Thursday, October 19, 2017

Virtual polygyny?

Polygyny is accepted to varying degrees around the world. In some countries it is permitted by law (dark blue) or by customary law (medium blue). Or the law may permit polygyny if the marriage has been performed in another country (light blue). (Wikicommons)

In my last two posts I presented evidence that repeated exposure to porn desensitizes the male brain, eventually causing atrophy in those areas that process erotic stimuli. In addition, porn seems to influence psychosexual development differently in young European American males and young African American males.

If porn is virtual polygyny, the male brain should tolerate porn overload to the extent it has coevolved with polygyny. The threshold for desensitization would therefore be higher in cultures with generalized polygyny (20-50% of all marriages) and lower in cultures with limited polygyny (less than 10%).

The ‘virtual polygyny’ hypothesis was first put forward by Shepher and Reisman (1985):

Pornography creates a world of polygynous fantasy, in which there are always sufficient consenting females who unhesitatingly display their naked bodies, or body parts, thus signaling their preparedness for immediate sexual intercourse.

This fantasy world is very different from real life, especially where monogamy is the norm:

Of course, this fantasy world of unlimited numbers of young, beautiful, seductive females, eagerly and enthusiastically engaging in every sort of sexual and violent activity, contrasts sharply with everyday reality. (Shepher and Reisman 1985, p. 107)

It is even different from real life in polygynous cultures:

No power struggle, no competition between males for sexual access to a specific female is involved, because mass production makes the pornographic dream easily available to everyone. (Shepher and Reisman 1985, p. 108).

As a result, porn leads to desensitization and a desire for more and harder porn:

The result of fantasy-directed expectations may be a deterioration of male-female relationships, perhaps a deterioration of heterosexual comradeship and even love. Surely many males become disillusioned with their female partners' ability to arouse them. Any consistent use of pornographic magazines could also find readers thus disillusioned with their partners and their own sexual performance. The consequence for males may be, among other dysfunctions, conditional impotence. (Shepher and Reisman 1985, p. 110)

[...] habitual viewing often seems to result in a loss of arousal. We have found then, not illogically, that pornography is pushed to seek novelty: oral sex, anal penetration, sex with children, bestiality, pseudolesbianism, and sadistic sexuality. What this extension of repertoire tends to do to the male-female relationship is not difficult to imagine. (Shepher and Reisman 1985, p. 110)

[...] Arguably, as pornography use grows, male-female relationships deteriorate, aggression against women increases, sexuality is pushed towards more and more extravagant forms, more and more detached from sexuality's basic function in human life. (Shepher and Reisman 1985, p. 112)

Given that the incidence of polygyny varies considerably among human cultures, could some human populations be less vulnerable than others to porn desensitization? The two authors seem to raise this question:

Among 847 human cultures, 708 (83.4%) were found to condone polygyny, 137 (16%) monogamy, and 4 (0.47%) polyandry' (Murdock 1967). About half of the polygynous cultures permit polygyny, but actually not many males are married polygynously. The other half practices systematic polygyny.

This is a classic sample of coevolution: natural selection working on the individual favors polygyny; cultural selection working on the group favors monogamy. The most "successful" cultures, in the sense of their having the largest populations (Europe, the Americas, Japan, China, India), are monogamous, and most individuals in polygynous cultures are monogamous as well. (Shepher and Reisman 1985, p. 112)

Shepher and Reisman (1985) don't pursue this line of reasoning. One reason may have been the view, common in evolutionary psychology, that human nature has evolved very little since the Pleistocene. Because the high incidence of polygyny in sub-Saharan Africa is associated with agriculture (African hunter-gatherers have a very low incidence), and because agriculture began to develop there only some six thousand years ago (Vansina 1994), Shepher and Reisman might have concluded that the male brain never coevolved with generalized polygyny in sub-Saharan Africa. But why, then, the reference to coevolution? I suspect they simply floated this idea in the hope that someone else would pick it up. Or perhaps they had discussed this idea at greater length in their original manuscript ...

Shepherd and Reisman were writing in the 1980s, at a time when porn desensitization was probably much less common than it is today. Malamuth and Billings (1986, p. 93) reviewed the literature at that time:

Varied studies conclude that repeated exposure to erotica will, under many circumstances, result in less sexual arousal to and reduced interest in such materials. These studies include both experimental and survey research.

The first clear experimental study demonstrating habituation was conducted as part of the research of the commission [on Obscenity and Pornography] (Howard, Reifler, & Liptzin, 1971). This study found that repeated exposure of male college students to erotica for 90 min a day, 5 days a week for 5 weeks, resulted in a reduction in sexual arousal to erotic stimuli as well as reduced interest in such pornography. Following 2 months of nonexposure, however, there was a recovery in sexual arousal to levels that were not significantly different from those prior to the repeated exposure procedure [...]

Malamuth and Billings (1986) noted that this study had been criticized on the grounds that the levels of exposure were not "realistic." Today, such levels are common. According to a study of 16-year-old boys in two Swedish towns, 10% of them viewed porn every day, and about a third of these frequent users viewed porn for more than ten straight hours several times a week (Mattebo et al. 2013). In addition, young men are viewing porn for much longer than five weeks.

With the current high levels of porn consumption, more research is needed on the ‘virtual polygyny’ hypothesis, especially on its prediction that some human populations are more vulnerable than others to erotic desensitization and atrophy.


Malamuth, N.M. and V. Billings. (1986). The functions and effects of pornography: Sexual communications versus the feminist models in light of research findings, in J. Bryant and D. Zillmann (eds) Perspectives on Media Effects, Hillsdale, New Jersey: Erlbaum.

