In summary, results of the present investigation indicate that, after controlling for prenatal and familial risk factors shown to increase risk for ADHD subtypes, conduct problems, and alcohol dependence symptomatology, most of the covariance among impulsivity, inattention, and conduct problems is due to common genetic influences; however, the observed comorbidity between conduct and alcohol problems is driven primarily by environmental influences common to both behaviors. Each phenotype is also under the influence of additional, unique genetic and/or environmental factors, suggesting that these externalizing disorders, while sharing some genetic and/or environmental influences, are not simply manifestations of the same underlying biological or environmental predisposition. Consistent with earlier reports, dimensions of ADHD were not significantly associated with alcohol problems in this adolescent female sample, and similar to earlier work (Knopik et al., 2005; 2006; 2009), prenatal and familial predictors of behavior, while significant, did not mediate genetic risk on externalizing behavior. Thus, while a variety of adolescent problem behaviors are significantly correlated, the structure of that association may differ as a function of phenotype (e.g., comorbid HYP/IMP and CDP vs. comorbid CDP
