The Natural History of Ashkenazi Intelligence
By Gregory Cochran, Jason Hardy, Henry Harpending, Department of Anthropology, University of Utah.
This paper elaborates the hypothesis that the unique demography and sociology of Ashkenazim in medieval Europe selected for intelligence. Ashkenazi literacy, economic specialization, and closure to inward gene flow led to a social environment in which there was high fitness payoff to intelligence, specifically verbal and mathematical intelligence but not spatial ability.
As with any regime of strong directional selection on a quantitative trait, genetic variants that were otherwise fitness reducing rose in frequency. In particular we propose that the well-known clusters of Ashkenazi genetic diseases, the sphingolipid cluster and the DNA repair cluster in particular, increase intelligence in heterozygotes. Other Ashkenazi disorders are known to increase intelligence. Although these disorders have been attributed to a bottleneck in Ashkenazi history and consequent genetic drift, there is no evidence of any bottleneck. Gene frequencies at a large number of autosomal loci show that if there was a bottleneck then subsequent gene flow from Europeans must ave been very large, obliterating the effects of any bottleneck. The clustering of the disorders in only a few pathways and the presence at elevated frequency of more than one deleterious allele at many of them could not have been produced by drift. Instead these are signatures of strong and recent natural selection. [...]
There are several key observations that motivate our hypothesis. The first is that the Ashkenazi Jews have the highest average IQ of any ethnic group, combined with an unusual cognitive profile, while no similar elevation of intelligence was observed among Jews in classical times nor is one seen in Sephardic and Oriental Jews today.
The second is that the Ashkenazim experienced very low inward gene flow, which created a favorable situation for natural selection.
The third is that they experienced unusual selective pressures that were likely to have favored increased intelligence. For the most part they had jobs in which increased IQ strongly favored economic success, in contrast with other populations, who were mostly peasant farmers. They lived in circumstances in which economic success led to increase reproductive success.
The fourth is the existence of the Ashkenazi sphingolipid, DNA repair, and other disease clusters, groups of biochemically related mutations that could not plausibly have reached their present high frequencies by chance, that are not common in adjacent populations, and that have physiological effects that could increase intelligence.
Other selective factors have been suggested. "Winnowing through persecution" suggests that only the smartest Jews survived persecution. Why this should be so is not clear. There was no similar outcome in other groups such as Gypsies who have faced frequent persecution (Crowe and Kolsti, 1991).
Another theory suggests that there was selective breeding for Talmudic scholarship. This seems unlikely to have been an important selective factor, since there weren't very many professional rabbis, certainly less than one ercent of the population. A selective force that only affects a tiny fraction of the population can never be strong enough to cause important evolutionary change in tens of generations. A plausible variant of the Talmudic scholarship model suggests that it was like a sexually selected marker and that rich families preferred to marry their daughters to males who excelled (Weyl and Possony, 1963; MacDonald, 1994) so that the payoff to telligence was indirect rather than direct as we suggest. Without detailed historical demographic information it will be difficult to evaluate this hypothesis. [...]
We describe two main clusters of Ashkenazi inherited disease, the sphingolipid cluster and the DNA repair cluster, reviewing evidence that these modulate early central nervous system development. A sample of Gaucher disease patients show a startling occupational spectrum of high IQ jobs, and several other Ashkenazi disorders, idiopathic torsion dystonia and non-classical adrenal hyperplasia, are known to elevate IQ.[...]
Ashkenazi Jews have the highest average IQ of any ethnic group for which there are reliable data. They score 0.75 to 1.0 standard deviations above the general European average, corresponding to an IQ 112-115. [...]
This fact has social significance because IQ (as measured by IQ tests) is the best predictor we have of succes in academic subjects and most jobs. Ashkenazi Jews are just as successful as their tested would predict, and they are hugely overrepresented in occupations and fields with the ighest cognitive demands. During the 20th century, they made up about 3% of the US opulation but won 27% of the US Nobel science prizes and 25% of the ACM Turin awards. Thev account for more than half of world chess champions. [...]
