The work, to be published in the November issue of the journal Psychological Science, is part of a new wave of research that embraces a more dynamic view of the relationship between genes and environment. Although older research treated nature and nurture as largely independent and additive factors, and saw people as the sum of their genetic endowments and environmental experiences, the emerging view allows that genes can influence the impact of experiences and experiences can influence the "expression," or activity levels, of genes.
Until recently, Turkheimer and others said, research had indicated that the "heritability" of IQ -- that is, the degree to which genes can explain the differences in IQ scores -- completely dominated environmental influences. That led some to call into question the value of programs such as Head Start, which are based on the assumption that by improving the childhood environment through extra attention, nutrition and care, a child's intellectual future could be improved.
But it turned out that virtually all those studies on the heritability of IQ had been done on middle-class and wealthy families. Only when Turkheimer tested that assumption in a population of poor and mostly black children did it become clear that, in fact, the influence of genes on IQ was significantly lower in conditions of poverty, where environmental deficits overwhelm genetic potential.
Specifically, the heritability of IQ at the low end of the wealth spectrum was just 0.10 on a scale of zero to one, while it was 0.72 for families of high socioeconomic status. Conversely, the importance of environmental influences on IQ was four times stronger in the poorest families than in the higher status families.
Turkheimer got around that shortage by tapping into data from the now defunct National Collaborative Perinatal Project, which started in the late 1960s. That study, funded by the National Institutes of Health, enrolled nearly 50,000 pregnant women, most of them black and quite poor, in several major U.S. cities. Researchers collected loads of data on the families and gave the children IQ tests seven years later.
Although the study was not designed to study twins, it was so big that many twins were born -- 623 pairs, to be exact, 320 of whom were successfully located by the original researchers and tested for IQ at age 7 in the 1970s. By culling through those test scores and the data on the families' socioeconomic status, Turkheimer was able to conduct one of the first analyses of the role of genes in IQ among the poor.
Marcus Feldman, a population geneticist at Stanford University who has studied gene-environment interactions, said the next big challenge is to find out what it is about socioeconomic status -- a measure that includes not only income but also parental education and occupational status -- that contributes to IQ, so social programs can more effectively boost those factors.
"SES is a surrogate for something that deserves further study," Feldman said. "A paper like this reemphasizes the importance of psychology and educational psychology and draws us somewhat away from genetics and back into the importance of the social sciences for understanding IQ. This says to me, let's spend the money and find out what it is about SES that makes the difference.
Still not a large enough experiment to keep me happy by any means, but I'm still ecstatic that people are studying how genetics and society intermix.