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Repeated Prisoner's Dilemma

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Last updated: 2026-07-18

The Repeated Prisoner's Dilemma

It is the iterated version of the classic dilemma.

Basic version (a single round):

Two prisoners are interrogated separately. Each can Cooperate (with the other) or Defect (betray them).

  • If both cooperate → both leave with a light sentence.
  • If one defects and the other cooperates → the defector goes free, the other receives the maximum sentence.
  • If both defect → both receive an intermediate sentence.

In a single round, the dominant strategy is to defect (even though both would be better off cooperating).

Repeated version (the one that matters): Many rounds are played in a row with the same opponent. Here everything changes.

Key results (Robert Axelrod, 1980s):

  • The most successful strategy in computerized tournaments was “Tit for Tat”:
  • Start by cooperating.
  • Copy what the other did in the previous round.
  • It is simple, clear, forgiving (does not hold a grudge forever), and retaliatory (punishes defection immediately).
  • More complex strategies such as “Generous Tit for Tat” (forgiving sometimes) work even better in noisy environments.

Relevance for adaptive altruism:

  • It demonstrates that reciprocity is an evolutionarily stable strategy.

  • One need not be “good” on principle, but follow a conditional norm: cooperate with those who cooperate and punish (or avoid) those who defect.

  • This explains why reciprocal altruism survives in populations.

Categories: Home -> Dictionary

Last updated: 2026-07-18

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