Marriage patterns drive fertility decline

Researchers Duncan Gillespie, Dr Virpi Lummaa and Dr Andrew Russell, from the University´s Department of Animal and Plant Sciences, studied Finnish church records from the 18th and 19th centuries, a time during which almost everyone married and divorce was forbidden, to trace the survival and marriage histories of 1,591 women.

They found that women aged 30-35 were the most likely to be married. Those that married wealthy husbands were married at a younger age but to relatively older men, thereby gaining the family size-benefits of wealth but also an increased risk of widowhood. This high chance of widowhood, coupled with low re-marriage prospects for older widows with children, limited the percentage of women in the population with the opportunity to reproduce at older ages.

In today´s society however, women do not start childbearing until an older age as marriage is often delayed, and casual or short-term relationships and divorce are more common. As a result, the natural selection maintaining young-age fertility might weaken and the relative strength of natural selection on old-age fertility could increase, something that could potentially lead to improvements in old-age fertility over many generations.

Duncan Gillespie from the University of Sheffield´s Department of Animal and Plant Sciences, said: "In today´s society, family-building appears to be increasingly postponed to older ages, when relatively few women in our evolutionary past would have had the opportunity to reproduce. As a result, this could lead to future evolutionary improvements in old-age female fertility.

"Childbearing within a relationship is still the norm in modern society, but at ages where fewer women have the chance to reproduce, we should expect the evolution of lower fertility."