US researchers have pinned down new differences in the brain chemistry of people with attention deficit hyperactivity disorder (ADHD).
They found ADHD patients lack key proteins which allow them to experience a sense of reward and motivation.
The Brookhaven National Laboratory study appears in the Journal of the American Medical Association. It is hoped it could help in the design of new ways to combat the condition.
For far too long there has been an assumption that children with ADHD are deliberately willful
Previous research looking at the brains of people with ADHD had uncovered differences in areas controlling attention and hyperactivity.
But this study suggests ADHD has a profound impact elsewhere in the brain too.
Researcher Dr Nora Volkow said: "These deficits in the brain's reward system may help explain clinical symptoms of ADHD, including inattention and reduced motivation, as well as the propensity for complications such as drug abuse and obesity among ADHD patients."
The researchers compared brain scans of 53 adult ADHD patients who had never received treatment with those from 44 people who did not have the condition.
All of the participants had been carefully screened to eliminate factors which could potentially skew the results.
Using a sophisticated form of scan called positron emission tomography (PET), the researchers focused on how the participants' brains handled the chemical dopamine, a key regulator of mood.
In particular they measured levels of two proteins - dopamine receptors and transporters - without which dopamine cannot function effectively to influence mood.
ADHD patients had lower levels of both proteins in two areas of the brain known as the nucleus accumbens and midbrain.
Both form part of the limbic system, responsible for the emotions, and sensations such as motivation and reward.
Patients with more pronounced ADHD symptoms had the lowest levels of the proteins in these areas.
It suggests that teachers need to make sure that school tasks are interesting and exciting, so that children with ADHD are motivated to remain interested Professor Katya Rubia
Dr Volkow said the findings supported the use of stimulant medications to treat ADHD by raising dopamine levels.
The findings also support the theory that people with ADHD may be more prone to drug abuse and obesity because they are unconsciously attempting to compensate for a deficient reward system.
Andrea Bilbow, of the ADHD charity ADDISS, said the study might help convince people who argue that ADHD is more to do with bad parenting than any concrete medical difference.
She said: "The findings of this new research will go a long way to helping us understand the presentation of symptoms but more importantly it may give teachers more of an idea of what interventions should be used in the classroom in order to accommodate children with ADHD.
"For far too long there has been an assumption that children with ADHD are deliberately wilful which has led to mismanagement and ultimately permanent exclusions from school."
Professor Katya Rubia, of London's Institute of Psychiatry, said: "This study widens our horizons. It shows that ADHD is not just about abnormalities in the attention systems of the brain, but also abnormalities in the motivation and emotion centres.
"It suggests that teachers need to make sure that school tasks are interesting and exciting, so that children with ADHD are motivated to remain interested."