yet do so well on those interactions to make up for the low level of exploitation from non-nice strategies. Cooperation becomes more complicated, however, as soon as more realistic models are assumed that for instance offer more than two choices of action, provide the possibility of gradual cooperation, make actions constrain future actions (path dependence), or in which interpret the associate's actions are is non-trivial (e.g. recognizing the degree of cooperation shown) Subsequent work In 1984 Axelrod estimated that there were "hundreds of articles on the Prisoner's Dilemma cited in Psychological Abstracts", and estimated that citations to The Evolution of Cooperation alone were "growing at the rate of over 300 per year". To fully review this literature is infeasible. What follows are therefore only a few selected highlights. Axelrod has a subsequent book, The Complexity of Cooperation, which he considers a sequel to The Evolution of Cooperation. Other work on the evolution of cooperation has expanded to cover prosocial behavior generally, and in religion, other mechanisms for generating cooperation, the IPD under different conditions and assumptions, and the use of other games such as the Public Goods and Ultimatum games to explore deep-seated notions of fairness and fair play. It has also been used to challenge the rational and self-regarding "economic man" model of economics, and as a basis for replacing Darwinian sexual selection theory with a theory of social selection. Nice strategies are better able to invade if they have social structures or other means of increasing their interactions. Axelrod discusses this in chapter 8; in a later paper he and Rick Riolo and Michael Cohen use computer simulations to show cooperation rising among agents who have negligible chance of future encounters but can recognize similarity of an arbitrary characteristic (such as a green beard). Whereas other studies have shown that the only Iterated Prisoner's Dilemma strategies that resist invasion in a well-mixed evolving population are generous strategies. When an IPD tournament introduces noise (errors or misunderstandings) TFT strategies can get trapped into a long string of retaliatory defections, thereby depressing their score. TFT also tolerates "ALL C" (always cooperate) strategies, which then give an opening to exploiters. In 1992 Martin Nowak and Karl Sigmund demonstrated a strategy called Pavlov (or "win–stay, lose–shift") that does better in these circumstances. Pavlov looks at its own prior move as well as the other player's move. If the payoff was R or