Reciprocal altruism has been proposed as a foundation of cooperation in humans. The core feature of reciprocity is the contingent relationship between acts of giving and receiving among social partners. However, contingency has remained largely an elusive concept with little empirical scrutiny. Food sharing in small-scale, nonmarket societies is a classic context for examining conditional cooperation. The debate concerning whether food sharing is a crucial component of household provisioning or a form of display geared toward personal benefit hinges on whether food is given conditionally. Several forms of contingency are defined here, and it is shown that a significant contingency relationship exists in food exchange among two groups of forager-agriculturalists, the Ache of Paraguay and the Hiwi of Venezuela. Exchange imbalances tend to favor lower-producing families, close kin, and nearby neighbors. These results have implications for understanding fairness in forager societies.

This study investigates the consequences of the human foraging niche and multiple dependent offspring on the optimal growth trajectory of humans. We test the hypothesis that the human pattern of slow human growth between age at weaning and puberty helps defer the compound energetic demand on parents with multiple dependents, by using growth and demographic data from two foraging societies, the Ache of eastern Paraguay and the Dobe Ju/'hoansi of Botswana and Namibia. We run simulations of observed and potential growth trajectories among sub-adults and their consequent energetic demands on parents given profiles of fertility, mortality, consumption and production. We find that either sub-adult production or food subsidies from other people must substantially increase in order to compensate for the dramatic increase in energetic demand on parents if offspring were to grow faster at younger ages. Our conclusion is that slow human growth followed by a rapid adolescent growth spurt may have facilitated rising human fertility rates and greater investments in neural capital.