Mattebo, M., T. Tyden, E. Häggström-Nordin, K.W. Nilsson, and M. Larsson. (2013). Pornography consumption, sexual experiences, lifestyles, and self-rated health among male adolescents in Sweden, Journal of Developmental and Behavioral Pediatrics, 34, 460-468.

Shepher, J. and J. Reisman. (1985). Pornography: A sociobiological attempt at understanding, Ethology and Sociobiology, 6, 103-114.  

Vansina, J. (1994). A slow revolution: Farming in Subequatorial Africa, Azania: Archaeological Research in Africa, 29-30(1), 15-26.

Thursday, October 12, 2017

The unexplored confound

First page of an underground porn comic, c. 1930s (Wikicommons). Pornography is now much more available and better in quality.

My last post presented a German study whose findings suggest that prolonged exposure to porn atrophies those portions of the male brain that process erotic stimuli (Kühn and Gallinat 2014). On the other hand, the arrow of causality might point the other way. Perhaps a man will seek out and view more porn if he already has less of the gray matter for sexual arousal.

The jury is still out. In this debate, we should keep in mind that people have argued against porn for different reasons. Refuting one argument doesn't necessarily refute the others.

Historically, porn has been condemned for three reasons:

It incites men to commit rape and other forms of sexual abuse. This is the oldest argument and is still used.

It objectifies women and causes men to treat women with less respect. This is the feminist argument of the 1960s and 1970s.

It desensitizes men to erotic images and causes them to seek more porn and harder porn to achieve the same effect. This argument is recent.  

The first argument has always been problematic. A recent review article concludes that "[i]t has been found everywhere it was scientifically investigated that as pornography has increased in availability, sex crimes have either decreased or not increased" (Diamond 2009).

The second argument likewise seems weak. A Danish survey found that porn actually seems to improve male attitudes toward the opposite sex:

The self-perceived effects of "hardcore" pornography consumption were studied in a large representative sample of young adult Danish men and women aged 18-30. Using a survey that included the newly developed Pornography Consumption Effect Scale, we assessed participants' reports of how pornography has affected them personally in various areas, including their sexual knowledge, attitudes toward sex, attitudes toward and perception of the opposite sex, sex life, and general quality of life. Across all areas investigated, participants reported only small, if any, negative effects with men reporting slightly more negative effects than women. In contrast, moderate positive effects were generally reported by both men and women, with men reporting significantly more positive effects than women. (Hald and Malamuth 2008)

Of course, this is self-report, which often reveals not what people think but rather what people think they're supposed to think. Self-report showed that Trump was going to lose the 2016 election. In a liberal society, male respondents might think twice before saying their attitudes toward women have worsened. For this reason, it's interesting that similar results have been obtained from a sample of university students in Indonesia—a conservative, Muslim majority country with strict anti-pornography laws (Mulya and Hald 2015).

With regard to the third argument, two American studies have presented findings that seem to contradict those of the German study. In the first one, male and female participants were exposed to erotic and neutral images, and their neurological response was measured during the first second of exposure. Hypersexuals (heavy consumers of porn) showed no signs of habituation or desensitization to erotic images in relation to neutral images (Prause et al. 2015).

The other American study likewise found no signs of desensitization:

Data from a large sample of men (N = 280) across similar studies were aggregated to test the hypothesis that consuming more VSS [visual sexual stimuli] was related to erectile problems. These men answered questions about their sexual behaviors and feelings, including their consumption of VSS, and viewed sexual films in the laboratory. Those who reported viewing more VSS in their own life reported higher sexual arousal to films in the laboratory. Self-reported erectile functioning with a partner was not related to the hours of VSS viewed weekly. Finally, those who viewed VSS more also reported higher desire for both partnered sexual behaviors and solo sexual behaviors. This pattern suggests that those who view more VSS likely have a higher sexual drive and experience a stronger sexual response to standardized VSS than those who view less VSS. Sexual arousal responsivity may not be impaired by viewing more VSS at home, as it actually was related to stronger desire and sexual arousal in two of the three relationships tested. (Prause and Pfaus 2015)

Some of these findings are compatible with those of the German study. The latter study looked for long-term effects of porn consumption and found evidence of atrophy in some areas of the brain. Viewing porn in a laboratory won't produce any measurable atrophy because the time spent viewing isn't long enough (the films varied in length from 20 seconds to three minutes). The same kind of objection holds for the other American study discussed above, which found that porn consumption didn't reduce sexual arousal during the first second of exposure to an erotic image. Perhaps porn consumption makes sexual arousal less sustainable even though the initial response remains as strong as ever.

On the other hand, it's harder to dismiss the self-report data from the second American study: men who viewed more porn had a higher level of sexual desire than those who viewed less. So what gives?

Perhaps the two groups of men were different to begin with. This is plausible because the pool of participants was much more diverse in this American study than in the German study. The latter study excluded participants with psychiatric, medical, and neurological disorders or with substance abuse problems, whereas the American study had much weaker exclusion criteria. Furthermore, the German study had German participants, whereas the American study had participants who were 53.3% white, 23.1% Hispanic, 16.0% black, and 7.6% other or unknown (Kühn and Gallinat 2014; Prause and Pfaus 2015).

No one seems to have considered that ethnic/racial background might be confounded with sexual arousal or hours spent viewing porn. It's not as if this kind of confound is unlikely. Brown and L'Engle (2009) found higher porn consumption by young African Americans than by young European Americans. Ybarra and Mitchell (2005) reported that "Hispanic youth were almost three times as likely to report online seeking versus offline seeking behavior versus otherwise similar youth of non-Hispanic ethnicity (p = .02)." Price and Miller (1984) found that African Americans were also more likely than European Americans to use sexual fantasy to achieve arousal.

