^'Cochrane & Harpending's [whose scientific paper informs Kesstrel's wiki link] reasoning is straightforward enough: If the gene mutations responsible for diseases in Ashkenazim didn’t confer some evolutionary selective advantage, they wouldn’t persist. Cochran and Harpending liken these defective genes to the genes in Africans that often deform hemoglobin. Carrying one copy of the gene, most research suggests, helps ward off malaria—surely an adaptive advantage. Two copies, however, cause sickle-cell anemia.
Cochran and Harpending reasoned the same must be true of the genes that cause illness among Ashkenazi Jews, particularly the four that cause mutations in the enzymes responsible for breaking down fats: Tay-Sachs, Niemann-Pick, Gaucher disease, and mucolipidosis type IV. Two copies cause devastating illness, but one, they speculate, mutely aids the carrier.
How? By enhancing intelligence. Without this extra edge, they hypothesize, the Ashkenazim would never have survived. The Jews “experienced unusual selective pressures that were likely to have favored increased intelligence,” they say. “Their jobs were cognitively demanding, since they were essentially restricted to entrepreneurial and managerial roles as financiers, estate managers, tax farmers, and merchants. These are jobs that people with an IQ below 100 essentially cannot do.”
“I have a stack of books, like four feet high, on all metabolic diseases,” Cochran tells me. “And the four sphingolipid diseases affecting Ashkenazi Jews”—the ones he and Harpending believe enhance intelligence—“are all in the same chapter. That’s like one in 100,000 odds. People could say it’s chance, I suppose—in the same way it’s chance that 27 percent of all of those guys go to Stockholm every year.”
There’s scant physical evidence for this assumption. But what the authors found was intriguing. Among the papers they unearthed were studies by Steven Walkley, a neuroscientist at Albert Einstein College of Medicine, that showed growth of additional dendrites in the tissues of humans and cats with Tay-Sachs and Niemann-Pick. They also cite a 1995 study in the Journal of Biological Chemistry that shows increased neural growth in the brains of rats with Gaucher disease. The authors decided to contact Ari Zimran, the head of the Gaucher Clinic at the Shaare Zedek Medical Center in Jerusalem. It turns out that 81 of his 255 working-age patients have jobs that require, by the author’s estimates, an IQ of at least 120. Twenty-three are engineers, and fourteen are scientists—a number that, if it were consistent with the Israeli workforce, should be just six.'^