We decomposed the total variance of the CBCL-OCS, TOCS total scores, and each of the TOCS OC trait dimensions identified in our factor analysis into genetic and environmental factors. Genetic variance could be attributable to additive effects (A), and/or dominance (non-additive) effects (D). Environmental variance was partitioned into common environmental (C) influences, which are shared by family members, and unique environmental (E) factors, which include measurement error. The within-pair additive genetic correlation (A) was set at 1 for MZ twins and 0.5 for DZ twins, whereas the dominance genetic correlation (D) was fixed at 1 for MZ twins and 0.25 for DZ twins29. The significance of the individual variance components was assessed by comparing the fit of the full models (ACE and ADE) to the nested sub-models (AE, CE, and E).
