Tit for tat

From EvoWiki

A type of trigger strategy, usually applied to a repeated Prisoner's Dilemma situation, in which an individual (called a player) responds with the same type of action her opponent just performed. A classical situation (game) is one in which player 1 can behave altruistically or selfishly, and player 2 will decide how to react based on player's 1 previous action. This will continue back and forth such that player 1 then decides how to react to player 2's action, whereupon player 2 decides, and then player 1 again, and so forth. In such a situation, a tit for tat strategy simply starts out being cooperative, and thereafter reciprocates altruistically if the other player has been altruistic, and behaves selfishly if the other has behaved selfishly. Tit for tat can be a remarkably successul strategy, because it experiences the benefits of mutual cooperation, yet does not get repeatedly exploited by selfish partners. For a brief discussion of the Prisoner's Dilemma, see the paragraph "reciprocal altruism" in the article on altruism.