AD-HKD is a relatively common disorder. Estimates of its prevalence vary, depending, to a great extent, on the diagnostic measures (e.g. questionnaires or interviews), age, and nature of the population studied (e.g. inner city or rural). Screening with behavioural questionnaires identifies between 10 and 20 per cent of the population as being affected. (69 Studies that use definitions based on DSM and that take into account some degree of pervasiveness and impairment, but allow comorbidity, report prevalence rates in the 5 to 10 per cent range.(61) When the more stringent ICD-based criteria are used and the diagnosis is restricted to the presence of the full syndrome without comorbid conditions, 1 per cent to 2 per cent prevalence rates are found (for a review see Swanson et a/.(l8)).

AD-HKD is more common in males. The ratio of males to females in community samples is approximately 3:1.(48> The prevalence of AD-HKD is highest among school-aged boys and lower among adolescents. In contrast, its prevalence among females seems lower, but is more stable across the range of ages (see Table

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Table 3 Prevalence rates of ADHD

In clinic samples, the combined subtype of AD-HKD is found in 60 per cent of cases; the predominantly inattentive subtype, in 30 per cent; and the hyperactive-impulsive subtype, in 10 per cent. (62> Higher lifetime rates of comorbid conduct, oppositional, bipolar, language, and tic disorders are found in those with the combined subtype.

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