Is Adult ADHD a Childhood-Onset Neurodevelopmental Disorder? Evidence From a Four-Decade Longitudinal Cohort Study

Abstract

Objective: 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. The authors report follow-back analyses of ADHD cases diagnosed in adulthood, alongside follow-forward analyses of ADHD cases diagnosed in childhood, in one cohort.

Method: Participants belonged to a representative birth cohort of 1,037 individuals born in Dunedin, New Zealand, in 1972 and 1973 and followed to age 38, with 95% retention. Symptoms of ADHD, associated clinical features, comorbid disorders, neuropsychological deficits, genome-wide association study-derived polygenic risk, and life impairment indicators were assessed. Data sources were participants, parents, teachers, informants, neuropsychological test results, and administrative records. Adult ADHD diagnoses used DSM-5 criteria, apart from onset age and cross-setting corroboration, which were study outcome measures.

Results: As expected, childhood ADHD had a prevalence of 6% (predominantly male) and was associated with childhood comorbid disorders, neurocognitive deficits, polygenic risk, and residual adult life impairment. Also as expected, adult ADHD had a prevalence of 3% (gender balanced) and was associated with adult substance dependence, adult life impairment, and treatment contact. Unexpectedly, the childhood ADHD and adult ADHD groups comprised virtually nonoverlapping 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.

Conclusions: The 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.

Figure 1

Childhood-ADHD and adult-ADHD groups comprised virtually non-overlapping sets.

Figure 2

The adult-ADHD and child-ADHD groups did not overlap. This was not simply because many childhood-ADHD cases just missed the 5-symptom threshold for adult diagnosis. Panels a and b show how many childhood-ADHD cases (N = 61) had each level of adult-ADHD inattention and hyperactive/impulsive symptoms. For comparison, panels c and d show how many adult-ADHD cases (N = 31) had each level of adult symptoms. (3 individuals who had ADHD as a child and as an adult are included in all graphs.) * 2 Study members with current schizophrenia at age 38 were excluded from the adult-ADHD diagnosis, per DSM5.

Figure 3

Few adult-ADHD cases had childhood onset before age 12 years. We used the available items rated by teachers at child ages 5, 7, 9, and 11: “very restless, often running about or jumping up and down, hardly ever still,” “squirmy fidgety child,” “poor concentration, short attention span.” Items were rated 0=does not apply, 1=applies somewhat, or 2=certainly applies, yielding a range from 0 to 6. Ratings were summed at each age, then averaged across ages 5, 7, 9, and 11. Panel a shows few adult-ADHD group members (N = 31) scored at least one symptom rated “2=certainly” by their teachers. For comparison, panel b shows that most childhood-ADHD group members (N = 61) scored more than one symptom rated “2=certainly.” (3 individuals who had ADHD as a child and as an adult are included in all graphs.)