In a recent post I presented some demographic data pointing to college as a classist institution. The high and escalating price tag excludes kids who can’t afford it, preventing them from acquiring the sheepskin that grants access to higher-paying jobs. The ensuing discussion began to unpack the links between money, habitus, aptitude, education, and employment. This post looks at the relationship between parents’ socio-economic status, which is a composite measure of income and education, and kids’ intelligence/aptitude.
Empirical studies have consistently found that about half the differences in individuals’ intelligence can be attributed to genetic factors: bright parents tend to produce bright offspring. Parenting, in contrast, seems to exert surprisingly little effect on children’s intelligence. However, parents’ socio-economic status is empirically correlated with intelligence: on average, kids from more well-to-do families tend to score higher on standardized IQ and aptitude tests than do kids from poor families — see this graph. Can we infer that, on average, rich people are intrinsically brighter than poor people, resulting in brighter offspring who are better suited for higher education and more likely to benefit from it?
Psychologist Eric Turkheimer and his students/colleagues at the U. of Virginia (my doctoral alma mater) have conducted several studies exploring this question. Here I describe two of them, both involving statistical analyses of existing longitudinal databases collected from sets of twins.
In this study Turkheimer et al. evaluated the results of IQ tests taken at age 7 by identical (monozygotic) and fraternal (dizygotic) twins. The study estimated the proportion of differences in IQ accounted for by genotype (monozygotes share 100% of the same genes, dizygotes share 50%), shared environment (which consists largely of parenting — twins are raised by the same parents at the same time), and nonshared environment (other aspects of kids’ lives, including neighborhood, school, peers, parents’ social networks, etc. — and presumably also uncaused emergent intelligence if there is such a thing). Using some fancy multivariate structural modeling techniques, the researchers demonstrated that the relative importance of these three correlates of IQ varied as a function of socioeconomic status. For the highest-SES kids, genotype was by far the most powerful correlate of IQ, with neither shared nor unshared environment adding any predictive power. For the lowest-SES kids, both shared and nonshared environment were strongly and separately correlated with IQ, whereas genotype was unrelated to IQ. In other words, for low-SES kids IQ is almost entirely a function of environment, while for high-SES kids IQ is almost entirely a function of heredity. The researchers conclude:
“Additive models of linear and independent contributions of genes and environment to variations in intelligence cannot do justice to the complexity of the development of intelligence in children… [T]he developmental forces at work in poor environments are qualitatively different from those at work in adequate ones.”
- “Genotype by Environment Interaction in Adolescents’ Cognitive Aptitude” (2006) — link available here
This study by Harden, Turkheimer & Loehlin replicated the 2003 study, using eleventh-grade twins’ SAT test scores as the measure of intellectual aptitude. Findings were similar to the prior study of 7-year-olds: the strong statistical impact of genetics on SAT scores appears only in the high-SES kids. In this study the shared environment accounted for practically none of the variance: this is consistent with other studies showing that whatever influence parenting has on younger children dissolves rapidly in adolescence. Unbundling the SES effect, the researchers found that the effect of parents’ income is stronger than their level of education. That is, kids’ genetic predisposition for academic aptitude is expressed more fully in rich families than in poor ones. The researchers conclude that their findings support a “social context as enhancement” hypothesis about genetic-environmental interaction:
“[D]ifferences among ‘normal’ environments are largely irrelevant for differences among children’s intelligence. Below a certain threshold of environmental quality, intelligence increases sharply with better environments… In contrast, above a certain threshold of environmental quality, the reaction plane is essentially flat: for any given genotype, better environments do not predict an increase in intelligence…, and for any given environment, genetic differences are equally well expressed… Thus any gene-environment interaction disappears above a threshold of environmental quality.”
These findings and their interpretations are fascinating in their own right. They unfold within a broader ecological model of development that’s more conceptually and methodologically sophisticated than traditional schemes in which nature, nurture, and culture are treated as separate and independent components that can simply be added together. There are practical implications as well, but I’ll leave those either to discussion or to a separate post.