Biometrical twin modeling was used to assess the relative contributions of genetic and environmental factors that influence liability to AN, MDD, and SA, and to estimate whether genetic and environmental factors are shared across traits. Additive genetic effects (A) are those genetic factors which influence a trait. Common environmental effects (C) are environmental influences which both members of a twin pair experience that increase similarity between twins. Unique environmental factors (E) make twins less similar; this term also includes measurement error. By comparing the intrapair correlation for MZ twins to the intrapair correlation for DZ twins, classic twin modeling assesses the contribution of: (1) additive genetic effects (i.e., heritability, a2); (2) common environmental effects (c2); and (3) unique environmental effects (e2) to the liability of a phenotype. The sum these effects, a2 + c2 + e2, accounts for 100% of the underlying variance of the phenotype.
