Scores on the Wechsler Intelligence Scale for Children

were analyzed in a sample of 7-year-old twins from the National Col-laborative Perinatal Project. A substantial proportion of the twins were raised in families living near or below the poverty level. Biometric analyses were conducted using models allowing for components attrib-utable to the additive effects of genotype, shared environment, and non-shared environment to interact with socioeconomic status (SES) measured as a continuous variable. Results demonstrate that the proportions of IQ variance attributable to genes and environment vary nonlinearly with SES. The models suggest that in impoverished families, 60 % of the vari-ance in IQ is accounted for by the shared environment, and the contri-bution of genes is close to zero; in affluent families, the result is almost exactly the reverse. Although the heritability of cognitive ability in childhood is well established (McGue, Bouchard, Iacono, & Lykken, 1993; Plomin, 1999), the magnitude, mechanisms, and implications of the heritability of I