For heritability analyses, we used twin pairs with complete data on the latent measure of nicotine withdrawal: adult monozygotic (MZ) pairs = 416, adult dizygotic (DZ) pairs = 318, adolescent MZ pairs = 92, adolescent DZ pairs = 62. First, tetrachoric and polychoric correlations for the categorical measures were estimated using Mx (Neale et al., 2002). Second, univariate genetic models (Eaves et al., 1978; Neale et al., 2002) were fitted for the categorical measure by the method of maximum likelihood using the program Mx (Neale et al., 2002). We estimated the proportion of the total variance in nicotine withdrawal that could be explained by additive genetic factors (A), environmental influences shared by members of a twin pair (C), and nonshared environmental influences (E). When shared environmental influences were estimated at zero, or in the case were MZ pairs correlation were more than twice as high as DZ twin correlations, non-additive genetic influences (D) were estimated. Non-additive genetic effects and shared environmental effects are confounded in twins reared together, respectively leading to DZ twin correlations less than one half the MZ
