The brains of hyperactive children are physically different from those of other infants.
Compelling evidence has been gathered during the largest magnetic resonance imaging study of juvenile brains of its kind, the AAAS heard on Sunday. Experts from the National Institute of Mental Health in Bethseda, Maryland, have revealed consistent differences in parts of the brain that are thought to be related to planned activity and ability to sustain attention, namely frontal, basal ganglia and cerebellar regions.
Judith Rapoport, the scientist who led the research, said that the work with 109 children also showed that sufferers had brains 5 per cent smaller than average, which points to a genetic root for the condition.
In addition, studies of identical twins where only one suffers from hyperactivity - a rare phenomenon - has shown that the hyperactive twin has a brain lesion, the other does not.
"There is increasing evidence that severe and persistent psychiatric disorders of childhood are secondary to relatively subtle abnormalities of brain development," Dr Rapoport said.
Attention deficit hyperactivity disorder is the most common mental disorder suffered by children in the US, making them inattentive and unable to control impulses.
Experts who believe observed anomalies may be the result of the widespread use of stimulant drug therapy have rubbished suggestions that differences in brain structure might be to blame for the condition.
Dr Rapoport's research has not yet pinpointed exactly how these observed differences are linked to the condition, but it is likely that they reveal an abnormal break- down in an early critical moment in the development of the foetal brain.
The observation does not provide experts with an objective way of diagnosing ADHD, which remains a matter of observing the child's behaviour.