Just as importantly, according to a recent review article (Collins et al. 2011), erotic content in music, movies, and magazines seems to have different impacts on the development of sexual behavior in young white and black Americans:

Brown and colleagues subsequently expanded on this work by linking exposure to sexual content in a broader variety of media to intercourse initiation and advances in noncoital behavior. They surveyed 1,017 North Carolina youth when they were 12-14 years old and again two years later. Sexual content exposure in television, music, movies, and magazines predicted advancing sexual behavior, even after other variables were controlled for statistically, but only among white youth, who comprised about half of the sample. No relationship was observed among African-American teens, who made up the other half of the study sample. 

Most recently, Hennessy and colleagues analyzed web surveys of 506 Pennsylvania teens aged 14-16 years at baseline and followed them annually for a total of three surveys. They examined television, music, movies, magazines, and video games with a sexual content exposure measure. Data were analyzed using growth curves, testing whether changes in exposure to sexual media over time are correlated with changes in sexual behavior during the same period. They found that changes in exposure to sexual content were associated with changes in behavior among white teens (the r = 0.46 correlation just missed statistical significance, perhaps due to the small sample), but there was no association among African-American youth.

Whether these differences between white and black Americans are innate or learned is beside the point. These differences exist, and they should be controlled in any study with a mixed pool of participants. Otherwise, it’s no longer clear which way the arrow of causality points. In this case, the American research team found that porn consumers have a higher level of sexual desire. They concluded that porn consumption increases a man’s desire for sex. Well, perhaps. Or perhaps there is a subset of men who consume lots of porn because they have a stronger desire for sex.

In any study of this sort, the pool of participants should be as homogeneous as possible, i.e., it should be limited to people who probably shared the same potential for sexual arousal when some of them began viewing porn.


Research on the harmful effects of porn is shifting to the hypothesis that excessive consumption desensitizes the male brain. It seems, however, that this effect varies from one man to another, both within racial/ethnic groups and between them. It may be that porn desensitization is a greater problem for men whose ancestors were overwhelmingly monogamous, i.e., from Eurasia. For them, porn has created something entirely novel that their ancestors never experienced and never had to adapt to: a virtual environment where a man can have sex with as many women as he wishes. In contrast, porn may be less problematic for men with ancestors from sub-Saharan Africa, where polygyny has long been the norm (Goody 1973; Pebley and Mbugua 1989).


Brown, J.D., and K.L. L'Engle. (2009). X-rated: Sexual attitudes and behaviors associated with U.S. early adolescents' exposure to sexually explicit media, Communication Research, 36, 129-151.

Diamond, M. (2009). Pornography, public acceptance and sex related crime: a review, International Journal of Law and Psychiatry, 32(5), 304-414.

Collins, R.L., S.C. Martino, and R. Shaw. (2011). Influence of New Media on adolescent sexual health: evidence and opportunities, Working Paper, Rand Health, April 2011

Dupanloup, I., L. Pereira, G. Bertorelle, F. Calafell, M.J. Prata, A. Amorim, and G. Barbujani. (2003). A recent shift from polygyny to monogamy in humans is suggested by the analysis of worldwide Y-chromosome diversity, Journal of Molecular Evolution, 57, 85-97.

Goody, J. (1973). The Character of Kinship, Cambridge: Cambridge University Press.

Hald, G.M. and N.M. Malamuth. (2008). Self-perceived effects of pornography consumption, Archives of Sexual Behavior, 37(4), 614-625.

Kühn, S. and J. Gallinat. (2014). Brain structure and functional connectivity associated with pornography consumption. The brain on porn, JAMA Psychiatry, 71(7), 827-834.  

Mulya, T.W. and G.M. Hald. (2014). Self-perceived effects of pornography consumption in a sample of Indonesian students, Media Psychology, 17(1), 78-101.

Pebley, A. R., & W. Mbugua. (1989). Polygyny and Fertility in Sub-Saharan Africa. In R. J. Lesthaeghe (ed.) Reproduction and Social Organization in Sub-Saharan Africa, Berkeley: University of California Press, pp. 338-364.

Prause, N. and J. Pfaus. (2015). Viewing sexual stimuli associated with greater sexual responsiveness, not erectile dysfunction, Sexual Medicine, 3(2), 90-98.

Prause, N., V.R. Steele, C. Staley, D. Sabatinelli, and G. Hajcak. (2015). Modulation of late positive potentials by sexual images in problem users and controls inconsistent with "porn addiction," Biological Psychology, 109, 192-199.

Price, J.H. and P.A. Miller. (1984). Sexual fantasies of Black and White college students, Psychological Reports, 54(3), 1007-1014.

Ybarra M. and K.J. Mitchell. (2005). Exposure to Internet pornography among children and adolescents: a national survey, CyberPsychology & Behavior, 8(5), 473-486.

Tuesday, October 3, 2017

The canary in the coal mine?

Magazine rack in a Japanese store (Wikicommons - Corpse Reviver)

Hugh Hefner's death has ended an era that actually ended around the turn of the millennium. Gone are the days of porn in limited supply. During my teen years Playboy wasn’t sold in my town. You had to go to a drugstore 15 kilometers away and buy it in person, while hoping the cashier wouldn't blab to others. Then you had to find a place to hide it. Videocassettes were starting to come on market, but they had to be bought even farther away, and there was still the problem of finding a hiding place.

Fast-forward to the year 2017. From my computer I can access porn of almost any description in almost any quantity. And the access is fast, free, and anonymous. There is no comparison to the world of my youth, and even less to the world of 1953, when Playboy made its debut. That same year King Farouk of Egypt was described as a "self-indulgent playboy" with "carloads of erotica" (Gunther, 1953, p. 205). Carloads? That's nothing. Today, anyone with an Internet connection can stash away thousands upon thousands of erotic images.

