Sex-Specific Vulnerabilities

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– by David C. Geary

Sex differences are common across species and often related to sexual selection, that is, they are found for traits that influence intrasexual competition for mates and intersexual discriminative mate choices (Andersson, 1994). Sexually selected traits are often exaggerated in size or are physiologically demanding (e.g., of cardiovascular fitness) and thus are more sensitive to stressor exposure than other traits. For instance, sex differences in armament or coloration (e.g., plumage) are often driven by male-male competition or female choice, but these sex differences become smaller when males are exposed to stressors. This is because stressor exposure affects the exaggerated male trait more strongly than the same trait in females, although the same differential sensitivity should occur for traits showing a female advantage. The key, universal stressors are well captured by the Horsemen of the Apocalypse – poor or inadequate nutrition (famine), disease (plague), and intense social competition (war; or predation for prey species).

The same principles apply to humans and can be used to identify age-, sex-, and trait-specific vulnerabilities (Geary, 2015). The basic idea can test tested for populations and individuals: Evolved sex differences are larger in healthy populations, and the magnitude of these differences becomes smaller as living conditional deteriorate. My colleagues and I tested this hypothesis using data from two large multi-national studies. The first was a British Broadcasting Corporation (BBC) study that included two spatial measures that typically favor men and the second was a study that included an emotion recognition measure (The Eyes Test) that typically favors women (Greenberg et al., 2022; Lippa et al., 2010). We confirmed the originally reported sex differences and then plotted their magnitude across countries that varied on the Human Development Index (HDI). Individuals living in low HDI countries are more likely to suffer from infectious diseases, poor nutrition and a larger overall disease burden than are individuals living in high HDI countries (Emadi et al., 2021).

In a prior study, we showed that men’s advantage in height and weight – both sexually selected traits related to male-male competition and female choice – increased with increases in HDI scores (Giofrè et al., 2025). Thus, sex differences in height can be used as a proxy for national differences in developmental health, with smaller differences indicating that large portions of the population are exposed to nutritional and disease stressors. Within low HDI countries, the sex difference in height is larger for wealthier individuals than for individuals growing up in poorer contexts. Using this pattern and height data from the BBC, we showed that the BBC data are biased toward healthier samples in lower HDI countries and thus will underestimate cross-national sex differences in spatial cognition.

Despite biased samples, we found that the magnitude of men’s advantage in spatial cognition increased with national increases in HDI scores, just as men’s advantage in height and weight increase with improvements in living conditions; women’s advantage in lifespan increases in these same countries and thus the effects are sex- and trait-specific (Medalia & Chang, 2011). However, women’s advantage in emotion recognition was stable across countries. It could be that men’s spatial abilities are more sensitive to poor early and current living conditions than women’s competence at recognizing emotions, but this is unlikely. Other studies suggest that women’s advantages in social cognition are compromised by extreme nutritional deficits and are smaller when they grow up in contexts with poor general living conditions (Bora & Köse, 2016). Our null finding for the Eyes Test could have been due to biased samples for lower HDI countries, but we were not able to assess this possibility due to the lack of height data in the corresponding study.

In any case, these results and others like them have theoretical and practical implications. Theoretically, these studies show that evolved human sex differences are not fixed, but rather their magnitude varies systematically with current living conditions and conditions during development, just like with other species (Geary, 2015). As living conditions improve and societies become more liberal and gender equal, sex differences for many traits become larger (Halsey & Geary, 2025; Herlitz et al., 2025). This pattern is now well documented and refutes strong social-constructivist theories on the origin of sex differences.

More practically, sex differences in trait-specific sensitivity can be used to identify at-risk populations and could be used diagnostically at the individual level (Geary, 2015). For instance, women have advantages in verbal memory, but these competencies decline more rapidly in women than men with the onset of dementia. Men’s well-developed visuospatial abilities are more severely compromised than their verbal abilities with exposure to various toxins. In other words, evolutionary theory and sex-specific vulnerabilities could be used to develop assessments that are more sensitive to various stressors or cognitive declines than assessments based on the assumption that deficits are equally diagnostic for women and men. By analogy, we would not identify growth deficits using height or weight charts averaged across boys and girls. Rather, any such diagnosis needs to be based on sex-specific norms and same may apply to many more traits that show sex differences.

Geary, D. C, Giofrè, D., Collaer, M., Lippa, R. A., & Halsey, L. (2026). The vulnerable male brain: Men’s spatial abilities are condition-dependent, sexually selected traits. Evolution and Human Behavior, 47(4), 106901.

References

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