Rather than trying to identify specific causal loci, the heritability-by-environment approach attempts to identify systematic differences in the genetic contribution to overall variance. As noted earlier, these models traditionally relied on twins and siblings to estimate the contribution of assumed genetic similarity to their observed phenotypic similarities. However, genome-wide data can also provide comparable estimates by comparing observed genetic similarity with observed phenotypic similarity of unrelated persons. The first column of Table 2 presents the genetic (Vg), environmental (Ve), and total phenotypic variance (Vp) for BMI among the third-generation participants of the FHS. The ratio of genetic variance to total variance provides an estimate of heritability. Estimated heritability of BMI is .52 and .60 for those without and with a college degree, respectively. This model is in line with the social push GxE model because the genetic factors are relatively more important for those in a relatively healthy environment. That is, the difference in heritability across the groups is due to differences in environmental variation rather than differences in the genetic effects. This can be seen in the components of
