Low levels of activity in specific circuits may help explain the seeming paradox of using stimulants like Ritalin to treat children who already seem overstimulated. In many children with A.D.H.D., these drugs can help the circuits function more normally.

“If you have a deficit in dopamine, it’s harder to concentrate on goal-oriented behavior,” Professor Pennington said. “The psychostimulants change the availability of dopamine in these same circuits.”

Although recent research has identified environmental factors that may increase the likelihood of developing the disorder, it is thought to have a stronger genetic component. Dr. Maximilian Muenke, chief of the medical genetics branch at the National Human Genome Research Institute, said that among identical twins, if one has A.D.H.D., the second has an 80 percent chance of having it as well. (Among fraternal twins, the comparable figure is 20 to 30 percent, the same as for any siblings.)

Dr. Muenke’s group published a paper last month identifying a gene, LPHN3, that is associated both with the disorder and with a favorable response to stimulants. But no one thinks that just one gene is responsible; just as attention is a complex phenomenon, so are the genetics of attention deficits.

When I asked Dr. Muenke whether genetic studies could someday play a role in treating the disorder, his reply was cautious. He spoke of eventually predicting which children will respond to specific medications, sparing families the frustration of switching from one medicine to another with no relief. He sounded more hopeful about the long-term prospects.

“I truly believe in the long run we will be able to develop personalized medicine for a child with A.D.H.D.,” he said, adding that when the specific underlying cause or causes are known, “this child will have a very specific treatment, whether this treatment is behavioral treatment alone or medication,” and the medication will be tailored to the child.