Monday, March 31, 2008

Questions on polygyny and the changing marriage market

A reader has raised a couple of questions. I’ll begin with the first one:

Let's suppose people from other parts of the world move to a tropical environment. Should we expect a shift toward polygynous habits within a couple of millennia under the influence of tropical agriculture?

People would shift toward more polygyny, but they would do so by pushing their envelopes of behavioral plasticity. A couple of millennia doesn’t seem to be enough time to shift the genetic goalposts.

Some 9,000 years ago, Amerindians began to develop agriculture in the tropical New World. These tropical agriculturalists, like the Yanomamo of Amazonia, show the predicted changes in behavior: higher female self-reliance in food production, lower paternal investment, and higher polygyny rates. Between 10% and 20% of all Yanomamo males are polygynous at any one time (Hames, 1995). Nonetheless, this is still lower than the 20-50% we see in sub-Saharan Africa.

It seems, then, that natural selection hasn’t had enough time to create a strong predisposition to polygyny in tropical Amerindians, at least not as strong as what we see in sub-Saharan Africa. In addition, their ancestors may have started off with a very low predisposition to polygyny, having entered the Americas by way of an Arctic environment that selected for high paternal investment. Even in the tropics, Amerindians still have an Arctic-adapted anatomy and this evolutionary conservatism may extend to behavior (Holliday, 1997, pp. 425-426).

In polygynous societies, women choose men. In societies with an unbalanced sex ratio (females>males wise) men choose women. How would you describe the actual state of our society according on this scale?

If we look at reproductive age brackets (20 – 40 years), there are more single men than single women. This is a relatively recent phenomenon and seems to date from the late 1980s. The 1992 U.S. census found the following ratios of single men to single women per age group:

Under 25 – 111
25 to 29 years – 128
30 to 34 years – 121
35 – 39 years – 109
40 – 44 years – 83
45 – 64 years – 64
65 years and over – 30

The imbalance is even greater among 20 to 40 year-olds if we look only at childless singles. It is greater still among white Americans (the black American population has a surplus of single women because of high mortality and incarceration rates among young black men).

Not long ago, the reverse was true. There were more single women than single men in all age groups. If a woman was still unmarried at 25, people considered her doomed to spinsterhood.

What happened? First, the death rate has fallen dramatically among young men. No major wars have occurred since 1945. Highway and workplace accidents have steadily declined. Alcohol is no longer the grim reaper that it once was. Today, so few young men are dying that we now have a serious shortage of organs for transplantation.

Second, divorce laws have liberalized throughout the Western world. It’s now much easier for a man to divorce his aging wife and marry a younger one. And this is the pattern we generally see with second marriages.

Third, it is economically easier, and socially more acceptable, for a woman to have children out of wedlock. Many of these single mothers then drop out of the marriage market, partly because they find it difficult to date men and be a parent at the same time. As well, men are often reluctant to care for children who are not their own, especially if (as is often the case) the woman is not interested in having any more.

In short, marriage-minded men are now in a seller’s market and not a buyer’s market. This is a big change, and one that has aroused surprisingly little comment from social scientists. Yet I see the evidence all around me—the thirty-something man who has a good job, no bad habits, and still no woman in his life. How come? Usually, he’ll duck the question and put on a brave face. Or he’ll blame himself: his low-paying job, his lack of personality, or his looks.

Yet these reasons were not operational when my parents were young—less than a half-century ago. If a man wasn’t married then, it was because he didn’t want to be. Or because he was a ‘bum’ or a ‘psycho’. How times have changed …


Hames, R. (1995). Yanomamö, Varying Adaptations of Foraging Horticulturalists, Originally prepared for Just in Time Anthropology series, Prentice Hall and Simon & Schuster, supplemental readings for Ember and Ember's Anthropology, 8th edition.

Holliday, T.W. (1997). Body proportions in Late Pleistocene Europe and modern human origins. Journal of Human Evolution, 32, 423-447.

