How Selection Happened

Our picture of how natural selection favored higher intelligence among European Jews in the Middle Ages relies upon three key observations. The first is that prosperous individuals had considerably more children, on average, than nonprosperous individuals in those days, as was then typical in most societies.46 Second, Ashkenazi jobs were cognitively demanding, since the members of this group 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 perform. Even low-level clerical jobs require an IQof something like 90.47 So, intelligence must have had greater rewards in those jobs than it does among farmers. This has to be true, really, since physical strength and endurance, which play a major part in success as a farmer, matter far less in finance and trade. If physical strength accounts for less of the variance, then cognitive and personality traits must account for more. Third, intelligence is significantly heritable. If the parents of the next generation are a little smarter than average, the next generation will be slightly smarter than the current one.

We can construct a scenario using IQ scores that illustrates this principle. We'll assume that parents of each generation averaged a single IQ point higher than the rest of the Ashkenazi adult population. In other words, let's suppose there was a modest tendency (mediated through economic success) for intelligent parents to have more surviving children than average parents— a tendency that certainly would not have been noticed at the time. If we assume a heritability of 30 percent for IQ, a very conservative assumption, then the average IQof the Ashkenazi population would have increased by about a third of a point (0.30 point) per generation. Over forty generations, roughly 1,000 years, Ashkenazi IQwould have increased by 12 points. If we assume that the Ashkenazim began with a typical European IQ_of 100 in the year AD 600, they would have reached an average IQof 112 by 1600, just about what we see in the Ashkenazim today. This picture is consistent with observations of high verbal and mathematical scores among Ashkenazi Jews, paired with average or lower-than-average visuospatial scores. Verbal and mathematical talent would have helped medieval businessmen succeed, whereas visuospatial abilities were irrelevant.

There may well have been some selection for IQ among Europeans in general over this period. Christian merchants in

London or Rotterdam may have experienced selective pressures similar to those of the Ashkenazi Jews, but there was an important difference between those merchants and the Jewish population: Christian merchant families intermarried. The mixing would have caused extensive gene flow with the general population, the majority of whom were farmers. It seems that if IQ increased in the general European population, there was a greater increase among the Ashkenazim.

Our hypothesis also explains why certain things didn't happen—in particular, why we don't see high IQscores and unusual intellectual achievement among other Jewish groups today. Although they, too, had very low rates of intermarriage, they never seem to have that high concentration of white-collar jobs that would have led to strong selection for verbal and mathematical intelligence. In part, this was because there were many more Jews in the Islamic world than in Christian Europe: With less persecution, there were more Jews than there were white-collar jobs. Our picture also explains why there's no real sign of unusually high intelligence among the Jews back in the days of the Roman Empire: The required events simply hadn't happened yet.

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