Ashkenazi Jewish intelligence
A 2005 scientific paper, "Natural History of Jewish Intelligence", proposed that Jews as a group inherit higher verbal and mathematical intelligence with somewhat lower spikes in spatial intelligence than other ethnic groups, on the basis of inherited diseases and the peculiar economic situation of Jews in the Middle Ages. Opposing this hypothesis are explanations for the congenital illnesses in terms of the founder effect, explanations of intellectual successes by reference to Jewish culture's promotion of scholarship and learning, and doubts about whether a group difference in intelligence really exists.
Evidence for a group difference in intelligence
One observational basis for inferring that Jews have high intelligence is their prevalence in intellectually demanding fields. While only about 2% of the U.S. population is of full Jewish descent, 27% of United States Nobel prize winners in the 20th century, 25% of Fields Medal winners, 25% of ACM Turing Award winners, 9 out of the 19 world chess champions, and a quarter of Westinghouse Science Talent Search winners have either full or partial Jewish ancestry. However, such statistics do not rule out factors other than intelligence, such as institutional biases and social networks. Undue weight is also given to the statistics because people of partial ancestry (half or less) are included, but only compared to the portion of the US population of full descent.
A more direct approach is to measure intelligence with psychometric tests. Different studies have found different results, but most have found above-average verbal and mathematical intelligence in Jews, along with below-average spatial intelligence.
The average IQ score of Jews has been calculated to be 112–115 (Cochran et al.), and 107–115 (Murray; Entine). A study found that Jews had only mediocre visual-spatial intelligence, while their verbal IQ (which includes verbal reasoning, comprehension, working memory, and mathematical computation) compensated for this with a high median of 125.6.
Assuming that today there is a statistical difference in intelligence between Jews and other ethnic groups, there still remains the question of how much of the difference is caused by genetic factors.
"Natural History of Intelligence"
"Natural History of Intelligence", a 2005 paper by Gregory Cochran, Jason Hardy, and Henry Harpending, put forth the conjecture that the unique conditions under which Jews lived in medieval Europe selected for high verbal and mathematical intelligence but not spatial intelligence. Their paper has four main premises:
- Today's Jews have a higher average mathematical and verbal IQ and an unusual cognitive profile compared to other ethnic groups, including Sephardi and Oriental Jews.
- From roughly 800 to 1650 CE, Jews in Europe were a mostly isolated genetic group. When Jews married non-Jews, they usually left the Jewish community; few non-Jews married into the Jewish community.
- During the same period, laws barred Jews from working most jobs, including farming and crafts, and forced them into finance, management, and international trade. Wealthy Jews had several more children per family than poor Jews. So, genes for cognitive traits such as verbal and mathematical talent, which make a person successful in the few fields where Jews could work, were favored; genes for irrelevant traits, such as spatio-visual abilities, were supported by less selective pressure than in the general population.
- Today's Jews suffer from a number of congenital diseases and mutations at higher rates than most other ethnic groups; these include Tay-Sachs, Gaucher's disease, Bloom's syndrome, and Fanconi anemia, and mutations at BRCA1 and BRCA2. These mutations' effects cluster in only a few metabolic pathways, suggesting that they arise from selective pressure rather than genetic drift. One cluster of these diseases affects sphingolipid storage, a secondary effect of which is increased growth of axons and dendrites. At least one of the diseases in this cluster, torsion dystonia, has been found to correlate with high IQ. Another cluster disrupts DNA repair, an extremely dangerous sort of mutation which is lethal in homozygotes. The authors speculate that these mutations give a cognitive benefit to heterozygotes by reducing inhibitions to neural growth, a benefit that would not outweigh its high costs except in an environment where it was strongly rewarded.
Other scientists gave the paper a mixed reception, ranging from outright dismissal to acknowledgement that the hypothesis might be true and merits further research.
In a television interview, Cochran said:
"It doesn't have to be extremely heritable for this [intelligence inheritance] to have happened, because you only need small changes in each generation, and there might be forty generations over 1000 years. So if [ Jews] increased a third of an IQ point per generation, that would almost certainly be enough to make this effect happen."
Other proposed genetic explanations
The enforcement of a religious norm requiring Jewish fathers to educate their sons, whose high cost caused voluntary conversions, might explain a large part of a reduction in the size of the Jewish population. Persecution of European Jews maybe have fallen disproportionately on people of lower intelligence.
