Previous behavior genetic research has demonstrated correlations between socioeconomic status (SES) and IQ; higher SES is associated with higher IQ while lower SES is associated with lower IQ. While these associations implicate a significant environmental component to intelligence, the heritability of IQ is also well documented. Further research demonstrates, however, that SES moderates IQ heritability in American populations; the heritability of intelligence is enhanced by higher SES and diminished by lower SES. Analyses by Tucker-Drob (2013) and Ritchie (2015) have identified developmental pathways underlying SES differences in the development of cognition and achievement. Here we advance this research by conducting similar longitudinal analyses using a genetically informative, longitudinal design. While previous studies evince a linear effect of SES on IQ, our analysis reveals a consistently nonlinear quadratic effect, with a negative relationship in which the slopes of IQ are greater at lower levels of SES. The effect is greatest for PIQ and Fluid IQ.
Analyzing data on 365 monozygotic and 376 dizygotic twin pairs from the Louisville Twin Study, we assess the quadratic effect of SES by IQ. The Louisville twins were administered full WISC tests up to five times between the ages of seven and 15. We examine the extent to which the effects of SES on subtest performance are mediated by g. We then decompose the relationship between SES and subtests into ACE components, and use these components to explore SES quadratic effects on stability and change in g and subtest performance over time. Finally, we develop two factor models of WISC subtests in which the rotation of the factors is determined by differential relations with SES rather than simple structure.
Works Cited
Ritchie, Stuart J., Timothy C. Bates, and Ian J. Deary. 2015. “Is Education Associated with Improvements in General Cognitive Ability, or in Specific Skills?” Developmental Psychology 51 (5): 573–82.
Tucker-Drob, Elliot M. 2013. “How Many Pathways Underlie Socioeconomic Differences in the Development of Cognition and Achievement?” Learning and Individual Differences 25 (June): 12–20.
