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This chapter examines social behaviors that are expressed conditional on the phenotype of others. David Queller argued that inclusive fitness analyses need to be done on a per-behavior basis, citing as an example the decision over whether to reproduce directly, and whether to aid a reproductive. Queller showed that inclusive fitness predictions are only sensible when one analyzes what an individual should do, given it finds itself in a particular behavioral role. The chapter first provides an overview of implicit and explicit conditionality and presents two classic examples: William D. Hamilton's greenbeard traits and Robert Trivers's theory of reciprocal cooperation. It also introduces an extension of Hamilton's rule to deal with explicitly conditional behaviors; this extension features a measure of phenotypic assortment that appears not to be the classic genetic relatedness of Hamilton's rule.

This chapter defends an essentially individualist model of the evolution of cooperation amongst Pleistocene foragers. The chapter argues that these foragers satisfied the conditions of the “folk theorem” of stable reciprocation-based cooperation. Their social groups were small and stable, with very high probabilities of future interaction. They were informationally transparent: reputation was very reliable. The rewards of cooperation were high, and the temptations to defect were rarely large. The costs of sanctioning defection were typically modest, and reputation was reliable enough and valuable enough to make investments in reputation through cooperating in controlling defection worth paying. Foragers cooperated because it was almost always in their individual interest to do so. The chapter argues against the idea that group selection drove the evolution of foraging cooperation; these views underestimate the costs of conflict; overestimate its benefits, and are not supported by the archaeological record. Third, the chapter shows that the selective environment changed at the Pleistocene-Holocene transition, as groups became sedentary and larger. These changes eroded the forager equilibrium, moreover group-group conflict is much more significant in the archaeological record from this time. While much remains unclear about the origins of farming and more complex social units, it is clear that the evolutionary basis of cooperation changed radically through this transition.