Abstract
The causes underlying sex differences in lifespan are strongly debated. While females commonly outlive males in humans, this is generally less pronounced in societies before the demographic transition to low mortality and fertility rates. Life-history theory suggests that reduced reproduction should benefit female lifespan when females pay higher costs of reproduction than males. Using unique longitudinal demographic records on 140,600 reproducing individuals from the Utah Population Database, we demonstrate a shift from male-biased to female-biased adult lifespans in individuals born before versus during the demographic transition. Only women paid a cost of reproduction in terms of shortened post-reproductive lifespan at high parities. Therefore, as fertility decreased over time, female lifespan increased, while male lifespan remained largely stable, supporting the theory that differential costs of reproduction in the two sexes result in the shifting patterns of sex differences in lifespan across human populations. Further, our results have important implications for demographic forecasts in human populations and advance our understanding of lifespan evolution.
Highlights
One little considered explanation for such variation in sex differences in human lifespan is the differing costs of reproduction affecting females and males at different times and in different populations
We found clear evidence that sexual dimorphism in adult lifespan changed across the transition of the population from high to low mortality and fertility rates
In contrast to previous studies, which tend to treat time as a confounding variable (e.g.38), we here made use of the fact that the study population went through dramatic changes in fertility and lifespan over the study period, and investigated how this influenced the sexual dimorphism in adult lifespan
Summary
One little considered explanation for such variation in sex differences in human lifespan is the differing costs of reproduction affecting females and males at different times and in different populations. Despite the generally lowered fertility after the demographic transition and rising rates of nulliparity in modern nations, large variation remains, and the average number of children varies between slightly over one in contemporary Europe[21] to around ten in the Hutterites, an Anabaptist group that practices communal living and shuns birth control[47] These previous findings suggest that, given the premise that reproduction is more costly in females than males, a recently reduced cost of reproduction has had a more pronounced effect on female life-history trade-offs, leading to a more female-biased longevity in modern populations. We predict that sexual dimorphism in adult lifespan should change over time if fertility patterns change
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