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I haven't read the other threads on this, but I'll go further than both of you and state that cooperating in the prisoner's dilemma does require either altruism or irrationality.

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I think it's rational self-interest.

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There are some other assumptions that have to be made in the finitely repeated prisoner's dilemma (most importantly, common knowledge of rationality), but including all the assumptions leads to a unique equilibrium where players defect on all rounds.

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How do you figure? If both players are rational, a tit-for-tat strategy (or slightly modified), ensures cooperation on all rounds. That is the rational strategy.