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The Nurture of Evolved Sex Differences
Why favorable conditions produce larger sex differences.
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Many human sex differences are now acknowledged, but their origin and practical importance continue to be vigorously debated [1, 2]. The default assumption among many social scientists and much of the lay public seems to be that any differences are largely (or perhaps entirely) the result of social factors, such as stereotypes or gender role expectations for boys and men and girls and women . For some, these beliefs are comforting because they provide a sense of control over matters that are important to them, and an expectation that with appropriate social policies and shifts in social mores, sex differences in culturally important outcomes (e.g., the numbers of women and men in computer science and engineering) will eventually disappear. One implication of this assumption is that sex differences that vary across time and place must per force be driven by social rather than biological factors.
However, the expression of many traits that facilitate reproductive competition for mates and drive mate choices have evolved to signal the underlying genetic and physical health of the individual, and thus their expression can vary across individuals, contexts, and time [4, 5]. For people, social factors, including formal laws (e.g., prohibition of polygynous marriages), informal social mores, and wealth and political developmental (e.g., broad legal rights) are also associated with variation in the magnitude of sex differences for multiple traits . But while these social and contextual factors can both restrict or facilitate the expression of biologically based sex difference, they do not create them.
Darwin’s  sexual selection, that is, the social dynamics that emerge with intrasexual competition for mates and intersexual choice of mating partners, is the primary source of sex differences across species [for review see 8]. Sexual selection results in the evolution of traits that support competition and choice, and the evolutionary emergence of sex differences for these traits, as illustrated in Figure 1.
These traits can be physical (e.g., body weight), ornamental (e.g., colorful plumage), behavioral (e.g., mating displays), or supported by brain and cognitive systems (e.g., bird song). The key result is trait exaggeration in one sex or the other. But this exaggeration can also create a vulnerability for the seemingly advantaged sex . Larger, exaggerated traits consume more cellular energy (and result in more oxidative stress and other cell damaging processes) to build, maintain, and express, making them especially vulnerable to energy and nutritional short falls, as well as to other stressors . By analogy, a poorly working furnace will result in a more rapid drop in ambient temperature in a 300-square-meter than a 100-square-meter house. Basically, the ability to fully express these traits depends on the overall condition of the individual, which is why they are called condition-dependent traits, and the condition of the individual will depend in part on social and ecological conditions.
The factors that sap the development and expression of these traits are well-captured by the Horsemen of the Apocalypse (Figure 2), that is, infection, famine, and intense social competition. Exposure to these conditions, as well as some man-made toxins, compromise exaggerated traits more than other traits and therefore reduces the magnitude of any associated sex differences [5, 10, 11]. There are, of course, individual differences within each sex in sensitivity to these stressors, such that some individuals are compromised more strongly than others, but the overall results are smaller sex differences for the population and more variability in the affected trait across individuals.
An example is provided by beak color in the male zebra finch (Taeniopygia guttata), which influences female mate choices. The color is a good indicator of the male’s current health and his ability to withstand stressors. Poor early nutrition  and intense social competition in adulthood  can result in larger decrements in males’ than females’ beak coloration that in turn signals poor health and compromised competitive ability. Similarly, exposure to certain toxins can have sex- and trait-specific effects. Bortolotti and colleagues  showed that exposure to PCBs (polychlorinated biphenyls) resulted in duller plumage coloration (influences female mate choices) in male but not female kestrels (Falco sparverius), and Jašarević and colleagues  showed that prenatal exposure to BPA (bisphenol A) disrupted male but not female spatial abilities (supports males’ searching for mates) in the deer mouse (Peromyscus maniculatus). In all these studies, typical sex differences were reduced or disappeared entirely with exposure to these stressors.
Turning closer to home, we find similar patterns in people, as is nicely illustrated by changes in the sex difference in height with changes in overall health. Among primates, larger males than females indicates an evolutionary history of physical male-male competition. Our male ancestors were larger than our female ancestors going back at least four million years , indicating a long history of such competition.
