There is growing evidence of substantial genetic overlap of alcohol and drug use disorders with both externalizing60-62 and internalizing disorders.63 For example, in a study of adolescent twins Button et al64 reported that 35% of the covariance between conduct disorder and drug use disorders could be attributed to common genetic sources and 46% to shared environmental influences. Using COT data, Knopik et al65 demonstrated that intergenerational links between maternal alcohol dependence and offspring ADHD may be mediated by pleiotropic genetic effects, again supporting the important role of genetic influences on both within individual comorbidity and familial aggregation.
