Thursday, September 24, 2009

Female face shape and sexual selection

Denise Liberton, an anthropologist at Pennsylvania State University, has been studying variation in human facial features. At an upcoming meeting of the American Society of Human Genetics, she’ll be presenting a comparative study of European and West African facial morphology. The main thrust of her presentation is that the shape of the face has differentiated among human populations in part through a selective force that acts primarily on women—and not on both sexes.

We found that several pairwise distances differed between the sexes. For example, the distance from the brow to nasal bridge was found to be more than 5% larger in females than males. We then tested for an interaction between sex and genetic ancestry by testing for differences in the slopes of the ancestry association between males and females. Although the pattern differed slightly between samples, after Bonferroni correction many correlations were the found to be same in both sexes. However, females in all three samples had many additional significant correlations that were not seen in males, while males had very few correlations that were not found in females. The results of these analyses suggest that selection on females is driving the differentiation in facial features among populations. (Liberton et al., 2009)

What is this selective force that acts mainly on female morphology and carries male morphology along in its wake? I suspect Denise Liberton has sexual selection in mind. If so, this finding would support Darwin’s belief that “the races of man differ from each other and from their nearest allies, in certain characters which are of no service to them in their daily habits of life, and which it is extremely probable would have been modified through sexual selection” (Darwin, 1936 [1888], p. 908).

Darwin was puzzled not only by the considerable physical differences separating humans from apes, but also by the considerable physical differences among human populations (Darwin, 1936 [1888], p. 530-531). He concluded that sexual selection was “the most efficient” cause of this differentiation (Darwin, 1936 [1888], p. 908).

Yet sexual selection usually acts on males in other mammals. The females are the ones who normally do the selecting. This is because they must take time out from the mate market for pregnancy, breastfeeding, and infant care. Meanwhile, the males never really leave the mate market, with the result that too many of them are competing for a limited number of available females.

This mammalian ‘law’ has influenced much writing about sexual selection in humans. According to Naomi Wolf, author of The Beauty Myth, "for women to compete with women through 'beauty' is a reversal of the way in which natural selection affects all other mammals" (Wolf, 1990, p. 3). She points to indigenous peoples in sub-Saharan Africa, Australia, and New Guinea as proof that the original human state was one of males vying for the attention of females.

I could cite other writers, but the gist of their argument is always the same. Authentic human nature is represented today by indigenous tropical peoples. They are what we were. Therefore, human nature is about polygynous males who devote little time and energy to raising their progeny and a lot to seducing the limited number of females. Women are thus the ones who have been sexually selecting.

In this kind of argument, we means ‘people of non-tropical origin,’ particularly those of European descent. Yet clearly this argument is false. We were not them for a long time. Europeans have an evolutionary history going back some 35,000 years on their continent. And this was when and where they evolved their current physical appearance: the shape of their face, the color of their skin, hair, and eyes; the length and form of their head hair. To understand why Europeans look the way they do, we should understand how their environment of sexual selection differed from that of tropical humans.

Ancestral humans were exposed to pressures of sexual selection that varied along a north-south axis. In the tropical zone, women could gather food year-round, thus making the cost of a second wife relatively low. With so many being scooped up, female mates were a limited resource. Too many men had to compete for too few women. The pressure of sexual selection was thus on men, with women being the ones who could pick and choose mates.

This situation reversed as humans moved away from the tropical zone. First, it became costlier for a man to provide for a second wife because women contributed less to the family food supply, the longer winters reducing opportunities for food gathering. Second, male mortality increased relative to female mortality because men had to hunt over longer distances. Together, these two trends resulted in too few men competing for too many women. This was particularly so on continental steppe-tundra, where women had almost no opportunities for food gathering and where men had to hunt wandering herds of herbivores over long distances (Frost, 2006; Frost, 2008).

Because of a geographic accident, i.e., a glacial mass over Scandinavia, it was in Europe during the last ice age (25,000 to 10,000 years ago), specifically on the northern and eastern plains, that continental steppe-tundra reached furthest to the south and covered the most territory during the time of modern humans. And this was when and where Europeans came to look European. They did not change in physical appearance because of climatic adaptation. The cause was a change in the direction and intensity of sexual selection: men were now selecting women, and to a much greater degree than elsewhere.


