Proposition 7. If a nice strategy cannot be invaded by a single individual, it cannot be invaded by any cluster of individuals either. - Robert Axelrod

The extraordinary success of TIT FOR TAT leads to some simple, but powerful advice: practice reciprocity. After cooperating on the first move, TIT FOR TAT simply reciprocates whatever the other player did on the previous move. This simple rule is amazingly robust. It won the first round of the Computer Tournament for the Prisoner's Dilemma by attaining a higher average score than any other entry submitted by professional game theorists. And when this result was publicized for the contestants in the second round, TIT FOR TAT won again. The victory was obviously a surprise, since anyone could have submitted it to the second round after seeing its success in the first round. But obviously people hoped they could do better-and they were wrong.

Tournament studies, ecological simulation, and theoretical analysis demonstrate: (1) A generous version of tit for tat is a highly effective strategy when the players it meets have not adapted to noise; (2) If the other players have adapted to noise, a contrite version of tit for tat is even more effective at quickly restoring mutual cooperation without the risk of exploitation; (3) Pavlov is not robust.

Just as important as getting cooperation started were the conditions that allowed it to be sustainable. The strategies that could sustain mutual cooperation were the ones which were provocable.