Twin model fitting was performed using the Mx software package (Neale et al., 2003). Mx was unable to estimate confidence intervals for all of the parameters in our best-fit models. Therefore, these models were re-fitted in Mplus (Muthen and Muthen, 2007). These analyses provided very similar estimates to those obtained with Mx, and we report the standard errors obtained in Mplus. With categorical dependent variables (as AD), Mplus uses a weighted least squares estimation procedure rather than maximum likelihood (as in Mx). Thus, the reported standard errors are not precise, but do provide a sense of the accuracy of our parameter estimation.
