Intraclass correlations (ICCs) for each trait and across traits were examined within MZ and DZ twins. The heritability of total OC traits and each OC trait dimension was estimated by structural equation modeling with age, sex, and respondent included as covariates using full information maximum likelihood (including pairs with incomplete data (n = 3)) in OpenMx27. For analyses using standardized z scores, age, sex, and respondent (parent or self) covariates were not included in the models because these factors were incorporated during z score calculation (see above). Saturated model fit was conducted to test the assumption of equality of means and variances between the MZ and DZ twins28. The goodness of fit parameters used to compare twin models were the likelihood-ratio chi-square statistic (χ2) and Akaike’s information criterion (AIC).
