Tuesday, April 7, 2020

COVID-19 update



A dying man, stoned on suspicion of spreading the plague - Felix Jenewein, 1899 (Wikicommons)



SARS-CoV-2, though novel, belongs to a long-existing group of respiratory pathogens: coronaviruses. Until the first appearance of SARS in 2002, these pathogens did little harm to their hosts, usually causing nothing worse than a common cold. So they may have coevolved with us. Furthermore, this coevolution may have taken different forms in different human populations and different cultural environments.

Coronaviruses infect lung tissue via a receptor, ACE2, that varies structurally not only between Asians and other human groups but also between different Asian groups. In particular, the Chinese population has fewer alleles that code for weak binding to the coronavirus S-protein (Cao et al. 2020). Different ACE2 alleles are also associated with differences in susceptibility to diabetic retinopathy, an eye disease with a distinct global pattern of prevalence: 22% in Italy, 23% in China, 30% in the United Kingdom, and 40% in the United States (Adams 2020).

This geographic pattern doesn’t exist because some populations have become more resistant to coronaviruses. Instead, the reverse seems to have happened: some populations have become more susceptible to coronavirus infection, perhaps as a means to prevent more serious pulmonary infections, like tuberculosis and pneumonic plague (Shekhar et al. 2017). Such an effect has been shown with γherpesvirus 68 and cytomegalovirus (Barton et al. 2007; Miller et al. 2019). This crude vaccination boosts the immune response through increased production of IFN-γ and increased activation of macrophages.

Historically, tuberculosis was especially common in crowded environments, where people lived in proximity not only to each other but also to domesticated animals (Comas et al. 2013). Such environments have existed continuously for the longest time in China, as well as in areas like the Indo-Gangetic Plain, the Fertile Crescent, and the Mediterranean Basin. Those areas are where people should be most susceptible to coronavirus infection.

This may explain why COVID-19 has been more severe in southern Europe than in northern Europe. It is surprising that infection tends to become less severe with latitude when one would expect the opposite: respiratory viruses spread more effectively under conditions of lower temperature, lower humidity, and lower solar UV.


Ongoing research?

These geographic differences have caught the interest of a molecular epidemiologist at the University of Hawai'i, Maarit Tiirikainen:

"There have been major differences in the rates of SARS-CoV-2 infection and the severe disease between the different geographic regions since the beginning of the COVID-19 pandemic, even among young individuals," Dr. Tiirikainen said. "Epidemiological studies-so-called Genome Wide Association Studies (GWAS)-indicate that populations carry different variants of the ACE2 gene. This variation in the gene coding for the ACE2 receptor may have an effect on the number of ACE2 receptors on the lung cells, as well as on how effectively the virus binds to the receptor. There may also be genetic differences in immune and other important genes explaining why some people get more sick than others."

She is collaborating with a genomics company, LifeDNA, in a study that will initially focus on Hawai'i's multiethnic inhabitants, specifically their diversity of ACE2 alleles in relation to the latest coronavirus (LifeDNA 2020 – h/t to Steve Sailer).


Parting thoughts

All humans can get infected by coronaviruses, but the infection tends to vary in severity from one population to another. This variance may reflect differences in genetic adaptation in different cultural environments.

Of course, adaptation may also be cultural. Because natural selection acts on the end result, and not on the means to that end, the means may be a purely learned algorithm, like adding spices to food or avoiding physical contact with strangers. One might not have understood why or how such practices worked, but they did work and would be passed on to subsequent generations, thus becoming the traditional way of doing things. Today, we’re likely to reject such practices as outmoded superstitions.

So be modern. Hug a stranger.


References

Adams N. (2020). Cracking the code to the 2019 novel coronavirus (COVID-19): Lessons from the eye. Eye Reports 6(1). 
https://eyereports.org/index.php/eyereports/article/view/97

Barton E.S., White D.W., Cathelyn J.S., Brett-McClellan K.A., Engle M., Diamond M.S., et al. (2007). Herpesvirus latency confers symbiotic protection from bacterial infection. Nature 447: 326-329. 
https://www.nature.com/articles/nature05762

Cao Y., Li L., Feng Z., Wan S., Huang P., Sun X., et al. (2020). Comparative genetic analysis of the novel coronavirus (2019-nCoV/SARS-CoV-2) receptor ACE2 in different populations. Cell Discovery 6(11). 
https://www.nature.com/articles/s41421-020-0147-1%3C/blockquote%3E 

Comas I., Coscolla M., Luo T., Borrell S., Holt K.E., Kato-Maeda M., et al. (2013). Out-of-Africa migration and Neolithic coexpansion of Mycobacterium tuberculosis with modern humans. Nature Genetics 45(10): 1176-1182.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3800747/

LifeDNA (2020). COVID-19: LifeDNA and University of Hawai’i Collaborate on Studying Why Certain Populations Are Hit Harder. Research focuses on ACE2 receptor, probing the role of genetics in both susceptibility to infection and severity of response April 2, University of Hawai'i Cancer Center 
https://www.uhcancercenter.org/about-us/newsroom/600-covid-19-lifedna-and-university-of-hawai-i-collaborate-on-studying-why-certain-populations-are-hit-harder

Miller H.E., Johnson K.E., Tarakanova V.L., Robinson R.T. (2019). γ-herpesvirus latency attenuates Mycobacterium tuberculosis infection in mice. Tuberculosis 116: 56-60. 
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6876742/

Shekhar S., Schenck K., Petersen F.C. (2017). Exploring host-commensal interactions in the respiratory tract. Frontiers in Immunology 8: 1971. 
https://www.frontiersin.org/articles/10.3389/fimmu.2017.01971/full

Tuesday, March 31, 2020

Affective empathy: a double-edged sword



March in Brooklyn (?) (Wikicommons - Amanda Hirsch). Can we learn to feel another person's pain or joy? Twin studies indicate that affective empathy is 52-57% heritable. The rest includes prenatal and postnatal influences that happen long before social learning begins.



