In a one-shot prisoner's dilemma, defection is rational. But life rarely consists of one-shot games. In repeated interactions, the shadow of the future changes everything - cooperation can emerge and be stable.

Robert Axelrod's famous tournaments proved that simple, cooperative strategies beat aggressive ones. Tit-for-tat - cooperate first, then mirror your partner's previous move - won by being nice, retaliatory, forgiving, and clear.

The Folk Theorem proves that almost any outcome can be sustained in repeated games if players are patient enough. Reputation becomes your most valuable asset - it determines whether others will cooperate with you.

In this topic, you'll learn why trust can be rational, how to build it strategically, when forgiveness beats punishment, and how to design relationships that sustain cooperation. Aristotle's philia - friendship based on mutual benefit - emerges naturally from repeated strategic interaction.