Childhood Socioeconomic Status Amplifies Genetic Effects on Adult Intelligence

Abstract

Studies of intelligence in children reveal significantly higher heritability among groups with high socioeconomic status (SES) than among groups with low SES. These interaction effects, however, have not been examined in adults, when between-families environmental effects are reduced. Using 1,702 adult twins (aged 24-84) for whom intelligence assessment data were available, we tested for interactions between childhood SES and genetic effects, between-families environmental effects, and unique environmental effects. Higher SES was associated with higher mean intelligence scores. Moreover, the magnitude of genetic influences on intelligence was proportional to SES. By contrast, environmental influences were constant. These results suggest that rather than setting lower and upper bounds on intelligence, genes multiply environmental inputs that support intellectual growth. This mechanism implies that increasing SES may raise average intelligence but also magnifies individual differences in intelligence.

When does socioeconomic status (SES) moderate the heritability of IQ? No evidence for g × SES interaction for IQ in a representative sample of 1176 Australian adolescent twin pairs

“…Significant C × SES interactions have been reported only the youngest samples (Tucker-Drob et al, 2011;Turkheimer et al, 2003). Studies of adolescents (Harden et al, 2007) and mature adults (T. C. Bates et al, 2013), however, indicate no evidence of C × SES interactions. Developmentally, the heritability of intelligence increases from infancy onwards (Haworth et al, 2010).…”

“…Gene×SES interactions have now been reported in US infants (Tucker-Drob, Rhemtulla, Harden, Turkheimer, & Fask, 2011) and children (Turkheimer, Haley, Waldron, D'Onofrio, & Gottesman, 2003), adolescents (Harden, Turkheimer, & Loehlin, 2007) and mature adults (T. C. Bates, Lewis, & Weiss, 2013). It is important to note that a number of non-significant findings have been reported in the US (Grant et al, 2010;Soden-Hensler, 2012) and null findings from outside the US, e.g., in the UK (Hanscombe et al, 2012) and Netherlands van der Sluis, Willemsen, de Geus, Boomsma, and Posthuma (2008).…”

“…For IQ, a genetic variant of this effect which we term a "Genomic Matthew effect" occurs when environments influence IQ development such that the genetically rich profit more from enriched environments than do those with reduced genetic potential(s). This implies that additional resources, often coupled with parental SES, act not to minimize gaps between children (Ceci & Papierno, 2005), but to amplify them (T. C. Bates et al, 2013).…”

“…Such national differences in the moderation of the heritability of IQ may provide insights into the mechanisms via which environments restrict or promote phenotypic ability. We previously predicted that, "in societies where provision of intellectual resources is universal (independent of income)…no G× SES effects [will be observed]" (T. C. Bates et al, 2013). For such ideas to be tested, analyses of heritability of IQ must be conducted in different nations and educational systems to assess the circumstances fostering the expression of talent.…”

umx: Twin and Path-based Structural Equation Modeling in R

“…In G⇥E models, the moderator is included in the means model, removing any heritable e↵ects it has on the DV of interest, and also moderates the A, C, and E path values (See Figure 9). A common application of this type of model has been to examine changes in heritability (and environmentality) across a range of values of a moderator such as, in human twin research, developmental stress or parental socio-economic status (Bates, Lewis, and Weiss 2013;Bates, Hansell, Martin, and Wright 2016).…”