As a result, the researchers say, over hundreds of years European Jews became more intelligent than their gentile countrymen.
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But increased intelligence may have come at a cost, with genetic diseases such as Tay-Sachs being side effects of genes that facilitate intelligence. The researchers say that it's highly unlikely that mutated genes responsible for these illnesses could have reached such high levels in Ashkenazim if they did not influence cognitive performance.
While the link is difficult to prove, there is some evidence that Gaucher disease does increase IQ. Around one in three people of working age who were patients of the Gaucher Clinic at the Shaare Zedek Medical Centre in Jerusalem had professions requiring an average IQ of more than 120 (the standard figure is 100). They included scientists, academics, physicians, and accountants.
So if natural selection is responsible for above-average intelligence in Ashkenazi Jews, could the same apply to other ethnic groups?
Co-author Gregory Cochran said, "The logical place to look for something similaran occupationally specialized, reproductively isolated group that might have been shaped by recent natural selectionwould be India. In particular, the Parsis."
Parsis are descendents of the Zoroastrians who emigrated from Persia to India in the 8th century. As in medieval Jewish communities in Europe, Parsis marry almost wholly within their own group. They are also economically successful with a long history as traders and businessmen.
Parsis are also especially vulnerable to diseases not seen in their neighbors, including Parkinson's disease, breast cancer, and tremor disorders.
Modern-day Ashkenazim are now far more likely to marry outside their ethnic group. Cochran says he would expect that both a tendency for higher IQ and associated genetic disorders to become less marked over time.
The idea that some ethnic groups are inherently more intelligent than others is controversial, and researchers are divided on the issue.
The Human Genome Project, completed in 2000, suggested that humans are 99.9 percent genetically identical across all races. At the time, the project's findings were hailed as evidence that the human genome is color-blind.
But the journal Psychology, Public Policy and Law recently published a study that reviewed research into the differences in average IQ between Asians, whites, and blacks.
J. Philippe Rushton, from the University of Western Ontario, and Arthur Jensen, an educational psychologist at the University of California, Berkeley, concluded these differences are 50 percent genetic in origin.
"Race differences show up by three years of age, even after matching on maternal education and other variables," Rushton said. "Therefore they cannot be due to poor education since this has not yet begun to exert an effect."
Writing in response to their findings, University of Michigan psychologist Richard E. Nesbitt said that "there is not a shred of evidence" in the research to suggest the IQ gap between races has a genetic basis.
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