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 have 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.
By the end of the 19th century, as a result of Russian persecution, there was massive Ashkenazi emigration from Eastern Europe to other areas of Europe, Australia, South Africa, the United States and Israel. Ashkenazim outnumbered Sephardim everywhere except North Africa, Italy, the Middle East and parts of Asia. Before World War II, Ashkenazim comprised 90% of world Jewry.
The destruction of European Jewry in World War II reduced the number of Ashkenazim and, to some extent, their numeric superiority over Sephardim. The United States became the main center for Ashkenazi Jews.
"Height variations within a population are largely genetic, but height variations between populations are mostly environmental, anthropometric history suggests."
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