Men appear to be evolving to be sexier, researchers have found.
According to a new paper in the journal Biology Letters, men are getting increasingly "sexy and formidable," gaining in height and weight at twice the rate that women are.
This means that sexual dimorphism—the physical differences between the males and females of a species—is increasing for humans, all thanks to improved standards of living over time.
"With cross-national and cross-generational improvements in living conditions, where environmental stressors recede, men's gains in height and weight are more than double those of women's, increasing sexual size dimorphism," the researchers wrote in the paper.
In many species, one sex is larger than the other. For example, male lions are larger than females, whereas in some species—including certain spiders and angler fish—females are larger than males.
Traits that enhance mating success like vibrant colors or larger body size in males often evolve due to female choice or male competition, evolving because females prefer them or because they help them fight off other males. Dimorphism can also develop from differing survival strategies between sexes, such as females needing better camouflage for nesting.
Humans likely evolved our sexual dimorphism—with males being taller and stronger than females, on average—due to physical competition between males for status and resource control. As a result, these traits are generally found more sexually appealing by women.
"Consequently, this is why men assess the formidability of potential opponents using physical cues, such as upper body strength, and women find taller, muscular men with a relatively large overall body mass (but not obese) particularly attractive," the researchers wrote.
However, being taller and more muscular comes with greater evolutionary costs, as a male must eat a lot and also be physically healthy both during childhood and as an adult to achieve and maintain such traits.
In our modern world, accessing food and avoiding disease is a lot easier than it was throughout much of human history.
The researchers describe in the paper how they analyzed World Health Organization (WHO) data on changes in height and weight of 135,000 people across 62 countries with changes in disease, as well as height data from elsewhere.
They found that as disease burden decreased, the height of men increased at over twice the rate in men compared to women.
"Our cross-national analyses suggest that as the social and ecological conditions of nations improve, including reductions in overall disease burden, people's height and weight increase, but more than twice as much in men as in women resulting in greater [sexual size dimorphism]," the researchers wrote.
"Based on our main finding of [sexual size dimorphism] being greater in more favorable environments, it is clear that the development and maintenance of gross morphology is more sensitive to living conditions in men than in women, at least in terms of height and weight."
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References
Giofrè, D., Geary, D.C., Halsey, L.G. (2024). The sexy and formidable male body: men's height and weight are condition-dependent, sexually selected traits. Biology Letters, 20. https://doi.org/10.1098/rsbl.2024.0565
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