Tit for tat is a highly effective strategy in game theory for the iterated prisoner's dilemma. It was first introduced by Anatol Rapoport in Robert Axelrod's two tournaments, held around 1980. An agent using this strategy will initially cooperate, then respond in kind to an opponent's previous action. Unless provoked, the agent will always cooperate. If provoked, the agent will retaliate. Some game theorists informally believe the strategy to be optimal, although no proof is presented.
