zygosity groups), 1 regression parameter for the effect of age on the means, 5 variances (5 sex by zygosity groups) and 5 covariances (for 5 sex by zygosity groups). In the cohorts without a DOS group, 4 means, 1 regression parameter for age, 4 variances and 4 covariances were estimated (13 parameters in total). The 4 or 5 covariances were standardized in each sex-by-zygosity group in order to obtain 4 or 5 twin correlations in each cohort. In addition, the 95 % confidence intervals for the twin correlations were computed. It was further tested whether the twin correlations could be constrained to be equal across sex (MZM = MZF and DZM = DZF = DOS).
