| This study investigated the genetic and environmental contributions to the etiology of Attention-Deficit/Hyperactivity Disorder (ADHD), Conduct Disorder (CD), and Oppositional Defiant Disorder (ODD) as well as the etiology of their comorbidity. Exploratory analyses were also conducted to test whether the genetic contributions to these disorders varied as a function of age, IQ, socioeconomic status (SES), and gender. The sample utilized for the current study included 269, 8--18 year-old, same sex, twin pairs. Probands were selected for extreme DSM-IV ADHD, ODD, or CD symptomatology and the DeFries and Fulker multiple regression technique was utilized to obtain heritability estimates. Analyses revealed that ADHD and ODD were primarily influenced by genetic factors, while CD seemed to be influenced by both genetic and environmental factors. The dimensions of ADHD (inattention and hyperactivity/impulsivity) were also found to be highly heritable and their overlap was largely mediated by shared genetic influences. The comorbidity of ADHD (and its dimensions) with other disruptive behavior disorders was largely mediated by shared genetic influences; however non-shared environmental factors also played a role. Exploratory analyses provided some indication that the inattention dimension of ADHD may be more heritable in older children and in children with higher IQ scores, while the hyperactivity/impulsivity dimension seemed to be more heritable in children from higher SES groups. Taken together, the results of this study provide further support for previous findings and highlight the need for further investigation into the effects of age, IQ, SES, and gender on the etiology of disruptive behavior disorders. |
