Our results support the hypothesis that MD, regular tobacco use and ND share a common liability. In males, the nature of the non-causal relationship between MD and regular tobacco use – for both cigarettes and snus – can be accounted for entirely by genetic factors shared by these traits. In females, genetic and environmental influences contribute to this covariance. Estimates of the genetic correlation between MD and regular tobacco use or ND are modest, ranging from ra = 0.13 to 0.30, indicating that the genetic liability to these traits is largely specific to each. However, genetic correlations are higher than environmental correlations (re = 0–0.17).
