The genetic correlation between MD and NDS (ra = 0.17) is comparable with that of MD and NDC (ra = 0.18). The unique environmental correlation between MD and NDS is low (re = 0.07). However, the coefficient of the regression path (β) between RUS and NDS is higher than that found for cigarette use (0.75 v. 0.37). Accordingly, much of the variance in NDS can be accounted for by RUS rather than being ND-specific. Ultimately, liability to RUS accounts for 57% of the total heritability and 51% of the total unique environmental variance of NDS. NDS-specific variance accounts for much of the remainder (40% of the total genetic variance and 49% of the total environmental variance); only 3% of the total genetic variance influencing NDS is shared with MD (Fig. 3), and only 0.4% of the total environmental variance is shared with MD.
