It is notable that SES was significantly associated with neuropsychological functioning at age 4–5 years among H/I, but not TD children. However, given that the H/I group evidenced lower SES at BL than the TD group, it is not possible to make causal inferences about the impact of SES on neuropsychological functioning. We speculate that the lack of an association between SES and neuropsychological functioning in the TD group may be due to the moderating effect of SES on child functioning. Specifically, the influence of lower levels of SES could be more evident than the influence of middle and high levels of SES. Since there are more children with lower levels of SES in the H/I group, this association was statistically significant in this group. In support of this hypothesis, Turkheimer, Haley, Waldron, Onofrio, & Gottesman (2003) found that among twins in lower SES families, greater variance in IQ was accounted for by the environment rather than by genetics, whereas among higher SES families, greater variance was accounted for by genes, leading them to propose that SES modified the heritability
