Richard Nisbett and colleagues have produced an erudite and accessible review of recent research on human intelligence (pdf here). The 300 references in the review put paid to the claims of Anneli Rufus and Andrew Sullivan that political correctness has killed off the study of intelligence. Researchers have been studying intelligence with vigor, including hot topics such as social class and racial differences in IQ.

The whole review is worth a careful read, but a particularly intriguing finding is that adopted children have significantly higher IQs than their non-adopted biological siblings. Most adoptions involve a child moving from a lower to a higher socioeconomic family environment. The effect of the environmental change is dramatic: 12 IQ points on average.

Family environment research offers plausible explanations for why adopted children have so much higher IQs than their genetically-similar siblings. Relative to children raised in poor or working class homes, children raised in middle and upper-income families hear several times as many words by the age of three, receive substantially more parental encouragement, have greater access to books and magazines, and are more likely to have parents who read to them.

It has long been known that the average IQ of poor children is lower than that of their wealthier counterparts. The optimistic implication of the “adoption effect” is that these differences are driven by environment rather than genetics. Public policies that facilitate the emergence of IQ-enhancing family environments throughout the population could therefore benefit a range of lower-income kids, rather than letting fate restrict the class-driven IQ bump to those kids who happen to get adopted by better off families.

[Cross-posted at The Reality-based Community]

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Keith Humphreys is a Professor of Psychiatry at Stanford University and served as Senior Policy Advisor in the White House Office of National Drug Control Policy in the Obama Administration. @KeithNHumphreys