Some say children keep you young, but it’s complicated
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For millennia, we have tried to understand why we age, with the ancient Greek philosopher Aristotle proposing it occurs alongside the gradual drying up of the internal moisture necessary for life.
In modern times, a leading idea known as the disposable soma hypothesis suggests that ageing is the price we pay for reproduction, with evolution prioritising the passing on of genes above all else. This creates a fundamental trade-off: the immense energy devoted to having and raising offspring comes at the cost of repairing DNA, fighting off illness and keeping organs in good shape.
This may particularly apply to women, who invest more in reproduction than men via pregnancy and breastfeeding. However, when scientists have tested this hypothesis by checking if women with more children live shorter lives, the results have been mixed: some studies support the idea, while others have found no effect.
“It is very difficult to disentangle what is just correlation [between having more children and a shorter life] and what is the underlying causation, unless you have a good, big dataset that covers several generations,” says Elisabeth Bolund at the Swedish University of Agricultural Sciences, who wasn’t involved in the study.
Euan Young at the University of Groningen in the Netherlands and his colleagues hypothesised that the inconsistency between studies exists because the cost of reproduction isn’t fixed – it depends on a 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 investigate this idea, the researchers analysed the parish records of more than 4500 Finnish women, spanning 250 years. These included the period of the Great Finnish Famine from 1866 to 1868, providing a means to gauge how hard times affect reproduction and longevity, says Young.
They found that among the women who lived before or after the famine or who didn’t have children during it, there was no significant association between the number of children they had and their lifespan. However, for the women who did have children during the famine, their life expectancy decreased by six months for every child they had.
The study builds on research published last year that used a dataset from a pre-industrial population in Quebec, Canda, monitored over two centuries, which showed this trade-off in mothers who were probably in poor health or under great stress, but didn’t 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 trade-off for mothers. “This very large dataset makes it feasible to account for confounding factors [such as genetics and lifestyle factors],” says Bolund. “The study gets us as close as we can to identifying causation without running a controlled experiment in the lab.”
The study also confirms the energetic demands 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, “lowering basal metabolism [the minimum number of calories your body needs to function at a basic level] and thus slowing or shutting down other important functions, resulting in a decline in health and shorter lifespans”, says Young. It also explains why previous studies sometimes found the trade-off only in lower socioeconomic groups, which were effectively always living in relatively resource-scarce environments, he says.
According to Bolund, the fact that this trade-off seems to occur in particularly tough circumstances, and when women typically had 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.
The costs of reproduction are now fairly low in Western societies, where the average number of children women give birth to has reduced considerably over the centuries, says Bolund. As a result, few women today will probably reach the threshold where the cost to their lifetime becomes obvious. Bolund and her colleagues’ research on a historical population in Utah, for instance, found this only appeared when women had more than five children – well below the 1.6 births that the average woman in the US is expected to have in her lifetime.
Other environmental factors may therefore become more significant 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 affect lifespan, says Bolund. The current longevity gap between men and women is probably a combination of the latter’s reduced reproductive costs compared with other times in history and lifestyle differences between the sexes.
Research also suggests that sex chromosomal differences are involved. “Sexes differ in a multitude of ways, beyond reproductive costs, so we need to conduct more research into how different factors contribute to sex-specific ageing,” says Young.
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