Recent cognitive evolution: The case of Ashkenazi Jews

 

Old Jewish cemetery in Prague (Wikicommons - Uoaei)

The 11th century seems to be the time when cognitive evolution began to accelerate among Ashkenazi Jews. In terms of mean cognitive ability, they would end up surpassing not only Christian Europeans but also other Jewish populations. 

 

 

In a previous post, I described how Christianity restarted cognitive evolution after a decline during Classical Antiquity, specifically by supporting the formation of monogamous families, by discouraging slavery, at least during the long period from 500 to 1500 AD, and eventually by creating the peace, order, and stability that allowed the middle class to expand and become dominant (Frost 2022).

 

I will now describe a parallel evolution that occurred under Judaism, particularly within its Ashkenazi branch, i.e., the Jewish communities of Central and Eastern Europe. This cognitive evolution is indicated by four lines of evidence:

 

·         A high polygenic score for alleles associated with educational attainment (Dunkel et al. 2019; Piffer 2019).

·         High incidences of nine neurological disorders of genetic origin: Tay-Sachs (two unrelated alleles), Gaucher's (five unrelated alleles), Niemann-Pick, and Mucolipidosis Type IV. These mutations affect the capacity of neural tissue to store sphingolipids, which are vital to the growth of neurons in the brain. All nine of them have arisen independently in the same metabolic pathway and have become unusually common in the same population over a relatively short time, perhaps a thousand years (Cochran et al. 2006; Diamond 1994).

·         A mean IQ that exceeds not only that of non-Jewish Europeans but also that of other Jewish groups. This cognitive advantage seems to be relatively recent, originating probably in the Middle Ages (Cochran et al. 2006).

·         A high proportion of Nobel Prize winners: 14% in the first half of the 20th century, 29% in the second half of the 20th century and, so far, 32% in the 21st century (Murray 2007, p. 30).

 

Within the Ashkenazi community, cognitive evolution was driven largely by specialization in trade, particularly in family-run businesses that operated in the increasingly dynamic economic environment of post-medieval Europe. This class of people, though proportionately smaller, would also contribute to the cognitive evolution of Christian Europeans. "They were not specialized craftsmen in life-trades with skills developed through long years of apprenticeship; they were semi-skilled family labour teams which set up in a line of business very quickly, adapting to shifts in market demand" (Seccombe, 1992, p. 182). The workforce was the household. In more successful households, the parents would have as many children as possible, and the children would marry earlier and start their own households earlier. In less successful ones, the children would postpone marriage or never marry.

 

This demographic model explains not only the cognitive evolution of Ashkenazi Jews but also their impressive population growth between the 16th and 19th centuries (Frost 2007). It is also the same model that Gregory Clark (2007) used to describe the mental and behavioral shift of the English population during medieval and post-medieval times: demographic success was closely linked to economic success, which in turn was linked to possession of cognitive ability and “middle-class” traits (low time preference, high impulse control, thrift, etc.).

 

For Ashkenazi Jews during the same time period, cognitive evolution was also assisted by a ban on polygyny. Around 1000 AD, a Jewish synod at Mainz, Germany forbade men to take more than one wife. The ruling was made for several reasons: (1) to reduce “quarreling” within the family; (2) to deter men from abusing their wives; (3) to prevent de facto divorce (i.e., abandonment of the first wife in her own household); and (4) to prevent the husband’s financial resources from being spread too thinly over several wives (Dinonline 2015). The ban on polygyny strengthened the reproductive importance of upper-class women by reducing female hypergamy. In particular, it prevented the classic case of a wealthy man taking a second wife who would be younger than the first but lower in social status … and thus less likely to have the mental and behavioral characteristics that had made him economically successful and able to afford polygyny.

 

The polygyny ban was accepted by Ashkenazi Jews but not by Sephardic Jews (Dinonlne 2015). This may explain why cognitive evolution was weaker among the latter than among the former. In addition, the Sephardim were operating within a much less dynamic economic environment, particularly during the period from the 16th to 19th centuries that saw so much population growth among the Ashkenazim. 

 

Reconstructing cognitive evolution

 

Until recently, it was possible to reconstruct cognitive evolution only by looking at present-day genomes and making inferences about the past. We can now look directly at past cognitive evolution by examining DNA from human remains. For example, when Woodley et al. (2017) compared DNA from sites across Europe and central Asia, they found a net increase between 4,560 and 1,210 years ago in the frequency of alleles associated with high educational attainment.

 

Unfortunately, that kind of longitudinal data is not yet available for Ashkenazi Jews. But we have a proxy: alleles that affect sphingolipid storage, specifically those for Tay-Sachs, Gaucher’s, Niemann-Pick and Mucolipidosis Type IV. An allele for Gaucher’s disease, and specific to Ashkenazi Jews, has been retrieved from the remains of 33 Jewish individuals who had lived in Erfurt, Germany during the 14th century (Waldman et al. 2022, p. 16).  Those individuals were not directly ancestral to modern Ashkenazi Jews; instead, both groups seem to descend from a common ancestral population that probably existed in the 11th century, when Jewish merchants first became established in Erfurt (Waldman et al. 2022; Wikipedia 2022a). That date is consistent with the estimated time of origin of the allele for Gaucher’s disease, which has been dated to a period between the 11th and 13th centuries (Colombo 2000).

