Multivariate path analysis of specific cognitive abilities data at 12 years of age in the Colorado Adoption Project

A parent-offspring multivariate conditional path model was fitted to specific cognitive abilities data from the Colorado Adoption Project (CAP) when the offspring were 12 years of age. The sample included 175 adoptees, 175 sets of adoptive parents, 175 biological mothers, 34 biological fathers, and 209 nonadopted children and their parents. Consistent with results obtained from multivariate genetic analyses of CAP data obtained at earlier ages, the effects of familial environmental transmission on individual differences in specific cognitive abilities were not significant. Assuming complete isomorphism (i.e., that the genetic and environmental etiologies of individual differences are the same) between the child and the adult measures, the heritability estimates for verbal, spatial, perceptual speed, and visual memory were .26, .35, .38, and .53, respectively. Although the heritability estimate for visual memory is somewhat higher than those for verbal, spatial, and perceptual speed abilities, these estimates are not significantly different. These estimates are higher than those obtained when the adoptees and controls were 4 years old (Rice et al., 1986, 1989); thus, heritabilities of specific cognitive abilities may increase as a function of cognitive development. Alternatively, genetic stability from childhood to adulthood may be greater from 12 years than from 4 years, which would be interpreted as greater heritability by the CAP parent-offspring design. The genetic correlations among the four measures were substantial, ranging from .27 between verbal and spatial abilities to .78 between spatial ability and perceptual speed. However, differences among these correlations are not significant, suggesting that their covariation may be due to general cognitive ability. Finally, estimates of bivariate-heritability indicate that on average about half of the phenotypic correlations among the four specific cognitive ability measures are due to genetic effects.