We detected significant sex differences for ND. Among cigarette smokers, ND was more highly heritable among women (a2 = 0.62) than men (a2 = 0.48). Furthermore, the proportion of genetic effects specific to ND – that is not shared with depression or regular use – was much greater in males (about 80%) than in females (49%). For both sexes, the remaining variance is shared mostly with regular cigarette use, and only a modest amount of the total genetic variance in ND is shared with depression. The difference between sexes in the proportion of ND-specific genetic effects is due mostly to the β path: for women, regular smoking and ND exhibit relatively high continuity, whereas additional, unidentified factors are relevant for men.
