Robert Axelrod

“The conditions for the evolution of cooperation tell what is necessary, but do not, by themselves, tell what strategies will be most successful. For this question, the tournament approach has offered striking evidence in favor of the robust success of the simplest of all discriminating strategies: TIT FOR TAT. By cooperating on the first move, and then doing whatever the other player did on the previous move, TIT FOR TAT managed to do well with a wide variety of more or less sophisticated decision rules. It not only won the first round of the Computer Prisoner’s Dilemma Tournament when facing entries submitted by professional game theorists, but it also won the second round which included over sixty entries designed by people who were able to take the results of the first round into account. It was also the winner in five of the six major variants of the second round (and second in the sixth variant). And most impressive, its success was not based only upon its ability to do well with strategies which scored poorly for themselves. This was shown by an ecological analysis of hypothetical future rounds of the tournament. In this simulation of hundreds of rounds of the tournament, TIT FOR TAT again was the most successful rule, indicating that it can do well with good and bad rules alike. TIT FOR TAT’s robust success is due to being nice, provocable, forgiving, and clear. Its niceness means that it is never the first to defect, and this property prevents it from getting into unnecessary trouble. Its retaliation discourages the other side from persisting whenever defection is tried. Its forgiveness helps restore mutual cooperation. And its clarity makes its behavioral pattern easy to recognize; and once recognized, it is easy to perceive that the best way of dealing with TIT FOR TAT is to cooperate with it.”
― Robert Axelrod, The Evolution of Cooperation: Revised Edition