This is a new sensory environment for humans. An analogy would be the increasing availability of food to native peoples in the far north. In the past their environment offered meat in limited amounts, and sometimes none was available. Hunters were thus highly motivated to seek out food and not let any go uneaten. Now fast-forward to the present. High-calorie snacks are available in any store, and they’re tasty with lots of salt, sugar, and fat—the very nutrients that were once in short supply. As a result, obesity is reaching epidemic proportions in the North.

No surprise really. And should we be surprised to learn that the increasing availability of porn today may have similarly adverse effects?

This question was addressed by a recent study on how porn consumption affects the male brain. Sixty-four men had their brains scanned, and the results were compared with the number of hours they spent viewing pornography per week. The results? Prolonged exposure to porn seemed to atrophy those portions of the brain that process erotic stimuli. The volume of gray matter was smaller in those subjects who viewed the most porn, and functional connectivity was likewise reduced. They seemed to require more porn (or harder porn) to achieve the same stimulation.

Taken together, one may be tempted to assume that the frequent brain activation caused by pornography exposure might lead to wearing and downregulation of the underlying brain structure, as well as function, and a higher need for external stimulation of the reward system and a tendency to search for novel and more extreme sexual material. This hypothesized self-perpetuating process could be interpreted in light of proposed mechanisms in drug addiction where individuals with lower striatal dopamine receptor availability are assumed to medicate themselves with drugs (Kühn and Gallinat 2014)

This interpretation is supported by a recent review of the literature:

Some internet activities, because of their power to deliver unending stimulation (and activation of the reward system), are thought to constitute supernormal stimuli, which helps to explain why users whose brains manifest addiction-related changes get caught in their pathological pursuit. [...] In short, generalized internet chronic overuse is highly stimulating. It recruits our natural reward system, but potentially activates it at higher levels than the levels of activation our ancestors typically encountered as our brains evolved, making it liable to switch into an addictive mode.

[...] previously established brain maps for "natural" sexuality cannot compare to the newly developed and continuously reinforced maps generated by continued compulsive watching of Internet pornography, and thus the addicted individual progresses to more explicit and graphic Internet pornography in order to maintain the higher level of excitement. (Love et al. 2015)

Of course, the arrow of causality might point the other way. Perhaps a man will seek out and view more porn if he already has less of the gray matter for sexual arousal. Only a longitudinal study can tell us which is causing what.

Let's suppose the first explanation is the right one. What can we do? Frankly, I'm pessimistic about legislative solutions. Give politicians the power to ban Internet porn (or Islamist extremism), and they'll use it to ban … the Alt-Right. Our political class lives in another age and sees reality through the lens of yesterday's issues and yesterday's priorities.

A second problem is that politicians try to ban child porn much more than the adult stuff. This is a classic case of going after a soft target that is secondarily important and perhaps not important at all. If porn has a desensitizing effect, it should cause pedophiles to lose interest in real children and focus on electronic images, since only the latter can be viewed in sufficient quantity to cause sexual arousal. So what’s the problem?

We should worry more about porn desensitization that disrupts relationships between adult men and women. A similar phenomenon has been noted with TV viewing: the more people watch TV, especially programming with romantic content, the more dissatisfied they feel with their marriages (Reizer and Hetsroni 2014). We may be becoming too good at creating virtual alternatives to reality.

The Japanese case

This desensitization may be most acute in Japan. In comparison to the United States, porn is more freely available there and less "compartmentalized":

Nudity is evident in both sexually identified and general circulation magazines. For instance, the weekly general-interest magazines will often include several photographs of nude women (plus ratings of local massage parlors). Although such magazines are oriented predominantly for businessmen, the inclusion of sexual photographs is also assumed to interest the business audience. In this regard, it is apparent that rigid boundaries do not exist for the publication of sexual material. In fact, nudity or sexual themes appear in Japanese teen magazines, sports magazines, fashion magazines, and so on. 

[...] Japanese public television is also different from its American counterpart. Overtly sexual material and nudity are permissible. For example, a Japanese television program known as the 11 P.M. Show can feature a strip tease, bare breasts and buttocks, reportage on massage parlors, expert authorities on sex, and so on. Similarly, the Japanese public-access movie channels can feature R-rated movies such as Emmanuelle (with some air-brushing). Finally, even Japanese commercials and advertisements have more flexibility in sexual content. (Abramson and Hayashi 2014, p. 179)

This ubiquitous porn is viewed by Japanese men, who are less polygynous than most human males and have lower blood levels of 5a-reductase—the enzyme that converts testosterone into the more physiologically active DHT (Ross et al. 1992). Lower testosterone activity seems to be an adaptation to monogamy and high paternal investment:

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. (Shur et al. 2008)

Some Japanese authors have reached this sort of conclusion, seemingly echoing J. Philippe Rushton (although the book in question predates his publications by several years):

Finally, a number of Japanese authors (e.g., Komatsu, 1974) have also suggested that Asian populations are less sexualized than Caucasian or black populations. They cite secondary sex characteristics (less public hair, smaller breasts, etc.) as evidence. Unfortunately, the data on sexual frequencies (intercourse, etc.) are not particularly reliable, and it is not clear whether competing responses (such as nurturance) are mediating sexual expression. (Abramson and Hayashi 2014, p. 182)

Japanese men and women may thus be especially vulnerable to porn consumption. More and more couples no longer have sex, the percentage rising from 31.9% in 2004 to 36.5% in 2008 and to 47.2% in 2016 (McCurry 2017; Moriki 2012). The major reasons given are "tired from work," abstention from sex after a birth, and "sex is too troublesome" (Moriki 2012). These reasons are probably proximal, and in any case it is far from clear why they would be growing in importance.

