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Tit for Tat

Summary

The simplest and most effective strategy for the iterated prisoner's dilemma, winner of Robert Axelrod's 1980 tournaments. Starts with cooperation, then copies the opponent's last move — immediately retaliating and immediately forgiving.

The Strategy

  1. Start with cooperation — always
  2. Copy opponent's last move — if they cooperated, cooperate; if they defected, defect
  3. No memory beyond last move — forgive immediately when opponent cooperates again

Why It Wins

Robert Axelrod identified four properties:

Property What It Does
Nice Never defects first — prevents unnecessary trouble
Retaliatory Immediately defects back — discourages persistent defection
Forgiving Returns to cooperation when opponent does — restores mutual benefit
Clear Predictable and intelligible — elicits long-term cooperation

Tournament Results

  • 14 programs competed in iterated prisoner's dilemma (200 rounds each)
  • Tit for Tat never won any individual game (can only lose or draw one-on-one)
  • Won the tournament by cooperating with enough players to get highest total score
  • Won again in second tournament (62 programs, unknown round count)
  • Generous Tit for Tat (occasionally forgiving defections) proved even more effective in chaotic conditions

Life Lessons

  • Being nice is a strength — leading with cooperation prevents escalation
  • Holding grudges is a weakness — forgiveness restores mutual benefit
  • Consequence must be proportional, consistent, and clear
  • A strategy always focused on winning can be the least effective at winning
  • "What makes cooperation possible is the fact that the players might meet again."

In AI Agent Design

Tit for Tat principles apply to multi-agent systems: - Agents that always defect end up in mutual destruction - Agents that always cooperate get exploited - Tit for Tat agents establish stable cooperation with other cooperative agents while defending against defectors

See Also