In a world of repeated dealings, what beats treachery? Not saintliness — a saint gets fleeced. The winner Axelrod found was almost embarrassingly simple: cooperate first, then do whatever the other did last time. Nice, provokable, forgiving. Slide the rounds and watch reciprocity out-earn both the pushover and the traitor.
Iterated Prisoner’s Dilemma, payoffs T=5>R=3>P=1>S=0. Tit-for-tat cooperates on round 1, then copies the opponent’s last move. Its strength is STABILITY, not domination: in an all-TFT world every pair reaps R each round, but a defector who invades only earns P after the first betrayal — so cooperation cannot be undercut. A fail-loud self-check throws unless TFT-vs-TFT (all cooperate) strictly out-scores an AllD mutant against TFT, i.e. AllD cannot invade — TFT is an ESS.
HONEST: TFT does NOT win every setting. In a raw one-shot round-robin, pure AllD can edge it out by savaging unconditional cooperators (AllC) — shown here. TFT wins ECOLOGICALLY: once naive cooperators are gone, TFT-rich populations reap R while defectors are stuck at P (Axelrod 1984). Reciprocity, not niceness, is what holds.