Darwin, C. (1936) [1888]. The Descent of Man and Selection in relation to Sex. reprint of 2nd ed., The Modern Library, New York: Random House.

Frost, P. (2008). Sexual selection and human geographic variation, Special Issue: Proceedings of the 2nd Annual Meeting of the NorthEastern Evolutionary Psychology Society. Journal of Social, Evolutionary, and Cultural Psychology, 2(4), pp. 169-191.

Frost, P. (2006). European hair and eye color - A case of frequency-dependent sexual selection? Evolution and Human Behavior, 27, 85-103.

Liberton, D.K., K.A. Matthes, R. Pereira, T. Frudakis, D.A. Puts, & M.D. Shriver. (2009).
Patterns of correlation between genetic ancestry and facial features suggest selection on females is driving differentiation. Poster #326, The American Society of Human Genetics, 59th annual meeting, October 20-24, 2009. Honolulu, Hawaii.

Wolf, N. (1990). The Beauty Myth. Toronto: Random House.

Thursday, September 17, 2009

Adoption and parental investment

It has long been known that children are likelier to be abused, neglected, or murdered by stepparents than by birth parents. This kind of genetic discrimination seems consistent with kin selection theory: parents are expected to care more for children who share kinship with them, as opposed to a purely legal and social relationship (Daly & Wilson, 1980).

If stepchildren are mistreated because they are not kin, we should see the same mistreatment of adopted children. To test this hypothesis, Gibson (2009) surveyed parents with at least one genetic and one adopted child over the age of 22, the idea being to compare the two groups of children for total parental investment. Contrary to expectation, the parents invested more in their adopted children than in their own:

This study categorically fails to support the hypothesis that parents bias investment toward genetically related children. Every case of significant differential investment was biased toward adoptees. Parents were more likely to provide preschool, private tutoring, summer school, cars, rent, personal loans and time with sports to adopted children (Gibson, 2009).

Why? One can imagine the parents making no distinction, but why would they discriminate against their own children? The answer seems to be that the adopted siblings made greater demands.

Adoptees were more likely than genetic offspring to have ever received public assistance, been divorced or been arrested. They also completed fewer years of schooling and were more likely to have ever required professional treatment for mental health, alcohol and drug issues.

… The current study may demonstrate cases where “the squeaky wheel gets the grease.” Summer school and private tutors are often remedial, and the fact that adopted children were more likely to receive them suggests they required them more often than genetic ones. The same can be said for rent, treatment and public assistance. Adoptees may have more difficulty establishing themselves relative to genetic children, and the fact that they divorce more often suggests they also have more difficulty staying established. Addiction and divorce may put adoptees in situations that require more parental investment. Parents provide more for adoptees not because they favor them, but because they need the help more often.
(Gibson, 2009)

For many behavioral traits, adoptees seem to differ genetically not only from their adoptive parents but also from the general population:

This supports other research showing that, compared to genetic children, American adoptees have a higher overall risk of contact with mental health professionals, specifically for eating disorders, learning disabilities, personality disorders and attention deficit hyperactivity disorder … They also have lower achievement and more problems in school, abuse drugs and alcohol more, and fight with or lie to parents more than genetic children …

… Adoptees may be genetically predisposed to negative outcomes at higher rates than the general population. Genetic factors clearly contribute to alcohol and drug addiction, as well as to some mental disorders like attention deficit hyperactivity disorder and schizophrenia …. An association between nonviolent criminality has been found between European adoptees and their genetic parents … Furthermore, research with Swedish adoptees suggests 55-60% of their educational performance is explained by genetic factors, and that the number of years of school adoptees complete is significantly related to how many years their genetic mothers completed ...
(Gibson, 2009).

All of this may explain why parents invest more in adopted children than in their own. But why do any parents adopt? Doesn’t such a decision, in itself, contradict kin selection theory?

The contradiction may be more apparent than real. Most adoptive parents have fertility problems and cannot have children on their own. Their only other option is to remain childless.

It may be that adopting fulfills a common instinct to reproduce and parents do it because it produces positive emotions. When people cannot have children biologically, adoption gives them a way to fulfill the “drive” to parent, maladaptive or not. (Gibson, 2009)


Daly, M., & M. Wilson. (1980). Discriminative parental solicitude: A biological perspective. Journal of Marriage and the Family, 2, 277-288.

