Concept

Tit for tat

Summary
Tit for tat is an English saying meaning "equivalent retaliation". It developed from "tip for tap", first recorded in 1558. It is also a highly effective strategy in game theory. An agent using this strategy will first cooperate, then subsequently replicate an opponent's previous action. If the opponent previously was cooperative, the agent is cooperative. If not, the agent is not. Tit-for-tat has been very successfully used as a strategy for the iterated prisoner's dilemma. The strategy was first introduced by Anatol Rapoport in Robert Axelrod's two tournaments, held around 1980. Notably, it was (on both occasions) both the simplest strategy and the most successful in direct competition. An agent using this strategy will first cooperate, then subsequently replicate an opponent's previous action. If the opponent previously was cooperative, the agent is cooperative. If not, the agent is not. This is similar to reciprocal altruism in biology. The term developed most concretely in Northern Ireland, to describe increasing eye for an eye mentality, amongst the Irish Republicans and Ulster Unionists. This can be seen with the Red Lion Pub bombing by the IRA being followed by the McGurk's Bar bombing, both targeting civilians. Specifically the attacks of massacres would be structured around the mutual killings of Protestant and Catholic communities, both communities being generally uninterested in the violence. This sectarian mentality led to the term "Tit for tat bombings" to enter the common lexicon of Northern Irish society. The success of the tit-for-tat strategy, which is largely cooperative despite that its name emphasizes an adversarial nature, took many by surprise. Arrayed against strategies produced by various teams it won in two competitions. After the first competition, new strategies formulated specifically to combat tit-for-tat failed due to their negative interactions with each other; a successful strategy other than tit-for-tat would have had to be formulated with both tit-for-tat and itself in mind.
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