I’ve been reading about the AEI seminar about IQ in Ashkenazic (of European ancestry) Jews with some interest (ScienceBlogling Razib raises a really good point that I hadn’t thought of too). A few years ago, I was asked to review a paper that dealt with this issue.
Let me state that I think IQ as a measure of intelligence doesn’t mean all that much, except when it is extremely low or extremely high. Nonetheless, it is a trait that we can measure–we should just be very careful about how much importance we place on IQ.
OK, back to the paper I was asked to review. It basically made the same argument that Entine makes. Ashkenazic Jews, because they had very low introgression from non-Ashkenazic populations, had a population structure where adaptive mutants for higher IQ could arise and be maintained given the appropriate selection pressure. The result is that Ashkenazic Jews have relatively higher frequencies (0.5-2.0%) of alleles (genetic variants) of genes that result in unusual neurological development (e.g., Tay-Sachs, Gaucher’s syndrome, other sphingophospholipids).
These alleles haven’t swept to fixation (i.e., occur at really high frequencies) because when they occur in the homozygous state (i.e., you have two copies of the allele) the effects are very bad–in Tay-Sachs, the child dies at a very young age. If heterozygous parents are too common (those who possess one allele), they will marry each other relatively frequently, and one-quarter of their offspring will die (this would be an example of negative-frequency dependent selection).
A corollary of this is that, if there were selection for alleles that confer higher IQ but confer fitness costs when homozygous, we would expect to see lots of loci with variant alleles at low frequencies, instead of one locus with a variant allele (with many different loci, the odds of a heterozygote marrying someone who is a heterozygote at the same locus are quite small). We do observe quite few loci with rare alleles with deleterious effects in Ashkenazic Jews.
What I’ve found frustrating in all of this is that these are testable hypotheses: we can screen for Tay-Sachs heterozygotes and give them IQ tests and compare them to homozygotes (obviously, you would have to control for demographic factors). Ideally, we would like to do twin studies, but I imagine, at this point, the few Ashkenazic twins out there are sick and tired of being enrolled in twin studies.
So, is anyone going to do these tests?
An aside: As an Ashkenazic Jew, I would love to be your genetically superior overlord (kidding), but as Dana Milbank put it:
Left unchallenged was the question of whether Jews are indeed smarter than others — even though it would have only required a walk down the hall to the office of new AEI visiting fellow Paul Wolfowitz, whose leadership on the Iraq war and conflicts of interest as head of the World Bank demonstrate that Jews are capable of questionable judgment.