Prisoner's
Dilemma

Two players each choose simultaneously to Cooperate or Defect — without knowing the other's choice. The payoffs create a tension: defecting is always the individually rational move, but mutual cooperation produces the best collective outcome. A single round has a clear dominant strategy (defect). Repeated across many rounds against the same opponent, the calculus changes — reputation and reciprocity start to matter.

Political scientist Robert Axelrod invited researchers to submit strategies to a round-robin computer tournament. Each strategy played 200 rounds against every other, and against itself. The 14 entries ranged from complex probabilistic programs to simple rules. The winner, by total score, was the shortest submission: Tit for Tat by Anatol Rapoport — four lines of BASIC. Cooperate on round 1, then copy the opponent's last move.

Axelrod ran a second tournament after publishing the results. Knowing TFT had won, 62 strategies entered. TFT won again.

Axelrod identified four properties shared by successful strategies:

• Nice — never defect first; avoid unnecessary conflict
• Retaliatory — punish defection quickly, or exploiters thrive
• Forgiving — resume cooperation after retaliation; avoid mutual lock-in
• Clear — be predictable enough that opponents can model you and cooperate

Strategies that lacked any one of these underperformed, even clever ones. The lesson: in an iterated game with no end date, cooperation can be individually rational.