Nearly 16 million American adults have been diagnosed with attention deficit hyperactivity disorder (ADHD), but evidence suggests that more than 30 percent of them don’t respond well to stimulant medications like Ritalin and Adderall. A new clinical trial provides a surprising explanation for why this may be the case: There are individual differences in how our brains circuits are wired, including the chemical circuits responsible for memory and concentration, according to a new study co-led by the University of Maryland School of Medicine (UMSOM) and performed at the National Institutes of Health (NIH) Clinical Center.