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The most successful strategy in the Iterated Prisoner's Dilemma. Following a tit-for-tat strategy entails doing to your partner whatever your partner did you you in the previous turn -- cooperating if s/he cooperated, and defecting if s/he defected. (In other words, if your partner was just nice to you, be nice back -- if your partner harmed you, strike back, but only once!)

Some memeticists propose that tit-for-tat strategies are the source of human ideas of fairness, justice, and, ultimately, morality. This is not, they say, because people consciously plan to carry out such strategies in their daily lives, but simply because tit-for-tat is the best you can do in a zero-sum game. (Of course, it has been argued {and quite well} that human life is not zero-sum.)