According to this hypothesis, lower socioeconomic status and greater exposure to social disadvantage during childhood leads to a decrease in the heritability of IQ, as compared to children raised in more advantaged environments.
[2][3] This hypothesized effect was first proposed by Sandra Scarr, who found support for it in a 1971 study of twins in Philadelphia, and these results were replicated by David C. Rowe in 1999.
A 2015 meta-analysis found that the effect was predominant in the United States while less evident in societies with robust child welfare systems.
[1] In Sandra Scarr's original work, she outlines the methods behind the study of genetic and environmental variance, using data from the 1960 US Census tract to estimate Socioeconomic status (SES),[6] and the Iowa Tests of Basic Skills to study genetic and environmental variations across children from advantaged and disadvantaged populations.
[7] One of the major limitations of Scarr's original work is that the sample of twins was separated into same-sex and opposite-sex as opposed to monozygotic (MZ) and dizygotic (DZ), a method which undermined the statistical power of this study.
[12] David C. Rowe's work looks at the influence of genetic and environmental factors specifically on the variation of verbal IQ, with data from the 1909 sibling pairs from the National Longitudinal Study of Adolescent Health.
[18] The original theory hypothesizes that higher SES will be associated with greater heritability of IQ, very few studies have addressed this interaction within adult populations.
[19] Evidence from Gottschling's 2019 study suggests that the influence of the SES of a family as a moderator for IQ heritability changes as people get older – with results showing the effect being less significant in adolescence and not shown present in adulthood.