This High IQ and corresponding high academic ability have been long known.[...]
Ashkenazi Jews have an unusual ability profile as well as higher than average IQ. The have high verbal and mathematical scores, while their visuospatial abilities are typical somewhat lower, by about one half a standard deviation, than the European average (Levinson, 1977; Levinson and Block, 1977).[...]
The Ashkenazi pattern of success is what one would expect from this abil istribution-great success in mathematics and literature, more typical results resentational painting, sculpture, and architecture.
It is noteworthy that non-Ashkenazi Jews do not have high average IQ test scores (Ortar,1967), nor are they overrepresented in cognitively demanding fields. This is important indeveloping any causal explanation of Ashkenazi cognitive abilities: any such theory must explain high Ashkenazi IQ, the unusual structure of their cognitive abilities, and the lack of these traits among Sephardic and Oriental Jews(Burg and Belmont, 1990; Patai, 1977) [...]
IQ tests predict a host of characteristic iduals including educational attainment, job performance, income, health, an non-obvious characteristics like susceptibility to Alzheimer's disease. In general t search for social and nutritional causes of IQ differences has not led to any convincin results and most workers now regard IQ as a biological rather than a social variable. [...]
IQ test scores are highly heritable, almost always greater than 0.5 when adult scor studied. Lower heritability estimates are found for children's IQ: the IQ of children does seem to reflect in part environmental influences like the social class of the home in whic the child is reared, but these influences disappear as the child matures and are essentially gone in adulthood. In the same way enrichment programs like Head Start cause a transient elevation in IO scores of children but these effects disappear as the child matures.[...]
With its high heritability, IQ should respond rapidly to directional selection... Assumine parents of the next generation have an average IQ one point above the population mean, the average IQ increases by 0.8 points per generation. In 20 human generations about 500 years, it would increase by 16 points-slightly more than the difference between average Ashkenazi IQ scores and average European IQ scores. Change of this magnitude over historical time is not at all implausible.
Detailed demographic data about early medieval Ashkenazim are lacking, but we c infer plausible parameters from the scarce information that we do have. First, their jo were cognitively demanding since they were essentially restricted to entrepreneurial managerial roles as financiers, estate managers, tax farmers, and merchants. These ar s that people with an IQ below 100 essentially cannot do. Even low-level clerical require something like an IQ of 90 (Gottfredson, 2003).[...]
Since strong selection for IQ seems to be unusual in humans (few populations have had most members performing high-complexity jobs) and since near-total reproductive isolation is also unusual, the Ashkenazim may be the only extant human population with polymorphic frequencies of IQ-boosting disease mutations, although another place to look for a similar phenomenon is in India. In particular the Parsi are an endogamous group with high levels of economic achievement, a history of long distance trading, business, and management, and who suffer high prevalences of Parkinson disease, breast cancer, and tremor disorders, diseases not present in their neighbors (see "The UNESC Parsi Zoroastrian Proiect". httD://www.unescoparzor.com).[...]
Rapid selection does not always yield efficient solutions. Overdominant mutations that increase heterozvgote fitness and harm homozygotes are favored in the short run, but over longer periods, modifier genes decrease the associated genetic load (Hammerstein, 1996). Sickle cell, the canonical example of overdominance in humans, is a response to a recent selective influence, since falciparum malaria in its present form is probably only a few thousand years old. We usually think of over dominant mutations as defenses against infectious diseases, and indeed many are, but rapid selection for metric traits other than disease resistance can also result in polymorphic frequencies of overdominant alleles. [...]
Selection for IQ among the Ashkenazim then would have had associated costs. First, genetic changes that aided fitness in a urban environment where most jobs had high IQ elasticity almost certainly reduced fitness in more typical environments, simply because change away from the optimum mix for a traditional environment. The expectation is that Ashkenazim would most likely suffer competitive disadvantage as peasant farmers or hunter-gatherers, for example.
