Tl;dr: The theory is that Ashkenazi Jews are more intelligent than other groups because they were forced into cognitively demanding occupations due to persecution, and the ones who were intellectually up to the challenge had more children, passing on their genes for intelligence.
This theory also explains why Ashkenazi Jews are more prone to genetic diseases – the genes for intelligence also cause serious genetic disease in homozygotes.
“This doesn’t seem to be due to any advantage in material privilege; Ashkenazi Jews frequently did well even in countries where they were persecuted. Nor is it obviously linked to Jewish culture; Jews from other regions of the world show no such advantage. So what’s going on?
Doctors have long noted that Ashkenazi Jews are uniquely susceptible to various genetic diseases. For example, they’re about a hundred times more likely to have Gaucher’s Disease, a hundred times more likely to get Tay-Sachs Disease, ten times more likely to have torsion dystonia, et cetera. Genetic diseases are so common in this population that the are official recommendation is that all Ashkenazi Jewish couples get screened for genetic disease before marriage. I’m Ashkenazi Jewish, I got screened, and I turn out to be a carrier for Riley-Day syndrome – three hundred times as common in Ashkenazi Jews as in anyone else.
Evolution usually gets rid of genetic diseases pretty quickly. If they stick around, it’s because they’re doing something to earn their keep. One common pattern is “heterozygote advantage” – two copies of the gene cause a disease, but one copy does something good. For example, people with two copies of the sickle cell gene get sickle cell anaemia, but people with one copy get some protection against malaria. In Africa, where malaria is relatively common, the tradeoff is worth it – so people of African descent have high rates of the sickle cell gene and correspondingly high rates of sickle cell anaemia. In other places, where malaria is relatively uncommon, the tradeoff isn’t worth it and evolution eliminates the sickle cell gene. That’s why sickle cell is about a hundred times more common in US blacks than US whites.
The moral of the story is: populations can have genetic diseases if they also provide a useful advantage to carriers. And if those genetic diseases are limited to a single group, we expect them to provide a useful advantage for that group, but not others. Might the Jewish genetic diseases provide some advantage? And why would that advantage be limited to Jews?
Most of the Jewish genetic diseases cluster into two biological systems – the sphingolipid system and the DNA repair system. This is suspicious. It suggests that they’re not just random. They’re doing something specific. Both of these systems are related to neural growth and neural branching. Might they be doing something to the brain?
Gaucher’s disease, one of the Ashkenazi genetic diseases, appears to increase IQ. CHH obtained a list of all of the Gaucher’s patients in Israel. They were about 15 times more likely than the Israeli average to be in high-IQ occupations like scientist or engineer; CHH calculate the probability that this is a coincidence to be 4×10^-19.
Torsion dystonia, another Ashkenazi genetic disease, shows a similar pattern. CHH find ten reports in the literature where doctors comment on unusual levels of intelligence in their torsion dystonia patients. Eldridge, Harlan, Cooper, and Riklan91848-9/abstract) tested 14 torsion dystonia patients and found an average IQ of 121; another similar study found an average of 117. Torsion dystonia is pretty horrendous, but sufferers will at least get the consolation prize of being really, really smart.
Moving from medicine to history, we find that Ashkenazi Jews were persecuted for the better part of a millennium, and the particular form of this persecution was locking them out of various jobs until the main career opportunities open to them were things like banker, merchant, and doctor. CHH write:
For 800 to 900 years, from roughly 800 AD to 1650 or 1700 AD, the great majority of the Ashkenazi Jews had managerial and financial jobs, jobs of high complexity, and were neither farmers nor craftsmen. In this they differed from all other settled peoples of which we have knowledge.
Jews who were particularly good at these jobs enjoyed increased reproductive success. Weinryb (1972, see also Hundert 1992) comments: “More children survived to adulthood in affluent families than in less affluent ones. A number of genealogies of business leaders, prominent rabbis, community leaders, and the like – generally belonging to the more affluent classes – show that such people often had four, six, sometimes even eight or nine children who reached adulthood. On the other hands, there are some indications that poorer families tended to be small ones…as an example, in a census of the town of Brody in 1764 homeowner households had 1.2 children per adult member while tenant households had 0.6.
Now we can start to sketch out the theory in full. Due to persecution, Jews were pushed into cognitively-demanding occupations like banker or merchant and forced to sink or swim. The ones who swam – people who were intellectually up to the challenge – had more kids than the ones who sank, producing an evolutionary pressure in favor of intelligence greater than that in any other ethnic group. Just as Africans experiencing evolutionary pressure for malaria resistance developed the sickle cell gene, so Ashkenazim experiencing evolutionary pressure for intelligence developed a bunch of genes which increased heterozygotes’ IQ but caused serious genetic disease in homozygotes. As a result, Ashkenazi ended up somewhat more intelligent – and somewhat more prone to genetic disease – than the rest of the European population.
If true, this would explain the 27% of Nobel Prizes and 50% of world chess champions thing.”
submitted by /u/FedeRivade