User:BenKovitz/Ashkenazi Jewish intelligence

Whether Ashkenazi Jews have higher intelligence than other ethnic groups has been an occasional subject of scientific controversy. Most recently, the 2005 paper "Natural History of Ashkenazi Intelligence" by Cochran, Hardy, and Harpending, argued on the basis of inherited diseases and the peculiar economic situation of Ashkenazi Jews in medieval Europe, that Ashkenazi Jews as a group now inherit higher verbal and mathematical intelligence than most other ethnic groups, along with lower spatial intelligence. Opposing this hypothesis are ambiguous statistical results regarding how much difference in intelligence really exists between Ashkenazi Jews and other groups, and causes that might explain the congenital illnesses and economic and intellectual successes of Ashkenazi Jews better than natural selection for cognitive traits. Such causes include the founder effect for the mutations that cause the congenital diseases, and elements of Jewish culture.

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


 * 1) Today's Ashkenazi Jews have a high average IQ and an unusual cognitive profile compared to other ethnic groups, including Sephardic and Oriental Jews.
 * 2) From roughly 800 to 1650 CE, 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 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.
 * 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, 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 anecdotally to correlate with exceptionally 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. The Cochran et al. hypothesis is not widely accepted as true, nor is there consensus that it's false. Present-day knowledge cannot definitively settle the question of whether Ashkenazi Jewish intelligence has a genetic origin.

Other evolutionary theories
Other suggested evolutionary accounts include: a long cultural history encouraging scholarship and learning; a contribution of talent in the study of Torah to social success in Jewish communities; the enforcement of a religious norm requiring Jewish fathers to educate their sons, whose high cost caused voluntary conversions, explaining a large part of a reduction in the size of the Jewish population; that historic persecution of European Jews fell disproportionately on people of lower intelligence.

Does a group difference in intelligence exist?
One basic question to be answered in assessing a genetic explanation of unusual intelligence in Ashkenazi Jews is whether today's Ashkenazi Jews really do, as a group, have unusual intelligence. Assessing intelligence, especially of ethnic groups, is notoriously difficult and subject to racist and political biases.

One observational basis for inferring that Ashkenazi Jews have high intelligence is their prevalance in intellectually demanding fields. From 1901–2010, 21.5% of Nobel prize winners were Jewish, while Jews make up a much smaller fraction of the population of the countries represented. For example, 36% of Nobel prize winners from the United States have been Jewish, while Jews make up 2.1% of the U.S. population. However, such statistics do not rule out factors other than intelligence, such as institutional biases and social networks.

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 Ashkenazi Jews, along with below-average spatial intelligence. Some studies have found IQ scores amongst Ashkenazi Jews to be a fifth to one full standard deviation above average. However, most studies of Jewish intelligence have used samples which were either poor representations of the whole population or were too small to give reliable results, and some studies have found Ashkenazi groups to have below-average intelligence.

Problems with the genetic explanation
Assuming that today there is a statistical difference in intelligence between Ashkenazi Jews and other ethnic groups, there still remains the question of whether the difference is caused by inheritance or environmental factors. The following are specific bases for doubt that genetic inheritance is the cause.

Problems found in studies of Ashkenazi genes
Most, though not all, of the Ashkenazi congenital diseases arose from genetic drift after a population bottleneck, and show no evidence of selective pressure of the kind called for in the Cochran, et al. paper. For 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 frequency of this mutation among Ashkenazi Jews today accords with genetic drift starting from a population bottleneck, not with selective pressure favoring its spread.

Though long thought to be a genetic isolate, recent studies have found that modern Ashkenazi Jews are more closely related to other European ethnic groups than to their Middle Eastern forebears. This throws doubt on premise #2 of Cochran et al.

Problems with reproductive advantage
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.

It's not clear that mathematical and verbal talent were the prime factors for success in the occupations that Jews were limited to. Social connections, social acumen, willingess to take risks, and access to capital (through inheritance, as above) likely played at least as great a role.

The Talmudic tradition
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 emphasis on scholarship came before the Jews turned from agriculture to urban occupations. This suggests that premise #3 of Cochran et al. might have the causal direction backward: mastery of written language due to the Talmudic tradition may have made the Jews well suited for financial and managerial occupations at the time when these occupations provided new opportunities. 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.