Mutations that increased heterozygote fitness in the unique environment experienced by the Ashkenazim (by increasing IQ, for example) while harming homozygotes could have become relatively common, just as sickle cell has. Our hypothesis is that many, perhaps most of the characteristic Ashkenazi genetic diseases fall into this category. Selection has imposed a heavy human cost - not crippling at the population level, cheaper than the malaria-defense mutations like sickle-cell and G6PD deficiencv. but tragic nonetheless.[...]
When we first see them in the historical record, the Ashkenazim were long-distanc merchants who traded with the Moslem world. This is the beginning of an occupat attern that is very different from those of other Europeans and from those of other Jewish groups, as well. The majority of Jews had already given up agriculture (Bottici and Eckstein, 2002), but the Jews of Islam, although urban, mostly worked in various crafts. The Ashkenazim, from their beginnings and for a long time, seldom had suc This pattern is detailed in Gross (1975, p. 147): "Two entirely different patterns i ractice of crafts and their place in Jewish life and society are discernible throughout t Middle Ages. One characterizes the communities in countries around the Mediterrane including in the south those in the continents of Asia and Africa, and in the north extending more or less to an imaginary demarcation line from the Pyrenees to the northern end of the Balkans. The other, in the Christian countries of Europe, was more o less north of the Pyrenees-Balkans line." and (p. 151) "North of the Pyrenees and Balkans crafts played a very small role as a Jewish occupation, from the incept Jewish settlement there."
The Ashkenazi population, established in northern France by the early 900s, prospered and expanded. They settled the Rhineland and England after the Norman Conquest. At first they were international merchants who acted as intermediaries with the Moslem world. As Moslems and Christians, especially Italians, increasingly found it possible to do business directly, Ashkenazi merchants moved more and more into local trade. When persecution began to be a serious problem and the security required for long-distance travel no longer existed, the Ashkenazim specialized more and more in one occupation, finance, left particularly open to them because of the Christian prohibition of usury. The majority of the Ashkenazim seem to have been moneylenders by 1100 AD (Ben-Sasson 1976; Arkin, 1975), and this continued for several centuries. Such occupations (sales, trade, finance) had high IQ demands, and we know of no other population that had suc large fraction of cognitively demanding jobs for an extended period.
In some cases, we have fairly detailed records of this activity. For example (Arkin, 1975 p.58), concerning the Jews of Roussilon circa 1270: "The evidence is overwhelming that this rather substantial group of Jews supported itself by money lending, to the virtual exclusion of all other economic activities. Of the 228 adult male Jews mentioned in the registers, almost 80 percent appear as lenders to their Christian neighbors. Nor were loans by Jewish women (mostly widows) uncommon, and the capital of minors was often invested in a similar manner. Moreover, the Jews most active as moneylenders appear to have been the most respected members of the community."
The Jews in this period were prosperous. Ben-Sasson points out (p. 401) that "Western Europe suffered virtual famine for many years in the tenth and eleventh centuries, there is no hint or echo of this in the Jewish sources of the region in this period. The city dweller lived at an aristocratic level, as befitted international merchants an honored local financiers." Their standard of living was that of the lower nobilitv (Roth, 2002)
Although prosperous, they were not safe. The first major crisis was the First Crusade, resulting in the death of something like a quarter of the Jews in the Rhineland. Religious hostility, probably exacerbated by commercial rivalries, increased, manifesting itself in the form of massacres and expulsions culminating in the expulsion of the Jews from most of Western Europe. They were expelled from England in 1290, from France in 1394, and from various regions of Germany in the 15th century. The expulsions had greater effect in the long run than massacres and persecutions. Jewish population growth rates were high due to prosperity and distaste for family limitation; so numbers tended to recover from attacks after a generation or two. But the potential for such population recovery decreased as Jews were excluded from more and more of Western Europe.
Many of the Jews moved east, first to Austria, Bohemia and Moravia, then to the Polish-Lithuanian Commonwealthe. The Polish rulers welcomed Jewish immigrants who could help modernize and reconstruct the country, which had been devastated by Mongol raids. Jews were welcome as urban investors and initiators of trade. [...]
