Thus far, we have presented the results of univariate biometric analyses of conduct disorder. However, it is well known that conduct disorder is often correlated with other types of disinhibitory behavior, including substance use. For example, conduct disorder diagnoses in early adolescence are associated with increased odds of DSM-IV nicotine dependence, alcohol abuse/dependence, and cannabis abuse/dependence diagnoses by age 18 (OR = 4.75–5.43) (Elkins et al., 2007). Multivariate behavioral genetic studies, which permit examination of the degree to which genetic and environmental influences contribute to covariance among multiple phenotypes, suggest that genetic factors account for the majority of the covariance among externalizing disorders (Kendler et al., 2003; Krueger et al., 2002; Slutske et al., 1998; Young et al., 2000). In one example of this from the population-based Minnesota Twin Family Study, conduct disorder, adult antisocial behavior, alcohol dependence, drug dependence, and low levels of constraint loaded onto a single, highly heritable (h2 = 0.81) additive genetic factor (Krueger et al., 2002). Of note, there were no unique genetic influences on conduct disorder. Rather, the additive genetic influences on conduct disorder
