Grim trigger
Cooperate for as long as the other side does, and if they ever fail to, refuse to cooperate for the rest of the relationship.
Often misread asDismissing grim trigger as unrealistic because real people do not punish for ever, and concluding that it therefore says nothing useful about real cooperation.
How to see it
Cooperate until betrayed, then never again
Grim trigger is the bluntest strategy in a repeated game: cooperate every round until the other side defects once, then defect for ever. Its job is to make the threatened punishment as large as a punishment can be, so that cooperation holds at the lowest level of patience any strategy could manage.
The checkIt is a benchmark rather than advice: it establishes the easiest conditions under which cooperation can be an equilibrium at all, which is what you need before asking whether gentler strategies also work.
Try itAsk whether you would actually carry out the punishment once provoked. In the prisoner's dilemma you would, because permanent mutual defection is itself an equilibrium of what remains. In a game where punishing costs you more than it costs them, check again before assuming the threat stands up.
Games that show it
Know these first
Easily confused with
Sources
- Friedman, J. (1971). Review of Economic Studies 38(1), 1-12.
- Mailath, G. & Samuelson, L. (2006). Oxford University Press.