 

A time of origin in the 11th century is also consistent with the founding of Jewish communities in what is now the Czech Republic and Poland:

 

We have already mentioned the existence of Jewish traders in Prague in the late tenth century. The biographies of St. Adalbert tell us that they trafficked in slaves. There was also in the early eleventh century, we will discuss further, a Jewish establishment at Przemysl, a town at the crossroads of two trading routes: Prague-Krakow-Kiev and Hungary-Kiev. The importance of this center is confirmed by the discovery, in the mid nineteenth century, of a great treasure of dirhams [Arab silver money] from the Iranian dynasty of the Samanids, dating from the first half of the tenth century. We will see that certain Hebrew documents from the 11th and 12th centuries also report the trade of Rhineland Jewish merchants with Poland. Gallus Anonymus, the famous Polish chronicler of the 11th century, relates that Queen Judith repurchased slaves in Poland from Jewish traders—which also proves the existence of this trade. A final confirmation of this phenomenon: the discoveries of Polish “treasures of silver” [hoards] from the 10th and 11th centuries, which have very many coins from the Rhineland towns of Western Europe. (Lewicki 1960, p. 232)

 

The importance of that trade is also indicated by the large number of Slavic words that appear in the works of contemporary Jewish authors from the Rhineland and even northern France (Lewicki 1960, pp. 236-237).

 

It looks like Ashkenazi communities entered an upward economic trend not long before the 11th century. This was a time when both the State and the Church began pacifying the social environment of Western Europe (Frost and Harpending 2015; Head 1992; Wikipedia 2022b). Trade thus became safer. In particular, slave merchants were able to establish long trade routes that ran from the lands of the pagan Slavs, across Western Europe, and into the Islamic world via Muslim Spain (Blumenkranz 1960; Korn 1971; Skirda 2010, pp. 83-120). Erfurt itself was one of several stops on a route from Bohemia to Spain (Skirda 2010, p. 115). It is likely, then, that the 11th century coincides with the time when cognitive evolution began to accelerate among Ashkenazi Jews.

 

After the 11th century, the slave trade began to lose importance, and other activities gradually took their place. There were a number of reasons. Trade with the Muslim world was disrupted by the Reconquista of Spain and by the Crusades in general. Throughout Western Europe, Jewish communities were accused of having Muslim sympathies and suffered persecution (Skirda 2010, pp. 104-105). Above all else, the Slavs were converting to Christianity, and their enslavement was becoming harder to justify.

 

Ashkenazi Jews thus shifted toward other activities. At first, they turned to trade with Central Asia via Kiev and the Black Sea (Skirda 2010, p. 105). In time, their interest focused on Europe, which was developing economically and offering many more opportunities. The result was a demographic expansion. From an estimated 25,000 in 1300, the Jewish population of Eastern Europe would grow to 50,000 by 1490, 250,000 after the mid-1600s, 910,000 by 1765, two and a quarter million by 1825, over five and a half million by 1880, and over eight and a half million by 1900 (DellaPergola 2001, p. 12).

 

References

 

Blumenkranz, B. (1960). Juifs et Chrétiens dans le monde occidental. Paris: Imprimerie nationale

 

Clark, G. (2007). A Farewell to Alms. A Brief Economic History of the World. Princeton University Press: Princeton.

 

Cochran, G., J. Hardy, and H. Harpending. (2006). Natural history of Ashkenazi intelligence. Journal of Biosocial Science 38(5): 659-693. https://doi.org/10.1017/S0021932005027069

 

Colombo, R. (2000). Age estimate of the N370S mutation causing Gaucher disease in Ashkenazi Jews and European populations: A reappraisal of haplotype data. American Journal of Human Genetics 66(2):692-697. https://doi.org/10.1086/302757

 

DellaPergola, S. (2001). Some fundamentals of Jewish Demographic History. In: S.DellaPergola, and J.Even (eds.) Papers in Jewish Demography 1997, (pp. 11-33), Jerusalem: The Hebrew University.

 

Diamond, J.M. (1994). Jewish Lysosomes. Nature 368: 291-292. https://doi.org/10.1038/368291a0

 

Dinonline. (2015). Marrying more than one wife: The decree of Rabbeinu Gershom — Then and today. November 18. https://dinonline.org/2015/11/18/marrying-more-than-one-wife-the-decree-of-rabbeinu-gershom-then-and-today/

 

Dunkel, C.S, M.A. Woodley of Menie, J. Pallesen, and E.O.W. Kirkegaard. (2019). Polygenic scores mediate the Jewish phenotypic advantage in educational attainment and cognitive ability compared with Catholics and Lutherans. Evolutionary Behavioral Sciences 13(4): 366-375.

https://psycnet.apa.org/doi/10.1037/ebs0000158

 

Frost, P. (2007). Natural selection in proto-industrial Europe. Evo and Proud, November 16

http://evoandproud.blogspot.com/2007/11/natural-selection-in-proto-industrial.html

 

Frost, P. (2022). When did Europe pull ahead? Evo and Proud, May 16

http://evoandproud.blogspot.com/2022/05/when-did-europe-pull-ahead.html

 

Frost, P. and H. Harpending. (2015). Western Europe, state formation, and genetic pacification. Evolutionary Psychology 13(1): 230-243. https://doi.org/10.1177%2F147470491501300114

 

Head, T.F. (1992). The Peace of God: Social Violence and Religious Response in France around the Year 1000. Cornell University Press.

