Having children plays a complicated role in the rate we age

For millennia we have tried to understand why we age, with the Greek philosopher Aristotle suggesting that this happens alongside the gradual drying up of the internal moisture necessary for life.

In modern times, a seminal idea known as the disposable soma hypothesis suggests that aging is the price we pay for reproduction, with evolution prioritizing the transmission of genes above all else. This creates a fundamental trade-off: the immense energy devoted to having and raising offspring comes at the expense of repairing DNA, fighting disease, and maintaining organs in good condition.

This may particularly apply to women, who invest more in reproduction than men through pregnancy and breastfeeding. However, when scientists tested this hypothesis by seeing whether women with more children lived shorter lives, the results were mixed: Some studies supported the idea, while others found no effect.

“It is very difficult to disentangle what is simply correlation [between having more children and a shorter life] and what is the underlying causality, unless you have a large dataset spanning multiple generations,” says Elisabeth Bolund of the Swedish University of Agricultural Sciences, who was not involved in the study.

Euan Young, of the University of Groningen in the Netherlands, and colleagues hypothesized that the inconsistency between studies exists because the cost of reproduction is not fixed: it depends on the mother’s environment. “In good times, this trade-off isn’t really visible. The trade-off only becomes apparent when times are tough,” says Young.

To study this idea, researchers analyzed church records of more than 4,500 Finnish women, spanning 250 years. These include the period of the Great Finnish Famine from 1866 to 1868, which makes it possible to assess the impact of difficult times on reproduction and longevity, says Young.

They found that among women who lived before or after the famine or who did not have children during it, there was no significant association between the number of children they had and their lifespan. However, for women who had children during the famine, their life expectancy decreased by six months for each child they had.

The study builds on research published last year that used a dataset from a pre-industrial population in Quebec, Canada, monitored for two centuries, that showed this trade-off in mothers who were likely in poor health or under great stress, but did not explore how this was affected by specific environmental conditions.

In contrast, Young’s team points to a specific catastrophic event as the driver that exposes the tradeoff for mothers. “This very large data set makes it possible to take into account confounding factors [such as genetics and lifestyle factors]”, says Bolund. “The study brings us as close as possible to identifying causation without conducting a controlled laboratory experiment.”

The study also confirms the energy needs of pregnancy and breastfeeding, which require hundreds of extra calories per day. During a famine, women can’t get this energy from food, so their bodies pay the price, “by lowering basal metabolism.” [the minimum number of calories your body needs to function at a basic level] and thereby slowing or stopping other important functions, leading to decline in health and shorter lifespan,” Young says. This also explains why previous studies have sometimes found a trade-off only in lower socioeconomic groups, who actually still lived in relatively resource-scarce environments, he says.

According to Bolund, the fact that this trade-off appears to occur in particularly difficult circumstances, and when women typically have many children, may partly explain why women generally live longer than men today, with girls born between 2021 and 2023 in the UK expected to live four years longer than their male counterparts.

Reproductive costs are now quite low in Western societies, where the average number of children women give birth to has declined significantly over the centuries, Bolund says. As a result, few women today are likely to reach the threshold where the cost of their lives becomes evident. Bolund and colleagues’ research on a historical population in Utah, for example, found that this only appeared when women had more than five children — well below the 1.6 births the average American woman is expected to have in her lifetime.

Other environmental factors could therefore become more important in explaining the lifespan gap between men and women. Men tend to be more likely to smoke than women and also drink more alcohol, which affects lifespan, says Bolund. The current longevity gap between men and women is likely due to a combination of the latter’s reduced reproductive costs compared to other times in history and lifestyle differences between the sexes.

Research also suggests that sex chromosomal differences are involved. “The sexes differ in multiple ways, beyond reproductive costs. So we need to do more research on how different factors contribute to sex-specific aging,” says Young.