Repeated Games & Tournaments

In a one-shot Prisoner's Dilemma, defection is the dominant strategy. But when the game is played repeatedly, cooperation can emerge. The shadow of the future changes the calculus — being nice today can pay off in future rounds.

Around 1980, political scientist Robert Axelrod ran a famous computer tournament, inviting experts to submit strategies for the iterated Prisoner's Dilemma. The winner? Tit-for-Tat — the simplest strategy of all: cooperate on the first move, then copy whatever the opponent did last round.

Key insight: Tit-for-Tat succeeds because it is nice(starts with cooperation), retaliatory (punishes defection immediately),forgiving (returns to cooperation if opponent does), and clear(opponent can easily understand its behavior).

• Tit-for-Tat — Cooperate first, then mirror opponent. Simple and effective.
• Grim Trigger — Cooperate until opponent defects once, then defect forever. Unforgiving.
• Pavlov — Win-stay, lose-shift: cooperate if both players made the same move last round, defect otherwise.
• Suspicious Tit-for-Tat — Like TfT but starts with defection. Struggles against itself.
• Tit-for-Two-Tats — Only retaliates after two consecutive defections. Very forgiving.
• Always Defect — Exploits cooperators but scores poorly against itself.
• Random — 50/50 coin flip each round. The baseline.