The biological basis of A.D.H.D. seems both real and complex: “There are structural differences in brain volumes, there are connections between brain regions reflected in white matter changes in people with A.D.H.D., there are chemical changes in some of the major transmitter systems in the brain,” Dr. Mahone said. “When the brain is asked to do something, the brains of children with A.D.H.D. do the same task but the brain is less efficient.”

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