In a previous post, I discussed how Western European societies used to postpone the age of first reproduction to the mid-20s. When this cultural pattern began is unsure, but it certainly preceded the Black Death of the 14th century and may have existed as early as the 9th century. It seems to have resulted from a combination of land scarcity and the rule of primogeniture, i.e., farms were kept intact and handed over to the eldest son when the parents died or retired. In such a situation, young couples had to wait until they had a farm of their own—and the means to start a family.
How late can a woman postpone having her first child and still be sure of perpetuating her lineage? In her mid 30s? That answer might be true today. In pre-modern societies, however, women had to start earlier. A recent study by Liu and Lummaa (2011) puts it no later than 30. Beyond 30, a woman would be faced with declines in both offspring quantity and offspring quality. Her risk of genetic extinction was proportionately higher.
In a study of rural Finnish parish records from the 18th and 19th centuries, Liu and Lummaa (2011) found that the women were 26 years old on average when they first gave birth. They had thus foregone the first decade of their reproductive life, something that would be unheard of in most traditional human societies. Yet they nonetheless managed to have 6.54 children on average.
Among these women, offspring quantity decreased with increasing age of first reproduction (AFR). This was partly because the time window for reproduction was narrower and partly because more of the reproduction was taking place during years of reduced fertility. Offspring quality held constant until the age of 30 and then too decreased.
The study defined “offspring quality” as the probability that one’s children will survive to adulthood and have children of their own. On average, 60% of the offspring survived to 15 years of age and 47% had children of their own. AFR did not affect offspring quality among mothers under 30. Over 30, higher AFR was associated with a lower probability of the children surviving to adulthood and a lower probability that the surviving children would have families of their own.
Why did fewer of these children survive to adulthood? The data provide no direct answers. One reason may be that older mothers tend to have children with lower birth weights, which in turn may lead to early death. Older mothers are also at risk of having children with birth defects.
And why did fewer of the surviving children have children of their own? Again, the data provide no direct answers. It may be that many of these children were physically or behaviorally compromised and thus less able to attract potential mates.
What does all of this mean for us today? In the short term, it means that a large part of the current population is headed towards genetic extinction. In the long term, there will be selection for increased fertility 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.” (Davis, 2011)
Davis, S. (2011). Marriage patterns drive fertility decline,
Liu, J. and V. Lummaa. (2011). Age at first reproduction and probability of reproductive failure in women, Evolution and Human Behavior, 32, 433-443.