To our knowledge, this is the only study that includes MD, regular tobacco use and ND, while accounting for the conditional nature of ND on tobacco use, in a single structural equation model. This design enables us to partition the genetic and environmental influences on ND into those that are ND-specific versus shared only with regular tobacco use, shared only with MD, or common to all three traits. Furthermore, we are able to compare these influences across the sexes, as well as between different types of tobacco use among males. Both the genetic and environmental components of this shared risk are modest, with genetic correlations slightly higher. These findings have potential clinical implications, in that individuals who regularly use cigarettes and/or snus might be at an increased risk for MD due to shared liability in addition to risk due to causal relationships among phenotypes. Furthermore, these results could be informative for gene-finding efforts, including genome-wide studies, since variants associated with a phenotype such as MD could be considered candidates for ND or regular tobacco use as well. Considered in the
