Brain Size Tied to Attention Deficit Hyperactivity Disorder. A new study found that the brains of children and adolescents in whom attention deficit hyperactivity disorder is diagnosed are on average 3 percent to 4 percent smaller in volume than those of children without the condition. There were no controls for race or IQ. Maybe lower IQ kids have smaller brains, and then low grades cause them to get misdiagnosed as having ADHD.
Update: Apparently there were some controls for IQ. A vocabulary test was used as a proxy for IQ.