Descriptive statistics for the substance use phenotypes and behavioral disinhibition are provided in Table 1. Figure 1 provides the biometric estimates, based on the twin sample alone (average age = 17 years), of the heritable, shared environmental, and nonshared environmental components of the correlation matrix among the five phenotypes. The variance in each phenotype, as well as the covariance among phenotypes, is largely due to heritable variation. However, there are also significant shared environmental effects, both on the variances and covariances. Finally, there are significant non-shared environmental effects, especially on the variances. Fit statistics for the ACE, ADE, and AE models are listed in Table 2. The ACE model fit best, followed by the AE and ADE models, according both to likelihood ratio tests and the AIC.
