Nearly 1 million U.S. children have potentially been misdiagnosed with attention deficit hyperactivity disorder just because they’re the youngest in their kindergarten class—meaning they’re also the most immature—according to a new study from Michigan State University economist Todd Elder. They’re more likely than older classmates to receive behaviour-modifying drugs like Ritalin, Elder reports in the upcoming issue of the Journal of Health Economics, which is concerning because long-term stimulant use may have long-term effects that aren’t entirely understood. It also wastes millions of dollars on unnecessary medications. ADHD is the most commonly diagnosed behaviour disorder in American children, with at least 4.5 million diagnosed under age 18, but there are no neurological markers for it. The study found that the youngest kindergartners were 60 per cent more likely to be misdiagnosed with ADHD than the oldest kids in their grade. By the fifth and eighth grades, the youngest were more than twice as likely to be prescribed with stimulants. Overall, about 30 per cent of 4.5 million children identified with ADHD are probably misdiagnosed, it found.