“Axelrod attributed the success of TFT [Tit For Tat] to four properties. It is nice, meaning that it is never the first to defect. The eight nice entries in Axelrod’s tournament were the eight highest ranking strategies. It is retaliatory, making it difficult for it to be exploited by the rules that were not nice. It is forgiving, in the sense of being willing to cooperate even with those who have defected against it (provided their defection wasn’t in the immediately preceding round). An unforgiving rule is incapable of ever getting the reward payoff after its opponent has defected once. And it is clear, presumably making it easier for other strategies to predict its behavior so as to facilitate mutually beneficial interaction.”
—The Stanford Encyclopedia of Philosophy. s.v., “Prisoner’s Dilemma.”.