London, May 19 (IANS): While it is well established that childhood attention-deficit/hyperactivity disorder (ADHD) may continue into adulthood, new research suggests that for some people the disorder does not emerge until after childhood.

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 in this study found that nearly 70 percent 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 the journal JAMA Psychiatry, these findings have important implications for our understanding of the disorder, as ADHD that onsets in adulthood could have different causes to its childhood version.

"We were very interested by this large 'late-onset' ADHD group, as ADHD is generally seen as a childhood-onset neurodevelopmental disorder,” said one of the researchers Jessica Agnew-Blais from King's College London.

"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,” Agnew-Blais said.

"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,” Agnew-Blais noted.

For the study, the research sample included more than 2,200 British twins from the Environmental Risk (E-Risk) Longitudinal Twin Study.

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.