Human cooperation is more than by-product mutualism

Correspondence: B. Palameta, Department of Psychology, St Thomas University, Fredericton, New Brunswick, E3B 5G3, Canada (email: palameta@stthomasu.ca). The study of cooperation between unrelated individuals received a major boost from Axelrod & Hamilton’s (1981) classic formulation of an evolutionarily stable solution (Tit for Tat, TFT) to the Prisoner’s Dilemma (PD). More recent models have shown that TFT is not always evolutionarily stable (Nowak & Sigmund 1993): indeed one can always find a mixed strategy capable of invading any pure strategy, including TFT (Dugatkin 1997). However, TFT has remained the most frequently investigated strategy in experimental studies of cooperation. Observations consistent with TFT have been reported for various animal species (Lombardo 1985; Milinski 1987; Dugatkin 1988; Milinski et al. 1990; Huntingford et al. 1994). Recently, Clements & Stephens (1995) disputed such claims on the grounds that (1) the payoff structures of the behaviours in question did not necessarily conform to PD, and (2) alternative explanations were not considered. Clements & Stephens (1995) recorded the behaviour of blue jays playing carefully controlled PD and mutualism games, and found stable cooperation only in the latter situation. Because the payoff structure of mutualism games ensures that cooperation is the best choice regardless of what your partner/ opponent does, the occurrence of cooperation in such games is simply a by-product of individual animals maximizing their own immediate rewards without reacting to each other’s behaviour at all (Roberts 1997). Thus there is no need to invoke reciprocity or TFT as a necessary route to cooperation. Clements & Stephens (1995) and Stephens et al. (1997) suggest that by-product mutualism is the most parsimonious explanation for the occurrence of cooperation in a number of species, including humans. Clements & Stephens (1995) go so far as to state that ‘there is no empirical evidence of nonkin cooperation in a situation, natural or contrived, where the payoffs are