1.Today’s Ashkenazi Jews have a high average mathematical and verbal IQ and an unusual cognitive profile compared to other ethnic groups, including Sephardi and Mizrahi 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.

http://en.wikipedia.org/wiki/Ashkenazi_Jewish_intelligence