Structural equation modelling of twin data was performed in the flexible matrix algebra program Mx (Neale et al., 2006), which employs maximum likelihood modelling procedures to determine the combination of A, C, and E that best explains the observed data. We fitted univariate and bivariate models (for more information about this model see Rijswijk et al., 2002) to the raw dichotomous data, where it is assumed that a normally distributed continuum of liability underlies the dichotomous observed categories. Age and sex effects were accounted for in the model by including them as covariates to adjust the thresholds.
