Psychiatric comorbidity associated with DSM-IV ADHD in a nonreferred sample of twins

To test the external validity of the dimensions and subtypes of DSM-IV attention-deficit/hyperactivity disorder (ADHD) by assessing the prevalence of psychiatric comorbidity.

Eight- to 18-year-old twins with ADHD (n = 105) and without ADHD (n = 95) were recruited through local school districts. Comorbid disorders were assessed by structured diagnostic interviews with the parent and child and by a behavioral rating scale completed by the child's classroom teacher.

Symptoms of inattention were associated with lower intelligence and higher levels of depression, whereas symptoms of hyperactivity-impulsivity were associated more strongly with symptoms of oppositional defiant disorder (ODD) and conduct disorder (CD). All DSM-IV subtypes were associated with higher rates of ODD and CD in comparison with controls, and the combined type was associated with more disruptive behavior disorder symptoms than the other 2 subtypes. The combined type and predominantly inattentive type were associated with more symptoms of depression than controls or the predominantly hyperactive-impulsive type.

These results provide support for the discriminant validity of the dimensions and subtypes of DSM-IV ADHD and suggest that clinicians should carefully screen for comorbid disorders as part of a comprehensive assessment of ADHD.