 

Korn, B.W. (1971). Slave Trade. Encyclopaedia Judaica 14: 1660-64. Jerusalem: MacMillan.

 

Lewicki, T. (1961). Les sources hébraïques consacrées a l'histoire de l'Europe centrale et Orientale et particulièrement a celle des pays slaves de la fin du IX^e au milieu du XIII^e siècle. Cahiers du Monde russe et soviétique 2(2): 228-41. https://doi.org/10.3406/cmr.1961.1466

 

Murray, C. (2007). Jewish Genius. Commentary. April: 29-35

 

Piffer, D. (2019). Evidence for recent polygenic selection on educational attainment and intelligence inferred from Gwas hits: A replication of previous findings using recent data. Psych 1: 55–75. https://doi.org/10.3390/psych1010005

 

Seccombe, W. (1992). A Millennium of Family Change. Feudalism to Capitalism in Northwestern Europe. London: Verso.

 

Skirda, A. (2010). La traite des Slaves. L’esclavage des Blancs du VIII^e au XVIII^e siècle. Paris: Les Éditions de Paris Max Chaleil.

 

Waldman, S., D. Backenroth, É. Harney, S. Flohr, N.C. Neff, G.M. Buckley, et al. (2022). Genome-wide data from medieval German Jews show that the Ashkenazi founder event pre-dated the 14th century. bioRxiv 2022.05.13.491805. https://doi.org/10.1101/2022.05.13.491805

 

Wikipedia. (2022a). Erfurt. https://en.wikipedia.org/wiki/Erfurt#Middle_Age

 

Wikipedia. (2022b). Peace and Truce of God. https://en.wikipedia.org/wiki/Peace_and_Truce_of_God

 

Woodley, M.A., S. Younuskunju, B. Balan, and D. Piffer. (2017). Holocene selection for variants associated with general cognitive ability: comparing ancient and modern genomes. Twin Research and Human Genetics 20: 271-280. https://doi.org/10.1017/thg.2017.37   

 

A virus that increases intelligence, and what else?

 

Caterpillar infected by baculovirus, before liquefaction (Wikicommons, Williams et al. 2017)

 

Higher IQ is associated with antibodies to cytomegalovirus, but only in adults whose IQ is already above a certain level. Which is the cause, and which is the effect?

 

Cytomegalovirus (CMV) belongs to the herpes family and is spread by contact with bodily fluids. Around 45 to 100% of the population may be infected, although most hosts are unaware that they are infected. It has one of the largest genomes among human viruses, a sign perhaps of its ability to manipulate its host:

 

With millions of years of coevolution within their hosts, CMVs, like other herpesviruses, encode numerous proteins that can broadly influence the magnitude and quality of both innate and adaptive immune responses. These viral proteins include both homologues of host proteins, such as MHC class I or chemokine homologues, and proteins with little similarity to any other known proteins, such as the chemokine binding protein. Although a strong immune response is launched against CMV, these virally encoded proteins can interfere with the host's ability to efficiently recognize and clear virus, while others induce or alter specific immune responses to benefit viral replication or spread within the host. (Miller-Kittreall and Sparer 2009)

 

CMV infection at birth leads to mental retardation (Andreou et al. 2021). Recently, however, it has been shown that infection later in life can lead to higher IQ. This is one of three findings from a Norwegian study on IQ and antibodies to CMV in adults. The study showed that CMV seropositivity was significantly associated with higher IQ in men who suffered from bipolar spectrum disorders and with lower IQ in women who suffered from schizophrenia spectrum disorders. CMV seropositivity was not associated with IQ in healthy controls (Andreou et al. 2021). The study’s authors were at a loss to explain the association between CMV seropositivity and higher IQ in bipolar men, all the more so because the number of bipolar men was small, only 35.

 

An association between CMV and IQ has now been found in healthy individuals. A Czech study has shown that IQ, especially verbal IQ, is higher in people with antibodies to CMV. Moreover, the IQ advantage decreases with decreasing levels of CMV antibodies, i.e., with increasing time since the CMV infection (Chvatálova et al. 2022).

 

Why do healthy Czechs show this association but not healthy Norwegians? It’s not because the Czechs were a larger sample. In fact, the Czech sample had 283 healthy individuals, and the Norwegian sample 474. The two samples did differ, however, in educational attainment. The Czechs were biology students at a university in Prague, whereas the Norwegians were randomly recruited from the Norwegian population register. The latter were also described as “Caucasians” living in Oslo. Oslo’s population is almost one third of immigrant origin, with Pakistanis forming the largest immigrant group. There are also large numbers of people from Sri Lanka, Turkey, Morocco, and Iraq (Wikipedia 2022). 

