There’s a long-standing debate about if and how parental socioeconomic status moderates the heritability of IQ. Research has often but not always found that heritability is lower in low-SES families. See Turkheimer and Horn’s excellent review for details (although some of Turkheimer’s own research on this is less than convincing).
Robert Kirkpatrick and colleagues have conducted what may be the best study on the question so far. They use a big Minnesota sample, comprising about about 2500 pairs of adolescent twins, non-twin biological siblings, and adoptive siblings, and investigate if SES moderates either genetic or environmental determinants of IQ.
Below are two graphs showing their main results, demonstrating how SES, measured as a composite of parental educational attainment, parental occupational status, and household income, influences the ACE variance components in the adolescent Minnesota sample. SES increases from left to right, and the values on the x-axis indicate the cumulative proportions of families at or below each SES level. The ACE components are additive heredity (A), shared or common family environment (C), and non-shared or individual-specific environment (E).
The top graph shows the unstandardized values of ACE components, based on the original IQ metric. Eyeballing the graph, the total unstandardized phenotypic variance appears to be about 140 in the lowest-SES families, while it’s about 200 in the highest-SES families. Taking square roots of these values, we get 11.8 and 14.1. These are the standard deviations of IQ, in IQ points, at the two extremes of the SES variable. This difference is mainly caused by the fact that genetic variance is about 75 at the lowest SES level while it’s about 130 at the highest SES level. The contributions of shared and unshared environmental variances are similar across SES levels. The results show that SES does moderate the heritability of IQ in this sample, albeit not very strongly — genes are more important than environment even in the most deprived family environments.
The bottom graph shows the same results but on a standardized metric where IQ variance is constrained to 1.0 across SES levels. The standardized genetic variance, or heritability, is about 55 percent at the lowest values of the SES variable, and a bit more than 60 percent at the top SES level, with the environmental variances following a corresponding but opposite pattern. Given that the total variance on the original IQ metric increases with SES, the unstandardized values are perhaps more interpretable.
What does it mean when heritability is lower at lower SES levels? One way to think about it is that impoverished environments prevent the realization of genetic potential. Somewhat perplexingly, Kirkpatrick et al. however found that heritability did not change as a function of trait level — i.e., dullness and smartness were equally due to genetic differences. Paradoxically, based on these results, it could also be argued that an individual’s genetic liability towards both high and low intelligence is less likely to be expressed in poor and uneducated families.
There are some other interesting results in the paper. For example, using the adoptee sample they were able to estimate the main effect of SES on IQ. The placement of an adoptee in a family with the lowest SES versus the very highest SES was associated with a 7 IQ point difference. While a causal interpretation of this effect depends on the untested assumption of random placement of adoptees in families, the magnitude sounds plausible to me.
What I think would facilitate the interpretation of SES-heritability interactions would be the use of latent factor models to conceptualize cognitive ability in these studies. When intelligence is defined as a single IQ score, as in Kirkpatrick’s study, it’s difficult to understand how any moderating effects actually operate. With a factor model of IQ, it would be possible examine if the same model applies throughout the SES range, that is, do SES differences cause differences in latent abilities, or is SES moderation actually due to the testing instrument “breaking down” at lower SES levels.
Kirkpatrick, R.M., et al. (2015). Replication of a Gene–Environment Interaction Via Multimodel Inference: Additive-Genetic Variance in Adolescents’ General Cognitive Ability Increases with Family-of-Origin Socioeconomic Status. Behav Genet., 45, 200-14.