TitforTat

Tit-for-tat is a simple strategy in the iterated prisoner's dilemma. It begins with cooperation and then copies the opponent's previous move: if the opponent cooperated in the prior round, you cooperate in the current round; if they defected, you defect in the current round. This makes tit-for-tat nice (it starts with cooperation), retaliatory (it punishes defection), and forgiving (it returns to cooperation if the opponent does).

Origin and prominence: The strategy gained widespread attention in game theory through Robert Axelrod's computer tournaments in the 1980s, as described in The Evolution of Cooperation (1984). Tit-for-tat performed unusually well against a variety of opponents, and its simple rule made it a focal point for studying how cooperative behavior can emerge in populations of rational agents.

Mechanics and implications: As a memory-one strategy, tit-for-tat is easy to implement and robust in many environments where interactions repeat over time. It tends to promote mutual cooperation when both players expect future interaction. However, it is sensitive to errors or noise: a single accidental defection can trigger a chain of retaliations, reducing overall payoffs. The strategy can struggle against opponents that defect for extended periods or against noisy environments where miscommunications occur.

Variants and criticisms: Generous tit-for-tat relaxes strict retaliation by occasionally forgiving defections to sustain cooperation in noisy settings. Other approaches, such as grim trigger, retaliate permanently after defection, which can deter cooperation. Critics note that tit-for-tat’s effectiveness depends on the structure of the interaction, such as the likelihood of future encounters and the presence of errors.

Applications: Tit-for-tat informs analyses of cooperation in economics, biology, and social science, and serves as a baseline strategy in multi-agent systems and experimental studies of the prisoner’s dilemma and related repeated games.