Genetic and environmental influences on vocabulary IQ: Parental education level as moderator

This article examines how parental education level moderates the genetic an d environmental contributions to variation in verbal IQ. Data are from 1909 non-Hispanic Whites and African American sibling pairs from the National L ongitudinal Study of Adolescent Health, which obtained nationally-based sam ples of identical (MZ) twins, fraternal (DZ) twins, full and half siblings, cousins (in the same household), and biologically unrelated siblings. In t he whole sample, the variance estimate for heritability (h(2) =.57, SE =.08 ) was greater than that for shared environment (c(2) =.13, SE =.04). Both h eritability and the shared environmental estimate were moderated, however, by level of parental education. Specifically, among more highly educated fa milies, the average h(2) =.74 (SE =.10) and the average c(2) =.00 (SE =.05) . Conversely, among less well-educated families, heritability decreased and shared environmental influences increased, yielding similar proportions of variance explained by genetic and environmental factors, average h(2) =.26 (SE =.15), and average c(2) =.23 (SE =.07).