That study, which tracked children for 10 years, takes the onus off the stimulant drugs used to treat the disorder, since it found the same size difference in ADHD children who were not given the medications. It also absolves parents of the blame fixed on them by some authorities, says Dr. Judith Rapoport, chief of the National Institute of Mental Health child psychiatry branch and lead author of a report appearing in tomorrow's Journal of the American Medical Association (news - web sites).

"There has been a lot of talk assuming this might be the parents' fault," says Rapoport. "Parents used to get a bum rap because they supposedly were not disciplining the children properly. This study supports a whole bunch of evidence suggesting that the condition is strongly biologically determined."

ADHD affects an estimated 3 percent to 5 percent of school-aged children, causing them to be overactive, impulsive and easily distracted. It is two to three times more common in boys than girls.

Starting in 1991, the researchers used magnetic resonance imaging (MRI) to look at the brains of 89 boys and 63 girls diagnosed with ADHD, comparing them with 139 young people without the condition. The images show the brain size of ADHD patients were 3 percent to 4 percent smaller than those of the unaffected group, a difference that persisted as the children grew older.

"There seems to be a general factor that has to do with growth of the brain, a fixed problem that happens early, possibly at birth," Rapoport says.

The finding of smaller brain size is not necessarily bad news for ADHD children, she adds, since about half the people diagnosed with the condition early in life eventually outgrow it.

And smaller brain size does not necessarily mean lower intelligence, says Carl M. Anderson, a development psychobiologist at McLean Hospital in Boston who is doing research along the same lines.

"Just because you have a smaller brain doesn't make you different in any way," says Anderson, who adds that a colleague, "a well-respected neurobiologist," was amused to learn he had a very small brain. "I don't think you can say categorically that these children are damaged for life," he says.

"The brain is amazingly adaptive," Anderson explains. "What this study says is that there is some factor, genetic probably, that makes the brains of these children smaller, probably in a complex interaction with dietary factors. Just having a smaller brain doesn't say anything about your intelligence."

Anderson praises the study as "unique in a number of regards. The first is that it is the largest study of brain size done across any disorder you can imagine."

Rapoport adds it is "the first study large enough to be believable."

The institute is doing a follow-up study, Rapoport says: "We are looking to see whether these abnormalities predict an outcome eight years later."

What To Do

More information about ADHD is available from the National Institute of Mental Health or the Center for Children and Youth with Disabilities.