While DSM-IV-TR estimates the prevalence rates for ADHD to range from 2 to 7% in school-age, rates as high as 17.1% have been reported in community surveys (Cohen et al., 1993). Rates for CD have been estimated to be as low as 0.9% for school-age children but as high as 8.7% in adolescents. The overall preva-lence of ODD varies across studies from 5.7 to 9.9%.
In school-age children, boys have higher rates than girls for all three disorders. In clinic settings, the ratio of boys to girls is about 9 : 1, but in community samples, this decreases to ap-proximately 3 : 1. Furthermore, teachers tend to identify fewergirls than boys as having ADHD symptoms. The combined type of ADHD is the most common subtype in both genders. How-ever, in the predominantly hyperactive–impulsive subtype of ADHD, the male to female ratio is approximately 4 : 1 while in the predominantly inattentive subtype the ratio falls to 2 : 1. In general, prevalence declines with age, but follow-up studies of children and adolescents indicate that the disorder frequently persists into adulthood. Longitudinal studies have reported rates of childhood cases that persist into adulthood to range from 4 to 75%. Factors that appear to predict the persistence of ADHD into adulthood include a positive family history for ADHD and the presence of psychiatric comorbidity, particularly aggression.
Among the AD-DBDs, approximately 90% of children with CD would also meet the criteria for ODD. Furthermore, 40% of children with ADHD also have ODD and 40% of children with ODD have ADHD. In terms of the comorbidity of the AD-DBD group with other diagnostic categories, it has been estimated that 15 to 20% of children with ADHD have comorbid mood disor-ders, 20 to 25% have anxiety disorders and 6 to 20% have learn-ing disabilities. Other conditions which may occur comorbidly with the AD-DBDs include Tourette’s disorder (TD), drug and alcohol abuse or dependence and mental retardation.
There is no single etiology for any of the AD-DBDs. It is likely that each of these disorders is heterogeneous. Nevertheless, a variety of studies using neurochemical markers, family-genetic analyses, patterns of comorbidity, and family studies have begun to delineate more homogeneous groups.