In summary, the results of the univariate model fitting suggested that there were qualitative sex differences in the genetic or shared environmental influences on CD but no quantitative sex differences. Whether these qualitative sex differences were attributable to sex-specific genetic influences (Full rA) or sex-specific shared environmental influences (Full rC) was difficult to resolve. In the Full rA model, the genetic correlation among opposite-sex twin pairs was initially estimated to be negative, likely because of insufficient covariance information in the data. This estimate was then constrained to be positive, resulting in the parameter being fixed at the lower bound (r = 0) of plausible values. In the Full rC model, the shared environmental correlation among opposite-sex twin pairs was estimated to be r = .23, which can be compared to the expected correlation, given no qualitative shared environmental sex difference, of 1. The Full rC model was selected as the best fitting model due to its slightly smaller AIC compared to the full rA model. Notably, estimates for A, C, and E obtained from the Full rC model matched the estimates obtained from separate models of each sex (results not shown).
