Comorbidity, the occurrence of multiple psychiatric conditions in one individual, is highly prevalent and has strong implications for prognosis and course of both treated and untreated conditions.47,48 Numerous studies have documented that use and misuse of different substances is highly correlated and that substance use and abuse/dependence are highly associated with externalizing psychopathology and, to a lesser extent, internalizing psychopathology. Multivariate twin studies can be used to model the extent to which the same genetic factors influence vulnerability across substances and psychopathology.
