“The chronological story that emerges from this analysis is the following. ALL D is the primeval state and is evolu-tionarily stable. But cooperation based on reciprocity can gain a foothold through two different mechanisms. First, there can be kinship between mutant strategies, giving the genes of the mutants some stake in each other’s success, thereby altering the payoff of the interaction when viewed from the perspective of the gene rather than the individual. A second mechanism to overcome total defection is for the mutant strategies to arrive in a cluster so that they provide a nontrivial proportion of the interactions each has, even if they are so few as to provide a negligible proportion of the interactions which the ALL D individuals have. Then the tournament approach described in chapter 2 demonstrates that once a variety of strategies is present, TIT FOR TAT is an extremely robust one. It does well in a wide range of circumstances and gradually displaces all other strategies in an ecological simulation that contains a great variety of more or less sophisticated decision rules. And if the probability that interaction between two individuals will continue is great enough, then TIT FOR TAT is an extremely robust one. It does well in a wide range of circumstances and gradually displaces all other strategies in an ecological simulation that contains a great variety of more or less sophisticated decision rules. And if the probability that interaction between two individuals will continue is great enough, then TIT FOR TAT is itself evolution-arily stable. Moreover, its stability is especially secure because it can resist the intrusion of whole clusters of mutant strategies. Thus cooperation based on reciprocity can get started in a predominantly noncooperative world, can thrive in a variegated environment, and can defend itself once fully established.”

这里聊了TIT FOR TAT如何能在恶劣的环境中生根，一种是亲属间的互惠，另一种就是群落都适用TIT FOR TAT可以入侵全部背叛的策略。当入侵成功后，其他人会发现TIA FOR TAT的策略优势而逐渐调整自己的策略成为TIT FOR TAT。

“TIT FOR TAT won the tournament because it did well in its interactions with a wide variety of other strategies. On average, it did better than any other rule with the other strategies in the tournament. Yet TIT FOR TAT never once scored better in a game than the other player! In fact, it can’t. It lets the other player defect first, and it never defects more times than the other player has defected. Therefore, TIT FOR TAT achieves either the same score as the other player, or a little less. TIT FOR TAT won the tournament, not by beating the other player, but by eliciting behavior from the other player which allowed both to do well. TIT FOR TAT was so consistent at eliciting mutually rewarding outcomes that it attained a higher overall score than any other strategy. So in a non-zero-sum world you do not have to do better than the other player to do well for yourself. This is especially true when you are interacting with many different players. Letting each of them do the same or a little better than you is fine, as long as you tend to do well yourself. There is no point in being envious of the success of the other player, since in an iterated Prisoner’s Dilemma of long duration the other’s success is virtually a prerequisite of your doing well for yourself.”

这点非常有启发。TIT FOR TAT赢了所有锦标赛，但从来没有比任何一个对手得过更高的分，因为TIT FOR TAT永远不会第一个背叛，但总得分却是最高的，因为TIT FOR TAT能让对局双方都往合作的方向发展。在不是零和游戏中，目标不是赢过或者嫉妒某一个人，而是做到自己最好。