Monday, March 24, 2008

Testosterone and human variation

My last few posts have presented the following hypothesis:

1. During the hunter-gatherer stage of prehistory, food gathering was less important further away from the equator. This was because the longer winters restricted gathering of fruits, vegetables, roots, etc.

2. At these higher latitudes, women had to shift from food gathering to food processing or to activities unrelated to food procurement. For a man, this raised the cost of providing for a second wife, making it prohibitive for all but the ablest hunters. Thus, with increasing distance from the equator, polygyny became less common and men provided more for their wives and children.

3. This north-south gradient accentuated with the advent of agriculture. In the tropical zone, year-round farming made women much more self-reliant in feeding themselves and their children. Conversely, it made polygyny much less costly for men, thereby increasing male-male competition for mates.

4. In these tropical agricultural societies, men and women initially adapted by pushing the envelope of behavioral plasticity. Over time, however, natural selection favored those genotypes that lay closer to the new behavioral mean, i.e., increased predisposition to polygyny, increased aptitude for male-male competition for mates, and decreased paternal investment.

5. This cultural and biological evolution seems to have gone the furthest among the agricultural societies of sub-Saharan Africa, where 20-50% of all sexual unions are polygynous. These societies typically dealt with the resulting wife shortage by giving priority to men who were at least ten years past puberty. Younger men (15-25 years old) could have sex lives only through surreptitious means or by seizing women during domestic or intertribal warfare.

6. This behavioral shift probably involved a number of genetic changes, including higher testosterone levels in young males. Raising these levels seems to lower the threshold for expression of polygynous behavior while raising the one for expression of paternal investment.

High testosterone levels are widely attested among populations that descend from sub-Saharan agriculturalists. When Ross et al. (1986) studied white and black American students (mean age = 20 yrs), they found mean testosterone levels to be 19% higher in the blacks than in the whites and free testosterone levels 21% higher. In a later study, Ross et al. (1992) found that young East Asian men had intermediate testosterone levels but less 5α-reductase—an enzyme that converts testosterone into the physiologically more active DHT.

Testosterone levels are higher in black boys than in white boys as early as the prepubertal age class (5-9 yrs) (Abdelrahaman et al., 2005). Black males seem to reach maximum t levels during adolescence and early adulthood (Ross et al., 1986; Winters et al., 2001). The black-white difference then shrinks after 24 years of age and is gone by the early 30s (Gapstur et al., 2002). It actually seems to reverse in later years (Nyborg, 1994, p. 111-113).

Broadly speaking, lifetime exposure to testosterone correlates with the incidence of prostate cancer and the highest incidences in the world are among African American men (Brawley and Kramer, 1996). Other populations of black African descent (i.e., West Indians and sub-Saharan Africans) exhibit lower incidences, but these have been shown to reflect underreporting and are probably just as high (Glover et al., 1998; Ogunbiyi and Shittu, 1999; Osegbe, 1997). It should be noted that the human body’s exposure to testosterone depends not only on blood serum levels but also on how well this hormone interacts with androgen receptors, which seem to be more numerous and more receptive in black subjects (Kittles et al., 2001).

Despite these findings, several studies have raised doubts about the existence of a black-white difference in t levels:

Ross et al. study

Ross et al. (1986) found that the black-white difference shrank to 15% for testosterone levels and to 13% for free testosterone levels after adjusting for time of sampling, age, weight, alcohol use, cigarette smoking, and use of prescription drugs. Thus, these factors may have artificially boosted testosterone levels in black subjects.

Of these factors, adjusting for weight and age actually widened the black-white difference. Adjusting for time of sampling narrowed it but only to a small degree. It was thus the lifestyle factors—smoking and drinking—that seem to be associated with high t levels in black subjects.

These lifestyle factors have been investigated by Martin et al. (2002), who found that pubertal development in males and females was associated with sensation seeking, smoking, and drinking, even after controlling for age. This resonates with my own experience. At school, boys who didn’t drink or smoke were derided as immature ‘sucks’. And strangely enough they looked less mature, being smaller and less hirsute. The smokers and drinkers were generally bigger and already sporting sideburns in Grade 8.

