Three different forms of sex-limited univariate and multivariate models were fitted to the data to account for the possibility of gender differences in the magnitude of genetic and environmental influences that have been seen in reference to smoking behaviors (Li et al., 2003), and inconsistently observed in large studies of alcohol dependence (Hardie et al., 2008; Heath et al., 1997; Hicks et al., 2007). The general sex-limitation model (Model I) posits that additive genetic effects may be common or specific to each sex. This is achieved by freely estimating the genetic correlation between opposite-sex twins instead of fixing it at 50%. Additionally, the model does not constrain the phenotypic variance or the magnitude of genetic and environmental effects across sexes. Alternatively, we also tested a restricted sub model of the general sex-limitation model, otherwise known as the “Common Effects Sex-Limitation Model” (Neale et al., 2003), that assumes that there are no sex-specific additive genetic factors but that the relative contribution of genetic and environmental factors differs between men and women (Model II). Finally, we tested a model that posits that