In our species, a major problem has been to create high-trust societies that encompass large numbers of people who are not closely related and yet have to deal with each other regularly. This problem hasn’t been resolved in most human populations—for the most part, people trust only family and close kin. Consequently, a market economy cannot realize its full potential: a lot of economic activity never happens because the low level of trust makes it too costly. This point is repeatedly made in the book India Unbound by Gurcharan Das:

[…] the social life of Indians revolves around the family or caste. It does not encompass the whole community. Perhaps this is why our streets are dirty when our homes are spotlessly clean. (Das 2002, p. 81)

A striking characteristic of Indian business is that it is family-owned and family-managed. […] (Das 2002, p. 265)

Whether businesses here can create managerial capitalism depends partly on Indian society’s ability to build “social capital.” Where strangers spontaneously trust each other and cooperate with each other, there is high social capital. Indeed, Tocqueville regarded this “art of association” as an essential virtue of American society because it moderated the American tendency toward individualism. Trust and cooperation are necessary in all market activity. Social capital can help companies make the transition from small family units to large, professionally run enterprises. High trust can dramatically lower transaction costs, corruption, and bureaucracy. (Das 2002, pp. 267-268)

The "large society problem" has been fully resolved only in two culture areas: Northwest Europe and East Asia. In general, the solution has been to weaken the importance of kinship in social relations and to strengthen impersonal forms of sociality that can bring everyone together, and not just closely related people. To be specific, the focus of empathy has been extended beyond the circle of close kin, and people become more attuned to universal social rules that exist independently of kinship obligations.

Northwest Europeans have transcended the ties of kinship to an unusual extent. North and west of a line running from Trieste to St. Petersburg, kinship ties have been relatively weak for at least a millennium. Almost everyone is single for at least part of adulthood, and many stay single their entire lives. In addition, households often have non-kin members, and children normally leave the nuclear family to form new households. This weak-kinship environment is associated with an equally unusual pattern of behavior: greater individualism, less loyalty to kin, and more willingness to trust strangers.

This is not so with East Asians, who still have strong kinship ties and are actually less individualistic than humans in general. Whereas a greater sense of self has helped Northwest Europeans transcend the limitations of kinship to build larger societies, East Asians have relied on a lesser sense of self to strengthen impersonal sociality within and beyond their circle of close kin. There is more emphasis on holistic attention, on social happiness rather than personal happiness, and on suspension of self-interest. Conversely, there is less emphasis on self-expression, self-esteem, and self-efficacy (Kitayama et al. 2014).


Empathy: cognitive versus affective

Empathy seems especially key to strengthening social relations beyond one's circle of close kin. It has two components. Cognitive empathy is the ability to understand the feelings of another person, and affective (or emotional) empathy is the ability to internalize those feelings and actually feel that person's pain or joy. Affective empathy is 52-57% heritable, and cognitive empathy 27% heritable (Melchers et al. 2016). This is in line with longitudinal studies on children: affective empathy remains stable as a child develops, while cognitive empathy progressively increases, perhaps through learning (Decety et al. 2017). 

Affective empathy, but not cognitive empathy, is sexually dimorphic: 

[...] females do indeed appear to be more empathic than males [but] [t]hey do not appear to be more adept at assessing another person's affective, cognitive, or spatial perspective" (Hoffman 1977).

Women are faster in recognizing facial expression, emotional body language, more sensitive to baby voice, more experientially reactive to negative, but not positive, emotional pictures compared to men. Men, on the other hand, seem to show better skills in cognitive empathy while women performed better in emotional empathy (Uysal et al. 2020).

This difference between men and women has been confirmed by a British study (Baron-Cohen and Wheelwright 2004), a largely Argentinean study (Baez et al. 2017), an Italian twin study (Toccaceli et al. 2018), and a Chinese study (Liu et al. 2018). The size of the difference varies, however, being slight in the British and Argentinean studies, large but not significant in the Italian study, and significant in the Chinese study. The sex difference in affective empathy largely accounts for the sex difference in aggression (Dryburgh and Vachon 2019). Women are also more likely to forgive, and this sex difference seems mediated by the sex difference in empathy (Witvliet et al. 2020).

Thus, affective empathy may have initially served to facilitate the relationship between a mother and her young children. This female adaptation may have a long evolutionary history among mammals: it has been shown that sensitivity to the pain of others is stronger in female mice than in male mice (Uysal et al. 2018).

As some human populations formed larger and more complex societies, natural selection may have gradually extended affective empathy to both sexes and to all social relationships. An analogy would be the gene-culture coevolution between lactose metabolism and dairy farming. The ability to digest lactose is lost after infancy by most humans but is lifelong in cultures where adults consume milk and other dairy products.


Northwest Europeans versus East Asians

Northwest Europeans and East Asians are similar in having high levels of empathy but differ in the relative importance of cognitive empathy versus affective empathy. Affective empathy is much more key to prosocial behavior among Northwest Europeans than among East Asians. This was the conclusion of Li et al. (2019):

Previous research has shown that affective empathy, rather than cognitive empathy, significantly predicts people's altruistic sharing behavior in economic games. However, most of these studies were conducted in Western populations. There might be cultural differences in the relations between empathy and altruism due to different levels of empathy between Western and Asian individuals. In this study, we measured different aspects of empathy in Chinese adults as well as their allocation offers in the dictator and ultimatum games. We found that cognitive empathy, but not affective empathy, was a significant predictor of adults' altruistic sharing behavior in the two economic games.

Similarly, Siu and Shek (2005) found that Chinese subjects had trouble distinguishing between cognitive empathy and affective empathy. They concluded that "Chinese people might not perceive the items from the two dimensions as too different in nature."

One might think that cognitive empathy would be worse than affective empathy as a basis for prosocial behavior. For instance, sociopaths are usually high in cognitive empathy: they know how another person feels in a given situation, but they use this knowledge to exploit and control that person. Wouldn't their resulting success eventually destroy social order? East Asian societies may have avoided this outcome through their low level of individualism and their correspondingly high level of social conformity. Kitayama et al. (2014) makes this point when discussing certain alleles of a gene, DRD4, that are associated with risk seeking and heavy drinking in the United States but not in East Asia. These alleles seem to increase the desire to emulate one's peers, and such emulation is more likely to favor dysfunctional behavior in the United States than in East Asia:

It might be the case that the 7R and 2R alleles are associated with greater acquisition of culturally sanctioned social orientations under generally favorable conditions of socialization, such as careful guidance and scaffolding of norm-congruous behaviors by socialization agents (e.g., parents, relatives, neighbors), but with markedly different, deviant behaviors (e.g., delinquency and risk proneness) under unfavorable social conditions or adversity, which might "reward" externalization or risk taking. (Kitayama et al. 2014)

These alleles seem to explain the weaker individualism and stronger social conformity of East Asians. When Kitayama et al. (2014) compared a sample of Euro-Americans with a sample of East Asians born in China, Korea, or Japan, they found that the East Asians were less individualistic than the Euro-Americans on a social orientation test, but this difference was limited to carriers of DRD4 alleles that increase dopamine signalling, i.e., 7- or 2-repeat alleles. Non-carrier East Asians were just as individualistic as non-carrier Euro-Americans (Kitayama et al. 2014)

Finally, we should keep in mind a serious shortcoming of affective empathy: you may become so overcome by your emotion that you can no longer accurately assess the target of your empathy. This point is made by Atkins (2014) in a review of several experimental studies of empathy in British and East Asian subjects:

Thus, it is possible that being in a highly emotionally empathic state may cloud the ability to accurately infer the emotions of a target due to the heightened emotions experienced in response to the suffering of another. In line with this reasoning, East Asians' lower level of emotional involvement might have freed cognitive resources to allow them to more accurately infer the emotions of targets.


