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Marriage patterns drive fertility decline
Researchers at the University of Sheffield have applied an evolutionary 'use it or lose it' principle when studying past marriage patterns to show that marriage can influence the evolution of age-patterns of fertility.
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 who 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 remarriage 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.