machines (that will eventually co-operate) explore the possibility that the other machine might be exploitable. Abreu and Rubinstein ( 1988 ) discuss this point in detail. Axelrod's ( 1997 , p. 21) new Complexity of Cooperation ignores the widespread criticism from game theorists discussed above. It recognises the unease registered by some biologists, but nevertheless reiterates the original claim that TIT-FOR-TAT embodies the essential features of a successful strategy for the indefinitely repeated Prisoners' Dilemma. But what of Axelrod's own discovery that evolution sometimes favours mean machines? Axelrod argues that mean machines did well only because they could exploit suckers. If his simulations had been run for longer, he claims that the mean machines which initially did well would have fallen by the wayside after eliminating the suckers on which their success depends. The intuition behind this argument is certainly valid for a population that initially consists only of strategies that always co-operate or always defect together with TIT-FOR-TAT, provided that no new strategies are allowed to intrude. If strategies that always co-operate dominate at the outset, then the strategy that always defects will do well initially, but then fade away altogether as the unconditional co-operators on which they prey become increasingly infrequent. But the intuition derived from this example doesn't extend to the case when unconditional defectors are replaced by TAT-FOR-TITs. If one starts the latter system off in the right basin of attraction, the final population will consist only of TAT-FOR-TITs. It is equally true that the final