Scientists have found differences in the brains of children with attention-deficit hyperactivity disorder.
ADHD is linked to brain abnormalities
University of California Los Angeles researchers found some areas of the brains of the children were smaller, but others had more grey matter.
Other studies have suggested that ADHD is linked to abnormalities in areas of the brain which control attention.
But the latest study suggests there are also structural changes in areas which control impulsive behaviour.
The researchers say they were able to combine the latest scanning technology with computer analysis to provide more detailed information about the differences in the brains of ADHD children.
ADHD is a serious behavioural disorder which experts estimate may affect up to 6% of children.
People with the condition have a poor attention span and tend to be impulsive and restless.
However, the underlying cause is still poorly understood.
The US researchers carried out scans on the brains of 27 children with ADHD, and 46 children with no signs of the disorder.
The ADHD children showed evidence of a reduction in the size two areas of the brain - one of each side - called the dorsal prefrontal cortices.
Similarly, there was evidence of a size reduction in the anterior temporal areas - also found on each side of the brain.
However, the scans also showed substantial increases in grey matter in large portions of the posterior temporal and inferior parietal cortices in children with ADHD.
Professor Bradley Peterson, of Columbia University, New York, who worked on the study, said the findings showed that abnormalities occurred not just in areas of the brain known to control attention, but also in regions which control impulsive behaviour.
"These findings may help us understand the sites of action of the medications used to treat ADHD, particularly stimulant medications.
"In conjunction with other imaging techniques, the findings may help us to develop new therapeutic agents given our knowledge of the cellular and neurochemical make-up of brain regions where we detected the greatest abnormalities."
Dr Mark Berelowitz, a child and adolescent consultant psychiatrist at the Royal Free Hospital, told BBC News Online the research helped give scientists a fuller picture of the physical causes of ADHD.
"This tells us more than we knew already about the parts of the brain that are affected by ADHD," he said.
"Children with ADHD have symptoms of over-activity, impulsivity and poor concentration, but previous scans have only highlighted differences in part of the brain related to one of these problem areas - attention.
"This suggested that we were either missing something in the brain, or we did not understanding the clinical problems sufficiently.
"Now the clinical condition and the brain imaging are beginning to join up and can explain one another."
Dr Berelowitz said that the study should also convince people who doubted that ADHD was a real clinical disorder.
"This should give food for thought to those who view ADHD as a 21st century construct."
However, he stressed that further studies were required to confirm the findings, and stressed that giving children with symptoms of ADHD brain scans would not be helpful at this stage.