unconditionally in the first period and behave cooperatively (altruistically) as long as the other agent does as well. If chances of meeting another reciprocal altruist are high enough, or if the game is repeated for a long enough amount of time, this form of altruism can evolve within a population.

This is close to the notion of "tit for tat" introduced by Anatol Rapoport, although there still seems a slight distinction in that "tit for tat" cooperates in the