A review of the subject

Atkins (2014) comes to several conclusions in his comparative review:

- When viewing a person suffering physical pain, British subjects report greater negative affect than do East Asian subjects.

-  When viewing a person suffering social pain, British subjects show greater empathic concern but lower empathic accuracy than do East Asian subjects.

- British subjects report greater empathic concern, but lower empathic accuracy than do Chinese subjects. Emotional expressivity predicts British but not Chinese empathic concern.

- Empathic concern explains differences between the two groups in donating, a measure of prosocial behavior.

- American subjects, more so than Japanese subjects, feel more affective empathy for one friend over another when the two friends are engaged in an intense disagreement.

In sum, East Asians have resolved the "large society problem" through a different psychological and behavioral package that places less emphasis on emotional involvement and more on restoration of social harmony.


References

Atkins, D. (2014). The Role of Culture in Empathy: The Consequences and Explanations of Cultural Differences in Empathy at the Affective and Cognitive Levels. Doctor of Philosophy (PhD) thesis, University of Kent.
https://kar.kent.ac.uk/47970/  

Baez, S., Flichtentrei, D., Prats, M., Mastandueno, R., García, A.M., Cetkovich, M., et al. (2017). Men, women...who cares? A population-based study on sex differences and gender roles in empathy and moral cognition. PLoS ONE 12(6): e0179336.
https://journals.plos.org/plosone/article/file?type=printable&id=10.1371/journal.pone.0179336  

Baron-Cohen, S. (2011). The Empathy Bell Curve. Phi Kappa Phi Forum; Baton Rouge 91(1): 10-12.
http://go.galegroup.com/ps/anonymous?id=GALE%7CA267422895&sid=googleScholar&v=2.1&it=r&linkaccess=abs&issn=15385914&p=AONE&sw=w  

Das, G. (2002). India Unbound. The Social and Economic Revolution from Independence to the Global Information Age. New York: Anchor Books.

Decety, J., K.L. Meidenbauer, and J.M. Cowell. (2017). The development of cognitive empathy and concern in preschool children: A behavioral neuroscience investigation. Developmental Science 2018;21:e12570. 
https://doi.org/10.1111/desc.12570  

Dryburgh, N.S.J., and D.D. Vachon. (2019). Relating sex differences in aggression to three forms of empathy. Personality and Individual Differences 151(1): 109526.
https://e-tarjome.com/storage/panel/fileuploads/2019-08-25/1566713640_E12864-e-tarjome.pdf 

Frost, P. (2017). The Hajnal line and gene-culture coevolution in northwest Europe. Advances in Anthropology 7: 154-174.
https://www.scirp.org/html/3-1590616_78813.htm  

Frost, P. (2015). Two paths. The Unz Review, January 24
https://www.unz.com/pfrost/two-paths/  

Hajnal, J. (1965). European marriage patterns in perspective: essays in historical demography. In D.V. Glass and D.E. Eversley (eds). Population in History. Chicago: Aldine Publishing, pp. 101-143.
https://www.taylorfrancis.com/books/e/9781315127019/chapters/10.4324/9781315127019-7  

Hallam, H.E. (1985). Age at first marriage and age at death in the Lincolnshire Fenland, 1252-1478. Population Studies 39(1): 55-69.
https://www.tandfonline.com/doi/abs/10.1080/0032472031000141276  

hbd chick (2014). Big summary post on the Hajnal Line. October 3
https://hbdchick.wordpress.com/2014/03/10/big-summary-post-on-the-hajnal-line/

ICA (2013). Research Themes - Marriage Patterns, Institutions for Collective Action
http://www.collective-action.info/_THE_MarriagePatterns_EMP  

Hoffman, M. L. (1977). Sex differences in empathy and related behaviors. Psychological Bulletin 84(4): 712-722. 
http://dx.doi.org/10.1037/0033-2909.84.4.712   

Kitayama, S., A. King, C. Yoon, S. Tompson, S. Huff, and I. Liberzon. (2014). The Dopamine D4 Receptor Gene (DRD4) Moderates Cultural Difference in Independent Versus Interdependent Social Orientation. Psychological Science 25: 1169-1177. http://pss.sagepub.com/content/25/6/1169.short 

Li, Z., J. Yu, and L. Zhu. (2019). Associations between empathy and altruistic sharing behavior in Chinese adults. The Journal of General Psychology 146(1): 1-16
https://www.tandfonline.com/doi/abs/10.1080/00221309.2018.1510826  

Liu, J., X. Qiao, F. Dong, and A. Raine. (2018). The Chinese version of the cognitive, affective, and somatic empathy scale for children: Validation, gender invariance and associated factors. PLoS ONE 13(5): e0195268. 
https://doi.org/10.1371/journal.pone.0195268 

Melchers, M., C. Montag, M. Reuter, F.M. Spinath, and E. Hahn. (2016). How heritable is empathy? Differential effects of measurement and subcomponents. Motivation and Emotion 40(5): 720-730. 
https://doi.org/10.1007/s11031-016-9573-7

Schulz, J.F., D. Bahrami-Rad, J.P. Beauchamp, and J. Henrich. (2019). The Church, intensive kinship, and global psychological variation. Science 366(707): 1-12. 
https://coevolution.fas.harvard.edu/files/culture_cognition_coevol_lab/files/sciencefull.pdf

Seccombe, W. (1992). A Millennium of Family Change. Feudalism to Capitalism in Northwestern Europe. London: Verso.
https://books.google.ca/books?id=MiTxtZI-pzUC&printsec=frontcover&hl=fr&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false

Siu, A.M.H. and D.T. L. Shek. (2005). Validation of the Interpersonal Reactivity Index in a Chinese Context. Research on Social Work Practice 15: 118-126.
http://rsw.sagepub.com/content/15/2/118.short  