It looks as though higher t levels cause boys to seek social environments where drinking and smoking is more common, probably because such environments provide more opportunities for male dominance and access to women. If we adjust for these lifestyle factors, we end up biasing the sample toward subjects with low t levels.

Rohrmann et al. study

Rohrmann et al. (2007) tested black Americans, white Americans, and Mexican Americans in three age classes: 20-44; 45-69; and 70+. In the 20-44 age class, white Americans had the lowest level of testosterone (5.17 ng/mL), black Americans an intermediate level (5.35), and Mexican Americans the highest level (5.55)

As noted above, the black-white difference in t levels shrinks after 24 years of age, is gone by the early 30s, and seems to reverse at older ages. So it should not show up in the average t levels of 20 to 44 year-olds. In addition, median age differed among the three groups of 20 to 44 year-olds, being 31.8 for the whites, 29.5 for the blacks, and 28.6 for the Hispanics. These age differences probably account for the higher t levels of the Mexican American subjects. Moreover, the youthfulness of the Mexican-American group is probably understated by the use of median ages rather than mean ages. If means had been used, the age differences would have probably been greater, given the more youthful age pyramid of the Hispanic-American population.

Finally, it is odd that the 45-69 year old black men had higher t levels (5.62) than the 20-44 year old black men (5.35). This is contrary to the literature on hormonal variation with age and suggests the authors may have massaged their data, perhaps by eliminating high testosterone outliers.

Ellison et al. study

Ellison et al. (2002) measured salivary testosterone in young men (15-30 years) from the United States, Congo, Nepal, and Paraguay. The American subjects had the highest t levels (335 pmol/l), followed by the Congolese (286), the Nepalese (251), and the Paraguayans (197).

But who were these young American subjects? They are simply identified as … young Americans—a demographic that is now less than 60% of European descent. In Boston, where the study was conducted, the youth population is now largely of African or part-African descent. In 2005, Boston public schools were 46% black and 31% Latino (mainly Puerto Ricans and Dominicans).

Perhaps a disproportionate number of subjects were white Harvard students. On the other hand, the authors state, "the USA participants were recruited by public advertisement, thus the potential for self-selection bias should be noted.” The demographic profile should therefore have resembled that of others who give blood in exchange for payment, i.e., disproportionately poor and non-white. In any event, the results are unusable without any indication of racial composition.

African studies

Several studies have found lower testosterone levels in African populations than in North Americans. This difference might be partly due to the effects of malnutrition or infectious diseases, notably among the Zimbabwean subjects studied by Lukas et al. (2004). The main reason, however, is that these studies used largely middle-aged or even elderly subjects. Lukas et al. (2004) report a mean age of 42.18 (S.D. = 13.75). Their scatter plot (Fig. 2) suggests a logarithmic decline in t values with age, but they had too few subjects below 25 for any meaningful analysis of that age group. The same criticism applies to a study by Campbell et al. (2003) of t levels in Ariaal pastoralists from northern Kenya. Those subjects had a mean age of 46.8 (S.D. = 14.3 years).

In addition, some of these studies were done on hunter-gatherers, like the !Kung of Namibia and the Ituri Forest pygmies of the Congo, who have low levels of polygyny and weak male-male competition for mates (e.g., Winkler and Christiansen, 1993). Their low t levels are thus to be expected. This factor may also partly explain why Ellison et al. (2002) found lower t levels in young Congolese than in young Americans. The Congolese subjects were Lese, an Ituri Forest Bantu population with high admixture from Pygmy hunter-gatherers.


Abdelrahaman, E., Raghavan, S., Baker, L., Weinrich, M., and Winters, S.J. (2005). Racial difference in circulating sex hormone-binding globulin levels in prepubertal boys. Metabolism, 54, 91-96.