By the logic above, variation in nutrition, disease risk, and social stressors represented by the Horsemen should result in variation in the magnitude of the sex differences in physical size, such as height. More precisely, height differences between the sexes should have increased over time as developed nations kept the Horsemen at bay with improvements in public health (among other factors) and be larger today in developed than in developing nations. Indeed, from 1900 to 1958, the sex difference in height increased 36 percent in Great Britain : In 1900, the average British man was 11 cm taller than the average woman, but this increased to 15 cm by 1958. For young adults in nutritionally stressed regions of Nigeria, men are 7.5 cm shorter than their better-nourished peers, whereas women are 3.2 cm shorter . The result is a sex difference in height that is 38 percent smaller than it would be if these adults had received better nutritional and medical care during childhood and adolescence.
Although much remains to be learned, there is evidence for similar sex-specific vulnerabilities in cognitive and behavioral traits. For instance, male-male competition is associated with more rough-and-tumble play (play fighting) for males than females during development across species . In keeping with a long evolutionary history of male-male competition, boys engage in rough-and-tumble play more frequently, with more vigor, and with greater zest than do girls. The highest rates occur in groups of unsupervised children and in safe contexts, where boys engage in various forms of playful physical assaults and wrestling 3 to 6 times more frequently than do same-age girls .
Barrett and colleagues [21, 22] demonstrated that chronic malnourishment through the prenatal and early preschool years undermined the rough-and-tumble and dominance-related play of boys more than girls. Overall, the most active and socially potent children were well-nourished boys and the least potent were malnourished boys, with girls somewhere in between the boys’ groups independent of the girls’ nutritional status.
It’s not just boys and men who are vulnerable to the Horsemen. Girls and women have advantages in folk psychology (sometimes called emotional intelligence), that is, in language, reading facial expressions and body language, and in making inferences about the thoughts and feelings of others (called theory of mind) [23, 24]. I’ve suggested that these advantages have evolved due to female-female competition through relational aggression (i.e., disrupting the reputation and social networks of competitors) and the benefits of forming and maintaining intense friendships that provide critical social and emotional support in adulthood .
The nutritional deficits associated with anorexia nervosa severely undermine these social competencies in women and more so than it does for men with similar nutritional deficits [26, 27, 28]. Moreover, these women’s social competencies improve if they recover normal weight. As with men’s height, women’s verbal memory, an aspect of their language and social competencies, improves more rapidity than that of men, resulting in a larger sex difference, as populations become healthier and wealthier .
The punch line is that favorable conditions, those that reduce risk of disease and poor nutrition and that keep social stressors in check, will result in larger sex differences in evolved traits. Ironically, these conditions are most common in wealthy, educated, industrialized, rich, and democratic (WEIRD) nations —those that promote gender equality. The irony follows from the belief that the promotion of gender equality and overall favorable conditions will reduce and eventually eliminate sex differences , but it does the exact opposite.
Social Constraints and Opportunities
The Horsemen of the Apocalypse are not the only factors that can influence the development and expression of sex differences. In many species, the pattern of sex differences, such as the intensity of male-male competition and the rigor of female choice, can vary with here-and-now social conditions, such as the number of competitors and prospective mates in the local community .
Social influences are even more important for people. Formal laws and informal social mores create constraints and opportunities that can substantively influence the expression of evolved biases. The imposition of legally imposed monogamy in WEIRD nations, for instance, reduces the intensity of male-male competition, resulting in less violence and crime, and intensifies female-female competition for high-status mates [25, 32]. These nations also create more social and economic niches and afford greater room for the expression of individual preferences and the expression of many sex differences.
As reviewed by Schmitt and colleagues , sex differences in many aspects of personality, self-esteem, and cognitive and psychological functioning are larger in WEIRD, gender equal countries. For instance, women are generally more cooperative and agreeable than men and men are more Machiavellian than women, on average. These differences are larger in more egalitarian countries. One potential reason is that religious prohibitions and proscriptions increase social cooperation and decrease self-serving behaviors in men and this in turn reduces the sex differences in these areas. The release of these prohibitions enables fuller expression of underlying differences; in this case, a decrease in men’s agreeableness and an increase in their use of Machiavellian social strategies .