Toccaceli, V., C. Fagnani, N. Eisenberg, G. Alessandri, A. Vitale and M.A. Stazi. (2018). Adult Empathy: Possible Gender Differences in Gene-Environment Architecture for Cognitive and Emotional Components in a Large Italian Twin Sample. Twin Research and Human Genetics 21(3): 214-226
https://doi.org/10.1017/thg.2018.19   

Uysal, N., U.M. Çamsari, M. ATEs, S. KandIs, A. Karakiliç, and G.B. Çamsari (2019). Empathy as a Concept from Bench to Bedside: A Translational Challenge. Noro psikiyatri arsivi, 57(1): 71-77. https://doi.org/10.29399/npa.23457 

Witvliet, C.V., L. M.R. Luna, J.L. VanderStoep, T. Gonzalez, and G.D. Griffin (2020). Granting forgiveness: State and trait evidence for genetic and gender indirect effects through empathy. The Journal of Positive Psychology 15(3): 390-399 
https://www.tandfonline.com/doi/full/10.1080/17439760.2019.1615108 

Tuesday, March 24, 2020

The myth of selective neutrality



Paleolithic tent (Wikicommons - Michal Mañas). Did Europeans lose haplogroup U because they were replaced by farmers from the south? Or because they needed less energy for body heat?



Blood group systems have long been used to reconstruct prehistory. A good example is the Diego antigen. One of its alleles, DI*A, has helped us chart the prehistory of indigenous peoples in the Americas. Among other things, we have learned that most of them originated in Siberia some 12,000 years ago. This is not the case with the Eskimo-Aleut and Na-Dene peoples, who seem to have entered North America later. 

It’s assumed here that the Diego antigen has mutated at a steady rate and that the mutations have displaced earlier ones at a steady rate. So this antigen can act as a clock. If two populations have separated from each other, we can estimate their time of separation by measuring the mean genetic difference between them at the Diego antigen.

The "clock" assumption has its limitations. Diego mutations are neither kept nor lost at a constant rate. Both processes can be slowed down or speeded up by natural selection: 

Our study also revealed a significant correlation between DI*A allele frequency and warm tropical conditions, domesticated crop type, and presence of disease-carrying vector species. The circumscribed areas defined by these factors compose a mosaic of specific biocenoses and pathocenoses. It is thus reasonable to consider natural selection in the distribution of human genetic polymorphisms. (Bégat et al. 2015)

It's widely believed that all blood groups have the same survival value, so differences between them should be "selectively neutral." That belief is mistaken. In fact, nothing in the genome is truly of neutral value, not even noncoding genes that supposedly do nothing. Even if a gene doesn't code for anything, it still affects the spatial configuration of genes on the chromosome, thus altering how one gene may regulate another. 

According to a recent study, 80% of our genome has some kind of function, even noncoding genes (The ENCODE Project Consortium 2012). Indeed, such genes may have disproportionately contributed to human evolution. Comparison of our genome with other primate genomes has shown that almost all human-specific deletions are in noncoding regions (Bae et al. 2015). Furthermore, DNA is mostly noncoding in human accelerated regions (HARs)—genomic regions that have been well conserved throughout vertebrate evolution but are strikingly different in humans, perhaps in ways that alter how coding genes regulate each other (Bae et al. 2015). This would be consistent with the belief that our ancestors evolved largely through new ways of regulating existing systems, particularly the pace and timing of development (King and Wilson 1975).


Loss of haplogroup U: population replacement or change in natural selection?

Let's now look at haplogroup U. This, too, is assumed to be "selectively neutral" and is used to reconstruct prehistory, specifically the replacement of hunter-gatherers by farmers in Europe. Haplogroup U is a group of mitochondrial genes that was widespread among Mesolithic hunter-gatherers throughout Europe and is now common only among the Sami of Finland and the Mansi of northwestern Siberia, both of whom were hunter-gatherers until recently (Derbeneva et al. 2002). Indeed, according to ancient mtDNA from central and western Europe, the population frequency of haplogroup U shows a sharp break at the time boundary between late hunter-gatherers and early farmers (Bramanti et al. 2009). That break strongly suggests that European hunter-gatherers were largely replaced by farmers spreading into Europe from the Middle East.

Yet things are not always as they seem. In Denmark, haplogroup U persisted at high frequencies long after the transition to farming, in fact as late as the Early Iron Age (Melchior et al. 2010). In Latvia and Ukraine it persisted into Neolithic times (Jones et al. 2017).

Perhaps haplogroup U disappeared because it ceased to be adaptive and was removed by natural selection. This haplogroup shifts the energy balance away from ATP synthesis and toward production of body heat—a useful cold adaptation for hunter-gatherers, who had to sleep in makeshift shelters and pursue game animals in all kinds of weather (Balloux et al. 2009; Montiel-Sosa et al. 2006). Farmers slept in a warmer environment and could more easily plan their outdoor activities.

This being said, the loss of haplogroup U was not the only genetic change across the Mesolithic-Neolithic divide. Were those other changes due to natives being replaced by farmers from the Middle East? Or was natural selection again responsible? Researchers have tried to exclude the second cause by examining how noncoding genes changed across the divide, on the assumption that such genes are generally non-functional and make no difference to one’s chances of survival and reproduction. As we've seen, that assumption is unfounded.

Clearly, some of this genetic change was due to natural selection. I mentioned the shift in energy balance, but there were others. Farmers had less need for odor recognition, monotony avoidance, and sensation seeking (Majid and Kruspe 2018; Zuckerman 2008). They also had to process reciprocal obligations with a larger number of people while interacting less, on average, with each person. All in all, farming did not impose the same demands on mind and body. Going from one way of life to the other required many physiological adjustments.

To explain the genetic divide between hunter-gatherers and farmers, we should also allow for founder effects. When bands of hunter-gatherers are given the opportunity, only a few will choose to become farmers. Because this minority is a small sample of the hunter-gatherer gene pool, the new farming population will differ genetically from the previous one in many random ways.


Conclusion

When reconstructing the past, particularly the transition from hunting and gathering to farming, we shouldn't interpret genetic change solely in terms of one population replacing another. Some of the change may also be due to a new regime of natural selection, as well as founder effects.

I once made this point to Greg Cochran, and his reply was that changes in natural selection couldn't possibly account for all of the genetic change we see in ancient DNA between late hunter-gatherers and early farmers. True, but that's not my point. Some population replacement did happen, but its magnitude is exaggerated by a methodology that attributes all genetic change to that one factor alone. 


