Extraversion is part of the male toolkit for mating success. It is especially useful in societies where a high incidence of polygyny means too many men must compete for too few women.
As a single man, I would spend close to $3,000 a year on dating. And that didn’t include things like buying a sportier-looking car. My behavior was also higher risk, in large part to impress women and show how ‘edgy’ I could be.
The mating game is costly. A British study found that men improve their sexual access to women at the cost of increased risk of hospitalization for accident or illness (Nettle 2005). There is thus a trade-off. If you invest more effort in chasing women, less is left over for taking care of yourself and any children you’ve fathered. You also risk early death.
The trade-off varies from one society to another. For a man, the mating game is less costly if the woman can provide for herself and her children with little male assistance. The cost may even be negative if she also provides for her man.
In such societies, it is in a man’s reproductive interest to mate with as many women as possible. The result? A ‘tragedy of the commons’ where the pursuit of self-interest ends up harming society as a whole. There won’t be enough women to go around, and many men will go a long time without a mate, or never find one at all.
This has long been the case with simple ‘horticultural’ societies in the tropical zone. The women feed themselves and their children with minimal male assistance because they can grow food year-round. And this food production is not appropriated by a State or a land-owning class.
Such societies remain simple in large part because intense sexual competition keeps them from evolving into more complex entities. The surplus males stir up endless conflict, if only because their sole access to women is through warfare, i.e., rape and abduction. There can never be pacification and, therefore, the formation of larger, more advanced societies.
How men and women adapt to high-polygyny environments
A high incidence of polygyny favors men with a different toolkit of physical and mental traits. Some personality traits, for instance, will be more advantageous than others. Such is the finding of a series of studies from rural Senegal, where 48% of men over 40 are polygynous.
Alvergne et al. (2009, 2010a, 2010b) found no correlation among Senegalese men between mating success and most personality traits, i.e., neuroticism, openness, and agreeableness. One trait, however, showed a strong correlation. This was extraversion, defined as “pro-social behavior which reflects sociability, assertiveness, activity, dominance and positive emotions.” Men with above-medium extraversion were 40% more likely to have more than one wife than those with below-medium extraversion, after controlling for age. Furthermore, this personality trait correlated with higher testosterone levels. Such a linkage suggests that extraversion is part of the male toolkit for mating success in a high-polygyny environment.
Of course, it may be that the relation of cause and effect is indirect. Extraversion helps men accumulate wealth, and wealthy men can afford to take second or third wives. Nonetheless, the correlation remained significant even after the researchers controlled for social class
Among Senegalese women, reproductive success correlated with neuroticism, i.e., a tendency “to be anxious, depressive, and moody.” Women with above-medium neuroticism had 12% more children than those with below-medium neuroticism, after controlling for age and marital rank. This personality trait may thus be part of the female toolkit for infant survival in an environment where women are almost solely responsible for parenting.
Could the cause and effect run in the other direction? Perhaps having more children makes a woman more neurotic. Yet neuroticism did not increase with age, whereas the number of children did. Furthermore, the correlation was stronger among rich women, who presumably had less reason to worry about child care.
On average, the Senegalese men were more oriented to polygyny and extraversion, but there was significant variation. Some seemed to be more monogamous and introverted.
Perhaps all human populations display this sort of variation in reproductive strategy, the differences among them being one of degree than of kind. Indeed, statistical differences can easily develop among human populations because the raw material for gene-culture co-evolution is already available. There is no need to wait for new mutations to come into existence.
What do the authors say about differences among human populations? They initially state, “Men in the present study had lower T [testosterone] levels than has been recorded for men from western societies using similar saliva assays” (Alvergne et al. 2009). Later on, in reviewing the literature, they qualify this statement:
It is worthy of note that inter-population differences in T levels were found to be more pronounced for young men (15-30 years) than for older men (45-60 years) (Ellison, 2003). The authors conclude that the differences between populations in patterns of T decline with age result from variations in peak levels during young adulthood and are thus highly dependent on the reproductive physiology of young males.
