Iterated prisoner's dilemma in an asocial world dominated by loners, not by defectors.

Cooperation among genetically unrelated individuals can arise when pairs of individuals interact repeatedly in the Prisoner's Dilemma. However, the conditions allowing the evolution of reciprocal cooperation become extremely restrictive as the size of the cooperative group increases, because defectors can exploit cooperators more efficiently in larger groups. Here we consider three strategies: Tit for Tat, defector, and loner. Loner beats defector in a non-cooperative world. However, a cooperative strategy Tit for Tat (TFT(0)) that stops cooperation after the first iteration when there is at least one defector in the group, can invade a world of loners, even in sizable groups, if both the TFT(0) and the defector strategies arise at the same frequency by mutation.