Gibson, K. (2009). Differential parental investment in families with both adopted and genetic children, Evolution and Human Behavior, 30, 184-189.

Thursday, September 10, 2009

The new marriage market and the future

High sex ratio cohorts are likely, virtually by definition, to have greater numbers of males who fail outright to establish stable couple relationships. Moreover, the relationships that many males do establish in high sex ratio periods may be tenuous and subject to jealousies, rivalries, and the threat of displacement by other, more highly “qualified” males. (Pedersen, 1991)

When Pedersen wrote the above almost two decades ago, he did not exactly fear the new marriage market of too many men chasing too few women. In fact, his prognosis was largely upbeat. There would be “lower divorce rates”, “greater marital stability”, “enhanced marital satisfaction for women”, “greater commitment by males to procurement of economic resources”, “greater willingness by men to engage in active parenting”, and “increase in fertility”. This generally rosy outlook, however, also included the prospect of increased male violence.

Has time proved Pedersen wrong? Well, he was wrong about the positive outcomes. Perhaps he was wrong here too. Look at regions where too many men chase too few women: East Asia, parts of India and, increasingly, the Western world. These regions generally have stability, order, and low crime rates. Conversely, too few men chase too many women in U.S. inner cities and the former Soviet Union.

But such comparisons may be confounded. Stable, orderly societies also tend to be those that value men and unwittingly seek to increase the sex ratio. One reason why East Asian societies are stable and orderly is that their men are good providers. Because they work and provide for their families, parents want to have as many sons as possible—because a son will take care of them in their old age. Hence the high rate of female feticide.

Similarly, one reason why Western societies are stable and orderly is because they value human life so much, even to the point of imposing restrictions on men that would be considered demeaning in other societies (wearing of seatbelts, regulations on cigarette smoking and alcohol consumption, etc.). Hence the low rate of male mortality.

Still, what evidence is there that high sex ratios do lead to male violence? We see men among us who stay celibate for a long time without endangering public order. There is the lifelong bachelor who leads a quiet productive life. There is the immigrant who remains single for 10 to 20 years while he saves up money, buys a home, and builds up his business.

These examples, however, differ on one point from the new society we’re entering—where over a third of all men will be frozen out of marriage and fatherhood. In the past, a lifelong bachelor was often a self-selected individual with a low sex drive and probably low testosterone levels. As for the immigrant who stays single for 10 to 20 years, he would eventually bring out a wife from the old country—typically 10 to 20 years his junior. Neither example violates the social contract we once offered young men: work hard, obey the law, and you will become a full-fledged member of society.

The problem begins when this social contract is no longer honored. Consider China, where the sex ratio has progressively risen since the mid-1980s—after the adoption of the one-child policy and the advent of prenatal sex testing. Today, 97% of all unmarried Chinese aged 28-49 are male. These men make up 72-75% of a large floating population that numbers between 100 and 150 million (Hudson & Boer, 2002, p. 29). Variously described as ‘migrants,’ ‘transients,’ or ‘bare branches,’ they account for a disproportionate share of crime, especially violent crime:

In Beijing, 44% of the crimes solved by the police were committed by transients. In Shanghai, this rate has been continually rising from 10% in the mid-1980s to 60%, even 80% in some districts, by 1995. … Moreover, our study found that many crimes committed by transient people are senseless and ruthless. An argument over a word can lead to a cold-blooded fight; burglars often kill the victims or witnesses on the scene if the offense is observed; highway robbery, rape, and kidnapping usually end with the victims’ death; and a complaint about the poor quality of goods sold by transient vendors can cause injury in a severe physical assault. (Hudson & Boer, 2002, p. 32)

A similar trend has appeared in the northwestern states of India, where sex ratios have reached particularly high levels:

Indeed there is a statistically significant relationship between violent crime rates and the sex ratio in Indian states. Sen notes that “extensive interdistrict contrasts … show a strong—and statistically very significant—relation between the female-male ratio in the population and the scarcity of violent crimes.
… The strongest correlation found was between murder and sex ratio, which were inversely related. As the authors note, “This correlation is very robust: no matter which other variables are included or excluded from the regression, we found that the female-male ratio remained highly significant, always with a negative sign. Further, the size of the coefficient of the female-male ratio is quite large” (Hudson & Boer, 2002, pp. 34-35)

Hudson and Boer (2002) argue that an excess of males increases not only violent crime and internal instability but also the probability that a country will go to war. For the government, war helps ease the tensions created by having too many single men, if only by killing them off. For a single man, war is a chance to enhance his social status and improve his access to women.

