Timing of natural menopause covaries with timing of birth of a first daughter: evidence for a mother-daughter evolutionary contract?
Age at natural menopause is characterized by significant variability, but the factors responsible for this observed variation are still not well recognized. Humans are cooperative breeders and non-reproducing grandmothers play important roles in raising children. We propose an evolutionary "mother-daughter contract" hypothesis that suggests that the oldest daughter helps her mother to raise younger siblings but, in return, expects her mother to cease her reproduction, shifting energy and time once her daughter's children are born. Data were collected by a questionnaire from 914 Polish postmenopausal women. From among those, 506 women, 44-98 years old, who had at least one child and who went through a natural menopause were included in the analysis. A woman's age at menopause was sensitive to the age at which she had her first daughter. The age of giving birth to the first daughter, even when she was not her first child, positively correlated with the age of the mother's menopause (N=332, p<0.02), while the age of giving birth to a first son did not have a statistically significant effect (N=332, p=0.36). Results of our study suggest that research on the menopausal transition should take into account mother-daughter relationships as potentially important determinants of the timing of menopause.