Prisoner's Dilemma: What Game Are You Playing?
Farnam Street (Shane Parrish), after Robert Axelrod · 2017
"The Prisoner's Dilemma shows that individually rational self-interest can produce a jointly worse outcome for everyone — but when the same game repeats indefinitely with an unknown end point, cooperation becomes far more likely to emerge and persist."
In the single-round version, both prisoners rationally defect (each serves 2 years) even though mutual cooperation (each serving just 1 year) would have been better for both — because neither can trust the other not to defect, and the cost of being the lone cooperator (3 years) is too high. Real-world versions of this same structure appear in oligopoly pricing (e.g., OPEC members individually tempted to overproduce oil despite a mutual agreement to restrict supply) and arms races.
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Tit For Tat
"In repeated (iterated) games, the most effective strategy is tit for tat: cooperate first, then simply mirror whatever your counterpart did last time — ideally with occasional forgiveness built in to escape cycles of mutual retaliation."