One of the more famous studies on the heritability of IQ is Eric Turkheimer and colleagues’ 2003 paper called Socioeconomic status modifies heritability of IQ in young children. According to Google Scholar, it has been cited more than 700 times. Based on a sample of 7-year-old twins, the study found that in impoverished families the shared environment accounted for about 60 percent of IQ variance while heritability was close to zero. In contrast, heritability was high and the effect of the shared environment nugatory in affluent families.
The literature on the interaction between socioeconomic status and IQ heritability is very mixed. Several studies besides Turkheimer’s find such interaction (although in no other study is it as extreme as in Turkheimer et al. 2003), but others, including some with the very best study designs, find none. I am not going to try to adjudicate between these contradictory findings at this time. Rather, I will show some interesting, hitherto unpublished (well, careful readers of Boetel and Fuerst’s The Nature of Race have seen them already) results pertaining to Turkheimer’s study and the question of race differences.
A popular belief is that Turkheimer’s findings have particular relevance for the dispute about the causes of the black-white IQ gap. For example, Malcolm Gladwell once wrote that genetic theories of the IQ gap are unsupported because, based on Turkheimer’s study, “it’s hopelessly naive to assume that the same rules apply to suburban, middle-class whites as apply to, say, urban, inner-city black families.”
It is quite natural to assume that Turkheimer’s study was indeed about interaction between race and heritability. The twin sample used, recruited between 1959 and 1965 for the National Collaborative Perinatal Project (NCPP), was 43% white and 54% black. The inevitable inference is that as heritability is zero in poor families, and blacks are disproportionately poor, genes can hardly have anything to do with the lower mean IQ of blacks.
However, Turkheimer et al. did not actually report results disaggregated by race. Their estimates of genetic and environmental influences on IQ were based on the whole sample. Therefore, the inference that heritability is low (if not zero) in blacks is not directly supported by the paper. As it happens, Kevin Beaver and colleagues have done a study on IQ heritability based on the same sample (Beaver et al. 2013). While race-specific ACE estimates were not included in that paper, either, they had been calculated and we obtained them from Kevin Beaver. The following table shows estimates for additive heritability (h2), shared environment (c2), and non-shared environment (e2) for 4- and 7-year-old black and white children in the NCPP sample, along with 95% confidence intervals:
As can be seen, point estimates are similar across races, and confidence intervals are mostly overlapping. Despite the strong SES-heritability interaction, there is no race-heritability interaction in this sample.
The sample of 7-year-olds used by Beaver et al. was somewhat different from that of Turkheimer et al., because the former included also non-twin siblings and half-siblings in their analysis. Moreover, estimation in Beaver et al. was based on maximum likelihood with robust standard errors, which resulted in wide CIs despite large Ns. However, we have a confirmation that heritability and environmentality estimates did not differ significantly by race in the analysis by Turkheimer et al., either (Eric Turkheimer, personal communication, October 4, 2013; he did not have precise estimates at hand, though).
The upshot is that while environmental deprivation may render genetic differences less important in the determination of children’s IQ, the typical black child in this large and downscale sample had apparently not been raised in deprived circumstances any more frequently than the typical white child in the sample. The lower IQs of blacks in this sample cannot therefore be put down to them having been exposed to environments less conducive to the expression of genetic variance in IQ than the environments experienced by whites.
Had the race-specific results been published by Turkheimer et al. back in 2003, we would have been spared much misleading speculation. But perhaps Turkheimer had his own reasons for withholding this information.