusing payment for merchandise, claiming defects in quality, failure to meet specifications, tardy delivery, or what-have-you. The great enforcer of morality in commerce is the continuing relationship, the belief that one will have to do business again with this customer, or this supplier, and when a failing company loses this automatic enforcer, not even a strong-arm factor is likely to find a substitute. (Mayer, 1974, p. 280)"
  • "It was the French practice to “let sleeping dogs lie” when in a quiet sector . . . and of making this clear by retorting vigorously only when challenged. In one sector which we took over from them they explained to me that they had practically a code which the enemy well understood: they fired two shots for every one that came over, but never fired first. (Kelly 1930, p. 18)"
  • "The TIT FOR TAT and the two-for-one strategy share something else as well. They both retaliate after a defection by the other. This observation leads to a general principle, since any collectively stable strategy which is willing to cooperate must somehow make it unprofitable for a challenger to try to exploit it."
  • "The live-and-let-live system was endemic in trench warfare. It flourished despite the best efforts of senior officers to stop it, despite the passions aroused by combat, despite the military logic of kill or be killed, and despite the ease with which the high command was able to repress any local efforts to arrange a direct truce."
  • "What made trench warfare so different from most other combat was that the same small units faced each other in immobile sectors for extended periods of time. This changed the game from a one-move Prisoner’s Dilemma in which defection is the dominant choice, to an iterated Prisoner’s Dilemma in which conditional strategies are possible."
  • "The introduction of raids completed the cycle of the evolution of the live-and-let-live system. Cooperation got a foothold through exploratory actions at the local level, was able to sustain itself because of the duration of contact between small units facing each other, and was eventually undermined when these small units lost their freedom of action."
  • "To provide discomfort for the other is but a roundabout way of providing it for themselves” (Sorley 1919, p. 283)."
  • "The original story is that two accomplices to a crime are arrested and questioned separately. Either can defect against the other by confessing and hoping for a lighter sentence. But if both confess, their confessions are not as valuable. On the other hand, if both cooperate with each other by refusing to confess, the district attorney can only convict them on a minor charge. Assuming that neither player has moral qualms about, or fear of, squealing, the payoffs can form a Prisoner’s Dilemma (Luce and Raiffa 1957, pp. 94-95)."
  • "As long as the interaction is not iterated, cooperation is very difficult. That is why an important way to promote cooperation is to arrange that the same two individuals will meet each other again, be able to recognize each other from the past, and to recall how the other has behaved until now. This continuing interaction is what makes it possible for cooperation based on reciprocity to be stable."
  • "one way to promote cooperation would be to make their interactions more frequent."
  • "A common reaction of someone caught in a Prisoner’s Dilemma is that “there ought to be a law against this sort of thing.” In fact, getting out of Prisoner’s Dilemmas is one of the primary functions of government: to make sure that when individuals do not have private incentives to cooperate, they will be required to do the socially useful thing anyway. Laws are passed to cause people to pay their taxes, not to steal, and to honor contracts with strangers. Each of these activities could be regarded as a giant Prisoner’s Dilemma game with many players."
  • "If the punishment for defection is so great that cooperation is the best choice in the short run, no matter what the other player does, then there is no longer a dilemma."