 

The last point would not be problematic if cognitive evolution had ended long ago among the common ancestors of Europeans, Middle Easterners, and South Asians. There is mounting evidence, however, for cognitive evolution in recent times. Ashkenazi Jews seem to have gained their cognitive advantage during the past 1,000 or so years, and the same seems to be true for the Parsis (Cochran et al. 2006; Dunkel et al. 2019; Frost 2021). There is even evidence for significant cognitive evolution in communities that are not normally thought of as ethnic groups, such as French Canadians in regions where British and American traders were historically few in number (Frost 2012).

 

Why does CMV seropositivity correlate with higher IQ?

 

The authors of the Czech study suggest that more intelligent people have more social contacts and are thus more likely to catch the virus: “we suggest that more intelligent subjects who have more social and sexual contacts—CMV is transmitted by close contacts, e.g. by kissing—might have a higher risk of encountering a CMV infection.” Yet there is no evidence that smarter people are more extraverted. In fact, they tend to be loners, if only because they have fewer people of their intellectual level to hang out with. The academic consensus seems to be that neither introversion nor extraversion correlates with intelligence (Saklofske and Kostura 1990).

 

Could the arrow of causality run in the other direction? Is it possible that CMV makes its host smarter? The authors reject that explanation because congenital CMV infection reduces intelligence. But maybe the effect is different in adults.

 

If CMV does increase the intelligence of adult hosts, the effect would be confined to those whose IQ is already above a certain level. The above two studies showed a significant increase only among university students, and not in a more mixed population with varying levels of educational attainment. But why did the latter study show significantly higher IQ in men with bipolar disorders and significantly lower IQ in women with schizophrenia? For the answer, we can turn to the results of a recent genome-wide association study: most of the alleles for schizophrenia are associated with lower intelligence, and most of the alleles for bipolar disorder are associated with higher intelligence (Smeland et al. 2020). Those results are partially confirmed by the findings of a prospective cohort study: “at least in men, high intelligence may indeed be a risk factor for bipolar disorder, but only in the minority of cases who have the disorder in a pure form with no psychiatric comorbidity” (Gale et al. 2013).

 

Behavior alteration?

 

Why would a virus try to make its host smarter? What would it gain? Perhaps the increase in intelligence is a side-effect. Perhaps the virus is trying to improve its chances of spreading to new hosts by altering the behavior of its current host (Cochran et al. 2000; Frost 2020).

 

Although behavior is more often altered by larger and more complex pathogens, particularly fungi, there are many viruses that engage in behavior alteration. For example, viruses from the baculoviridae family will infect a caterpillar and make it hyperactive to spread their viral progeny over a wider area. Or the caterpillar will be made to climb to the top of a plant and dissolve itself through overproduction of enzymes, thus becoming a mass of tasty goo for ingestion by potential hosts (Han et al. 2015; Williams et al. 2017). Rabies is another behavior-altering virus: it makes its host more aggressive and thus more likely to bite potential hosts.

 

Although CMV infects a wide range of people, it seems to target a smaller subgroup for behavior alteration, i.e., individuals with intelligence above a certain threshold, and men more than women. If we look at the epidemiological data, we see that male homosexuals are especially susceptible. A study at a venereal disease clinic found that antibodies to CMV were present in 94% of the male homosexual patients and 54% of the male heterosexual patients. “The data suggest that sexual transmission is an important mode of spread of CMV among adults and that homosexual men are at greater risk for CMV infections than are heterosexual men” (Drew et al. 1981). Another study has identified passive anal sex as the most effective means of transmission: “Of seven sexual practices investigated, only passive anal-genital intercourse correlated with the acquisition of cytomegalovirus infection (p =0.008)” (Mintz et al. 1983).

 

Which is the cause and which is the effect? Does passive anal sex facilitate CMV infection? Or does CMV infection facilitate the desire for passive anal sex? The answer may be ‘yes’ to both questions. Sometimes ‘the cause’ and ‘the effect’ are two sides of the same coin.

 

This virus may indeed be the ‘gay germ’ that Greg Cochran has written about. Or one of them. Male homosexuality probably has several causes, and the microbial cause probably involves more than one pathogen.

 

 

References

 

Andreou, D., K.N. Jørgensen, L.A. Wortinger, K. Engen, A. Vaskinn, T. Ueland, R.H. Yolken, O.A. Andreassen, and I. Agartz. (2021). Cytomegalovirus infection and IQ in patients with severe mental illness and healthy individuals. Psychiatry Research 300:113929. https://doi.org/10.1016/j.psychres.2021.113929  

 

Chvatálová, V., B. Šebánková, H. Hrbáčková, P. Tureček, L. Příplatová, and J. Flegr. (2022). Differences in cognitive performance between cytomegalovirus-infected and cytomegalovirus-free students. PsyArXiv, 5 May 2022. https://doi.org/10.31234/osf.io/jbvky  

 

