Tit for tat
Tit for tat is an English saying meaning "equivalent retaliation". It is also a highly effective strategy in game theory
Game theory
Game theory is a mathematical method for analyzing calculated circumstances, such as in games, where a person’s success is based upon the choices of others...

 for the iterated prisoner's dilemma. It was first introduced by Anatol Rapoport
Anatol Rapoport
Anatol Rapoport was a Russian-born American Jewish mathematical psychologist. He contributed to general systems theory, mathematical biology and to the mathematical modeling of social interaction and stochastic models of contagion.-Biography:...

 in Robert Axelrod
Robert Axelrod
Robert M. Axelrod is an American political scientist. He is Professor of Political Science and Public Policy at the University of Michigan where he has been since 1974. He is best known for his interdisciplinary work on the evolution of cooperation, which has been cited in numerous articles...

's two tournaments, held around 1980. An agent
Agent (grammar)
In linguistics, a grammatical agent is the cause or initiator of an event. Agent is the name of the thematic role...

 using this strategy will initially cooperate, then respond in kind to an opponent's previous action. If the opponent previously was cooperative, the agent is cooperative.