ADHD may emerge after childhood for some people, according to new study

Attention-deficit/hyperactivity disorder (ADHD) is a developmental disorder marked by inattention, hyperactivity and impulsivity and is one of the most common behavioural disorders in children. It is widely believed that adult ADHD is the continuation of the disorder from childhood.

However, researchers from the Institute of Psychiatry, Psychology & Neuroscience (IoPPN) at King's found that nearly 70 per cent of the young adults with ADHD in their study did not meet criteria for the disorder at any of the childhood assessments. Adults with this 'late-onset' ADHD had high levels of symptoms, impairment and other mental health disorders.

Published in JAMA Psychiatry, these findings have important implications for our understanding of ADHD, as ADHD that onsets in adulthood could have different causes to childhood ADHD.

Findings from this UK cohort are confirmed by evidence for adult-onset ADHD world-wide: a study from Brazil will be published by JAMA Psychiatry alongside this research, which also identified a large proportion of adults with ADHD as not having the disorder in childhood. Both the UK and Brazilian studies support previous findings from a New Zealand cohort.

The research sample in the King's College London study included more than 2,200 British twins from the Environmental Risk (E-Risk) Longitudinal Twin Study. Symptoms of childhood ADHD were measured at the ages of 5, 7, 10 and 12 through mother and teacher reports. Young adults were interviewed at the age of 18 to assess ADHD symptoms and any associated impairments, as well as the existence of other mental health disorders.

As the study was a cohort of twins, the researchers were also able to examine the genetic basis of ADHD. They found that adult ADHD was less heritable than childhood ADHD, and that having a twin with childhood ADHD did not place individuals at a higher risk of developing late-onset ADHD.

Dr Jessica Agnew-Blais from the IoPPN at King's College London said: 'We were very interested by this large 'late-onset' ADHD group, as ADHD is generally seen as a childhood-onset neurodevelopmental disorder. We speculated about the nature of late-onset ADHD: the disorder could have been masked in childhood due to protective factors, such as a supportive family environment. Or it could be entirely explained by other mental health problems. Alternatively, late-onset ADHD could be a distinct disorder altogether. We think it is important that we continue to investigate the underlying causes of late-onset ADHD.

'Although ADHD occurs in approximately 4 per cent of adults, relatively few adults receive a diagnosis or treatment for the disorder. It is crucial that we take a developmental approach to understanding ADHD, and that the absence of a childhood diagnosis should not prevent adults with ADHD from receiving clinical attention.'

Professor Louise Arseneault, also from the IoPPN at King's College London, said: 'Our research sheds new light on the development and onset of ADHD, but it also brings up many questions about ADHD that arises after childhood. How similar or different is 'late-onset' ADHD compared with ADHD that begins in childhood? How and why does late-onset ADHD arise? What treatments are most effective for late-onset ADHD? These are the questions we should now be seeking to answer.'