Cochran, G.M., P.W. Ewald, and K.D. Cochran. (2000). Infectious Causation of Disease: An Evolutionary Perspective. Perspectives in Biology and Medicine 43 (3): 406-48.

https://doi.org/10.1353/pbm.2000.0016  

 

Cochran, G., J. Hardy, and H. Harpending. (2006). Natural history of Ashkenazi intelligence. Journal of Biosocial Science 38: 659-693, https://doi.org/10.1017/S0021932005027069  

 

Drew, W.L., L. Mintz, R.C. Miner, M. Sands, and B. Ketterer. (1981). Prevalence of Cytomegalovirus Infection in Homosexual Men. The Journal of Infectious Diseases 143(2): 188–192. https://doi.org/10.1093/infdis/143.2.188

 

Dunkel, C.S., M.A. Woodley of Menie, J. Pallesen, and E.O.W. Kirkegaard. (2019). Polygenic scores mediate the Jewish phenotypic advantage in educational attainment and cognitive ability compared with Catholics and Lutherans. Evolutionary Behavioral Sciences 13(4): 366-375. https://doi.org/10.1037/ebs0000158  

 

Frost, P. (2012). Tay-Sachs and French Canadians: A case of gene-culture co-evolution? Advances in Anthropology 2(3): 132-138. http://dx.doi.org/10.4236/aa.2012.23016  

 

Frost, P. (2020). Are Fungal Pathogens Manipulating Human Behavior? Perspectives in Biology and Medicine 63(4): 591-601. https://doi.org/10.1353/pbm.2020.0059   

 

Frost, P. (2021). Commentary on Fuerst et al: Do Human Populations Differ in Their Mental Characteristics? Mankind Quarterly 62(2). http://doi.org/10.46469/mq.2021.62.2.9  

 

Gale, C., G. Batty, A. McIntosh. et al. (2013). Is bipolar disorder more common in highly intelligent people? A cohort study of a million men. Molecular Psychiatry 18: 190–194. https://doi.org/10.1038/mp.2012.26  

 

Han, Y., S. van Houte, G.R. Drees, M.M. van Oers, and V.I. Ros. (2015). Parasitic Manipulation of Host Behaviour: Baculovirus SeMNPV EGT Facilitates Tree-Top Disease in Spodoptera exigua Larvae by Extending the Time to Death. Insects 6(3): 716–731. https://doi.org/10.3390/insects6030716  

 

Miller-Kittrell, M., and T.E. Sparer. (2009). Feeling manipulated: cytomegalovirus immune manipulation. Virology Journal 6: 4 (2009). https://doi.org/10.1186/1743-422X-6-4  

 

Mintz, L., W.L. Drew, R.C. Miner, and E.H. Braff. (1983). Cytomegalovirus infections in homosexual men. An epidemiological study. Annals of Internal Medicine 99(3):326-9. https://doi.org/10.7326/0003-4819-99-3-326

 

Saklofske, D. H., and D.D. Kostura. (1990). Extraversion-introversion and intelligence. Personality and Individual Differences 11(6): 547-551. https://doi.org/10.1016/0191-8869(90)90036-Q  

 

Smeland, O.B., S. Bahrami, O. Frei, A. Shadrin, K. O'Connell, J. Savage, K. Watanabe, F. Krull, F. Bettella, N.E. Steen, T. Ueland, D. Posthuma, S. Djurovic, A.M. Dale, and O.A. Andreassen. (2020). Genome-wide analysis reveals extensive genetic overlap between schizophrenia, bipolar disorder, and intelligence. Molecular Psychiatry 25(4):844-853. https://doi.org/10.1038/s41380-018-0332-x  

 

Williams, T., C. Virto, R. Murillo, and P. Caballero. (2017). Covert Infection of Insects by Baculoviruses. Frontiers in Microbiology 17(8): 1337. https://doi.org/10.3389/fmicb.2017.01337

 

Wikipedia (2022). Oslo – Demographics. https://en.wikipedia.org/wiki/Oslo#Demographics

When did Europe pull ahead?

 

Medieval market – Nicole Oresme (15th century) (Wikicommons)

 

In terms of GDP per capita growth, northwest Europe began to surpass the rest of the world during the 14th century: before the conquest of the Americas, the invention of printing, the Atlantic slave trade, and the Protestant Reformation.

 

 

In a recent post, Steve Sailer asks why the European world pulled ahead of the non-European world between 1000 and 1500 AD:

 

[…] much of the non-European world entered a sort of cultural recession well before Europeans directly interfered with them. If you look at, say, Charles Murray’s 2003 book Human Achievement, several major non-European civilizations appear to have lost momentum in making progress in the arts and sciences over roughly the time period of 1000 or maybe 1250 to 1500. (Sailer 2022)

 

For Steve, the reason was the collapse of the Mongol Empire during the 14th century. That century was indeed a turning point for Europe, particularly for England and Holland:

 

These North Sea economies experienced sustained GDP per capita growth for six straight centuries. The North Sea begins to diverge from the rest of Europe long before the 'West' begins its more famous split from 'the rest.'