Interview with Grégoire Canlorbe

I was recently interviewed by Grégoire Canlorbe, a young French author and scholar. The interview covers a variety of topics and can be read in its entirety (in two parts) at American Renaissance:

https://www.amren.com/features/2020/03/how-did-whites-get-their-appearance/ 
https://www.amren.com/features/2020/03/why-are-human-groups-so-different/

Une traduction française est disponible sur le site Evopsy de Philippe Gouillou :

http://www.evopsy.com/concepts/coevolution-frost.html
http://www.evopsy.com/concepts/hbd-frost.html


References

Bae, B-I., D. Jayaraman, and C.A. Walsh. (2015). Genetic changes shaping the human brain. Developmental Cell 32: 423-434. 
https://www.sciencedirect.com/science/article/pii/S1534580715000787

Balloux F., L.J. Handley, T. Jombart, H. Liu, and A. Manica. (2009). Climate shaped the worldwide distribution of human mitochondrial DNA sequence variation. Proceedings of the Royal Society B. Biological Sciences 276: 3447-3455. 
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2817182/

Bégat, C., Bailly, P., Chiaroni, J., & Mazières, S. (2015). Revisiting the Diego Blood Group System in Amerindians: Evidence for Gene-Culture Comigration. PloS one 10(7), e0132211.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4493026/

Bramanti, B., M.G. Thomas, W. Haak, M. Unterlaender, P. Jores, K. Tambets, I. Antanaitis-Jacobs, M.N. Haidle, R. Jankauskas, C.J. Kind, et al. (2009). Genetic discontinuity between local hunter-gatherers and Central Europe's first farmers. Science 326: 137-140.
http://roceeh.mediatis.de/fileadmin/download/Publications/Bramanti_Sci09_Meso_Neo.pdf

Derbeneva, O.A., E.B. Starikovskaya, D.C. Wallace, and R.I. Sukernik, (2002). Traces of early Eurasians in the Mansi of Northwest Siberia revealed by mitochondrial DNA analysis. American Journal of Human Genetics 70: 1009-1014. 
https://www.sciencedirect.com/science/article/pii/S0002929707603085

Jones, E.R., G. Zarina, V. Moiseyev, E. Lightfoot, P.R. Nigst, A. Manica, et al. (2017). The Neolithic transition in the Baltic was not driven by admixture with early European farmers. Current Biology 27(4): 576-582.
https://www.sciencedirect.com/science/article/pii/S0960982216315421

King, M-C, and A.C. Wilson. (1975). Evolution at two levels in humans and chimpanzees. Science 188: 107-116.
http://hydrodictyon.eeb.uconn.edu/people/schwenk/KingWilsonHumansChimps75.pdf

Majid, A., and N. Kruspe. (2018). Hunter-gatherer olfaction is special. Current Biology 28: R108-R110. 
https://www.sciencedirect.com/science/article/pii/S0960982217316160

Melchior, L., N. Lynnerup, H.R. Siegismund, T. Kivisild, and J. Dissing. (2010). Genetic diversity among ancient Nordic populations. PLoS One 5(7): e11898
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2912848/

Montiel-Sosa, F., E. Ruiz-Pesini, J.A. Enriquez, A. Marcuello, C. Diez-Sanchez, J. Montoya, D.J. Wallace, and M.J. López-Pérez, (2006). Differences of sperm motility in mitochondrial DNA haplogroup U sublineages. Gene 368: 21-27.
http://cnc.cj.uc.pt/BEB/private/pdfs/2007-2008/RepBiology/ExtraBibliog/MontielSosa2006.pdf

The ENCODE Project Consortium. (2012). An integrated encyclopedia of DNA elements in the human genome. Nature 489: 57-74 
https://www.nature.com/articles/nature11247

Zuckerman, M. (2008). Genetics of Sensation Seeking. In J. Benjamin, R.P. Ebstein, and R.H. Belmaker (Eds) Molecular Genetics and the Human Personality, (pp. 193-210). Washington D.C.: American Psychiatric Publishing Inc.
https://books.google.ca/books?id=mfANqS-SnwgC&printsec=frontcover&hl=fr&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false



Monday, March 16, 2020

From here it's all downhill





Polish IQ scores for three subtests: Similarities, Vocabulary, and Arithmetic (Wikicommons - Pedros)

Throughout the West the Flynn effect is coming to an end. More and more people are bumping up against a ceiling that is getting lower and lower.



The Flynn effect is ending throughout the West. After a century of steady increase, average IQ scores are levelling off and even starting to fall in Scandinavia, England, and Austria (Dutton 2016; Flynn 2007, p. 143; Rindermann 2018, pp. 85-88; Teasdale and Owen 2005). 

But how real was that increase anyway? Read the popular literature from a hundred years ago. Look at the size of the vocabulary and the complexity of the plots. Also look at what people had to know when graduating from elementary school. IQ scores increased during the 20th century largely because people became more familiar with taking standardized tests and thinking in terms of standardized answers to standardized questions. They had no choice. This was a time when people were staying increasingly longer in school. As higher education became the rule for everyone—first high school and then college and university—good grades became the goal for everyone, and not just for a small elite. 

So the average phenotype has been pushed to the limit by compulsory secondary education and by social pressure to pursue tertiary education. Meanwhile, the average genotype has been trending in another direction. 

- Reaction time has lengthened, i.e., people are taking longer on average to process the same information. In Great Britain, mean reaction time has risen by 13 points since the Victorian era (Woodley et al. 2013). This finding may be due to better sampling of the general population over time (hbd*chick 2013); however, a Swedish study found the same lengthening of reaction time, particularly in cohorts born since the 1970s (Madison 2014; Madison et al. 2016). 

- The genetic basis for intelligence has declined, as shown by a progressive decline in the "polygenic score" of alleles associated with educational attainment. In Iceland, this score has fallen since the cohort born in 1910 (Kong et al. 2017). Among Euro Americans, it fell between the 1931 and 1953 birth cohorts (Beauchamp 2016).

In sum, the average phenotype and the average genotype have been moving in opposite directions. This point is made by Hong (2020):

In general, more educated women delay the onset of childbearing and have fewer children overall compared to less educated women. This pattern is very robust in both developed and developing countries, and various theories have been proposed to explicate the intrinsic, potentially causal relationship between education level and fertility.

[...] The phenotype of EA [educational attainment], on the other hand, may experience a very different type of selective pressure. A large literature in sociology shows that educational attainment and socio-economic status are associated, and cultural evolutionary theory predicts that humans readily copy the behaviors of those who are perceived as more prestigious or successful. Although educational attainment cannot be "copied" in a literal sense, individuals who pick models with high EA are likely to be more motivated in learning and committed to their academic studies, and as a result become more likely to obtain higher EA themselves. Thus, genetic fitness and cultural fitness of the same trait (EA) invites selection in opposite directions [...].

