The evolution of contingent cooperation

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.