1. Evidence for a Group Difference in Intelligence

The most direct evidence of a difference in intelligence among Ashkenazi Jews comes from psychometric tests. Different studies have found different results, but most have found well above-average verbal and mathematical intelligence in Ashkenazi Jews, along with slightly below-average spatial intelligence,^[1][2] producing an average IQ score in the range of roughly 107 to 115, well above the general mean of 100.^[3][4][5][6] Cochran et al. calculated an average IQ of 112–115,^[7] and Murray & Entine found 107–115.^[6][8][9] A 1954 study found that 24 of the 28 (86%) children in the New York public school system who had an IQ of 170 or higher were Jewish.^[6] One study found that Ashkenazi Jews had only near-average visual-spatial intelligence, about IQ 98, while a 1958 study of yeshiva students demonstrated that they had an extraordinary high verbal intelligence (which includes verbal reasoning, comprehension, working memory, and mathematical computation) as their median verbal IQ was found to be nearly 126.^[5]

Another kind of evidence is that Ashkenazi Jews have had success disproportionate to their small population size in a variety of intellectually demanding fields, such as science, technology, politics, law, and commerce.^[10][11] Only about 2% of the U.S. population is of full Ashkenazi Jewish descent,^[7] but 27% of United States Nobel prize winners in the 20th century,^[3][7] 25% of the winners of the Fields Medal (the top prize in mathematics),^[8] 25% of ACM Turing Award winners,^[7] a quarter of Regeneron Science Talent Search winners,^[8] and 38% of the Academy Award-winning film directors^[5] have either full or partial Ashkenazi Jewish ancestry. Jews comprise up to one third of the student populace at Ivy League universities,^[10] and 30% of the U.S. Supreme Court's law clerks.^[11] In Great Britain, Jews are overrepresented among Nobel prize winners by a factor of 8.0.^[3] In Hungary in the 1930s, Ashkenazi Jews comprised 6% of the country's population, but 55.7% of physicians, 49.2% of attorneys, 30.4% of engineers, and 59.4% of bank officers.^[5]

2. Proposed Genetic Explanations

Assuming that there is a statistical difference in intelligence between Ashkenazi Jews and other ethnic groups, there still remains the question of how much of the difference is caused by genetic factors.^[12]

2.1. "Natural History of Ashkenazi Intelligence"

"Natural History of Ashkenazi Intelligence",^[7] a 2005 paper by Gregory Cochran, Jason Hardy, and Henry Harpending, put forth the conjecture that the unique conditions under which Ashkenazi Jews lived in medieval Europe selected for high verbal and mathematical intelligence but not spatial intelligence. Their paper has four main premises:

1. Today's Ashkenazi Jews have a higher average mathematical and verbal IQ and an unusual cognitive profile compared to other ethnic groups.
2. From roughly 800 to 1650 AD, Ashkenazi Jews in Europe were a mostly isolated genetic group. When Ashkenazi Jews married non-Jews, they usually left the Jewish community; few non-Jews married into the Jewish community.
3. During the same period, laws barred Ashkenazi Jews from 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. Given the high heritability of IQ, 800 years is more than sufficient time for the selective pressure on verbal and mathematical intelligence to produce a 16-point increase in IQ.
4. Today's Ashkenazi Jews suffer from a number of congenital diseases and mutations at higher rates than most other ethnic groups; these include Tay–Sachs disease, Gaucher's disease,^[13] Bloom's syndrome, and Fanconi anemia, and mutations at the BRCA1 and BRCA2 genes. 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.^[14]

2.2. 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.^[15] Persecution of European Jews may have fallen disproportionately on people of lower intelligence.^[14]

2.3. Criticism of the Genetic Explanations

In medieval Ashkenazi 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 is not clear that mathematical and verbal talents 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.^[12]

On the other hand, controversial^[16] research by Gregory Clark concluded that social mobility has been consistently low but non-negligible throughout history, and that social mobility was no lower in previous centuries than it has been in recent times.^[17]

In the history of Jewish culture, 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.^[12] Similar cultural traditions continue to the present day, possibly providing a non-genetic explanation for contemporary Ashkenazi Jews' high IQs and prevalence in intellectual fields.^[12]

Some genetic studies have suggested that most Ashkenazi 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.^[12][18] In one example, the mutation responsible for Tay–Sachs disease arose in the 8th or 9th century, when the Ashkenazi Jewish population in Europe was small, just before they spread throughout Europe. The high frequency of this disease among Ashkenazi Jews today might simply be the result of their not marrying outside their group, not because the gene for Tay–Sachs disease confers an advantage that more than makes up for the fact that the disease usually kills by age three.^[12] However, an examination of the frequencies and locations of the genes for 21 Ashkenazi Jewish congenital diseases suggested that six of them do appear to result from selective pressure, including the mutation for Tay–Sachs disease.^[18] 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.^[19]

Evolutionary psychologist Steven Pinker suggested that "the most obvious test of a genetic cause of the Ashkenazi advantage would be a cross-adoption study that measured the adult IQ of children with Ashkenazi biological parents and gentile adoptive parents, and vice versa," but noted, "No such study exists, so [Cochran]'s evidence is circumstantial."^[20]

The methods and findings of Cochran et al and some of the other studies have been disputed by scientists such as Harry Ostrer, who said: “It’s bad science – not because it’s provocative, but because it’s bad genetics and bad epidemiology.”. Critics point to studies from the beginning of the 20th century that concluded that Jews have lower-than-average IQ, and to the fact that other ethnic groups in the U.S. - such as Chinese - also display higher-than-average scores on IQ tests.^[16]

3. Proposed Cultural and Historical Explanations

One type of explanation for higher intelligence in Ashkenazi 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.^[21] Unlike the surrounding cultures, most Jews, even farmers,^[7] 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.^[12]

Other proposed cultural explanations:

• Talmudic scholarship was so respected in European Ashkenazi Jewish ghetto society that outstanding (though often financially poor) scholars were highly sought after as husbands for the daughters of even the wealthiest merchants, who could afford to support the married couples. A father who made it possible for the groom to devote himself to Talmud study was performing a mitzvah. This attitude provided selection pressure in favor of intellectual aptitude, and enhanced social mobility.^[14][22]
• Ashkenazi Jews (as well as other ethnic Jews) were marginalized by discrimination, and therefore had to put more effort to survive and be outstanding.^[23]
• The rise of Islamic civilization created demand for educated professionals with intellectual skills. According to Eckstein and Botticini, between 750 and 900 AD, nearly all the Jews in Mesopotamia and Persia left farming and moved to the big cities of the Abbasid Caliphate, where they specialized in jobs more lucrative than farming. Jews had a clear advantage in these professions as a result of centuries of literacy.