Structural equation modeling was used to examine G-E interplay between the EXT composite and 6 environmental risk factors. First, we used univariate biometric models to estimate the relative genetic and environmental effects on each variable54. These models assume variance in each measure is attributable to 3 independent sources: additive genetic (A), shared environment (C), and nonshared environment (E). Estimates of the ACE variance components are derived by comparing the similarity of members of MZ twin pairs relative to that of members of DZ twin pairs, given that MZ twins share all their genes and DZ twins share on average 50% of their segregating genes (A effects are present if rMZ > rDZ). Shared environmental effects refer to environmental effects that increase similarity among family members (C effects are present if rDZ > ½ rMZ) while nonshared environmental effects (including measurement error) contribute to differences between members of a twin pair (E effects are present if rMZ < 1.0).