There is some indication that the Japanese realize that porn is becoming a problem. But it’s not clear where this realization will lead. In this, as in many other things, they follow the lead of other Western countries, especially the United States. Concern about porn is thus limited to child porn.


Abramson, P.R. and H. Hayashi. (1984). Pornography in Japan: Cross-cultural and theoretical considerations, in N.M. Malamuth and E. Donnerstein (eds) Pornography and Sexual Aggression, pp. 173-184, Orlando: Academic Press.

Gunther, J. (1953). Inside Africa, New York: Harper & Brothers.

Kühn, S. and J. Gallinat. (2014). Brain structure and functional connectivity associated with pornography consumption. The brain on porn, JAMA Psychiatry, 71(7), 827-834.

Love, T., C. Laier, M. Brand, L. Hatch, and R. Hajela. (2015). Neuroscience of Internet pornography addiction: a review and update, Behavioral Sciences, 5(3), 388-433.

McCurry, J. (2017). Record numbers of couples living in sexless marriages in Japan, says report, The Guardian, February 14

Moriki, Y. (2012). Mothering, co-sleeping, and sexless marriages: implications for the Japanese population structure, The Journal of Social Science, 74, 27-45.  
Reizer, A. and A. Hetsroni (2014). Media exposure and romantic relationship quality: A slippery slope? Psychological Reports, 114(1), 231-249.

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.

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.

Tuesday, September 26, 2017

Tales from old bones

Around three thousand years ago Bantu began to spread east and south from the Nigeria/Cameroun border, eventually replacing the original inhabitants of eastern and southern Africa. Those people no longer exist. Only the DNA in their skeletal remains are left to speak for them.

When scientists began to retrieve ancient DNA from human remains, they succeeded only at sites in the temperate and arctic zones. It seemed impossible to retrieve any at tropical sites, apparently because warm year-round temperatures soon reduce DNA to a meaningless molecular jumble.

This problem seems to be solved. Two years ago, DNA was successfully retrieved from 4,500 year old remains in Ethiopia. Now, we have ancient DNA from several sites across eastern and southern Africa over a range of dates from 10,000 to 400 years ago (Skoglund et al. 2017).

Vanished peoples

This new study shows that eastern and southern Africans have changed a lot since the time of the ancient Greeks. As far north as Tanzania, the continent was once home to peoples related to the Hottentots (now called Khoisans, Khoe-Sans, or simply San)—short in stature, gracile in body build, and light yellowish brown in color. From Zanzibar north, people were of mixed Middle Eastern and Cushitic origin—sort of like present-day Ethiopians but with more Arab ancestry.

What happened to these peoples? They were either replaced or absorbed by Bantus moving in from the west, although it now looks like they were replaced a lot more than they were absorbed. No trace of them remains in Malawi's gene pool:

Population replacement by incoming food producers appears to have been nearly complete in Malawi, where we detect little if any ancestry from the ancient individuals who lived ~8,100-2,500 BP. Instead, present-day Malawian individuals are consistent with deriving all their ancestry from the Bantu expansion of ultimate western African origin. (Skoglund et al. 2017)

The original inhabitants were related to present-day Khoisans but had significantly diverged from them:

Notably, the Khoe-San-related ancestry in ancient individuals from Malawi and Tanzania is symmetrically related to the two previously identified lineages present in the San [...], estimated to have diverged at least 20,000 years ago [...], implying that this was an ancient divergent branch of this group that lived in eastern Africa at least until 1,400 BP. (Skoglund et al. 2017)

This is in line with previous DNA findings from the Fwe (a Bantu group of southwestern Zambia), particularly the presence of Khoisan admixture that resembles nothing in present-day Khoisans:

[It is possible] that the Fwe intermarried with a Khoisan group whose genetic composition differed from that of the populations included in molecular anthropological investigations to date. [...] it is plausible that the Fwe ancestors interacted with a Khoisan community that differed genetically from those still settled in southern Africa today, which was ultimately replaced by the newcomers. (Barbieri et al. 2013)

Aside from these scattered fragments of DNA, we also have the testimony of ancient observers. Two tenth-century Arab geographers state that "in the outer reaches of the land of the Zanj there are cool highlands in which live white Zanj" (Lewis 1990, p. 121, n. 3). The Zanj are the dark-skinned peoples of east Africa and the term 'white' is better translated by 'lighter-skinned.' (The words 'black' and 'white' are often used in a relative sense in Arabic). The highlands might be the Drakensberg Escarpment of South Africa. 

Encounters with the archaic Other

Modern humans arose some 80,000 years ago in eastern Africa through a series of population expansions that culminated twenty thousand years later in a big bang that spread outward in Africa and then into the Middle East, Europe, and Asia (Watson et al. 1997). There, they encountered more archaic hominins: Neanderthals and, farther east, Denisovans. There was some intermixture. How much? Some have argued that modern Europeans and Asians are 3.4 to 7.9 percent admixed (Lohse and Frantz 2013). Most still opt for a lower figure of 1.5 to 2.1 percent (Prüfer et al. 2014).

But it wasn't only in Eurasia that modern humans encountered Neanderthal-like groups. Archaic hominins were present in Africa itself, some being relatively close to modern humans, and some more distantly related.

