Taken at face value, the latest figures on attention deficit hyperactivity disorder (ADHD) suggest a growing epidemic in the United States. According to the Centers for Disease Control, 15 percent of high school children are diagnosed with ADHD. The number of those on stimulant medication is at 3.5 million, up from 600,000 two decades ago. ADHD is now the second most common long-term diagnosis in children, narrowly trailing asthma.