Cost structures and socioecological conditions impact the fitness outcomes of human alloparental care in agent-based model simulations

Alloparental care—care provided to children who are not one's offspring—is ubiquitous across human populations. Empirical research reveals socioecological variation in who cares for children, but less attention has been paid to the type of care provided. To better understand the fitness outcomes of different forms of alloparental care, or allocare, we categorize such care into two broad forms based on economic cost structures: additive cost and declining marginal cost allocare. Additive cost allocare requires alloparents to pay equal costs for each child to whom care is provided, while declining marginal cost allocare entails reduced costs for additional children beyond the first. Given this general typology, we investigate how fitness is impacted by the type of allocare provided in socioecological conditions of scarcity or abundance. Results of an agent-based model indicate that allocare has fitness benefits in nearly all circumstances, but the impact of cost structures depends on resource availability. In contexts of abundance, the cost structure of allocare does not matter as individuals' reproductive success is instead constrained by fertility and mortality more than the availability of resources or time. In conditions of scarcity, however, the greatest increases in reproductive success are achieved when allocare has a declining marginal cost structure. This is due to an economy of scale permitting alloparents to scale up their care at discounted rates. Consequently, we expect allocare practices to exhibit these patterns cross-culturally: in contexts of scarcity allocare is anticipated to be focused on practices with declining marginal cost structures and to be much less variable than in contexts of abundance. We discuss several ethnographic examples that are consistent with the overall findings of our simulations, and we conclude with recommendations for future modeling and empirical work on allocare.