The latest DNA study has confirmed that modern humans intermixed with at least one archaic group as they expanded into western Africa:

The possible basal western African population lineage would represent the earliest known divergence of a modern human lineage that contributed a major proportion of ancestry to present-day humans. Such a lineage must have separated before the divergence of San ancestors, which is estimated to have begun on the order of 200-300 thousand years ago. (Skoglund et al. 2017)

This archaic ancestry is visible in human remains found at the Iwo Eleru rock shelter, in southwestern Nigeria, and dated to approximately 16,300 BP:

Our analysis indicates that Iwo Eleru possesses neurocranial morphology intermediate in shape between archaic hominins (Neanderthals and Homo erectus) and modern humans. This morphology is outside the range of modern human variability in the PCA and CVA analyses, and is most similar to that shown by LPA individuals from Africa and the early anatomically modern specimens from Skhul and Qafzeh. (Harvati et al., 2011)

Archaic ancestry is also visible in present-day West Africans, particularly in their teeth: 

[...] compared to other world populations, Africans south of the Sahara Desert are distinct dentally — especially in their expression of nine high- and two low-frequency morphological features. [...] the same nine high-frequency traits are also ubiquitous in the dentitions of extinct hominids and many extinct and extant non-human primates.  
[...] The presence and, indeed, prevalence (see next section), of high-frequency Sub-Saharan dental traits in fossil and recent hominoids—some of which are probably direct ancestors of modern humans, suggests they have been around for a long time.  
[...] A final ancestral feature found with some regularity in Sub-Saharan Africans, relative to other modern groups, is polydontia. Numerous cases of extra incisors, third premolars, and fourth molars have been noted [...] In one study (Watters, 1962) the incidence reached 2.5-3% in several hundred west Africans; many of the extra teeth were fully formed and erupted. "Typical" mammals exhibit three incisors and four premolars (Jordan et al., 1992). Polydontia is also found in living non-human primates. (Irish, 1998)

How much archaic ancestry do sub-Saharan Africans have today? The latest DNA study is silent on this point. Any answer can only be approximate, there being no reconstructed genome of this Neanderthal-like population. Moreover, there was probably more than one such population within Africa. Watson et al. (1997) attribute 13% of the sub-Saharan gene pool to a population that expanded some 111,000 years ago—when Skhul-Qafzeh hominins entered the Middle East from Africa. Those hominins were anatomically modern, or almost so, but culturally Neanderthal. Hammer et al. (2011) estimate that about 2% of the sub-Saharan African genome comes from a much more divergent population that split off from the ancestors of modern humans some 700,000 years ago. That admixture entered the sub-Saharan gene pool about 35,000 years ago, perhaps in Central Africa, since pygmy groups from that region have the most.

It looks like the proportion of archaic ancestry is higher in sub-Saharan Africans than in other modern humans. This is to be expected because of the broader range of archaic populations in Africa, including some that were almost modern anatomically and behaviorally. Admixture with them would have been likelier.

Admixture: good, bad, or neither?

Some alleles have successfully introgressed from archaic hominins, thus helping our ancestors adapt to new climates and new diets (Racimo et al. 2015). In general, however, we should not expect such alleles to perform as well in the body of a modern human as they did in the body of an archaic hominin. It's like taking a part from a Chevy and installing it on a Subaru. It might work, but I wouldn't count on it. 

If we look at Neanderthal admixture in the Eurasian genome, we see that natural selection has tended to remove functional genes, while leaving the non-functioning ones alone. 

Neanderthal ancestry decreases in proximity to functional elements in all populations [...] as does Denisovan ancestry in Oceanians [...] most likely reflecting greater selection against Neanderthal ancestry in low B statistic regions. Power to detect archaic ancestry is elevated close to regions of linked selection due to a reduction in the rates of incomplete lineage sorting caused by the lower effective population size in these regions, so these results are not artifacts of reduced power. Thus, similar processes appear to have worked to remove Neanderthal and Denisovan ancestry near genes. (Sankararaman et al. 2016)

Archaic admixture is also associated with reduced male fertility:

Our study provides new evidence in support of the hypothesis that reduced male fertility may be a common feature of admixture between human populations diverged by at least a half million years, a hypothesis that was previously suggested based on genetic patterns associated with the hybridization between Neanderthals and modern humans.

[...] One line of evidence for reduced fertility in male hybrids is that the proportion of archaic ancestry in modern humans is significantly reduced on chromosome X compared to the autosomes. This is suggestive of reduced male fertility as loci contributing to this phenotype are concentrated on chromosome X in hybrids of other species. We confirm an extreme reduction of Neanderthal ancestry on chromosome X (16%-34% of the autosomes depending on the population) and find a quantitatively similar reduction of Denisovan ancestry (21% of the autosomes in Oceanians).

The second line of evidence in support of the hypothesis of reduced fertility in hybrids is that there is a reduction of archaic ancestry in genes that are disproportionately expressed in testes, a known characteristic of male hybrid fertility (Sankararaman et al. 2016)

In sum, archaic admixture did provide modern humans with some ready-made alleles that have helped them adapt to new climates and new diets, but this advantage hardly applies to Africa. There, modern humans were already adapted to the local climate and diet. Archaic admixture couldn't have done much to help them adapt, since the new environments they faced were cultural ones of their own making.


Barbieri, C., A. Butthof, K. Bostoen, and B. Pakendorf. (2013). Genetic perspectives on the origin of clicks in Bantu languages from southwestern Zambia, European Journal of Human Genetics, 21(4), 430-436.  

Hammer, M.F., A.E. Woerner, F.L. Mendez, J.C. Watkins, and J.D. Wall. (2011). Genetic evidence for archaic admixture in Africa, Proceedings of the National Academy of Science (USA), 108(37), 15123-15128,

Harvati, K., C. Stringer, R. Grün, M. Aubert, P. Allsworth-Jones, C.A. Folorunso. (2011). The Later Stone Age Calvaria from Iwo Eleru, Nigeria: Morphology and Chronology. PLoS ONE 6(9): e24024. doi:10.1371/journal.pone.0024024  

Irish, J.D. (1998). Ancestral dental traits in recent Sub-Saharan Africans and the origins of modern humans, Journal of Human Evolution, 34, 81-98.  