As they had in Western Europe, the Jews of Poland had a very unusual occupational profile. The very first to immigrate were mainly moneylenders, but that soon change They became tax-farmers, toll-farmers, estate managers, and they ran mills and taverns. According to Weinryb (1972) in the middle of the fourteenth century, "About 15 percent of the Jewish population were earners of wages, salaries and fees. The rest were independent owners of business enterprises." They were the management class of the Polish-Lithuanian Commonwealth. Besides literacy, success in those specialized occupations depended upon skills similar to those of businessmen today, not least the abilitv to keep track of complex transactions and money flows.
Eventually, as the Ashkenazi population of the Polish-Lithuanian Commonwealth creased, more and more Jews became craftsmen - there are after only so many managerial and financial slots. Still, for 800 to 900 years, from roughly 800 AD to 1650 or 1700 AD, the great majority of the Ashkenazi Jews had managerial and financial jobs, jobs of high complexity, and were neither farmers nor craftsmen. In this they differed from all other settled peoples of which we have knowledge.
Jews who were particularly good at these jobs enjoyed increased reproductive success. Weinryb (1972, see also Hundert 1992) comments: "more children survived to adulthood in affluent families than in less affluent ones. A number of genealogies of business leaders, prominent rabbis, community leaders, and the like generally belonging to more affluent classes - show that such people often had four, six, sometimes even eight or nine children who reached adulthood. On the other hand, there are some indications that poorer families tended to be small ones. It should also be added that overcrowding, which favors epidemics was more prevalent among the poorer classes. In short, the number of children surviving among Polish Jews seems to have varied considerably from one social level to another." He goes on to suggest that wealthier Jews were less crowded as they lived in bigger houses, they could keep their houses warmer, they could afford wet-nursesthey had better access to rural refugia from epidemics. [...]
Societies reward different behavioral traits. In some times and places successful warriors and soldiers have had high status, in others merchants, in still others bureaucrats as in ancient China. There were societies in premodern Europe in which merchants and businessmen ranked near the top, but this was atypical. To the extent that status and wealth were inherited rather than earned, the correlation between cognitive traits and reproductive success in elite croups may have been quite weak.
In almost every case elite groups experienced substantial gene flow with other, much larger groups that were not subject to the same selective pressures. This means that the selective pressures experienced by such groups were diluted, spread out into the general population. Christian merchants in London or Rotterdam may have experienced selective pressures similar to those of the Ashkenazi Jews, but they intermarried: there was extensive gene flow with the general population, the majority of whom were farmers.
The selection pressures experienced by farmers were probably quite different: most likely cognitive skills did not have as high a correlation with income among farmers that they did among ndividuals whose occupations required extensive symbol manipulation, such as as moneylenders, tax farmers, and estate managers.
The Ashkenazi occupational pattern was different from that of the Jews living in the Islamic world. The Jews of Islam, although reproductively isolated, did not have the concentration of occupations with high IQ elasticity. Some had such job in some of the Arab world, in some periods, but it seems it was never the case that most did. In part this was because other minority groups competed successfully for these jobs - Greek Christians, Armenians, etc., in part because Moslems, at least some of the time, took many of those jobs themselves, valuing non-warrior occupations more highly than did medieval Christians. In fact, to a large extent, and especially during the last six or seven hundred years of relative Moslem decline, the Jews of Islam tended to have "dirty" jobs (Lewis, 1984). These included such tasks as cleaning cesspools and drying the contents for use as fuel - a common Jewish occupation in Morocco, Yemen, Iraq, Iran, and Central Asia. Jews were also found as tanners, butchers, hangmen, and other disagreeable or despised occupations. Such jobs must have had low IQ elasticity; brilliant tanners and hangmen almost certainly did not become rich.
The suggested selective process explains the pattern of mental abilities in Ashkenazi Jews - high verbal and mathematical ability but relatively low spatio-visual ability. Verbal and mathematical talent helped medieval businessmen succeed, while spatio-visual abilities were irrelevant.
