According to modern evolutionary theory, the fitness of individuals consists of both their personal reproductive success and the reproductive success of those with whom they share genes in common. It follows that one of the most biologically costly events possible is the death of a child. This study investigated the grief intensity of bereaved parents and their immediate families using ratings made by 263 bereaved parents. Predictions were derived from sociobiological tenets relating to parental investment, paternal uncertainty, and the propagation potential of both parents and children. Consequently, it was found that mothers grieved more than fathers, healthy children were grieved for more than unhealthy children; male children were grieved for more than female children; health of child and sex of child interacted such that the pattern of grief intensity obtained was healthy male greater than healthy female = unhealthy female = unhealthy male; similar children were grieved for more than dissimilar children; maternal grandmothers grieved more than either maternal grandfathers or paternal grandmothers, who in turn grieved more than paternal grandfathers; and mothers' siblings grieved more than fathers' siblings.