Brawley, O.W. and Kramer B.S. (1996). Epidemiology of prostate cancer. In Volgelsang, N.J., Scardino, P.T., Shipley, W.U., and Coffey, D.S. (eds). Comprehensive textbook of genitourinary oncology. Baltimore: Williams and Wilkins,

Campbell, B., O’Rourke, M.T., and Lipson, S.F. (2003). Salivary testosterone and body composition among Ariaal males. American Journal of Human Biology, 15, 697-708.

Ellison, P.T., Bribiescas, R.G., Bentley, G.R., Campbell, B.C., Lipson, S.F., Panter-Brick, C., and Hill, K. (2002). Population variation in age-related decline in male salivary testosterone. Human Reproduction, 17, 3251-3253.

Gapstur, S.M., Gann, P.H., Kopp, P., Colangelo, L., Longcope, C., and Liu, K. (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.

Glover, F., Coffey, D., et al. (1998). The epidemiology of prostate cancer in Jamaica. Journal of Urology, 159, 1984-1987.

Kittles, R.A., Young, D., Weinrich, S., Hudson, J., Argyropoulos, G., Ukoli, F., Adams-Campbell, L., and Dunston, G.M. (2001). Extent of linkage disequilibrium between the androgen receptor gene CAG and GGC repeats in human populations: implications for prostate cancer risk. Human Genetics, 109, 253-261.

Lukas, W.D., Campbell, B.C., and Ellison, P.T. (2004). Testosterone, aging, and body composition in men from Harare, Zimbabwe. American Journal of Human Biology, 16, 704-712.

Martin, C.A., Kelly, T.H., Rayens, M.K., Brogli, B.R., Brenzel, A., Smith, W.J., and Omar, H.A. (2002). Sensation seeking, puberty, and nicotine, alcohol, and marijuana use in adolescence. Journal of the American Academy of Child & Adolescent Psychiatry, 41, 1495-1502.

Nyborg, H. (1994). Hormones, Sex, and Society. The Science of Physiology. Westport (Conn.): Praeger.

Ogunbiyi, J. and Shittu, O. (1999). Increased incidence of prostate cancer in Nigerians. Journal of the National Medical Association, 3, 159-164.

Osegbe, D. (1997). Prostate cancer in Nigerians: facts and non-facts. Journal of Urology, 157, 1340.

Pettaway, C.A. (1999). Racial differences in the androgen/androgen receptor pathway in prostate cancer. Journal of the National Medical Association, 91, 653-660

Rohrmann, S., Nelson, W.G., Rifai, N., Brown, T.R., Dobs, A., Kanarek, N., Yager, J.D., Platz, E.A. (2007). Serum estrogen, but not testosterone levels differ between Black and White men in a nationally representative sample of Americans. The Journal of Clinical Endocrinology & Metabolism, 92, 2519-2525

Ross, R.K., Bernstein, L., Lobo, R.A., Shimizu, H., Stanczyk, F.Z., Pike, M.C., and Henderson, B.E. (1992). 5-apha-reductase activity and risk of prostate cancer among Japanese and US white and black males. Lancet, 339, 887-889.

Ross, R., Bernstein, L., Judd, H., Hanisch, R., Pike, M., and Henderson, B. (1986). Serum testosterone levels in healthy young black and white men. Journal of the National Cancer Institute, 76, 45-48.

Winkler, E-M., and Christiansen, K. (1993). Sex hormone levels and body hair growth in !Kung San and Kavango men from Namibia. American Journal of Physical Anthropology, 92, 155-164.

Winters, S.J., Brufsky, A., Weissfeld, J., Trump, D.L., Dyky, M.A., and Hadeed, V. (2001). Testosterone, sex hormone-binding globulin, and body composition in young adult African American and Caucasian men. Metabolism, 50, 1242-1247.

