The common vulnerability theory posits an underlying vulnerability factor predisposes the clustering of disorders. Twin studies support the common vulnerability theory and demonstrate common genetic influence shared between nicotine dependence and many comorbid disorders. Twin studies show that nicotine and alcohol dependence share genetic influences (Golub and Johnson, 2001; Hettema et al., 1999; True et al., 1999; Young et al., 2006). Twin studies also show common genetic influence for both nicotine dependence and major depressive disorder (Kendler et al., 1993; Lyons et al., 2008), and an overlapping genetic risk for nicotine dependence, conduct disorder, and antisocial personality disorder (ASPD; Fu et al., 2007). This work provides evidence that some genetic variants may increase the risk of developing nicotine dependence, other substance dependence as well as other psychiatric disorders.
