Our estimates of the contributions of genetic and unique environmental factors to MD are nearly identical to those obtained when modeling the relationship between MD, RUC and NDC (a2 = 0.31, e2 = 0.69). The heritability of RUS is quite similar to that of RUC (a2 = 0.66 and 0.70, respectively). The heritability of NDS is 0.62. The genetic correlation between MD and RUS (ra = 0.22) is slightly higher than that for cigarette use in males. As with MD and RUC, the unique environmental covariance between MD and RUS could be set to 0. The remaining variance in RUS and NDS is accounted for by unique environmental factors.
