In 1902, Dr. G. F. Still, a researcher who worked with children with behavior disorders, theorized that the behavior problems were the result of neurological problems, not bad parenting. For the next six decades, researchers used neurological testing to search the brain for structural abnormalities that might explain the symptoms.

During the 1970s, researchers began looking more closely at symptoms of hyperactivity and concluded that the disorder appeared to be caused by cognitive disabilities of memory and attention problems. Around this time, scientists also discovered that attention problems could exist without hyperactivity and were likely to persist into adulthood.