The prisoner’s dilemma is a standard example of game theory. It shows why two completely rational individuals might not cooperate, even if it appears that it is in their best interests to do so.

The following example was formalised by Albert W. Tucker, who coined the term the “prisoner’s dilemma”, presenting the game as follows:

Two criminals are arrested and imprisoned. Each prisoner is in solitary confinement with no means of communicating with the other. The prosecutors lack sufficient evidence to convict the pair on the principal charge, but they have enough to convict both on a lesser charge. Simultaneously, the prosecutors offer each prisoner a bargain. Each prisoner is given the opportunity either to betray the other by testifying that the other committed the crime, or to cooperate with the other by remaining silent. The offer is:

• If A and B each betray the other, each of them serves two years in prison
• If A betrays B but B remains silent, A will be set free and B will serve three years in prison (and vice versa)
• If A and B both remain silent, both of them will only serve one year in prison (on the lesser charge).

Because betraying a partner offers a greater reward than cooperating with them, purely rational self-interested prisoners will betray the other, meaning the only possible outcome for two purely rational prisoners is for them to betray each other.

The interesting part of this result is that pursuing individual reward logically leads both of the prisoners to betray, when they would get a better reward if they both kept silent. In reality, humans display a systemic bias towards cooperative behaviour in this and similar games, despite what is predicted by simple models of “rational” self-interested action.

The prisoner’s dilemma game can be used as a model for many real world situations involving cooperative behaviour. In casual usage, the label “prisoner’s dilemma” may be applied to situations not strictly matching the formal criteria of the classic or iterative games: for instance, those in which two entities could gain important benefits from cooperating or suffer from the failure to do so, but find it difficult or expensive—not necessarily impossible—to coordinate their activities.

An edited edition of the page found at https://en.wikipedia.org/wiki/Prisoner%27s_dilemma