IQ, Covariance, and Heritability

I’ve been meaning to get to this very interesting paper (pdf) about the heritability of IQ–it supports something I’ve long suspected about IQ. The abstract (boldface mine):

To further knowledge concerning the nature and nurture of intelligence, we scrutinized how heritability coefficients vary across specific cognitive abilities both theoretically and empirically. Data from 23 twin studies (combined N = 7,852) showed that (a) in adult samples, culture-loaded subtests tend to demonstrate greater heritability coefficients than do culture-reduced subtests; and (b) in samples of both adults and children, a subtest’s proportion of variance shared with general intelligence is a function of its cultural load. These findings require an explanation because they do not follow from mainstream theories of intelligence. The findings are consistent with our hypothesis that heritability coefficients differ across cognitive abilities as a result of differences in the contribution of genotype-environment covariance. The counterintuitive finding that the most heritable abilities are the most culture-dependent abilities sheds a new light on the long-standing nature-nurture debate of intelligence.

The key part is genotype-environment covariance. What the hell is that? Well (boldface added):

Genotype by environment covariance: certain genotypes (combinations of SNPs) are more likely to be found in certain environments. In other words, children ‘smart genes’ will be more likely to be reared in ‘smart environments.’

…For example, if I don’t realize that children who have a slightly higher IQ score are more likely to be raised in environments that dramatically increase IQ (e.g., better nutrition, less lead exposure, better educational access, parents with better educations, etc.), then I would conclude, erroneously, that these SNPs [genetic differences] have a much greater contribution to IQ than they otherwise do.

By the way, that last scenario sounds like a society where one’s economic success is heavily determined by academic performance, including test taking, which then will improve the academic success of your children.

In other words, China (and, increasingly, the U.S.) [the study enrolled Chinese patients, hence the emphasis on China].

Scott Barry Kaufman describes a related developmental mechanism that happens within generations (boldface mine):

They discovered two main findings. First, in samples of both adults and children, they found that the greater the cultural load, the greater the test was associated with IQ…This finding is actually quite striking, and suggests that the extent to which a test of cognitive ability correlates with IQ is the extent to which it reflects societal demands, not cognitive demands.

Second, in adults, the researchers found that the higher the heritability of the cognitive test, the more the test depended on culture. The effects were medium-to-large, and statistically significant…

Instead, the best explanation may require abandoning some long held assumptions in the field. The researchers argue that their findings are best understood in terms of genotype-environment covariance, in which cognitive abilities and knowledge dynamically feed off each other. Those with a proclivity to engage in cognitive complexity will tend to seek out intellectually demanding environments. As they develop higher levels of cognitive ability, they will also tend to achieve relatively higher levels of knowledge. More knowledge will make it more likely that they will eventually end up in more cognitively demanding environments, which will facilitate the development of an even wider range of knowledge and skills. According to Kees-Jan Kan and colleagues, societal demands influence the development and interaction of multiple cognitive abilities and knowledge, thus causing positive correlations among each other, and giving rise to the general intelligence factor.

These two mechanisms combined would lead us to overestimate the effects of the traditional model of heritable IQ: that there are global ‘smart’ genes. Note two things. First, this does not imply that there can’t be any additive genetic variance in IQ (‘global smartness’), simply that we are overestimating it. Second, these mechanisms do not mean that IQ is solely ‘cultural.’ Rather:

What these findings do suggest is that there is a much greater role of culture, education, and experience in the development of intelligence than mainstream theories of intelligence have assumed. Behavioral genetics researchers– who parse out genetic and environmental sources of variation– have often operated on the assumption that genotype and environment are independent and do not covary. These findings suggests they very much do.

For the Andrew Sullivans and Charles Murrays out there, it’s also worth noting that the test that separate blacks and whites (in the U.S.) as well as the rich versus poor are the most are also those most affected by genotype-environment covariance. This is a good thing, since it means there’s a lot we can do to close the black-white and poverty gaps in terms of educational performance.

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4 Responses to IQ, Covariance, and Heritability

  1. Min says:

    Research dating back to around 1960 indicated that the difference between Black and White children on questions phrased as, “A is to B as C is to ___” was no longer in evidence when the questions were phrased as “A goes with B like C goes with ___.” There was clearly a cultural and linguistic defect in those questions. But did the standardized tests change the wording of those questions?

  2. ottilie says:

    Even in the way we teach narrow sense/broad sense heritability in population genetics in biology 101… instructors often convey an inaccurate notion that genetic + environmental variance =100% (for any type of trait), and that each of these factors is fixed, and the total variance and heritability are fixed through time, so the task is just to estimate the ratio of the two. Actually – you can change environmental variance just by changing the group that you’re looking at (are you looking at plant height in a uniform field, or across a broader area where there are wet and dry areas, different soil types). Just by changing the environmental variance of your sample, you were able to change the heritability calculation.

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