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 context of varied estimates of genetic correlation across sexes, cultures and cohorts, our results are consistent with the hypothesis that MD, tobacco use and ND share a statistically significant genetic and/or environmental liability, but do not preclude the existence of causal factors.
