Definition
Tit-for-tat (TFT) is a strategy for the iterated Prisoner's Dilemma: on the first move, cooperate; thereafter, do whatever the opponent did on the previous move. Submitted by Anatol Rapoport, the strategy won Robert Axelrod's 1980 round-robin tournament against more elaborate entrants — and won the rematch when participants knew what to beat.
The four properties that characterize tit-for-tat and other winning iterated-PD strategies are:
- Niceness — never the first to defect.
- Retaliation — punish defection immediately.
- Forgiveness — return to cooperation as soon as the opponent does.
- Clarity — the rule is simple enough that the opponent can recognize and adapt to it.
Why it matters
How it works
Tit-for-tat embodies reciprocity in its purest form. Cooperate when others cooperate; defect when others defect; do not initiate conflict; do not hold grudges. The rule's strength is twofold. First, it is robust: it does not depend on knowing the opponent's identity, history, or strategy. Second, it is invasion-resistant: a TFT-playing population is hard to invade by any exploiter strategy.
The strategy fails against unconditional defectors (it accepts one sucker payoff on the first move) and in highly noisy environments (one mistake can lock two TFT players into mutual defection). Most practical implementations introduce some forgiveness — Generous-TFT cooperates after defection with some probability ε, breaking the deadlock.