… the worst-case scenario implies that China may have close to 40 million young adult bare branches to spare in twenty years, and that the government may at that point ardently wish to see them give their lives in pursuit of a national interest. The alternative is to allow them to remain a threat to national interest, which may increasingly be seen as an untenable policy position by the government (Hudson & Boer, 2002, pp. 36-37)

Another scenario is that China will export single males and import single females. In recent years, a popular destination for Chinese emigrants has been Africa, partly because of its lax immigration laws and partly because of its wealth of undeveloped raw resources. This Chinese diaspora is estimated to range in size from 500,000 to 750,000 (Mohan & Kale, 2007). To the extent that single males take this route, there will inevitably be intermarriage that may alter China demographically when they go home.

Such a scenario may resemble what is already happening in South Korea, where 13.6% of all marriages now involve foreign brides (Lee et al., 2006). At first, these brides were largely ethnic Koreans from China. Preferences are now shifting toward Southeast Asians:

Another 18 percent of foreign wives are from other Asian countries, especially Vietnam, the Philippines and Thailand. According to one international marriage agency, provincial bachelors prefer Southeast Asian women despite their darker skin (racial differences are still subject to stares and comment here, as any expatriate knows) because ethnic Korean women from China have acquired a reputation of melting away into the cities to work after using their new husband only to get a visa. (source)

In one rural county, 3 out of 10 marriages are with foreign women, mostly Southeast Asians. Likewise, a growing proportion of school-age children are of mixed origin:

In North Jeolla province there are 755 biracial students, 700 in elementary schools, 44 in middle schools and 11 in high schools. Four out of eight new students entering Mupung Elementary School in Muju next year are biracial. (source)

Some projections suggest that children of mixed parentage will account for 30% of South Korean births by 2020 (Wikipedia – Immigration to South Korea).

This trend has not gone unnoticed in the ‘other’ Korea. The North Korean press has recently condemned it as “an unpardonable bid to negate the homogeneity of the nation, make South Korea multiracial and Americanize it.”

Both of these trends will probably spread to China, i.e., growing numbers of international marriages and increasing nationalist opposition. That country has an even worse gender imbalance and has adopted the same Western ethos of autonomous individuals being free to pursue their own happiness—even when such pursuits impose costs on everyone else.

The main difference is that China’s standard of living is lower than South Korea’s. Chinese bachelors will probably look for wives in even poorer places, particularly Cambodia, Indonesia, and Africa.


Hudson, V.M. & A.D. Boer. (2002).
A surplus of men, a deficit of peace. Security and sex ratios in Asia’s largest states, International Security, 26, 5-38.

Lee, Y-J., D-H. Seol, & S-N. Cho. (2006).
International marriages in South Korea: the significance of nationality and ethnicity, Journal of Population Research, November 1, 2006.

Mohan, G., & D. Kale. (2007). The invisible hand of South-South globalisation: Chinese migrants in Africa, A Report for the Rockefeller Foundation prepared by The Development Policy and Practice Department, The Open University, Milton Keynes, MK7 6AA, UK

Pedersen, F.A. (1991). Secular trends in human sex ratios: Their influence on individual and family behavior, Human Nature, 2, 271-291.

Thursday, September 3, 2009

Young men in a sellers market

Except for frontier areas, single women used to outnumbered single men on the American marriage market. This situation has reversed since the late 1970s and early 1980s because of falling male mortality and the rising numbers of older men divorcing and remarrying (Pedersen, 1991). Similar reversals have occurred throughout the Western world.

In this new environment, more women are hesitating to commit, knowing that the penalty of being ‘left on the shelf’ is much less than before. Many, in fact, are playing the market and postponing marriage as late as possible. One result is a rising age of first marriage—almost 30 in most Western countries.