Occupational segregation also increases in WEIRD, gender equal countries, presumably due to underlying differences in preferences for working with and helping people as contrasted with working with things . Girls’ and women’s greater interest in other people and relationships follows from their greater investment in children and their need to develop BFF (best friends forever) relationships that serve as a source of social and emotional support. Boys’ and men’s greater interest in things likely follows from an evolutionary history of tool making, most of which is done by men.
Stoet and I found there were proportionally (relative to the number of women and men in college) fewer women than men studying and working in non-organic science, technology, engineering, and mathematics (STEM) fields, such as computer science, in gender-equal Norway and Finland than in Algeria . In fact, the pattern was found throughout the world, whereby wealth and gender equality were associated with proportionally fewer women entering these fields. Women in less wealthy and less gender equal countries appear to pursue these types of degrees for economic reasons. As economic niches widen and countries become wealthier and more liberal, women (and men) pursue careers that are better aligned with their interests.
In a follow-up study, we examined the occupational aspirations of nearly half a million adolescents across the 80 developing and developed nations that participated in the 2018 Programme for International Student Assessment (PISA) of academic competencies . In this assessment, students were asked, “What kind of job do you expect to have when you are about 30 years old?”, which we termed occupational aspirations. As shown in Figure 4, there was not a single country in which girls were as interested in non-organic STEM fields (e.g., engineering) or blue-collar things-oriented occupations (e.g., carpenter) as were boys, and not a single country in which boys were as interested in people-oriented occupations (e.g., teacher) as were girls. There was nonetheless considerable cross-national variation in the magnitude of these differences.
Across countries (median), there were about 4 boys for every girl aspiring to a things-oriented occupation, and about 3 girls for every boy aspiring to a people-oriented occupation. In keeping with our earlier finding for STEM degrees, for every girl who aspired to enter a things-oriented STEM occupation, there were 5 boys. Again, the ratio was larger in gender-equal countries. In Morocco and the United Arabic Emirates, respectively, there were 1.5 and 1.7 boys for every girl aspiring to a things-oriented STEM occupation, as compared to 4.5 and 4.8 boys to every girl in gender-equal Sweden and Norway. These patterns mirror those found one hundred years earlier .
These differences were larger in adolescents from blue-collar backgrounds. Many girls from higher-income families aspired to white-collar occupations that were neither clearly things- or people-oriented (e.g., accountant, manager) or were higher-level people-oriented occupations (e.g., physician). The latter findings are consistent with changes in women’s occupational choices from 1972 to 2010 in the U.S., where there was an increase in women working in professional occupations but there was not a shift to more engagement with male-typical blue-collar or white-collar things-oriented occupations .
In other words, there are stable sex differences across time and place in many occupational aspirations and choices that likely result from deeper differences in interests in people and relationships as contrasted with an interest in working with things. In WEIRD countries there has also been secular changes that improved women’s educational and occupational opportunities. These improvements, however, are concurrently associated with larger sex differences in aspirations for and segregation into things-oriented and people-oriented occupations. As noted, these amplified sex differences are not restricted to occupations, and emerge in many social, behavioral, and cognitive traits.
The critical point here is that change in the magnitude of sex differences across time and place are part and parcel of the expression of evolved biases, and not necessarily evidence that these traits are largely or solely caused by social and cultural factors. To be sure, social (e.g., prohibition of polygynous marriages) and cultural (e.g., overall wealth, personal liberties) factors can and do have substantive influences on human behavior and well-being. These social and cultural factors can modify the expression of sex differences, but they do not create them de novo.
David C. Geary is a Curators’ Distinguished Professor in the Department of Psychological Sciences and Interdisciplinary Neuroscience Program at the University of Missouri. Among other things, he studies sex differences and has written several books on the topic, including three editions of Male, female: The evolution of human sex differences.