We develop the hypothesis that the unique demography and sociology of Ashkenazi in medieval Europe selected strongly for intelligence. The usury laws provided a unique niche, and Ashkenazi literacy 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, although there is direct evidence only for Gaucher disease. Other Ashkenazi disorders like torsion dystonia and the disorders of steroid synthesis 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 in the genes of any bottleneck. Further, 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.

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