Genetic epidemiological studies have been undertaken to address the possibility that MD and tobacco use traits (initiation of tobacco use, progression to regular use, and ND) share a common liability influenced by genetic and/or environmental factors. Kendler et al. (1993) tested a causal model, in which smoking causes MD or vice versa, and a non-causal model, in which the association between these traits is due to shared genetic and/or environmental liability. Their results supported a non-causal model in which shared liability was due predominantly to genetic factors. Other twin studies have also identified modest to moderate levels of genetic and/or environmental correlation between regular tobacco use or ND and depression or depressive symptoms, in both adolescents (McCaffery et al. 2008) and adults (Fu et al. 2002; Korhonen et al. 2007; Lyons et al. 2008). Kendler et al. (1993) reported a substantial genetic correlation between MD and regular smoking in women (ra = 0.56). While a similar estimate has been reported among adolescent females (ra = 0.62, McCaffery et al. 2008), lower estimates have also been reported (ra = 0.17–0.25, Korhonen
