A long-running study followed a whole birth cohort in Dunedin, New Zealand, from birth to age 38. Researchers compared people who met ADHD criteria as children with people who met ADHD symptom criteria as adults. The two groups barely overlapped. Most adults with ADHD had no history of the disorder in childhood (90%). Out of 61 children diagnosed with ADHD, only 3 still met the criteria at age 38, and those 3 made up just 10% of the 31 adults who qualified. The adults who met ADHD criteria had shown only mildly raised symptoms before age 12 (0.33 standard deviations). The childhood-ADHD group, by contrast, had shown extremely raised symptoms in the same period (1.60). The adult group also did not carry the raised genetic risk score linked to childhood ADHD, while the childhood group did. Because of this, the authors suggest that adult ADHD diagnosed by symptoms may not always be a condition that starts in childhood. They caution that they did not predict these results in advance, so other studies need to check them. The adult group was small (31 people), ADHD was not measured during the participants' twenties, and the cohort was largely untreated for ADHD.
Source: Terrie E. Moffitt, Renate Houts, Philip Asherson, et al.. Is Adult ADHD a Childhood-Onset Neurodevelopmental Disorder? Evidence From a Four-Decade Longitudinal Cohort Study. American Journal of Psychiatry (2015). doi:10.1176/appi.ajp.2015.14101266