The original study is here. For those of you who, like me, can't get through more than a couple of pages of that (not sure what that says about my Ashkenazi IQ....), the Economist does a nice job of explaining the theory. To the best of my understanding, here's how it goes: because of Ashkenazi Jews' occupations and social circumstances in the Middle Ages, genes which enhanced intelligence were favored. Ie. there was natural selection of people with higher IQs. The problem is that the genes which favor intelligence are also -- according to this theory, at least -- connected to disease. Diseases like Gaucher's, Tay Sachs and Niemann-Pick, which Jews are particularly susceptible to,
all involve extra growth and branching of the protuberances that connect nerve cells together... That would yield better linkage between brain cells, and might thus lead to increased intelligence. Indeed in the case of Gaucher's disease, the only one of the three in which people routinely live to adulthood, there is evidence that those with full symptoms are more intelligent than the average. An Israeli clinic devoted to treating people with Gaucher's has vastly more engineers, scientists, accountants and lawyers on its book than would be expected by chance.'And although I didn't get very far with the original study, the bit I did get through added that 'several other Ashkenazi disorders, idiopathic torsion dystonia and non-classical adrenal hyperplasia are known to elevate IQ.' It also adds that 'there is no similar elevation of intelligence' among Sephardi Jews, who do not have the same susceptibility to these genetic diseases as Ashkenazi Jews.
As Steven Pinker, himself no intellectual slouch, and Orthomom note, "It would be hard to overstate how politically incorrect this paper is."
I'd add three things. First of all, the fact that this paper -- or the very idea of certain groups having higher or lower IQs, full stop -- may be politically incorrect is no reason not to discuss this; an adult society does not simply hide/censor ideas and truths simply because they are uncomfortable. What's important is what you do with these ideas/truths, if anything.
Second, let's take this all with a big grain of salt; this theory has a long way to go before it is proven. It was clear from The Economist summary of the theory, which was littered with 'may's, 'might's and 'suggests,' that a concrete link between these diseases and intelligence is at the moment nothing more than speculation.* Some more objections in the NYT article on the subject, and I'm sure that astute Bloghead readers with better scientific backgrounds than me will analyze it all thoroughly.
Third, it will be a sad day when the Jewish genius, which most of us are inclined to brag about once in a while, turns out to be the flip side of a curse.
One other point: the study gives a plausible explanation for why there are so many Jewish nobel prize winners etc. It's not that the average Jew is so much cleverer than anyone else -- the Ashkenazi average IQ is only a few points higher than the general average IQ. But with a standard deviation of 15, this means there is also a higher proportion of Ashkenazi Jews with an IQ over 140 than other groups. Now there's an argument even I can understand...
*I'd be interested to hear whether French Canadians, another group particularly susceptible to Tay Sachs, also have higher than average IQ (Torontonians, don't answer that).
(Hat tip: Tamara of Fourth Rabbi)