Sex Differences in Vulnerability

Psychologists typically think of vulnerabilities in terms of emotional or behavioral disorders, such as anxiety or issues with attentional or behavioral control. I have proposed that vulnerabilities can also be viewed from the perspective of evolutionary biology, in particular, sexual selection.

The latter involves competition for mates and discriminative mate choices, and it often results in the exaggeration of the traits that facilitate the ability to compete for mates or are attractive to the opposite sex. The peacock’s tail is a good example of an exaggerated, sexually selected trait.

One result of trait exaggeration is the evolutionary emergence of sex differences for these traits. The interesting thing about them is that they are condition-dependent, that is, their development and expression are sensitive to social and ecological conditions. These traits are particularly sensitive to the Horsemen of the Apocalypse, that is famine (poor nutrition), plague (chronic disease), and war (intense social competition), as well as to man-made toxins. As exposure to these stressors increases, it becomes increasingly difficult to develop exaggerated traits and thus their size or color pattern (or whatever the case may be) becomes less exaggerated on average.

As these traits become smaller with deteriorating conditions, the magnitude of any associated sex differences shrinks. In other words, the magnitude of evolved sex differences can fluctuate from one context to another and can change across historical time within the same context. Although much remains to be learned, many human sex differences follow this pattern: They are larger in low-risk contexts (e.g., low frequency of infectious diseases) and become smaller as conditions deteriorate.

Height provides a good example of condition dependence because it is easily measured and likely reflects an evolutionary history of male-male physical competition and women’s preference for men who are taller than average. If height is a condition-dependent, sexually selected trait that is analogous to the peacock’s tail, then men should be taller than women throughout the world but, at the same time, the magnitude of this sex difference should vary in predictable ways across populations and across generations.

Sex differences should be the largest in populations with low exposure to the Horsemen of the Apocalypse and to toxins, and the differences should be smaller in contexts with higher exposure to these stressors. These predicted patterns are indeed what we find.

The sex difference in height is the largest in the healthiest nations, and growing up in stressful contexts (e.g., chronic poor nutrition) compromises the height of men more than women and thereby reduces the magnitude of the sex difference. The point is well illustrated by historical changes in developed nations during the 20th century, a period marked by substantive gains in overall health.

From 1900 to 1958 in Great Britain, Kuh et al. found a 1.09 cm (0.43 inches)/decade increase in men’s height as compared to a 0.36 cm (0.14 inches)/decade increase for women. The average British man at the beginning of the 20th century was 11 cm (4.33 inches) taller than the average woman, but this increased to 15 cm (5.91 inches) by 1958, a 36% increase in less than three generations. The same pattern is found in developing nations today. For young adults in nutritionally-stressed regions of Nigeria, for instance, men are 7.5 cm (2.95 inches) shorter than their better-nourished same-sex peers, whereas women are 3.2 cm (1.26 inches) shorter. The result is a sex difference in height that is 38% smaller than it would be if these adults had received better nutritional and medical care during childhood and adolescence.

The sex differences in folk psychology and folk physics that I discussed in previous posts show a similar pattern. Language is one component of folk psychology and girls and women have advantages in many of the basic features of language comprehension and production.

One resulting prediction is that girls’ language development will be more vulnerable to stressors than boys’ language development. A supporting example is found with the effects of prenatal cocaine exposure on language competencies. Malakoff et al. (1999), for instance, examined natural language development in prenatally exposed 2-year-olds and found a developmental lag relative to unexposed children. Critically, the gap between exposed children and their same-sex peers was 3- to 6-times larger for girls than for boys. The authors concluded that “the effect of cocaine-exposure is qualified by gender, and is stronger for girls than for boys” (Malakoff et al., 1999, p. 173).

Another example is found for women with anorexia nervosa, that is, women under severe nutritional distress. These women show broad deficits in folk psychology (e.g., interpreting body language) and smaller deficits in other areas (e.g., folk physics) that recover when they regain a normal weight.

Boys’ and men’s competencies in folk physics show the same sex-specific vulnerabilities. An example is provided by the accidental exposure of thousands of people in Taiwan to PCB (polychlorinated biphenyl) contaminated cooking oil, including 74 women who were pregnant at the time or became pregnant soon thereafter. Their children’s health and aspects of their cognitive development, including spatial reasoning (part of folk physics), were followed for years. Children who were prenatally exposed to PCB were compared to demographically matched peers from 6 to 9 years of age, inclusive.

These comparisons revealed that spatial reasoning abilities were compromised in boys who were exposed, but not girls. For unexposed children, boys scored higher on spatial reasoning than did girls in most years, as is typically found, whereas toxin-exposed girls outperformed exposed boys in most years.

There are a number of studies showing that men’s visuospatial abilities are more vulnerable to toxin exposure than are other abilities (e.g., verbal abilities). An example is provided by a study of male Finnish factory workers’ level of aluminum exposure (through welding) and performance on a variety of cognitive measures. While controlling demographic and other factors, the primary deficits were “in tasks requiring working memory, particularly that relating to processing of visuospatial information” (Akila et al., 1999, p. 632). Men’s occupational exposure to lead has also been well studied, with mixed or weak evidence for exposure-related declines in basic visual memory.

The evidence is more consistent for exposure-related declines in more complex spatial abilities. Haenninen et al. (1978), for instance, found that higher than normal lead levels in the blood were associated with deficits in men’s spatial cognition and psychomotor dexterity, but not in verbal abilities. These authors concluded the “performances that were most affected by lead were dependent on visual intelligence and visual-motor functions” (Haenninen et al., 1978, p. 688).

I describe many other examples elsewhere (see references). There are two important points here. The first is that evolved sex differences are not immutable and, in fact, are expected to vary across contexts and time in predictable ways. The second is that an acceptance of evolutionary influences on human sex differences can lead to the development of measures that are sensitive to sex-specific vulnerabilities and thus can be used to better understand and potentially ameliorate them.