In this chapter we report results from model-fitting analyses of data from the Colorado Adoption Project using the simple sibling and parent–off-spring models developed in the preceding chapter as well as extending the model to include age-to-age genetic correlations and to consider the multivariate case. As discussed in Chapter 7, model fitting has several advantages over less sophisticated approaches to the interpretation of behavioral genetic data: It yields appropriate parameter estimates given the assumptions of a model; it provides standard errors for these parameter estimates; and it provides goodness-of-fit tests to aid in the evaluation of alternative models.

Sibling model

A simple model can be used to represent the sibling adoption design because the essence of this design lies in the comparison between two correlations: the correlations for adoptive and nonadoptive siblings. The sibling model, illustrated in the path diagram of Figure 7.3, was applied to the adoptive- and nonadoptive-sibling correlations presented in Chapter 6. The model involves only three parameters: heritability and shared and nonshared environment. As specified in Equations (7.4) and (7.5), the model assumes that the observed nonadoptive-sibling correlation is a function of half the heritability of the trait and of shared environmental influence; the adoptive-sibling correlation arises only from shared environmental influence – in the absence of selective placement, heredity does not contribute to the resemblance of adoptive siblings.