The evolution of cooperation among animals, either within or between species, presents an obvious problem for Darwinists. Darwin himself recognised this. He thought that social behaviour, as shown by social insects, could be explained by family selection. He also remarked that his theory would be disproved if it could be shown that some property of one species existed solely to ensure the survival of another; by implication, mutualism between species must be explained by indirect benefits to the individuals performing cooperative acts.

Despite these essentially correct insights, little progress was made in analysing the selective forces responsible for the evolution of cooperative behaviour for 100 years after the publication of The Origin of Species. For many biologists, it was enough that a trait could be seen to favour the survival of the species, or even of the ecosystem. It is clear from the writings of J.B.S. Haldane and R.A. Fisher that they knew better than this. The decisive turn to the significance of kinship in social behaviour, however, was by Hamilton (1964). Perhaps inevitably, the elegance of the idea of inclusive fitness and the prevalence of genetic relationship between members of social groups tended to distract attention from other selective forces, and in particular from mutualistic effects; that is, from the fact that two (or more) individuals may cooperate because it benefits both to do so. However, the significance of mutualistic effect was not lost sight of (e.g. Michener, 1974; West-Eberhard, 1975), and Trivers (1971) pointed out the possibility of delayed reciprocation.