 

[...] we can pin point the beginning of this 'little divergence' with greater detail. In 1348 Holland's GDP per capita was $876. England's was $777. In less than 60 years time Holland's jumps to $1,245 and England's to 1090. The North Sea's revolutionary divergence started at this time. (Greer 2013b)

 

This process began before the European conquest of the Americas, the invention of printing, the creation of modern finance institutions, the Atlantic slave trade, or the Protestant Reformation. None of these can be proper explanations for this "little divergence." (Greer 2013a; see also Thompson 2012 and Hbd *chick 2013).

 

The divergence began within a part of Europe that was much less affected by the rise and fall of the Mongol Empire. Moreover, if we compare southern Europe with North Africa during the same period, we see the same divergence that we see more generally between Christian Europe and the rest of the world. Yet North Africa was never conquered by the Mongols.

 

It looks like internal causes were responsible for the divergence between Christian Europe and the rest of the world. Those causes seem to have their point of origin in northwest Europe during the long period from 500 to 1500 AD. In that region, the Western Church consolidated a pre-existing pattern of small, nuclear households, weak family ties, and residential mobility, thus strengthening a mindset of individualism and impersonal sociality (Frost 2020; Schulz et al. 2019). Then, from 1000 AD onward, the Western Church strove to pacify social relations (Frost and Harpending 2015). Those two factors—an individualistic mindset operating in a pacified social environment—allowed the market economy to expand into all areas of life and eventually replace kinship as the main organizing principle of society (Frost 2020; Macfarlane 1978; Weber 1930).

 

The expansion of the market economy went hand in hand with the expansion of the middle class. In England, this class began to expand in the twelfth century and would gradually replace the lower classes through downward mobility. By the 1800s, its lineages accounted for most of the English population. English society thus became more middle class in its values: "Thrift, prudence, negotiation, and hard work were becoming values for communities that previously had been spendthrift, impulsive, violent, and leisure loving" (Clark 2007, p. 166). The same process took place elsewhere in Western Europe and more generally throughout Europe to varying degrees and over different timescales (Frost 2019, p. 176).

 

In sum, between 500 and 1500 AD the Western Church created a system of social reproduction that would have far-reaching demographic, behavioral, and economic consequences. To understand that system, one must understand not only the Bible but also the writings of early and medieval Christianity, as well as the pagan Germanic elements it incorporated (Russell 1994). Finally, one must understand the preceding system, and its failings.

 

The pre-Christian world: demographic and cognitive decline

 

Ancient DNA from Greece suggests that mean cognitive ability began to decline at some point during Classical Antiquity (Woodley of Menie et al. 2019). A similar decline probably happened throughout the Mediterranean basin and the Middle East of that time.

There were three main causes:

 

·         A decline in fertility and family formation, particularly among the upper classes (Caldwell 2004; Hopkins 1965; Roetzel 2000, p. 234);

·         A corresponding increase in female hypergamy, often by freed slaves, which reduced the reproductive importance of upper-class women (Perry 2013);

·         An increase in the slave population, particularly foreign slaves (Harris 1999). This ongoing influx disrupted the process of local cognitive evolution. Even if there had been demographic overflow from the upper classes, that overflow could not have replaced the lower classes, since those classes were being replaced from external sources.

 

Christianity and Islam both tried to correct the ruinous demographic state of the ancient world. Islam succeeded in reversing negative population growth but failed to restart cognitive evolution. In some ways, it made such evolution more difficult. Islam increased female hypergamy by permitting male polygamy, thus further reducing the reproductive importance of upper-class women (van den Berghe 1960). Foreign slaves were also imported on a larger-scale than in antiquity, thus further disrupting local cognitive evolution (Lewis 1990). Finally, the upper classes tended to congregate in urban areas, where the death rate was higher.

 

Before the 20th century, population growth had been sluggish in the Muslim world. Wherever Muslims coexisted with Christians, the latter community was often the one that grew at a faster pace. This was the case in the Balkans:

 

By the end of the eighteenth century the Muslim population had entered a period of comparative economic and moral decline. Several explanations have been offered for this development. Certainly the fact that the Muslim population provided the soldiers contributed to its ultimate weakening. Their concentration in towns also made them more susceptible to the ravages of plague and other diseases. Turkish customs, particularly the practice of polygamy, played a part. This process of decay was clearly illustrated in the eighteenth century in the changing demography of the Balkan towns where Christian and national elements formed an increasingly larger proportion of the population (Jelavich and Jelavich, 1977, pp. 6-7)

 

Christianity, especially Western Christianity, succeeded not only in promoting population growth but also in restarting cognitive evolution, specifically by supporting the formation of monogamous families, by discouraging slavery, at least during the long period from 500 to 1500 AD, and eventually by creating the peace, order, and stability that allowed the middle class to expand and become dominant. The rise of Christian Europe actually began before its expansion into the Americas and Asia. The latter was, in fact, a consequence of the former.

 

References

 

Caldwell, J.C. (2004). Fertility control in the classical world: Was there an ancient fertility transition?  Journal of Population Research 21:1. https://doi.org/10.1007/BF03032208  

 

Clark, G. (2007). A Farewell to Alms. A Brief Economic History of the World. Princeton University Press: Princeton.

