Twin models were fitted using Maximum likelihood estimation procedures operationalized in Mx (Neale, 1999). In addition to parameter estimates and 95% confidence intervals (CI), the program provides Chi-square (χ2) goodness-of-fit statistics that are used to determine how well different models fit the data. Nested models were compared using a Chi-square difference test (i.e., the difference in minus twice the log-likelihood (−2LL) estimates for two nested models is distributed as a Chi-square with degrees of freedom (df) equal to the difference in degrees of freedom between the two models). Nested and non-nested models were also compared using the AIC (Akaike, 1987); lower AIC values indicate a better or more parsimonious model fit. Statistical significance of individual path coefficients were determined by constraining the path to zero and conducting a Chi-square difference test and an examination of the change in the AIC statistic. However, given the limitations of the classical twin design (Keller and Coventry, 2005), we focus primarily on the parameters and parameter space indicated in the best-fitting sex-limitation models, thus providing the best interpretation of our findings.