As long as the genetic decline is offset by better and longer education, there is nothing to worry about. We may be learning more slowly, but we're spending more time learning. As Courtiol et al. (2016) argue: "the jockey has become more skilled as the power of the horse dwindles."


What if the horse collapses?

Unfortunately, this situation cannot go on forever. The phenotype (“jockey”) and the genotype (“horse”) are not independent entities; in fact, the former is developed within the limits of the latter. As the upper limit gets lower and lower, more and more of us will be bumping up against that ceiling. In other words, "at some point genetics will become the limiting factor in determining the phenotype of EA, which may eventually decline as a result of natural selection" (Hong 2020).

That point seems to be ... about now. In the West, the Flynn effect has exhausted itself, having used up the wiggle room of education. From here it's all downhill.  As Dutton et al. (2016) note in their review of the literature:

Eventually, the ceiling of this ability would be reached and the losses would start to reveal themselves on IQ tests. In addition, Woodley and Fernandes (2016) have shown that the Flynn Effect is not primarily occurring on general and heritable intelligence factor g, whereas the negative Flynn Effect does seem to occur on g. The negative Flynn Effect displays a Jensen Effect, and is mainly occurring on the more heritable abilities.


References

Beauchamp, J.P. (2016). Genetic evidence for natural selection in humans in the contemporary United States. Proceedings of the National Academy of Sciences. 113(28): 7774-7779.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4948342/

Courtiol, A., F.C. Tropf, and M.C. Mills. (2016). When genes and environment disagree: Making sense of trends in recent human evolution. Proceedings of the National Academy of Sciences 113(28): 7693-7695.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4948334/

Dutton, E., D. van der Linden, and R. Lynn. (2016). The negative Flynn Effect: A systematic literature review. Intelligence 59: 163-169.
https://www.gwern.net/docs/iq/2016-dutton.pdf

Flynn, J.R. (2007). What is Intelligence? Beyond the Flynn Effect. Cambridge University Press.
https://books.google.ca/books?id=qvBipuypYUkC&printsec=frontcover&hl=fr&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false

Hbd*chick (2013). A response to a response to two critical commentaries on woodley, te nijenhuis and murphy. May 27
http://hbdchick.wordpress.com/2013/05/27/a-response-to-a-response-to-two-critical-commentaries-on-woodley-te-nijenhuis-murphy-2013/

Hong, Z. (2020). Modelling the on-going natural selection of educational attainment in contemporary societies. Journal of Theoretical Biology 493: 110210
https://www.biorxiv.org/content/10.1101/605311v2.full

Kong, A., M.L. Frigge, G. Thorleifsson, H. Stefansson, A.I. Young, F. Zink, G.A. Jonsdottir, A. Okbay, P. Sulem, G. Masson, D.F. Gudbjartsson, A. Helgason, G. Bjornsdottir, U. Thorsteinsdottir, and K. Stefansson. (2017). Selection against variants in the genome associated with educational attainment. Proceedings of the National Academy of Sciences 114(5): E727-E732.

Madison, G. (2014). Increasing simple reaction times demonstrate decreasing genetic intelligence in Scotland and Sweden, London Conference on Intelligence. Psychological comments, April 25
#LCI14 Conference proceedings
http://www.unz.com/jthompson/lci14-questions-on-intelligence/   

Madison, G., M.A. Woodley of Menie, and J. Sänger. (2016). Secular Slowing of Auditory Simple Reaction Time in Sweden (1959-1985). Frontiers in Human Neuroscience, August 18
https://www.frontiersin.org/articles/10.3389/fnhum.2016.00407/full  

Rindermann, H. (2018). Cognitive Capitalism. Human Capital and the Wellbeing of Nations. Cambridge University Press.

Teasdale, T.W., and D.R. Owen. (2005). A long-term rise and recent decline in intelligence test performance: The Flynn Effect in reverse. Personality and Individual Differences 39(4): 837-843.
https://doi.org/10.1016/j.paid.2005.01.029   

Woodley of Menie, M.A., and H.B.F. Fernandes. (2016). Showing their true colours: Possible secular declines and a Jensen effect on colour acuity — More evidence for the weaker variant of Spearman's Other Hypothesis. Personality and Individual Differences 88: 280-284.
https://www.sciencedirect.com/science/article/abs/pii/S0191886915005826

Woodley, M.A., J. Nijenhuis, and R. Murphy. (2013). Were the Victorians cleverer than us? The decline in general intelligence estimated from a meta-analysis of the slowing of simple reaction time. Intelligence 41: 843-850. 
https://pdfs.semanticscholar.org/e8cc/634169c7c5d3e4738fe08091c86177be1380.pdf

Monday, March 9, 2020

The ghosts of West Africa



Bushmen in the Kalahari (Wikicommons, Andy Maano). When recorded history began, in Sumer and Egypt, black Africans were absent from most of Africa, even from most of West Africa. The lands south of the Sahara were largely home to various hunter-gatherers who were small, almost childlike in build, and light reddish-brown in color. 



Most Americans think of native Africans as black and of white Africans as recent intruders; and when they think of Africa's racial history they think of European colonialism and slave trading. But very different types of peoples occupied much of Africa until as recently as a few thousand years ago.

When Jared Diamond penned those words, analysis of ancient DNA was years away. Even when it began, there was a feeling that such analysis would always be impractical in Africa or anywhere else in the tropics. The climate is too warm for that stuff to last thousands upon thousands of years.

Apparently not. DNA has been retrieved from the remains of four individuals at a site in Cameroon, two of them going back 8,000 years and the other two 3,000 years. The main finding? The individuals were most similar to Pygmies, who still exist as isolated groups of hunter-gatherers in the Congo basin. There was no genetic similarity to the Bantu peoples who now predominate throughout central, eastern, and southern Africa (Lipson et al. 2020).

This finding is no surprise. Linguistic evidence has shown that the Bantu are all descended from a group of farming peoples who, some two to three thousand years ago, began to expand eastward and southward from what is now the Cameroon-Nigeria border. 