Lohse, K., and L.A.F. Frantz. (2013). Maximum likelihood evidence for Neandertal admixture in Eurasian populations from three genomes, Populations and Evolution, 1307, 8263  

Prüfer, K., F. Racimo, N. Patterson, F. Jay; (2014). The complete genome sequence of a Neandertal from the Altai Mountains, Nature, 505(7481), 43-49.

Racimo, F., S. Sankararaman, R. Nielsen, and E. Huerta-Sanchez. (2015). Evidence for archaic adaptive introgression in humans, Nature Reviews Genetics, 16(6), 359-371.  

Sankararaman, S., S. Mallick, N. Patterson, D, Reich; et al. (2016). The combined landscape of Denisovan and Neanderthal ancestry in present-day humans, Current Biology, 26(9), 1241-1247.  

Skoglund, P., J.C. Thompson, M.E. Prendergast, A. Mittnik; et al. (2017). Reconstructing prehistoric African population structure, Cell, 171(1), 59-71  

Watson, E., P. Forster, M. Richards, and H-J. Bandelt. (1997). Mitochondrial footprints of human expansions in Africa, American Journal of Human Genetics, 61, 691-704.  

Saturday, September 16, 2017

An idea abandoned by its father

Italian wall lizard (Podarcis sicula) (Credit: Charles J. Sharp). Five mating pairs were taken from one island to another, and over the next thirty generations the transplanted population became remarkably different from the parent population.

Unlike other animals, we adapt not only to natural environments but also to cultural environments of our making. We thus direct our own evolution. At the same time we are busy redesigning our cultural environment, the latter is just as busy redesigning us. Like the natural environment, it favors the survival and reproduction of those who best fit in.

This concept of gene-culture coevolution began with anthropologist Claude Lévi-Strauss in a 1971 lecture:

... Among early humans, biological evolution may have selected for pre-cultural traits like upright posture, manual dexterity, sociability, symbolic thinking, and ability to vocalize and communicate. It was culture, however, once it came into being, that consolidated these traits and propagated them. When cultures specialize, they consolidate and favor other traits, like resistance to cold or heat in societies that have willingly or unwillingly had to adapt to extreme climates, like dispositions to aggressiveness or contemplation, like technical ingenuity, and so on. In the form these traits appear to us on the cultural level, none can be clearly linked to a genetic basis, but we cannot exclude that they are sometimes linked partially and distantly via intermediate linkages. In this case, it would be true to say that each culture selects for genetic aptitudes, which then, via a feedback loop, influence the culture that had initially helped to strengthen them.

Credit is usually given, however, to geneticist Luigi Luca Cavalli-Sforza. In 1976, he developed the first mathematical models for gene-culture coevolution with another geneticist, Marcus Feldman, and in 1978-1979 he spoke on this subject to a cultural evolution class at Stanford (Feldman & Cavalli-Sforza 1976; Stone & Lurquin 2005, p. 108). Two of his students were Robert Boyd and Peter Richerson, who later wrote a seminal book on gene-culture coevolution (Boyd & Richerson 1985). In the mid-1980s, he decided to test this concept in the field by investigating the cultural and genetic bases of artistic talent among the Inuit:

One of the most remarkable phenomena in the contemporary Canadian Arctic is the presence of highly-acclaimed art forms — carving in stone and ivory, and printing on paper. The question we ask is: how can we account for the wide-spread distribution of such talent in a small dispersed population? (Berry & Cavalli-Sforza 1986, p. 2)

To answer this question, he organized a joint project with psychologist John W. Berry at Queen's University and anthropologist Bernard Saladin d'Anglure at Université Laval. The Inuit were chosen for study because their high rate of adoption made it possible "to distinguish cultural from biological inheritance by studying correlations of adopted children with foster relatives on one hand and biological relatives on the other" (Berry & Cavalli-Sforza 1986, p. 5). Also, until recently, Inuit had lived off the land and, as such, had "abilities [that] are considered to be adaptive to a nomadic and hunting life style" (Berry & Cavalli-Sforza 1986, p. 3). Berry argued that the artistic talent of the Inuit came from certain mental skills that helped them during hunting.

Hunters, by this way of thinking, require good visual acuity, keen disembedding skills and a well-developed sense of spatial orientation. To hunt successfully, the hunter must be able to discern the object of the quest (which is often embedded in a complex visual landscape), then disembed the object, and finally return to home base. In contrast, agriculturalists need not develop these particular skills, but rather they need to invest in other areas of development, such as conservation (in both the economic and the Piagetian senses) and close social interactions. (Berry 2008, p. 3)

The project fell through. Cavalli-Sforza said he had to quit because of illness. Neither of his biographies, however, mention any illness during that time period (Frost 2014). Interestingly, his American biography ascribes his interest in culture at that time to a desire to disprove the existence of mental differences between human populations:

Yet another source of his interest in culture was the idea that the concept of human cultural learning was a valid weapon against racist arguments that differences between people (for example, different IQ scores among ethnic groups) were due to biologically determined "racial" differences. (Stone & Lurquin, 2005, p. 86)

The reality was a bit different. He believed in the importance of culture, but not as an entity separate and distinct from biology. This put him in opposition not only to the racists he denounced in the 1960s but also to the antiracists who increasingly viewed him with suspicion from the late 1980s onward.

With Cavalli-Sforza out of the picture, research on gene-culture coevolution languished over the next quarter-century. This field of research needed a high-profile champion in academia, and all of the possible candidates were either unable or unwilling.  Cavalli-Sforza never was suited for the job, being too timid and, frankly, too easy to blackmail. (Do you really think his wartime research on anthrax involved only mice?)