Monday, March 17, 2008

Testosterone and paternal investment

As modern humans moved north into environments with longer winters, women were less able to feed themselves and their children through food gathering. They thus became more dependent on food from their male partners. For men, this greatly increased the cost of having a wife and children, thus making polygyny prohibitively expensive for all but the ablest hunters.

Initially, this situation came about by men and women pushing their respective envelopes of behavioral plasticity. It may not have been the happiest of situations, but circumstances left no other choice. Over time, however, natural selection should have improved things by favoring men who were less predisposed to polygyny and more predisposed to provide for their wives and children.

How? Apparently, by lowering testosterone levels in men once they’ve entered a pair bond. This has been shown by findings recently presented at this year’s annual meeting of the American Association of Physical Anthropologists. According to Shur et al. (2008):

Numerous studies reveal a negative correlation between testosterone concentration and paternal care in diverse mammals including nonhuman primates and humans. Several researchers suggest that spousal investment accounts for the lower testosterone of married men compared to unmarried men, but findings that the lowest testosterone levels are observed in married men with children implicate paternal care as particularly relevant. Thus testosterone reduction may reflect a facultative shift in male reproductive strategy from intrasexual competition and copulation to care of young.

This hypothesis was tested with wild olive baboons, among whom lactating females form close “friendships” with their male partners.

In contrast to control males, male friends experienced a decrease in testosterone level coinciding with the birth of their female friends’ infants. Male friends also maintained a lower basal testosterone level than did control males during the lactation period of their female friends. Testosterone levels in male friends increased gradually corresponding with developing infant independence.

This finding may explain the marked differences in testosterone levels we see in humans, particularly between tropical and non-tropical populations. These levels seem to decrease wherever men compete less keenly for mates (because polygyny is less common) and wherever they invest more in parenting. Lowering the level of testosterone seems to lower the threshold for expression of paternal investment.

If the testosterone level has fallen in some populations because of selection for paternal investment, we should see evidence of such selection elsewhere, e.g., altered spatial distribution of testosterone receptors in the brain, more mental space dedicated to parenting behavior, etc.


Shur, M.D., Palombit, R.A., and Whitten, P.L. (2008). Association between male testosterone and friendship formation with lactating females in wild olive baboons (Papio hamadryas anubis). Program of the 77th Annual Meeting of the American Association of Physical Anthropologists, p. 193.

Monday, March 10, 2008

The path to civilization?

As hunter-gatherers moved into non-tropical environments, women had to give up food gathering, this kind of food procurement being untenable during winter. So women shifted to food processing, such as butchery and carcass transport, or to activities unrelated to food, such as shelter building, garment making, leather working, transport of material goods, etc. (Waguespack, 2005).

This was a key advance in cultural evolution. Among tropical hunter-gatherers, everyone is primarily dedicated to food procurement. All other activities are secondary. Among temperate and arctic hunter-gatherers, women no longer procure food during much or most of the year. They can thus dedicate themselves to a broader range of activities beyond those of hunting and gathering.

Paleoanthropologists usually attribute this cultural advance to a chain of events initiated by the advent of agriculture: 1) farmers were able to produce a food surplus that could be stored for future use; 2) this food surplus fell under the control of high-status individuals; 3) these individuals bolstered their power and prestige by giving food to underlings in exchange for various services (waging war, keeping written records, creating works of art, building roads, temples, monuments, aqueducts, etc.); 4) the recipients were thus freed from the daily quest for food and could fully dedicate themselves to other tasks; 5) these tasks were developed to a degree that would have been impossible in an economy devoted primarily to food procurement (Testart, 1982).

Except, except … even before agriculture, some of these tasks were already being developed far beyond the immediate needs of food procurement. We see this especially with the construction of large structures, the so-called megaliths. In eastern Turkey (Nevali Cori and Göbekli Tepe), large circular ceremonial complexes have been dated to 11,000 BP, when agriculture was at most incipient. In Ireland (Carrowmore), some megaliths have been dated to 7400 BP. This would have been in the Mesolithic and long before agriculture (Megaliths – Wikipedia; Rowley-Conwy, 1995).

