Child Fostering and Repartnering Among Tsimane Mothers: Evidence from a Competing Risks Survival Analysis

Abstract

Child fostering is widespread in much of the world, yet the evolutionary mechanisms shaping fostering decisions remain poorly understood. One possibility is that fostering can be strategically leveraged to improve mothers’ reproductive prospects after union dissolution by facilitating repartnering opportunities. In this study, we test whether Bolivian Tsimane mothers are more likely to have another child after fostering out one or more of their offspring, and whether this association differs depending on whether the subsequent birth occurs with the same partner or with a new partner. We use longitudinal demographic and residency data collected among Tsimane horticulturalists, including > 10 000 births to > 2000 mothers, and ~70 000 residence observations between 1999 and 2021. We analyze mothers’ birth hazards using a discrete-time competing-risk survival model, distinguishing between births with the same father and births with a different father. Our results show that fostering is associated with higher rates of subsequent childbearing, and that this association varies by paternal identity. These findings suggest that fostering may play a strategic role in shaping women’s reproductive trajectories and partner transitions in this population.