The implications of an additional source of evidence in support of a common liability for substance use disorders in the realm of gene finding efforts in the context of other comorbid psychiatric disorders is significant for the field. Although externalizing problems were not the focus of this study, others have identified common genetic and environmental factors between externalizing problems, such as conduct disorder, attention deficit hyperactivity disorder, and substance use disorders (Iacono et al., 2008). If it is the case that childhood disruptive disorders and antisocial tendencies are early risk factors for future substance problems, identifying how much of the shared liability across substances overlaps with these disorders will help to further our understanding of addiction processes.
