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modern · born 1943
Axelrod ran a tournament to find the best strategy for the repeated prisoner's dilemma. The winner was almost embarrassingly simple: be nice, retaliate, forgive, and never be the first to defect.
Facts reviewed 2026-07-11.
Robert Axelrod is an American political scientist who wanted to answer one of the oldest questions in social science with an experiment: when does cooperation emerge among self-interested actors who cannot rely on a central authority? Rather than argue about it, he decided to test it, by staging a computer tournament.
He invited game theorists from around the world to submit strategies for playing the repeated prisoner's dilemma, where the same two players face the cooperate-or-defect choice over and over. The strategies played each other hundreds of times, and Axelrod tallied the scores.
The winner, submitted by the psychologist Anatol Rapoport, was the simplest program entered: Tit for Tat. It cooperates on the first move, then does whatever the opponent did last time. Axelrod ran the tournament again in 1980 with more entrants who knew the first result, and Tit for Tat won again. From the results he distilled the properties of successful strategies: be nice (never defect first), be provocable (retaliate against defection), be forgiving (return to cooperation once the opponent does), and be clear (be easy to read so cooperation can take hold).