= Grim trigger
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= Grim trigger strategy
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A <grim trigger> is a history-dependent <pure strategy> in a <repeated game> that switches permanently to punishment after a trigger event. In the <iterated prisoner's dilemma> above, the trigger is any player's observed defection, and punishment is <always defect>. Unlike <Tit for tat>, <grim trigger> never restores cooperation after the opponent cooperates again. Whether mutual <grim trigger> is a <subgame perfect equilibrium> depends on the stage <payoffs>, <discount factor>, and monitoring assumptions; it is not an unconditional property of the rule. See https://ocw.mit.edu/courses/6-254-game-theory-with-engineering-applications-spring-2010/5a158a7558165d0e22b5b27fcfa01713_MIT6_254S10_lec15.pdf#page=12[Ozdaglar, lecture 15, slides 12–15].
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