The advent of agriculture is often portrayed as the key moment when humans set out on the path to civilization. Could this interpretation be mistaken? Could we be looking at just another consequence of a cause that lies earlier in time? Perhaps both agriculture and civilization originated in a technological revolution that began once women had moved out of food gathering and into a much wider range of tasks. Perhaps women were the ones who acquired the skills that eventually took humans beyond a subsistence level of existence.


Rowley-Conwy, P. (1995). Making first farmers younger: The West European evidence. Current Anthropology, 36, 346-353.

Testart, A. (1982). The significance of food storage among hunter-gatherers: residence patterns, population densities and social inequalities. Current Anthropology, 23, 523-537.

Waguespack, N.M. (2005). The organization of male and female labor in foraging societies: Implications for early Paleoindian archaeology. American Anthropologist, 107, 666-676.

Monday, March 3, 2008

Why didn't women hunt?

Among hunter-gatherers, women became less self-reliant in feeding themselves and their children with increasing distance from the equator. This was because longer winters restricted food gathering, thus forcing women to turn to men for food procured through hunting.

But why didn’t these women switch from gathering to hunting? They did, to a limited extent. In non-tropical environments, many would take part in fishing and capture of small animals. Some would even hunt big game if no men were available, as in the case of widows. Among the Inuit: “This type of role exchange did occur traditionally under extraordinary conditions (Kemp, pers. comm.) but normally only males were regarded as potential providers” (Schrire and Steiger, 1974, p. 179).

Why males were so regarded is a subject of debate. Evolutionary psychologists will argue that women are better at resolving certain spatial tasks than men are, and vice versa. Women excel at recognizing food items within complex arrays of vegetation, whereas men excel at tracking moving objects over long distances and performing the mental transformations needed to stay on track over time and space (Eals and Silverman, 1994). Cultural anthropologists, on the other hand, will point to social conventions, notably menstrual taboos, that forbid women to touch hunting gear or weapons. Both positions may be right. If women are less proficient at hunting, cultural selection will favor taboos that steer them away from that activity.

Whatever the reason, according to a comparative study of 71 hunter-gatherer societies, women do not switch to hunting, at least not appreciably so, when opportunities to gather food disappear. They instead become more involved in food processing or in activities unrelated to food (shelter building, garment making, etc.).

What has been established so far is that as the percentage of meat in the diet increases, a concordant shift in both the types of resources women procure and their degree of involvement in manufacturing activities occurs. First, plant-gathering activities tend to focus on high post-encounter returns from items such as fruits and roots, and processing-intensive plant foods like seeds and nuts are generally avoided. Second, the average amount of female time spent in the procurement of food decreases with the proportion of meat in the diet. Third, female participation in nonsubsistence activities increases in societies with hunting-dominated subsistence economies. Although many of the examined tasks such as burden carrying and butchery are likely to be associated with subsistence, they are not directly involved with food procurement. I interpret these general relationships to reflect a substantial difference in the organization of labor between predominately hunting-based versus predominately gathering-based forager economies. As female labor is increasingly oriented to tasks other than direct food procurement and especially activities that facilitate hunting, male hunters may have potentially more time and energy to devote to resource acquisition. (Waguespack, 2005, p. 671).

In short, when women can no longer maintain their self-reliance through food gathering, they don’t shift to other forms of food procurement. Instead, they intensify the pair bond with their male partners, transforming it into one of mutual dependence.


Eals, M. and I. Silverman. (1994). The hunter-gatherer theory of spatial sex differences: Proximate factors mediating the female advantage in recall of object arrays. Ethology and Sociobiology, 15, 95-105.

Schrire, C., and Steiger, W.L. (1974). A matter of life and death: An investigation into the practice of female infanticide in the Arctic. Man, 9, 161‑184.

Waguespack, N.M. (2005). The organization of male and female labor in foraging societies: Implications for early Paleoindian archaeology. American Anthropologist, 107, 666-676.