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About half of children diagnosed with ADHD - or attention-deficit hyperactivity disorder - seem to grow out of it by the time they're in their 20's. A new study may explain why. It found that some regions of the brain in children with ADHD tend to mature later than normal.

NPR's Jon Hamilton has more.

JON HAMILTON: Scientists studying the developing brain usually have to rely on MRI scans that are like single snapshots. They only capture what's happening at a particular moment in time. But a team at the National Institute of Mental Health has spent years taking multiple snapshots of children's brains at regular intervals as they grow up. Now, they've assembled these images into time-lapse movies that show how the brain matures. One sequence is following children with ADHD.

Philip Shaw is the investigator who led the team. He pulls up a movie on his laptop.

Dr. PHILIP SHAW (Lead Investigator, National Institute of Mental Health): You can see a wave of brain maturation going from this middle line and the frontal. And the last bit to mature are these bits in the middle, on the side, at the front, which are important for the control of action and attention.

HAMILTON: Action and attention are key functions that are disrupted in children with ADHD. Shaw's team studied more than 400 children, half of them with ADHD. The team measured changes in the thickness of the brain's gray matter or cortex. Shaw says that for all the kids, the cortex followed the same sequence of change.

Dr. SHAW: It starts off relatively thin in early childhood. It gets thicker then reaches its peak cortical thickness and then it gets thinner throughout adolescence.

HAMILTON: But Shaw says when they compared the two groups, the timing of these changes was different.

Dr. SHAW: The big finding was the difference between the kids with ADHD and those who didn't have the disorder in the age at which they reached this milestone of peak cortical thickness. Healthy kids had a peak at around age 7 or 8, kids with ADHD, a couple of years later around the age of 10.

HAMITON: The delay in this developmental milestone was most apparent in the area of the brain that controls action and attention.

Judith Rapoport, another scientist involved in the study, says the scan showed that the brains of many of the kids with ADHD did eventually catch up.

Dr. JUDITH RAPOPORT (Scientist, National Institute of Mental Health): It's long been known that some kids, even if they seem very hyperactive and are a lot of trouble in early grades seem to grow out of it and do extremely well as their grandmother said all along they would.

HAMILTON: But it's not clear from the scans why some kids with ADHD continue to have problems. Researchers suspect it's because their brains remain immature.

Rapoport says teachers, doctors and investigators could do more for these children if they were able to identify them at the earliest possible age.

Dr. RAPOPORT: If we could know earlier on in the game which are the children who are going to grow out of it, one could reassure the parents, one could take all those resources that are being wasted, apply them to the ones who are going to not do as well.

HAMILTON: The clues may come from studying the brains of infants. Nearly all of the children in this study were at least 5 when they had their first MRI. Younger kids are hard to diagnose and less likely to lie still for a brain scan.

Even so, Stewart Mostofsky of Kennedy Krieger Institute in Baltimore says researchers need to find a way to look at younger brains.

Dr. STEWART MOSTOFSKY (Research Scientist, Kennedy Krieger Institute): The real important question is what is happening earlier in brain development and brain maturation that is contributing to the onset of the disorder, the development of this disorder.

HAMILTON: And that may involve events that take place even before birth. The new study appears in the online edition of the Proceedings of the National Academy of Sciences.

Jon Hamilton, NPR News.

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