Prisoner's dilemma
The prisoner's dilemma is a game in which individually rational choices lead to a worse outcome for everyone than cooperation would.
Two rational individuals, each pursuing their own interest, end up worse off than if they had cooperated. The prisoner's dilemma is the most famous illustration that rationality and good outcomes can part ways.
The prisoner's dilemma is a game in which two players, each acting in their own rational self-interest, reach an outcome worse for both than if they had cooperated. It is the canonical example in game theory of how individually sensible choices can produce collectively poor results.
The setup
In the classic story, two suspects are interrogated separately. Each can stay silent, cooperating with the other, or betray the other, defecting. If both stay silent, both get a light sentence. If both betray, both get a heavy one. But if one betrays while the other stays silent, the betrayer goes free and the silent one suffers most. Whatever the other does, each individual is better off betraying, so both betray, and both end up worse off than if both had stayed silent.
Why cooperation fails
The tragedy is that defection is the dominant strategy, the best choice regardless of what the other does, yet mutual defection is worse for both than mutual cooperation. The pursuit of individual advantage leads to a collectively inferior outcome that neither wanted. This captures a vast range of real situations, arms races, price wars, overfishing, pollution, where each party's rational self-interest undermines the common good.
Escaping the dilemma
What rescues cooperation in the real world is usually repetition and the shadow of the future. When the game is played many times, players can reward cooperation and punish betrayal in later rounds, making cooperation rational after all, the logic behind strategies like tit for tat. Reputation, communication, trust, and binding agreements likewise help parties escape the trap that a single, isolated encounter sets.
The prisoner's dilemma is among the most influential ideas in social science because it exposes a deep and uncomfortable truth: that individual rationality and collective welfare can diverge sharply. It explains why cooperation is so valuable and so fragile, and why so much of human institution-building is, at bottom, machinery for escaping the dilemma.