More intriguing is the discovery of admixture from an extinct West African people. These were hunter-gatherers who shared common ancestry with the Pygmies of central Africa and the Khoisans of southern Africa; however, they had intermixed much more with an archaic hominin that had diverged from ancestral modern humans at about the same time as the Neanderthals:

The West African clade is distinguished by admixture from a deep source that can be modelled as a combination of modern human and archaic ancestry. The modern human component diverges at almost the same point as Central and southern African hunter-gatherers and is tentatively related to the deep source that contributes ancestry to the Mota individual, and the archaic component diverges close to the split between Neanderthals and modern humans (Lipson et al. 2020)

This suggests that the Bantu expansion was the second leg of an earlier expansion of farming peoples who had first replaced the hunter-gatherers of West Africa. This is in line with the thinking of George Murdock, an American anthropologist who argued that black Africans originated with the spread of agriculture from the Niger's headwaters, near the Mali-Guinea border. This region was the cradle of the Sudanic food complex: sorghum, pearl millet, cow pea, and other crops.

Murdock’s scenario is supported by linguistic evidence. Speakers of proto-Niger-Congo broke up around 10,000 years ago, and the oldest group appears to be proto-Mande speakers, whose descendants inhabit the Niger's headwaters (Blench 1984, pp. 128-129; Ehret 1984; Murdock 1959, pp. 44, 64-68). Farming itself seems to have begun later. According to Harris (1976, p. 352), “the problem of dating must be left in abeyance, but it is clear that some form of seed-crop cultivation was underway in the interior at least by the second millennium B.C.”

It looks like a stable population of hunter-gatherers took shape on the Niger’s headwaters around 10,000 years ago. They gradually became proto-agricultural, i.e., more sedentary and better able to manage their food sources. By 4,000 years ago, they had become true farmers and were entering a phase of sustained demographic expansion that would see them colonize the banks of the Niger farther and farther downstream until they reached the rain forest in southern Nigeria. As they adapted to this new environment, they reached a modus vivendi with the Pygmy inhabitants, at first as tenants and then as de facto landowners who took over more and more of the land. Meanwhile, the Pygmies were pushed back farther and farther into the forest until they were no more.

In sum, farming can support a much larger population, and it was this demographic advantage that enabled farming peoples to replace hunter-gatherers, first in West Africa and eventually throughout almost all of sub-Saharan Africa.


Memories of the first West Africans

Those hunter-gatherers are remembered in the traditions of West Africa: 

Pygmies may have been the first inhabitants of Côte d'Ivoire. In their oral tradition, most of the present-day peoples, in particular the Dan-Yacouba, recount that their ancestors, on arriving in the country, found "little red men" whom they pushed back into the forest. Others speak of "little brown men", who had supernatural powers and to whom presents were given to win them over. (Mantongouine 2012)

According to some authors like Allou and Gonnin, the presence of these mysterious beings appears in the oral traditions. They are presented as short beings about 1m 44 to 1m 55 according to J.N. Loucou, with reddish skin, abundant hair, and feet pointing backward. They appear in almost all of the regions of prehistoric Côte d’Ivoire in the sense that almost all of the oral traditions of Côte d’Ivoire’s ethnic groups affirm that they found pygmies in the area before they became established. (Afri 2013; see also Gonnin and Allou 2006; Loucou 1984, p. 18)

Everywhere, but mainly in the countries from which the Pygmies have long disappeared, the Blacks who are considered to be the oldest occupants of the land say that it does not really belong to them and that, when their distant ancestors, coming from the East, established themselves, they found it in the possession of little men with reddish complexions and large heads who were the real natives and who, in exchange for fulfilment of certain agreements, permitted the Negroes who first arrived on a piece of land to enjoy its use and cultivate it. Eventually, those little men disappeared, but the memory of them has persisted. (Delafosse 1922, p. 14)

The Mano of Liberia say that the forested area used to contain only “talking chimpanzees.” These small creatures, called Lam, inhabited the area when the Mano first came. A Lam and his family would live in a hole in the ground (Riddell 1970, p. 27).


Year-round farming, polygyny. and increased stature and robustness

In addition to their means of subsistence, this expanding population of farmers differed from the hunter-gatherers in another way: a much higher rate of polygyny. Farming, especially year-round farming, makes women more self-reliant in feeding themselves and their children, thus cutting the costs, for a man, of having a second wife (van den Berghe 1979, p. 65). The result is a high polygyny rate: 20-50% of all marriages in sub-Saharan farming societies (Bourguignon and Greenbaum 1973, p. 51; Goody 1973; Pebley and Mbugua 1989; Welch and Glick 1981; White 1988).

If some men have more wives, others have to do without. In general, men must compete more keenly with each other for access to women. When such rivalry intensifies in nonhuman species, there is selection for larger, stronger, and more muscular males. This may explain the physical robustness of polygynous farming peoples in sub-Saharan Africa.

This point was studied by Butovskaya et al. (2015) in their study of two East African peoples: the polygynous Datoga and the monogamous Hadza. Datoga men were larger and more robust than Hadza men. They also scored higher on measures of physical aggression, verbal aggression, anger, and hostility. In fact, the two groups differed fundamentally in their attitudes toward aggression:

There is a negative attitude toward aggression among the Hadza but not among the Datoga. In situations of potential aggression, the Hadza prefer to leave. In contrast, aggression is an instrument of social control — both within the family and in outgroup relations — in Datoga society. Datoga men are trained to compete with each other and to act aggressively in particular circumstances.  (Butovskaya et al. 2015).

The two groups also differed at the androgen receptor gene, with the polygynous Datoga more often having an allele that correlated in men with aggressiveness and number of children fathered. Thus, through a process of gene-culture coevolution, a highly polygynous culture has produced a different sort of man, both mentally and physically.

There are other explanations for the diminutive and less robust appearance of African hunter-gatherers. O'Dea (1994) has argued that Pygmies are smaller and less robust because they are less exposed to sunlight in the rain forest and thus less able to synthesize vitamin D and maintain a large and strong skeleton. But how would this theory explain the small, gracile appearance of the Khoisan hunter-gatherers of the Kalahari, who live in an open environment with high solar radiation?


Darker skin

The polygyny rate correlates with darkness of skin, even after you control for latitude (Manning et al. 2004). This is particularly so in sub-Saharan Africa, where highly polygynous farming peoples are noticeable darker than the largely monogamous Pygmy and Khoisan hunter-gatherers. The reason may be a widespread mental association between gender and skin color. Because women are naturally lighter-skinned than men, traditional cultures tend to associate light skin with femininity and dark skin with masculinity (van den Berghe and Frost 1986). There is thus a selective compromise between natural selection for darker skin as a protection against solar radiation and sexual selection for lighter skin as a criterion of femininity (or darker skin as a criterion of masculinity). 

Because unmated women of any kind are scarce in a polygynous society, there is weaker sexual selection for women with lighter skin. This may be why farming peoples are noticeably darker-skinned in sub-Saharan Africa (Frost 2008).


