Stag hunt
Also known as: Assurance game
Cooperate for a big shared prize, but only if you trust the other to cooperate too.
What it means
A coordination game, drawn from Rousseau, in which two hunters do best by jointly pursuing a stag but each can defect to catch a hare alone for a smaller, safer payoff. It has two pure equilibria — both hunt stag (payoff-dominant) or both hunt hare (risk-dominant) — so the dilemma is trust, not greed: cooperation pays most but exposes you if your partner bolts. Unlike the prisoner's dilemma, mutual cooperation here is an equilibrium; the obstacle is the fear that the other will play it safe. It matters as the canonical model of trust, assurance, and the fragility of socially efficient cooperation, from team projects to collective security.
Examples
Two colleagues can land a big joint win by both committing fully, but each is tempted by a safe solo task if unsure the other will show up.
Two friends agree to run at 6am; each would rather sleep than turn up alone in the rain, so a single doubt about the other's alarm sinks a plan both preferred.
Two firms would both profit from one shared charging standard, but each keeps its proprietary plug rather than gamble on the other switching — the safe hare, the forgone stag.
First described in Rousseau (1755); formalized in game theory.