  The concept of reciprocal altruism is closely related to the Tit-for-Tat strategy in the iterated Prisoner's Dilemma (IPD) from game theory. In the IPD, players interact on multiple occasions, and are able to adjust their behaviour depending on what their opponent has done in previous rounds. There are two possible strategies, co-operate and defect; the payoff matrix (per interaction) is as in section 2.1 above. The fact that the game is iterated rather than one-shot obviously changes the optimal course of action; defecting is no longer necessarily the best option, so long as the probability of subsequent encounters is sufficiently high. In their famous computer tournament in which a large number of strategies were pitted against each other in the IPD, Axelrod and Hamilton (1981) found that the Tit-for-Tat strategy yielded the highest payoff. In Tit-For-Tat, a player follows two basic rules: (i) on the first encounter, cooperate; (ii) on subsequent encounters, do what your opponent did on the previous encounter. The success of Tit-for-Tat was widely taken to confirm the idea that with multiple encounters, natural selection could favour social behaviours that entail a short-term fitness cost. Subsequent work in evolutionary game theory, much of it inspired by Axelrod and Hamilton's ideas, has confirmed that repeated games permit the evolution of social behaviours that cannot evolve in one-shot situations (cf. Nowak 2006); this is closely related to the so-called 'folk theorem' of repeated game theory in economics (cf. Bowles and Gintis 2011). For a useful discussion of social behaviour that evolves via reciprocation of benefits, see Sachs et al. 2004.