Lately, there seems to have been a renewal of interest, as seen in this review article on "Human biological and psychological diversity":

Humans migrated out of Africa at least 50,000 years ago and occupied many different ecological and climatological niches. Because of this, they evolved slightly different anatomical and physiological traits. For example, Tibetans evolved various traits that help them cope with the rigors of altitude; similarly, the Inuit evolved various traits that help them cope with the challenges of a very cold environment. It is likely that humans also evolved slightly different psychological traits as a response to different selection pressures in different environments and niches. One possible example is the high intelligence of the Ashkenazi Jewish people. Frank discussions of such differences among human groups have provoked strong ethical concerns in the past. We understand those ethical concerns and believe that it is important to address them. However, we also believe that the benefits of discussing possible human population differences outweigh the costs. (Winegard et al. 2017)

This article is a good read, and I was intrigued by its examples of fast evolution, particularly the Italian wall lizards. Five mating pairs were taken from one island to another, and over the next thirty generations the transplanted population became remarkably different from the parent population. The lizards were now larger, had shorter hind limbs, and could bite with much more force. Even more remarkably, they had a new morphological trait: a cecal valve—a muscle between the large and small intestines that slows down food movement and allows digestion of cellulose. This is an adaptation to the abundance of plant food on that island, but it is surprising that an entirely new trait could evolve so fast.

As far back as Darwin, biologists have described evolutionary change as slow. This is true only when organisms live in slowly changing environments. Transplant them into a very different one, and they will evolve very fast. This has been especially true for modern humans, who over the past 50,000 years have spread into a wide range of natural environments from the tropics to the arctic and into an even wider range of cultural environments:

Humans, like many animals, actively alter their environment, which changes the selection pressures they face (Laland et al. 2001; Laland and Sterelny 2006). In fact, humans may be the paradigmatic example of a niche-creating species, using brains rather than brawn to conquer the world (Baumeister 2005; Pinker 2010). Across the globe, humans devised distinctive cultural systems to cope with their environments, creating vastly different selective regimes from one culture to another. (Winegard et al. 2017)

Humans are indeed niche creators who have speeded up their own diversification. Nonetheless, they aren't alone in diversifying so fast. For example, some animal and plant species have spread into a wide range of new habitats since the last ice age, thereby giving rise to many new populations. Whether these recent populations are "sibling species," "subspecies," or "races"—the distinction is often arbitrary—their example can help us understand our own genetic diversity (Frost 2011).

These populations, like our own, seem to have evolved much more anatomically than they have genetically. Their anatomies are often distinct from each other, with no overlap, yet their genomes overlap considerably—there is far more genetic variation within each population than between them. So they are easier to tell apart by their appearance than by their genes. 

Why this discordance between genes and anatomy? Genes don't lie, do they? To make sense of this puzzle, we need to understand three points:

  • When a gene has different "alleles" or versions of itself, these alleles vary in their degree of similarity, some performing very differently and others identically or almost so—often because the gene itself is little more than "junk DNA.
  • Population boundaries separate not only different populations but also different natural or cultural environments. This is especially true for humans. The cultural environment usually differs, even when the natural environment is the same.
  • If the alleles of a gene perform differently, some of them will be more useful to one population than to another because they do better in one environment than in another. The more differently they perform, the more their frequencies will differ across population boundaries, with some alleles being more common in some populations than in others. Conversely, if the alleles perform identically, they will do equally well in all environments and tend to be equally common in all populations. To the extent that different alleles are present within a single population, the reasons will be more stochastic and less related to the usefulness of any one allele. Genetic variation within a population is therefore disproportionately due to alleles that perform similarly.

So genetic variation between populations differs qualitatively from genetic variation within each population. The first kind matters a lot more than the second kind. There are exceptions to this rule, e.g., balanced polymorphisms, founder effects, genetic drift, but that's the general picture. So when you read that genes vary far more within human populations than between them, you should keep in mind that we see the same pattern with many sibling species that are nonetheless anatomically and behaviorally distinct. This pattern tells us only that the populations in question are very young. It doesn't tell us how different they really are from each other, since real evolutionary change can happen very fast—as we saw with the Italian wall lizards. 


Berry, J.W. (2008). Models of Ecocultural Adaptation and Cultural Transmission: The Example of Inuit Art, paper presented at the conference Adaptation et socialisation des minoritiés culturelles en région, June 3-4, Quebec City.

Berry, J.W., and L.L. Cavalli-Sforza. (1986). Cultural and genetic influences on Inuit art. Report to Social Sciences and Humanities Research Council of Canada, Ottawa.

Boyd, R. and P.J. Richerson. (1985). Culture and the Evolutionary Process, Chicago: Chicago University Press.

Feldman, M.; Cavalli-Sforza, L. (1976). Cultural and biological evolutionary processes, selection for a trait under complex transmission, Theoretical Population Biology, 9: 238-59.

Frost, P. (2014). L.L. Cavalli-Sforza. A bird in a gilded cage, Open Behavioral Genetics, March 28,

Frost, P. (2011). Human nature or human natures? Futures, 43: 740-748.  
Lévi-Strauss, C. 1971). Race et culture, conférence de Lévi-Strauss à l'UNESCO le 22 mars 1971 (Audio). Polit'productions  

Stone, L. and P.F. Lurquin. (2005). A Genetic and Cultural Odyssey. The Life and Work of L. Luca Cavalli-Sforza, New York: Columbia University Press.

Winegard, B., B. Winegard, and B. Boutwell. (2017). Human biological and psychological diversity, Evolutionary Psychological Science, 3(2): 159-180.