In a world of self-interest, cooperation emerges in repeated Prisoner’s Dilemma through Tit for Tat. Cooperate on the first move, then copy exactly what the opponent did in the previous round. Tit for Tat is nice, retaliatory, forgiving, and clear. It thrives with cooperators, protects against cheaters, and recovers from temporary defections. Be nice until given a reason not to, punish bad behavior but forgive, and keep it simple.