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Full Idea
The successful TIT FOR TAT strategy (for the iterated prisoner's dilemma) says cooperate on the first round, then do whatever the other player did in the previous round.
Gist of Idea
TIT FOR TAT says cooperate at first, then do what the other player does
Source
William Poundstone (Prisoner's Dilemma [1992], 12 'TIT')
Book Ref
Poundstone,William: 'Prisoner's Dilemma' [OUP 1992], p.240
A Reaction
There are also the tougher TWO TITS FOR A TAT, and the more forgiving TIT FOR TWO TATS. The one-for-one seems to be the main winner, and is commonly seen in animal life (apparently). I recommend this to school teachers.