Dozens of clever programs entered this contest. The four-line entry beat them all
In 1980, political scientist Robert Axelrod invited game theorists to submit computer programs to compete in a round-robin tournament of the iterated Prisoner's Dilemma, where the same two 'players' face each other over many rounds instead of just one. Sophisticated strategies running dozens of lines of code went up against the shortest submission of all, just four lines long. Which strategy came out on top?
Reveal the answer
The four-line program won, both in Axelrod's first tournament and a larger rematch. Submitted by game theorist Anatol Rapoport, it was called Tit for Tat: cooperate on the first move, then simply copy whatever the opponent did last time. Its success helped launch decades of research into how cooperation can emerge among self-interested players without any central enforcement.
— Robert Axelrod, The Evolution of Cooperation — 1984; tournament results first published 1980
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