Sources of Fitness Interdependence Associated with Shared Fate and Cooperation in a Small-scale Horticultural Society

Cooperation often relies on people’s ability to discern in whom to invest their limited resources. One solution to this partner choice dilemma entails estimating one’s fitness interdependence with others. Previous studies indicate that estimates of fitness interdependence (i.e., shared fate) motivate cooperation, but the sources of information that contribute to shared fate remain an elusive question. We examine links among ten sources of interdependence (i.e., attributes and experiences that yoke partners’ fitness), shared fate, and cooperation among the Mayangna, a small-scale horticultural society in Nicaragua. While eight sources of interdependence showed positive bivariate associations with shared fate, only relatedness, commensality, and shared subsistence activities were uniquely associated with higher shared fate. Moreover, shared fate (1) was associated with more cooperation across seven fitness-relevant domains, (2) statistically mediated associations between relatedness and cooperation, and (3) had strong effects on cooperation (i.e., forgoing money to buy rice for a partner). Results indicate that shared fate arises from sources of fitness interdependence and cooperation is proximally guided by shared fate. Estimates of fitness interdependence by way of shared fate may therefore offer a simple solution to partner choice dilemmas: Help partners if one has a positive stake in their welfare.