There have been fewer studies examining the genetic comorbidity between substance involvement and internalizing disorders. For instance, shared genes only explain a modest proportion of the covariance between smoking and depressive symptoms during adolescence,72 however, the impact of shared genetic vulnerability gains prominence during adulthood.73,74 There also appears to be a genetic link between cannabis misuse and vulnerability to major depression in adult samples. Twins who used cannabis were more likely to also report depression and suicidality compared with co-twins who never used cannabis.75 However, Fu and colleagues argue that genetic covariation between depression and cannabis abuse/dependence is attributable to comorbid antisocial personality disorder, albeit, perhaps to a greater extent in men.76