Criticism of the genetic explanations
In medieval society, wealth, social status, and occupation were largely inherited. The wealthy had more children than the poor, but it was difficult for people born into a poor social class to advance or enter a new occupation. Leading families held their positions for centuries. Without upward social mobility, genes for greater talent at calculation or languages would likely have had little effect on reproductive success. So, it's not clear that mathematical and verbal talent were the prime factors for success in the occupations to which Jews were limited at the time. Social connections, social acumen, willingness to take risks, and access to capital through both skill and nepotism could have played at least as great a role.
Genetic studies have suggested that most Jewish congenital diseases arose from genetic drift after a population bottleneck, a phenomenon known as the founder effect, rather than from selective pressure favoring those genes as called for by the Cochran, et al. hypothesis. To take one example, the mutation responsible for Tay-Sachs disease arose in the 8th or 9th century, when the Jewish population in Europe was small, just before they spread throughout Europe. The high frequency of this disease among Jews today might simply be the result of their not marrying outside their group, not because the gene for Tay-Sachs confers an advantage that more than makes up for the fact that the disease usually kills by age three. However, an examination of the frequencies and locations of the genes for 21 Jewish congenital diseases suggested that six of them do appear to result from selective pressure, including the mutation for Tay-Sachs. There is still no evidence one way or the other about whether the reason for this is increased intelligence for commercial skills or something else.
Evolutionary psychologist Steven Pinker suggested that "[t]he most obvious test of a genetic cause of the advantage would be a cross-adoption study that measured the adult IQ of children with biological parents and gentile adoptive parents, and vice versa", but noted, "No such study exists, so [Cochran]'s evidence is circumstantial."
Another type of explanation for higher intelligence in Jews is differences in culture which tend to promote cultivation of intellectual talents.
For example, after the destruction of the Second Temple in 70 CE, Jewish culture replaced its emphasis on ritual with an emphasis on study and scholarship. Unlike the surrounding cultures, most Jews, even farmers, were taught to read and write in childhood. Talmudic scholarship became a leading key to social status. The Talmudic tradition may have made the Jews well suited for financial and managerial occupations at a time when these occupations provided new opportunities.
The emphasis on scholarship came before the Jews turned from agriculture to urban occupations. This suggests that premise #3 of Cochran et al. may have the causal direction backward: mastery of written language enabled Jews to thrive in finance and international trade rather than the other way around. Similar cultural traditions continue to the present day, possibly providing a non-genetic explanation for contemporary Jews' high IQs and prevalence in intellectual fields. Preoccupation with Torah and Talmud study keeps alive a certain intellectual acumen, attuned to weighing situations and opinions.
Other proposed cultural explanations:
- European  Jewish women would only have children with Jewish males who had high status or wealth.
- The fact that Jews have been a minority group in most of the places they lived, their community always helped other Jewish fellows, and with collaboration managed to preserve their position and their holdings.
- Jews (as well as other ethnic Jews) were marginalized by pogroms and discrimination, and therefore had to put more effort to survive and be outstanding.
- The encounter with Islam forced the Jews to strengthen the literacy revolution that had taken root centuries earlier and made it more challenging for Mizrahi and Sephardic Jews to integrate in society.
- Genetic studies on Jews and Medical genetics of Jewish people
- History of the race and intelligence controversy
- Race and intelligence
- List of Jewish Nobel laureates
- Scientific racism
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In private conversations with dozens of geneticists, almost no one was willing to categorically rule out the theory that Jews may have received a genetic gift of intelligence as recompense for the extraordinary number of brain diseases. "Yes, selection for intelligence is credible," Hebrew University's Joel Zlotogora, one of the few scientists willing to be quoted, told me. "For me everything is credible. I think the founder effect is true for only some disorders, but not for all of them, and there must be something else. There could very well be positive selection." At least for now most scientists are reluctant to embrace positive selection as an explanation of Jewish intelligence, although it remains a respected theory. David Goldstein, a geneticist at Duke University and author of the book Jacob's Legacy: A Genetic View of Jewish History, called the theory "tantalizing, circumstantial, politically incorrect in the extreme... [but] cannot be ruled out."
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