The heritability and comorbidity of attention deficit hyperactivity disorder (ADHD) with conduct disorder (CD), oppositional defiant disorder (ODD), and executive function (EF) deficits were examined in 224 child twins (140 monozygotic and 84 dizygotic). The Coolidge Personality and Neuropsychological Inventory for Children (Coolidge, 1998), a standardized, 200-item, Diagnostic and Statistical Manual of Mental Disorders (4th ed.; American Psychiatric Association, 1994) aligned, parent-as-respondent inventory, assessed psychopathology. Structural equation model fitting revealed that the individual scale heritabilities were substantial: .82 for ADHD, .74 for CD, .61 for ODD, and .77 for EF deficits. The results of the multivariate twin analyses suggest that ADHD shares most of its genetic liability with CD, ODD, and EF deficits. Thus, the findings argue for a common biological risk underlying these commonly comorbid externalizing behavior problems and cognitive deficits. The residual genetic variance provides preliminary support for additional genetic influences underlying CD, ODD, and EF that are independent of ADHD.