Pair-bonding modifies the age-specific intensities of natural selection on human female fecundity.

In many animals, including humans, the ability of females to reproduce depends not only on their survival to each age but also on being pair-bonded to a mate. Exposure of the genetic variation underlying fecundity to natural selection should therefore depend on the proportion of females both alive and pair-bonded. In spite of this, female "marital" status is seldom considered to impact the strength of selection on age-specific fecundity. We used marriage-history data of preindustrial Finns who experienced conditions of natural mortality and fertility to investigate how assortative mating by age and socioeconomic status affected female fitness and underlay age-specific female marriage patterns. The probability that a female was married peaked at age 30-40 years; females who married in their early 20s to high-socioeconomic-status husbands had the highest levels of lifetime reproductive success. Greater age difference between the pair, which is typical for females who are married to high-socioeconomic-status husbands, increased the likelihood of widowhood occurring premenopause, adding to declines in the proportion of genetic variation exposed to selection with age. Using the age schedule of female marriage, we present an indicator of selection intensity on within-pair-bond fecundity. Our results suggest that the decline in selection intensity after age 30 years is a factor in the evolutionary maintenance of female reproductive senescence and menopause.