Prisoner's dilemma

A strategic game in which two players independently choose whether to cooperate or defect. Each has an incentive to defect, even though both receive a better outcome if they cooperate.

Nash equilibrium: A strategy profile in which no player can improve their payoff by changing strategy alone. Mutual defection is the game's Nash equilibrium, though mutual cooperation pays more.
Tragedy of the commons: The depletion of a shared resource when individuals pursue their own benefits. It extends the dilemma's incentive conflict to shared resources and many participants.
Stag hunt: A coordination game in which mutual cooperation yields the best outcome, but success depends on others cooperating. Unlike the dilemma, cooperation can be a best response when the other player cooperates.
Merrill Flood: An American mathematician and operations researcher who conducted early experiments on strategic games. His 1950 experiments with Melvin Dresher helped bring the game into practical study.
Tit for tat: A repeated-game strategy that begins by cooperating and then copies the opponent's previous move. It demonstrated how reciprocal responses can sustain cooperation across repeated encounters.
Dominant strategy: A strategy that yields at least as good an outcome as any alternative, whatever other players choose. Defection is dominant for each player in the one-shot dilemma.
Arms race: A competition in which rivals build military capability in response to one another. Mutual escalation can resemble repeated defection despite the costs both sides bear.
Chicken game: A game in which each player prefers the other to yield, while mutual refusal is the worst outcome. Its incentives differ: neither player has a dominant strategy to yield or stand firm.
Melvin Dresher: An American mathematician whose research contributed to early game theory and operations research. He devised the strategic game later popularized as the prisoner's dilemma.
Robert Axelrod: An American political scientist known for research on cooperation and the evolution of norms. His tournaments and analysis showed how simple strategies could thrive in repeated dilemmas.

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