Despite a prevailing assumption that adult ADHD is a childhood-onset neurodevelopmental disorder, no prospective-longitudinal study has described the childhoods of the adult-ADHD population. We report follow-back analyses of ADHD cases diagnosed in adulthood, alongside follow-forward analyses of ADHD cases diagnosed in childhood, in one cohort.
Participants belonged to a representative birth cohort of 1,037 individuals born in Dunedin, New Zealand in 1972-73 and followed to age 38, with 95% retention. Symptoms of ADHD, associated clinical features, comorbid disorders, neuropsychological deficits, GWAS-derived polygenic risk, and life impairment indicators were assessed. Data sources were participants, parents, teachers, informants, neuropsychological testing, and administrative records. Adult ADHD diagnoses used DSM5 criteria, apart from onset-age and cross-setting corroboration, which were study outcomes.
As expected, the childhood-ADHD group showed 6% prevalence, male excess, childhood comorbid disorders, neurocognitive deficits, polygenic risk, and, despite having outgrown their ADHD diagnosis, residual adult life impairment. As expected, the adult-ADHD group showed 3% prevalence, gender balance, adult substance dependence, adult life impairment, and treatment contact. Unexpectedly, the childhood-ADHD and adult-ADHD groups comprised virtually non-overlapping sets; 90% of adult-ADHD cases lacked a history of childhood ADHD. Also unexpectedly, the adult-ADHD group did not show tested neuropsychological deficits in childhood or adulthood, nor did they show polygenic risk for childhood ADHD.
Findings raise the possibility that adults presenting with the ADHD symptom picture may not have a childhood-onset neurodevelopmental disorder. If this finding is replicated, then the disorder's place in the classification system must be reconsidered, and research must investigate the etiology of adult ADHD.