Genetic models were fitted by the method of maximum likelihood, using the structural equation modeling program Mx (Neale et al., 2003). For each phenotype, the thresholds (assessed as z-scores on the underlying standard normal distribution), were adjusted for lag time between age at first use and age at the time of report (see Section 2.5.4). A series of sub-models examining the statistical significance of A, C and E were compared to the full model to derive the best-fitting univariate models. Sub-models were tested by calculating the difference between the −2 log likelihood fit of the full model and nested sub-model, which is distributed as chi-square for the given degrees of freedom.
