Mean ratings of the 11 psychopathology dimensions were residualized on age, age-squared, and age × sex. Univariate biometric models were used to decompose observed phenotypic variance into variance attributable to additive genetic factors (A), dominance genetic factors (D), environmental factors shared by the twins (C), and environmental factors not shared by the twins plus measurement error (E), and to account for sibling interaction/rater bias (S).27 A correlated factors multivariate biometric model27 was used to estimate both the variances for genetic and environment components for each psychopathology phenotype and the 11 × 11 covariance matrices among the genetic, and among the environmental, components of variance.28 Model estimation for the 11 psychopathology dimensions was based on the observed 22 × 22 phenotypic covariance matrices for the 11 phenotypes across the two co-twins for each type of twin pair.