Archaic admixture in West Africa

The ancient DNA study is also consistent with evidence that a partially archaic population used to live in West Africa. One piece of evidence is a skull from a Nigerian site (Iwo Eleru), which is only about 16,300 years old and yet is intermediate in shape between the skulls of modern humans on the one hand and the skulls of Neanderthals and Homo erectus on the other (Harvati et al. 2011; Stojanowski 2014). Furthermore, genomic analysis shows an apparently higher level of Neanderthal ancestry in the Yoruba of Nigeria than in the Luhya of Kenya. This admixture seems to come from a Neanderthal-like population that once lived in West Africa (Hawks 2012).


Conclusion

In the fifteenth century, Europeans discovered a continent whose inhabitants would have looked quite different a millennium earlier. Two tenth-century Arab geographers reported 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 “white Zanj” were probably the Khoisan hunter-gatherers who once inhabited the inland plateau of southern Africa.

If we could rewind history, we would see true black Africans retreating progressively to West Africa and then to the area of the Niger’s headwaters. This leads us to a strange conclusion. When recorded history began, in Sumer and Egypt, black Africans were absent from most of Africa, even from most of West Africa. Perhaps they didn’t yet exist anywhere. The lands south of the Sahara were largely home to various hunter-gatherers who were small, almost childlike in build, and light reddish-brown in color. 


References

Afri, A. (2013). Existait-il des peuples en Côte d’Ivoire avant le XVIIIème siècle ?
http://anicetafri.over-blog.com/existait-il-des-peuples-en-cote-d’ivoire-avant-le-xviiième-siÈcle

Blench, R. (1995). Recent developments in African language classification and their implications for prehistory. In T. Shaw, P. Sinclair, B. Andah, and A. Okpoko (Eds.) The Archaeology of Africa (pp. 126-138). London: Routledge.

Butovskaya M.L., O.E. Lazebny, V.A. Vasilyev, D.A. Dronova, D.V. Karelin, A.Z.P. Mabulla, et al. (2015). Androgen receptor gene polymorphism, aggression, and reproduction in Tanzanian foragers and pastoralists. PLoS ONE 10(8): e0136208.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4546275/ 

Delafosse, M. (1922). Les Noirs de l’Afrique. Paris: Collection Payot. 
https://www.herodote.net/Textes/delafosse_noirs_afrique.pdf

Diamond, J. (1994). How Africa Became Black. Discover, February 1
https://www.discovermagazine.com/planet-earth/how-africa-became-black

Ehret, C. (1984). Historical/linguistic evidence for early African food production. In J.D. Clark and S.A. Brandt (Eds.) From Hunters to Farmers: The Causes and Consequences of Food Production in Africa (pp. 26-35). Berkeley: University of California Press.

Frost, P. (2008). Origins of black Africans, Evo and Proud, February 10
http://evoandproud.blogspot.com/2008/02/origins-of-black-africans.html

Gonnin, G. and R.K. Allou. (2006). Côte-d’Ivoire : les premiers habitants. Abidjan: Les éditions du CERAP.

Goody, J. (1973). Polygyny, Economy and the Role of Women, in J. Goody (Ed.) The Character of Kinship, Cambridge: Cambridge University Press, pp. 175-190.

Harris, D.R. (1976). Traditional systems of plant food production and the origins of agriculture in West Africa. In J.R. Harlan, J.M.J. De Wet, and A.B.L. Stemler. (ed.) Origins of African Plant Domestication, (pp. 311-356), The Hague: Moulton.
https://books.google.ca/books?id=tGOtFegfro4C&lr=&hl=fr&source=gbs_navlinks_s

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
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0024024  

Hawks, J. (2012). Which population in the 1000 Genomes Project samples has the most Neandertal similarity? John Hawks Weblog, February 8
http://johnhawks.net/weblog/reviews/neandertals/neandertal_dna/1000-genomes-introgression-among-populations-2012.html  

Lewis, B. (1990). Race and Slavery in the Middle East. An Historical Enquiry. New York: Oxford University Press.

Lipson, M., I. Ribot, S. Mallick, et al. (2020). Ancient West African foragers in the context of African population history. Nature 577: 665-670.
https://reich.hms.harvard.edu/sites/reich.hms.harvard.edu/files/inline-files/Shum_Laka_published_online_0.pdf

Loucou, J-N. (1984). Histoire de la Côte-d’Ivoire. Tome 1 : La formation des peuples. Abidjan: Centre d’édition et de diffusion africaine (CEDA).

Manning, J.T., P.E. Bundred, and F.M. Mather. (2004). Second to fourth digit ratio, sexual selection, and skin colour. Evolution and Human Behavior 25(1): 38-50.
https://www.sciencedirect.com/science/article/abs/pii/S1090513803000825

Mantongouine. (2012). L'histoire de la Côte d'ivoire 
http://mantongouine.free.fr/index.php?option=com_content&view=article&id=66:lhistoire-de-la-cote-divoire&catid=34:description

Murdock, G.P. (1959). Africa. Its Peoples and Their Culture History. New York: McGraw-Hill.

O'Dea, J.D. (1994). Possible contribution of low ultraviolet light under the rain-forest canopy to the small stature of Pygmies and Negritos. Homo 44(3): 284-7.

Pebley, A. R., and Mbugua, W. (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.

Riddell, J.C. (1970). Labor Migration and Rural Agriculture among the Gbannah Mano of Liberia. Dissertation, Department of Anthropology, University of Oregon.
https://scholarsbank.uoregon.edu/xmlui/bitstream/handle/1794/22554/Riddell_Labor%20Migration%20and%20Rural%20Agriculture.pdf?sequence=1&isAllowed=y

Stojanowski, C.M. (2014). Iwo Eleru's place among Late Pleistocene and Early Holocene populations of North and East Africa. Journal of Human Evolution 75: 80-89.
http://www.sciencedirect.com/science/article/pii/S0047248414000876

van den Berghe, P.L. (1979). Human Family Systems. An Evolutionary View. New York: Elsevier.

van den Berghe, P.L., and P. Frost. (1986). Skin color preference, sexual dimorphism and sexual selection: A case of gene-culture co-evolution? Ethnic and Racial Studies 9(1): 87-113.
https://www.tandfonline.com/doi/abs/10.1080/01419870.1986.9993516

Welch, C.E., and Glick, P.C. (1981). The incidence of polygamy in contemporary Africa: A research note. Journal of Marriage and the Family 43:191-193.

White, D. R. (1988). Rethinking polygyny. Co-wives, codes, and cultural systems. Current Anthropology 29: 529-572.