What is grim trigger strategy?

Grim trigger strategy is a concept used in game theory which is often applied in economics, political science, and other related fields.

In this strategy, a player will cooperate with his/her opponent until the opponent decides to deviate from the cooperative behavior. Once that happens, the player will punish the opponent by also deviating from the cooperative behavior, and continue to do so for the remainder of the game.

The name, "grim trigger" comes from the fact that once a player triggers this strategy, they will continue to pursue it grimly, as they will gain more utility from punishing the opponent rather than cooperating.

The Grim trigger strategy is commonly used in situations where there is no opportunity for communication or coordination between players, and carries the risk of leading to a suboptimal outcome for both parties involved.