In an experiment testing whether hyperactivity is due to a brain abnormality, the brain activity of 25 hyperactive adults was compared to the brain activity of 25 adults who were not hyperactive  The tests revealed that the hyperactive adults had much less brain activity in the premotor cortex, a region of the brain believed to control action, than did the nonhyperactive adults
The experimenters concluded that diminished activity in the premotor cortex is one cause of hyperactivity
The test was performed only on adults because even though the method by which the test measured brain activity is harmless to adults, it does require the use of radiation, which could be harmful to children.