Fig. 1.

Structural equation model (SEM) for a basic univariate twin analysis (ACE model) extended with three covariates (denoted as $x_{11},x_{12}$ and $x_{13}$ for the first and as $x_{21}$, $x_{22}$ and $x_{23}$ for the second twin of a family). Y denotes the phenotypic values of the first (Y1) and second (Y2) twin and A refers to additive genetic influences for the first (A1) and second (A2) twin, which are correlated 0.5 in dizygotic twins and 1 in monozygotic twins. E1 and E2 denote unique-environmental influences of the first and second twin respectively and are assumed to be uncorrelated. C, common-environmental influences, are the same for all family members. Double-headed arrows denote (co-)variances. The path coefficients, ${\mathit{\beta }}_1$, ${\mathit{\beta }}_2$, ${\mathit{\beta }}_3$, a, c and e represent regression coefficients that express the estimated effect of the respective influences