(Alvergne et al. 2009)
The decline in T levels with age is sharper among polygynous Senegalese men than among monogamous Senegalese men. In short, higher T levels in young adulthood mean lower ones later in life:
When men get older than 50, a reversed pattern is observed, with polygynously married men having lower T levels than monogamously married men.
(Alvergne et al. 2009).
This may account for the authors’ initial statement that T levels were lower in Senegalese men than in Western men. The Senegalese subjects were 38.3 years old on average.
A similar reversal with age has been noted in U.S. studies. African Americans have a clear testosterone advantage over Euro-Americans only from puberty to about 24 years of age. This advantage then shrinks and eventually disappears at some point during the 30s (1). The pattern then seems to reverse at older ages (Ellis & Nyborg 1992; Gapstur et al. 2002; Nyborg 1994, pp. 111-113; Ross et al. 1986; Ross et al. 1992; Tsai et al. 2006; Winters et al. 2001).
No one really knows why. We know that too much testosterone early in life causes long-term harm, e.g., increased risk of prostate cancer. Perhaps there is also degradation of the body’s capacity to produce testosterone.
A hen is an egg’s way to make another egg. I remember being told that as a child. Perhaps we should now say: A man is a sperm’s way to make more sperm.
H/T to Tod.
1. Tsai et al. (2006) found that baseline levels of both total and bioavailable testosterone were significantly higher in African Americans than in Euro-Americans with a median age of 33-34. Ellis and Nyborg (1992) found that African Americans had a slight but still significant (p=0.028) testosterone advantage over Euro-Americans among subjects with a median age of 38. It is difficult to identify the ‘tipping point’ because both studies used pools of subjects with wide age ranges.
Alvergne, A., M. Jokela, C. Faurie, and V. Lummaa. (2010). Personality and testosterone in men from a high-fertility population, Personality and Individual Differences, 49, 840-844.
Alvergne, A., M. Jokela, and V. Lummaa. (2010). Personality and reproductive success in a high-fertility human population, Proceedings of the National Academy of Sciences, 107, 11745-11750.
Alvergne, A., C. Faurie, and M. Raymond. (2009). Variation in testosterone levels and male reproductive effort: Insight from a polygynous human population, Hormones and Behavior, 56, 491-497.
Ellis, L. and H. Nyborg. (1992). Racial/ethnic variations in male testosterone levels: a probable contributor to group differences in health, Steroids, 57, 72-75.
Gapstur, S.M., P.H. Gann, P. Kopp, L. Colangelo, C. Longcope, and K. Liu. (2002). Serum androgen concentrations in young men: A longitudinal analysis of associations with age, obesity, and race. The CARDIA male hormone study. Cancer Epidemiology, Biomarkers & Prevention, 11, 1041-1047.
Nettle, D. (2005). An evolutionary approach to the extraversion continuum: Evolution and human behaviour. Evolution and Human Behavior, 26, 363-373.
Nyborg, H. (1994). Hormones, Sex, and Society. The Science of Physiology. Westport (Conn.): Praeger.
Ross, R.K., L. Bernstein, R.A. Lobo, H. Shimizu, F.Z. Stanczyk, M.C. Pike, and B.E. Henderson. (1992). 5-apha-reductase activity and risk of prostate cancer among Japanese and US white and black males. Lancet, 339, 887-889.
Ross, R., L. Bernstein, H. Judd, R. Hanisch, M. Pike, and B. Henderson. (1986). Serum testosterone levels in healthy young black and white men. Journal of the National Cancer Institute, 76, 45-48.
Tsai, C.J., B.A. Cohn, P.M. Cirillo, D. Feldman, F.Z. Stanczyk, A.S. Whittemore. (2006). Sex steroid hormones in young manhood and the risk of subsequent prostate cancer: a longitudinal study in African-Americans and Caucasians (United States), Cancer Causes Control, 17, 1237–1244.
Winters, S.J., A. Brufsky, J. Weissfeld, D.L. Trump, M.A. Dyky, and V. Hadeed. (2001). Testosterone, sex hormone-binding globulin, and body composition in young adult African American and Caucasian men. Metabolism, 50, 1242-1247.