In human families, the expected levels of involvement in child care differ for paternal and maternal grandparents (mostly due to paternity uncertainty). We tested how living with paternal versus maternal grandparents (or living without any grandparents) influenced fertility parameters of familes and the biological condition of children. 592 families having in total 2264 children from the Mogielica Human Ecology Study Site in rural Poland took part in the study. Women (at least 45 years of age) who during their reproductive years lived with parents-in-law were found to have significantly more children (average 5.8 children per woman) than women who lived with their own parents (average 3.9). Higher fertility of women who lived with their in-laws was achieved through extending their reproductive lifespans: they gave birth to their last child, on average, at the age of 36.7, which is 4.5 years later than did women who lived with their own parents (32.3 years). Average age of first reproduction was the same in both groups (23.5 years). Average weight and length of children at birth did not vary significantly among groups. One may speculate that paternal grandparents, who, because of paternity uncertainty, have a lower expected relatedness to each of their son’s children, maximize their inclusive fitness when the number of their grandchildren is higher. In contrast, the maternal grandparents benefit more by promoting the biological quality of their daughter’s children, rather than their numbers. The psychological and biological mechanisms through which such effects are achieved are under study.

Supported by the Center for Human and Primate Reproductive Ecology of Yale University and by the Polish National Centre for Science.