The Three-Toed Sloth on IQ and genetics

Cosma Shalizi explains it all for you.

If, like me, you’re heartily tired both of

* People who dogmatically deny either that some sort of generalized cognitive ability is measurable, and that IQ testing is a decent though imperfect proxy for that measurement, or that different human population groups with different genetic heritages might have different distributions of cognitive ability


* People who use the fact of intergroup differences in average cognitive capacity and the possibility that they are partly genetically mediated to justify indifference to the facts about how much worse off, on average, the descendants of slaves in this country are than the rest of us

and if you’re curious about the actual science involved, then you want to read this post from the always-interesting Three-Toed Sloth, aka Cosma Shalizi of CMU.

It turns out that what I had always regarded as an obvious point has actually only been in the literature since 1997: twin studies, even homozygous twin studies, don’t in fact eliminate common environmental factors, because even children adopted at birth (which most aren’t) still share nine months of prenatal environment. On the other hand, I can no longer find the paper encompassing what I thought was the smoking-gun finding that population genetics matters: IQ exhibits hybrid vigor, so that given parental IQs the child’s predicted IQ is noticeably deflected upward if the parents share few ancestors. (That is, the child of an IQ 120 Nigerian and an IQ 120 Norwegian will tend to have a higher IQ than the child if two IQ 120 Nigerians or two IQ 120 Norwegians.) What I especially like about that finding is how implausible it makes the blanket charge of “racism” directed at students of IQ; could there be anything more anti-racist than the claim that race-mixin’ is eugenic?

Footnote I spotted one anachronism: referring to Glenn Loury as a “conservative thinker” is soooooooooooooooo 1990. His liberal friends, who used to try to talk him out of palling around with Bill Bennett, are now trying to remind him that GL the Conservative (whose IQ was at least 2 sigma higher than, for example, Charles Murray’s) actually had some useful points to make.

Second footnote IQ denialism, though it’s not identical with evolutionary-psychology denialism (what one Red Blogger calls “neck-down Darwinism”) has some of the same roots; they’re both natural, though unwise, reactions to the attempt to pretend that racism has a scientific basis.

h/t Andrew Sullivan

Author: Mark Kleiman

Professor of Public Policy at the NYU Marron Institute for Urban Management and editor of the Journal of Drug Policy Analysis. Teaches about the methods of policy analysis about drug abuse control and crime control policy, working out the implications of two principles: that swift and certain sanctions don't have to be severe to be effective, and that well-designed threats usually don't have to be carried out. Books: Drugs and Drug Policy: What Everyone Needs to Know (with Jonathan Caulkins and Angela Hawken) When Brute Force Fails: How to Have Less Crime and Less Punishment (Princeton, 2009; named one of the "books of the year" by The Economist Against Excess: Drug Policy for Results (Basic, 1993) Marijuana: Costs of Abuse, Costs of Control (Greenwood, 1989) UCLA Homepage Curriculum Vitae Contact: