An unresolved issue in research on child survival is the extent to which familial mortality risk in infancy is due to biological influences net of sociodemographic and economic factors. We examine the effect of consanguinity on early childhood mortality in an Old Order Amish settlement by using the inbreeding coefficient, an explicit measure of the degree of relatedness in one's ancestry. Inbreeding has a net positive effect on neonatal and postneonatal deaths. We find social, demographic, and population-based sociocultural explanations for this effect among the Amish population which is known to experience certain genetically transmitted defects associated with mortality.