The genetic correlation between MD and NDC was 0.18 for males and 0.30 for females; the corresponding environmental correlations were lower (0.14 for males and 0.17 for females). The total genetic variance of NDC can be partitioned into its different sources: that which is specific to NDC (path a3,3); that which is shared only with RUC (a function of paths β and a2,2); and that which is shared with MD both independent of RUC (path a3,1) and through genetic factors shared between MD and RUC (a function of paths b and a2,1). Because the β path differs between the sexes (0.37 for males and 0.63 for females) the proportion of genetic and environmental variance specific to NDC varies considerably between the sexes: 79% of the genetic influences on NDC are specific to dependence in males, contrasted with 49% in females. Consequently, far less of the genetic variance affecting NDC is shared with regular smoking in males (20%) than in females (47%). Variance shared between NDC and MD accounts for very little of the total genetic variance influencing NDC (<5% for both sexes, Fig. 3), but cannot be removed from the model.
