The Great Divergence
By David C. Geary and Lewis G. Halsey
As women’s social, occupational, and political engagement expanded in Western nations, social scientists anticipated a great psychological and behavioural convergence of men and women. As Alice Eagly and colleagues wrote, “The demise of many sex differences with increasing gender equality is a prediction of social role theory.” Their perspective is that most sex differences result from adherence to socially imposed norms and expectations. These norms and expectations have changed over the past several generations, as women entered educational, occupational, and political spheres that had been historically dominated by men. Today, in most highly developed nations, women are now the majority in higher education and are on track to become the majority in some formerly male-dominated and high-status professions, such as medicine. Even if one might argue that the gains in gender equality are not complete, social-role theory tells us that we should therefore see the sexes acting and thinking more similarly.
But like many almost-utopian academic predictions, social role and related theories have hit the wall of reality. Instead of convergence of the sexes, we have something closer to a great divergence. To the surprise and dismay of social scientists, many psychological and behavioural sex differences are larger in more gender-equal countries—precisely the opposite of their predictions—which has dealt a major blow to the plausibility of sociological theories of the origin of sex differences. As reviewed by David Schmidt and colleagues, then Marco Balducci, and most recently, Agneta Herlitz and colleagues, the divergence is broad and deep, including aspects of personality, emotional expressiveness, mental health, cognition, and occupational choices and preferences, among others. To be sure, there has been convergence in some areas (for example, intimate partner violence) and no change in others, but the trend is clear—women and men are becoming more different in important ways. Many of the expanding differences are small to moderate for individual traits, but the tableau created is of a substantive divergence.
Expanding liberal mores and a greater range of social and occupational niches enable a fuller expression of individual and sex differences in personality and preferences, among other traits. We are therefore better able to see the true magnitude of innate differences between the sexes in thinking, behaving, and choosing. By analogy, in the US in the 1960s, most families seemed to be similarly entranced by the Ed Sullivan show. But this convergence in viewing habits was driven by a lack of alternatives (there were just three television stations to choose from) rather than a lack of differences between households in their underlying interests. We can see these individual differences better today now that there are hundreds of viewing options available through cable television, streaming, and online entertainment.
One counterargument proposed to explain the great divergence is that more egalitarian societies have stronger gender stereotypes and that these are driving at least some of this divergence. This is a last-ditch attempt to rescue social roles and other blank-slate theories as explanations of sex differences. The problem is that there is a kernel of truth to many stereotypes, so stronger stereotypes could just as easily arise from, instead of causing, larger sex differences in gender-equal countries.
Not Only Society but Also Biology
It’s not just an expansion of options and a loosening of social mores that undergirds the great divergence. Some of the expanding sex differences are likely driven by biological changes. These changes are not down to evolutionary selection but rather a fuller expression of already-evolved sex differences, with this fuller expression occurring in individuals who grew up and live in more favourable environments—put simply, adequate calories and nutrition, low levels of disease, and low levels of social stress.
This pattern follows from Darwin’s theory of sexual selection, that is, competition with members of the same sex for potential mates and discriminating mate choices (female choice of male partners). These dynamics result in the evolutionary amplification in one sex or the other of the physical (e.g., larger size), behavioural (e.g., courtship displays), or cognitive (e.g., birdsong) traits that facilitate competition and choice. The exaggerated nature of such traits makes them more costly to build and maintain, which in turn makes them effective and honest indicators of the bearer’s condition and physical health. By analogy, building and keeping warm a 200-square-metre house is more costly than for a 100-square-metre house and any resource deficits will more strongly compromise the bigger property—in winter, if money and heating oil run short, the larger home will be the colder one.
The key idea is that many biologically based sex differences are greatest under environmentally good conditions, while these traits are less different between the sexes if conditions are or were poorer during childhood and adolescence. This prediction is based on mating competition but holds for any traits that are more impressive in one sex than the other, including behavioural and cognitive characteristics. It has been known for a while that the environment can influence the development and expression of evolved traits, but it is only very recently that this approach has been applied to human sex differences. We and other researchers are on the case, and the findings so far are compelling.
Height provides a clearcut, incontrovertible example of how favourable environments affect sex differences in our biology. As dating apps amply demonstrate, women are attracted to taller and heavier, muscular men. So, in the dating game, taller and stronger-looking men have an advantage, gaining more attention from the fairer sex, as well as being more formidable in the eyes of would-be competitors. This explains why, consistently across societies, on average men are substantially taller and heavier than women—they’ve evolved to be by the process of sexual selection.
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