Grim Trigger

Grim trigger is a strategy in game theory for a repeated game, such as an iterated prisoner's dilemma. Initially, a player using grim trigger will cooperate, but as soon as the opponent defects (thus satisfying the trigger condition), the player using grim trigger will defect for the remainder of the iterated game. Since a single defect by the opponent triggers defection forever, grim trigger is the most strictly unforgiving of strategies in an iterated game.

To employ a grim trigger strategy effectively, it is important that the player communicate their intentions before the first defection takes place. Once a defection occurs, cooperation cannot be restored. 'Grim Trigger' is essentially the nuclear weapon of strategies--it threatens mutual destruction to ensure cooperation and is therefore a more effective threat than tactic. To successfully force cooperation using a 'grim trigger' strategy is it important that opponents believe in the credibility of your commitment to the strategy--whether its true or not.