“We all know that people are not angels, and that they tend to look after themselves and their own first. Yet we also know that cooperation does occur and that our civilization is based upon it. But, in situations where each individual has an incentive to be selfish, how can cooperation ever develop?” “The approach of this book is to investigate how individuals pursuing their own interests will act, followed by an analysis of what effects this will have for the system as a whole. Put another way, the approach is to make some assumptions about individual motives and then deduce consequences for the behavior of the entire system (Schelling 1978). The case of the U.S. Senate is a good example, but the same style of reasoning can be applied to other settings.”

人总是先思考自身利益的，那人们为什么又会合作呢？当然是因为合作长期带来的利益大于个人的利益。

“The definition of Prisoner’s Dilemma requires that several relationships hold among the four different potential outcomes. The first relationship specifies the order of the four payoffs. The best a player can do is get T, the temptation to defect when the ther player cooperates. The worst a player can do is get S, the sucker’s payoff for cooperating while the other player defects. In ordering the other two outcomes, R, the reward for mutual cooperation, is “assumed to be better than P, the punishment for mutual defection. This leads to a preference ranking of the four payoffs from best to worst as T, R, P, and S. The second part of the definition of the Prisoner’s Dilemma is that the players cannot get out of their dilemma by taking turns exploiting each other. This assumption means that an even chance of exploitation and being exploited is not as good an outcome for a player as mutual cooperation. It is therefore assumed that the reward for mutual cooperation is greater than the average of the temptation and the sucker’s payoff. This assumption, together with the rank ordering of the four payoffs, defines the Prisoner’s Dilemma.”

囚徒困境专业版定义。首先，一方背叛一方合作的收益背叛方是T, 合作方是S，T是最大值，S是最小值。当双方都合作时，收益是R，双方背叛时收益是P，R是大于P但小于T，而P是大于S的。同时囚徒困境并不能通过互相剥削来获得更大的收益，具体来说就是T+S<2*R.

“If the game is played a known finite number of times, the players still have no incentive to cooperate. This is certainly true on the last move since there is no future to influence. 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 of any sequence of plays that is of known finite length (Luce and Raiffa 1957, pp. 94-102), This reasoning does not apply if the players will interact an indefinite number of times. And in most realistic settings, the players cannot be sure when the last interaction between them will take place. As will be shown later, with an indefinite number of interactions, cooperation can emerge. The issue then becomes the discovery of the precise conditions that are necessary and sufficient for cooperation to emerge.”

这里描述了一个很有意思的场景。如果是有限次的囚徒困境，理智双方是不可能达成合作的：最后一次肯定是要选择背叛的，因此倒数第二次因为大家都明白最后一次是选择背叛，所以当下选择合作并不能换来未来的收益，所以也是选择背叛，以此类推。但当人们不确定未来还要进行多少次合作、背叛选择的话，那合作就会产生。