<P> In addition to the general form above, the iterative version also requires that 2R> T + S, to prevent alternating cooperation and defection giving a greater reward than mutual cooperation . </P> <P> The iterated prisoner's dilemma game is fundamental to some theories of human cooperation and trust . On the assumption that the game can model transactions between two people requiring trust, cooperative behaviour in populations may be modeled by a multi-player, iterated, version of the game . It has, consequently, fascinated many scholars over the years . In 1975, Grofman and Pool estimated the count of scholarly articles devoted to it at over 2,000 . The iterated prisoner's dilemma has also been referred to as the "Peace - War game". </P> <P> If the game is played exactly N times and both players know this, then it is always game theoretically optimal to defect in all rounds . The only possible Nash equilibrium is to always defect . The proof is inductive: one might as well defect on the last turn, since the opponent will not have a chance to later retaliate . Therefore, both will defect on the last turn . Thus, the player might as well defect on the second - to - last turn, since the opponent will defect on the last no matter what is done, and so on . The same applies if the game length is unknown but has a known upper limit . </P> <P> Unlike the standard prisoner's dilemma, in the iterated prisoner's dilemma the defection strategy is counter-intuitive and fails badly to predict the behavior of human players . Within standard economic theory, though, this is the only correct answer . The superrational strategy in the iterated prisoner's dilemma with fixed N is to cooperate against a superrational opponent, and in the limit of large N, experimental results on strategies agree with the superrational version, not the game - theoretic rational one . </P>

In the nash equilibrium of a one-shot simultaneous move prisoner's dilemma game