Adoption Data and Two Related Hypotheses

Data on the Wechsler Intelligence Scale for Children-Revised (WISC-R) from a now classic adoption study (Capron & Duyme, 1989, 1996) were used to examine the hypothesized relationship between diverse cognitive tests’ g loadings and the degree to which the scores on each of the tests is influenced by the socioeconomic status @ES) of the biological parents of the adopted children (a genetic effect) as contrasted with the SES of the adoptive parents (an environmental effect). The analysis shows that the genetic effect is reflected by psychometric g to a greater degree than is the environmental effect, a finding consistent with the hypothesis that the g factor largely reflects the genetic component of variance in cognitive tests. These data also extend previous findings on Spearman’s hypothesis that the standardized mean white-black (W-B) difference on various tests is directly related to the tests’ g loadings. It was found that the profile of the mean W-B differences on various subtests of the WISC-R (in the U.S. standardization sample) is more similar to the profile of genetic effects on the subtest scores than to the profile of environmental effects, as measured in the adoption study.