 

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Red is beautiful: Perceived femininity of skin color in an African population

 

Perceived masculinity and femininity of facial skin color. Cameroonian women rated faces of Cameroonian men, and Cameroonian men rated faces of Cameroonian women (Fiala et al. 2022, Supplementary Material).

 

 

 

Women are the fair sex. They are paler than men, who conversely are ruddier and browner. Today, that sexual dimorphism is hardly noticed in Western societies, having been overwhelmed by much larger differences of race and ethnicity and further obscured since the 1920s by the tanning fad (Segrave 2005). But it was noticed earlier. Wherever the visual arts developed—ancient Egypt, the Greco-Roman world, early South and East Asia, Mesoamerica—female figures were given a lighter hue and male figures a darker one (Capart 1905, pp. 26-27; Eaverly 2013; Frost 2010, pp. 35-81; Pallottino 1952, pp. 34, 45, 73, 76-77, 87, 93, 95, 105, 107, 115; Soustelle 1970, p. 130; Tegner 1992; Wagatsuma 1967).

 

Sexual dimorphism in skin pigmentation

 

Skin color was first measured objectively in the 1930s, when spectrophotometers became commercially available. By measuring how much light the skin reflects across the visible spectrum, and how much it absorbs, one could identify its pigments and quantify their relative importance. Edwards and Duntley (1939) concluded that male and female complexions differ because of differing concentrations of melanin (brown), hemoglobin (red), and carotene (yellow).

 

Castration keeps men from acquiring their distinctive complexion:

 

One of the outstanding characteristics of a human male castrate is the paleness of the skin. After treatment with androgenic hormone, however, the individual takes on a darker and more ruddy hue. This observation suggests that the skin of the castrate is deficient in melanin and blood, and that the androgenic hormone increases the content of these substances in the integument. (Edwards et al. 1941)

 

Estrogen has similar but much weaker effects, which are further reduced by the other female hormone, progesterone. Ovariectomy thus has much less impact on female skin than castration has on male skin (Edwards and Duntley 1949). The sex hormones seem to alter skin pigmentation not only through ongoing transient effects but also through permanent organizational effects before birth and at puberty.

 

A hormonal causation is also suggested by the digit ratio. This is the length of the index finger divided by the length of the ring finger, and it tells us the relative proportions of estrogens to androgens in body tissues during development. In adults, the digit ratio correlates with lightness of female skin but not with lightness of male skin (Manning et al. 2004).

 

Sexual dimorphism in tanning capacity

 

Men and women likewise differ in tanning capacity. Men tan more than women even when both are equally exposed to the sun. This was shown in a New Guinea study of three body sites: skin on the unexposed upper inner arm; skin on the exposed forearm; and time spent in the sun. Despite identical sun exposure, the men were darker than the women, and more so on exposed skin (Harvey 1985). The same finding appears in another New Guinea study, whose author ruled out the possibility of the women being less exposed, "as in most parts of New Guinea the adult females are responsible for most of the food cultivation and are therefore exposed almost continuously to sunlight." (Walsh 1964).

 

Differences between human populations

 

Skin color is more sexually dimorphic where people are medium-colored and less so where they are very fair or very dark (Frost 2007; Madrigal and Kelly 2007). This sexual dimorphism cannot fully express itself in a very fair population because female skin encounters a physiological limit when it lightens after puberty. In a very dark population, male skin likewise encounters a physiological limit to further darkening.

 

Are there other population differences? Has this sexual dimorphism evolved differently in different populations?

 

Apparently. A recent paper shows that this sex difference differs qualitatively between Europeans and sub-Saharan Africans. When Fiala et al. (2022) measured the skin color of individuals from the Czech Republic and Cameroon, they found that women had fairer skin in both groups. But their skin was fairer in different ways. Among the Czechs, female skin was less red than male skin, in line with previous studies on European or Euro-American subjects. Among the Cameroonians, however, female skin was redder than male skin, and also more yellow.

 

Adaptation to the natural environment?

 

Is the redder complexion of African women an adaptation to the natural environment? If the skin is better supplied with blood, does it better cope with UV radiation, heat load, skin injuries, or some other aspect of a tropical environment? Let’s examine these three factors, while keeping in mind that they would have to be more fitness-reducing for African women than for African men.

 

UV radiation. The yellow pigment of skin (carotene) does provide some protection from UV (Stahl et al. 2012). So it’s plausible that African women compensate for having less melanin in their skin by having more carotene. On the other hand, there is no evidence that the red pigment of skin (hemoglobin) provides UV protection.

 

Heat load. When more blood is flowing to the skin, heat is radiated away more easily from the body (Hertzman 1959). It may be, then, that the increased redness of African female skin serves to disperse body heat in warmer climates. Nonetheless, we still have to explain why heat load is more fitness-reducing for African women than for African men.

 

Skin injuries. When more blood is flowing to the skin, wounds heals faster because more leukocytes can reach skin tissues and fight potential infections (Mathieu et al. 2006). Again, we still have to explain why this factor would matter more for African women than for African men. Coetzee et al. (2012) raise a similar objection: if ruddiness is attractive because it indicates physical health, why is it considered unattractive in the case of European women or African men?

