Researchers at Oregon Health & Science University estimate the true percentage of people with Attention-Deficit/Hyperactivity Disorder, or ADHD, in the United States is approximately 3.5% of the population — substantially lower than many common estimates. The study, published this month in the Journal of the American Academy of Child and Adolescent Psychiatry (JAACAP), also found that more than half of children with ADHD had a co-occurring mental disorder.