Prisoner's Dilemma

What Is the Prisoner's Dilemma?

The prisoner's dilemma is a game used by researchers to model and investigate how people decide to cooperate—or not. 

Imagine that Prisoner A and Prisoner B are charged with a crime and detained separately, and each has the chance to give a confession. Neither prisoner knows what the other will choose to do. There are several possible outcomes:

  • If only one prisoner confesses, the confessor is set free, but the non-confessor serves a three-year sentence.
  • If neither prisoner confesses, both serve a one-year sentence.
  • If both confess, both serve two years.

Should Prisoner A keep quiet, or betray Prisoner B by admitting to the crime? This is the classic version of the prisoner's dilemma.

Many other versions of the dilemma have been created, including ones in which each participant stands to gain more or less depending on their decisions, as well as other set-ups in which multiple games are played sequentially. The structure of the prisoner's dilemma was described by mathematicians in the 1950s and has since been used as a model for real-world situations and applied in domains such as psychology, economics, and political theory.

The prisoner's dilemma also encapsulates the conflict that can arise between individual and collective interests: In many scenarios (think arms races or the use of limited natural resources), incentives encourage individual people or countries to make self-serving choices, even though the broader community would benefit from cooperation.

How Is the Prisoner's Dilemma Used?

The prisoner's dilemma, like other games used by researchers, can illuminate how people make decisions about cooperation and betrayal under different conditions. In the classic version of the dilemma, the "rational" choice, given all the possible outcomes, is to betray the other player. But adding additional rounds of the game—and thus the potential for retaliation by one who is betrayed—can change the equation.

Psychologists continue to use the prisoner's dilemma (and similar games) to explore dynamics such as how memory affects adaptive decision-making and whether intuitive thinking boosts cooperation.

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