ABSTRACT
A major motivation for carrying out a study based on adoptive families is that it permits comparisons of family members who are and are not genetically related. Thus, an adoption study can provide evidence of the relative contribution of shared genes and shared environments in producing familial resemblance on a trait. Major focus of the Texas Adoption Project was on intellectual abilities. Tests of general intelligence were administered to all available members of the adoptive families in the initial study. The chapter describes the means and standard deviations of IQs on the individually administered IQ tests for various groups in the original and follow-up studies. On the whole, biologically related family members show higher correlations on IQ test subscales than do biologically unrelated individuals who share family environments via adoption. Among the subscales showing this pattern most clearly were measures involving spatial abilities—Block Design on the Wechsler tests and Formboard on the Beta.
