Internal and external validity of attention-deficit hyperactivity disorder in a population-based sample of adults.

Background: Follow-up studies of childhood ADHD have shown persistence of the disorder into adulthood, but no epidemiological data are yet available.
Method: ADHD DSM-IV symptoms were obtained by self-report in an adult population-based sample of 1813 adults (aged 18-75 years), that was drawn from an automated general practitioner system used in Nijmegen, The Netherlands. The structure of ADHD symptoms was analysed by means of confirmatory factor analyses. Other data used in this report are the General Health Questionnaire (GHQ-28), information about the presence of three core symptoms of ADHD in childhood, and about current psychosocial impairment.
Results: The three-factor model that allowed for cross-loadings provided the best fit in the entire sample. This result was replicated across gender and age subsamples. Inattentive and hyperactivity symptom scores were significantly associated with measures of impairment, even after controlling for the GHQ-28. Subjects with four or more inattentive or hyperactive-impulsive symptoms were significantly more impaired than subjects with two, one and no symptoms. The prevalence of ADHD in adults was 1.0% (95% CI 0.6-1.6) and 2.5% (1.9-3.4) using a cutoff of six and four current symptoms respectively, and requiring the presence of all three core symptoms in childhood.
Conclusions: These results support the internal and external validity of ADHD in adults between 18 and 75 years. ADHD is not merely a child psychiatric disorder that persists into young adulthood, but an important and unique manifestation of psychopathology across the lifespan.