Psychiatric Comorbidity Associated With zy DSM-IV ADHD in a Nonreferred Sample of Twins ERIK G. WILLCUTT, PH.D., BRUCE F. PENNINGTON, PH.D., NOMITA A. CHHABILDAS, M.A., MELANIE C. FRIEDMAN, M.A., zyxwvu AND JOSEPH ALEXANDER, B.A. zyx ABSTRACT Objective: zyxwvutsrqpo To test the external validity of the dimensions and subtypes of zyxwvu DSM-IV attention-deficitlhyperactivity disorder (ADHD) by assessing the prevalence of psychiatric comorbidity. Method: Eight- to 18-year-oldtwins 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. Results: 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-IVsubtypes 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. Conclusions: These results provide support for the discriminant validity of the dimensions and subtypes of DSM-IVADHD and suggest that clinicians should carefully screen for comorbid disorders as part of a comprehensive assessment of ADHD. J. zyxwvu Am. Acad. Child Adolesc. Psychiafr): 1999,38(11 ):I 355-1 362. Key Words: attention-deficiVhyperactivity disorder, comorbidity, DSM-IV, validity. zyxwv DSM-Wattention-deficidhyperactivity disorder (ADHD) (American Psychiatric Association, 1994) is the most recent conceptualization of a diagnosis that has evolved extensively over the past 2 decades. DSM-IZI (American PsychiatricAssociation, 1980) described separate diagnoses of attention deficit disorder with hyperactivity (ADD+H) and attention deficit disorder without hyperactivity (ADDNO). ADDNO was characterized by symptoms of inattention and impulsivity in the absence of significant hyperactivity, whereas ADD+H included elevations in all 3 domains. However, because few studies of the validity of this distinction had been published by the time diagnostic Accepted May 26 1999. Dr. Wil&unis with the University zyxwvutsr of Colorado at Boulder Institutefor Behav- ioral Genetics; Dr. Pennington, Ms. Chhabihs, Ms. Friedman,and M EAkxander are with the Universiv of Denver Department of Pkychology. This study was supported by NICHDgrants HD-11681 and HD-27802 and NIMHgrants F32 MH12100, MH00419, and MH38820. The authors thank the school staff andfdmilies that participated in the study. Reprint requests to Dr. W i l h t t , University of Colorado at Boulder Institute for Behavioral Genetics, Campus zyxwvutsrqpon Box 447, Boulder, CO 80309; e-mail: willrun@ Colorado. edu. 0890-8567/99/3811-13550 1999 by the American Academy of Child and Adolescent Psychiatry. criteria were developed for DSM-ZZI-R (American Psychi- atric Association, 1987), a more parsimonious model was adopted that combined the DSM-ZZZ domains of hyperac- tivity, impulsivity, and inattention to form a single dimen- sion of symptoms for the diagnostic category of ADHD. DSM-ZZZ-R relegated A D D N O to the diagnosis of undif- ferentiated ADHD, a residual diagnostic category without specific diagnostic criteria. Continuing uncertainty about the appropriate concep- tualization of ADHD provided the impetus for numerous studies of the factor structure and external correlates of ADHD. In contrast to the unidimensional conceptualiza- tion described in DSM-ZZZ-R, these studies consistently showed that symptoms of ADHD comprised 2 distinct dimensions, one consisting of symptoms of hyperactivity- impulsivity (hyp-imp) and the other comprising symp- toms of inattention (e.g., Bauermeister, 1992; Pelham et al., 1992). Further evidence against the single-dimension model was provided by studies indicating that ADD+H and ADDIWO were associated with different profiles of psychiatric comorbidity. Specifically, children with ADD+H were at higher risk for oppositional defiant disorder (ODD) and conduct disorder (CD) and exhib- J. AM. ACAD. CHILD ADOLESC. PSYCHIATRY, 38:11, NOVEMBER 1999 1355