Although shared genetic and environmental liabilities influence MD, regular tobacco use and ND, causal pathways might also contribute to the association among these traits. The details of causal hypotheses and how they might apply to our results are beyond the scope of this study. Briefly, evidence suggests that nicotine can have detrimental effects on neurotransmitter systems and neural integrity (e.g. white-matter lesions) (Ding et al. 2003; Malone et al. 2003), which could in turn have an impact on depressive symptoms (Balfour & Ridley; 2000; Tiemeier, 2003). Alternatively, individuals experiencing depressed mood might use nicotine as a form of self-medication (Parrott, 2006), in which case depressive symptoms could lead to tobacco use rather than vice versa.
