Tit-for-Tat

A simple reciprocal strategy — cooperate first, then mimic the opponent's previous action.

Ask the Game theory assistant 1 min read · Updated September 9, 2026

Definition

Strategy in repeated Prisoner's Dilemma: $a_1=C$ , $a_t=a_{t-1}^{(\text{opp})}$ for $t>1$ .

Niceness (never defects first), retaliation, forgiveness (single punishment), and clarity are its defining traits.

Simple, provocable, forgiving, and clear — the four virtues that made it win Axelrod's tournaments.

Encourages cooperation without being exploitable by defectors.

Axelrod's 1980 tournaments: Tit-for-Tat outperformed more complex entries in a round-robin of the repeated PD.

Generous and contrite variants handle noisy environments better.

Supports cooperation in the infinitely repeated PD as a subgame-perfect equilibrium when $\delta\ge(T-R)/(T-P)$ .

In noisy environments TFT falls into retaliation spirals; Generous TFT (Nowak-Sigmund) repairs this.

Emerges in biology, diplomacy, and online reputation.

Remains a canonical benchmark in multi-agent learning.

Assembled from the ReLU.chat curated knowledge base. These explanations are concise on purpose; check the sources for anything important.