 

Adaptation to the social environment?

 

Alternatively, the redder complexion of African women may have evolved as an adaptation to the social environment, specifically for gender recognition. In a study using Euro-American participants, people could tell whether a facial photo was male or female, at a rate much higher than chance, even when the image was blurred and provided no useful information other than the degree of redness (Tarr et al. 2001). Sexual dimorphism in skin color has two components: hue (degree of brownness and redness) and luminosity (degree of contrast between lightness of facial skin and darkness of lip/eye area). Hue is the fast channel for gender recognition. If the face is too far away or the lighting too dim, the mind will switch to the slower but more accurate channel of luminosity (Dupuis-Roy et al. 2009; Dupuis-Roy et al. 2019; Jones et al. 2015; Nestor and Tarr 2008a; Nestor and Tarr 2008b; Tarr et al. 2001; Tarr, Rossion, and Doerschner 2002). This gender cue may serve not only to tell men and women apart but also to modify male behavior by reducing aggressiveness and stimulating feelings of care and protection (Frost 2011).

 

African women maintain this gender cue through a different mix of skin pigments. They have more carotene in their skin, and thus a yellower complexion, to offset the loss of UV protection due to having less melanin. Unlike European women, they also have more blood in their skin and thus a redder complexion. Why is this? Perhaps increased redness does not visually alter dark skin in the same way that it visually alters light skin. When redness is increased, the dark skin of African women may look lighter and the light skin of European women may look darker. The social environment has thus favored lighter female skin in both populations, but the mix of pigments is different.

 

This gender cue was studied by Fiala et al. (2022) in their Czech/Cameroonian study. Cameroonian women were asked to rate facial photos of Cameroonian men, and Cameroonian men were asked to rate facial photos of Cameroonian women. The results showed a significant correlation between skin color and perceived masculinity/femininity:

 

The slope between perceived masculinity and colour (higher scores along all three CIELab dimensions, meaning basically lighter skin that allows both redness and yellowness to stand out) of Cameroonian men was negative - 0.29 (CI: - 0.52, - 0.05). In Cameroonian women, the slope between perceived femininity and colour was conclusively positive 0.52 (CI: 0.27, 0.76).

 

[…] More masculine men in the Cameroonian sample have therefore darker, less red, and less yellow skin colour.

[…] More feminine women thus have a lighter, yellower, and redder skin than less feminine women.

 

There was no such correlation among the Czechs. This second finding seems to contradict previous findings that facial skin color is used for gender recognition (see above). In those studies, however, the participants were Euro-American or Euro-Canadian, and they were not necessarily conscious of the visual cues they were using. At least on a conscious level, the sex difference in skin color has lost its social significance within the Western world, largely because of the growing importance of racial/ethnic differences in real life and in the virtual life of advertising and the mass media. In addition, the naturally lighter complexion of women has often been reduced or eliminated through deliberate tanning.

 

Ethnographic data

 

When we were preparing our joint paper on skin color preference, Pierre van den Berghe examined the Human Relations Area Files, a cross-cultural database. He found a strong association in traditional societies between femininity and lightness of skin color: the ideal woman was described as “white” in Europe, the Middle East, and East Asia, as “golden” in Southeast Asia, and as “red” in sub-Saharan Africa (van den Berghe and Frost 1986). We were somewhat surprised to see this idealization of female redness in different ethnographic accounts:

 

Tallensi (Ghana) ‑

“In skin colour they vary from black through chocolate brown to bronze, which the natives call “red” (bon‑ze'e) and regard as the most attractive bodily hue.” (Fortes 1945, p. 7)

 

Hausa (Nigeria) ‑

“Light skin colour, referred to as “red”, ranks high in the Hausa criteria of beauty; many variations of colour, from black to a very light reddish brown are seen.” (Smith 1965, p. 264)

 

Igbo (Nigeria) ‑

“In Ibo culture, however, these yellowish or reddish complexions are considered more beautiful than the darker, ‘blacker,’ complexions.” (Ardener 1954, pp. 71-72)

 

Somali (Somalia) ‑

“Men appreciate women of good height and stature, with good hips and breasts, and plump but not fat.  A reddish tinged skin is thought highly of in preference to a dark dull black.” (Lewis 1962, p. 13)

 

This ideal is explained at some length by Lugira (1970, pp. 34-35) with respect to the Ganda people of Uganda:

 

The Ganda concept of skin pigmentation considers light coloured complexions to be differing shades of white.  A dark brown skin colour is said to be — eruyeru, that is, somewhat white.  A really brown‑reddish‑yellow person is said to be mweru = white, which in comparison would be considered to be blonde; and this in the Ganda aesthetic language is considered as red = myufu, the most perfect skin pigmentation. (Lugira 1970, pp. 34‑35)

 

So the question remains open. Female skin may be redder in Africa because of selection by the natural environment, perhaps as a means to reduce heat load or facilitate wound healing. There is also evidence, however, for selection by the social environment.

 

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