Child-rearing is incredibly burdensome for humans. Human children are entirely dependent upon human adults for their survival and stay "in the nest" longer than any other species. The presence of a grandmother in a family provides a source for additional child care, assistance during illnesses and pregnancies, and other child-rearing advantages. It's much easier for a woman to care for her children and produce additional healthy offspring if help is available. In tribal societies, the entire tribe may be the source of child-rearing assistance, but in western society, it has usually been the grandmother who has performed the role.

Bruce