Because people are getting married later, the ratio of single men to single women under 30 looks almost balanced. This near-parity has lulled some writers, like Glowsky (2007), into thinking that intense competition for women is a problem only for middle-aged men. Actually, single men under 30 face even fiercer competition. Using age-preference data, Ni Bhrolchain and Sigle-Rushton (2005, pp. 44, 46) have estimated that two single men are competing for each single woman at the youngest ages:

… Among men, average partner supply is 0.4 at age 17, reaches and goes above 1.0 at around 30 in the US and at 45 in England and Wales (though also, briefly, at age 30) and then rises to 2.0 (US) and 1.6 (E&W) by age 60. On these estimates, a 50-year-old American man had around the same number of potential partners as an American woman of 20 in 1990 ....

… In pure demographic terms, then, and taking these figures at face value, men and women of the same age encounter quite dissimilar levels of partner supply at most ages. In 1990-91, average availability for women far exceeds that for men at younger ages and the reverse is true at older ages. In both countries [United States and Great Britain] in 1990-91, unmarried women aged 20-24 had between 34% and 163% more potential partners on average than did men, and those aged 25-29 between 8% and 28% more.

It might be argued that these estimates overstate the gender imbalance. After all, some young men are uninterested in female companionship because they’re asexual, homosexual, or psychologically immature. This objection was tested by Ni Bhrolchain and Sigle-Rushton (2005, p. 53) through a survey of dating agency clients:

If men experience partner shortages at the prime ages of male marriage, we would expect to find young men over-represented in the dating agency client population. This is indeed the case. Between ages 21 and 40 the age-specific sex ratio among Dateline clients is between 1.4 and 2.6, compared with sex ratios of between 1.1 and 1.3 among the unmarried in England and Wales in 1996.

The new ‘seller’s market’ contrasts with what young men used to face as recently as the early 1970s. Yet this change in fortune has triggered few alarm bells among the commentariat. How come? One reason is that many people blame the sexual revolution of the 1960s on that era’s low ratio of single men to single women. The argument is that a surplus of single women makes men sexually irresponsible. To ensure stable families, we therefore need a surplus of single men. Such a surplus would not make women sexually irresponsible, since women are naturally inclined to form stable, long-lasting relationships.

This view was notably advanced by Pedersen (1991). He predicted that the recent shift to a higher sex ratio among singles would mean less divorce, less illegitimacy, less marital instability, higher birth rates, and greater commitment by men to getting an education and pursuing a career. These presumed benefits caused many, particularly on the political right, to see the new marriage market as a godsend. As recently as 2004 the Wall Street Journal ran an opinion piece that praised the wife shortage and its beneficial influence on the American family (Wilson, 2004).

There is another reason for the complacency of the commentariat. Singleness is a topic where older women do most of the talking and writing. For them, the man shortage is what really matters.

That the adverse marriage market position of young men has been overlooked may be due to the traditional tendency in demography to examine mainly female marriage and partnership. One author pointed to the marriage market difficulties of older women as being the “real” marriage squeeze (Veevers 1988), but in doing so ignored the partner shortages experienced by young men — an issue of greater demographic significance since it occurs at and before the prime ages of male marriage. Davis and van den Oever (1982) suggest that the surplus of unmarried men at young ages is of little importance since most will ultimately marry. (Ni Bhrolchain & Sigle-Rushton, 2005, pp. 59)

From a broader societal viewpoint, we should worry more about the lack of wives for younger men under 35 than about the lack of husbands for older women over 45. The latter are no longer able to have children and are simply seeking companionship. In contrast, the former risk being denied not only companionship but also a role in reproduction.


Glowsky, D. (2007). Why do German men marry women from less developed countries? SOEP papers on Multidisciplinary Panel Data Research #61

Ni Bhrolchain, M. & W. Sigle-Rushton. (2005). Partner supply in Britain and the U.S. Estimates and gender contrasts, Population, 60, 37-64.

Pedersen, F.A. (1991). Secular trends in human sex ratios: Their influence on individual and family behavior, Human Nature, 2, 271-291.

Wilson, J.Q. (2004). Sex Matters. Will too many boys make China and India aggressive militarily? The Wall Street Journal, Tuesday, July 13, 2004