Lineage-based differences in grandparental investment according to adverse early life experiences of grandchildren

Evolutionary theory predicts a downward flow of investments from older to younger generations that represents individuals’ efforts to maximize their inclusive fitness. Maternal grandparents and maternal grandmothers in particular have been consistently found to show the highest investment (e.g., time, care and resources) in their grandchildren. However, grandparental investment may depend on varying social and environmental conditions affecting the development of children, modifying the benefits and costs of grandparental investment. Using population-based survey data of English and Welsh adolescents, the present study investigates whether grandparental investment responds to adverse early life experiences (AELEs) of grandchildren. In contrast to current literature that considers grandparental investment in response to AELEs from the perspective of increased investment to meet the increased need, we predict that higher number of AELEs are associated with reduced grandparental investment as increased AELEs tend to reduce the reproductive value of grandchildren. Moreover, we predict that those grandparents who already invest less (i.e., paternal grandparents) react more strongly to elevated AELEs compared to those grandparents who invest the most (i.e., maternal grandparents and maternal grandmothers in particular). We found support for our predictions that maternal grandparents (maternal grandmothers in particular) showed investment in grandchildren that was unrelated to their grandchildren’s AELEs. In contrast, paternal grandparents reduced their investment in grandchildren in cases of increased AELEs. These findings were largely robust to measurement error in the of AELEs and confounding due to omitted shared causes.