Nearly 1 million American children may have been misdiagnosed with attention deficit hyperactivity disorder, or ADHD, not because they have real behaviour problems but because they were the youngest in their kindergarten, researchers say.

Children who are the youngest in their grades are 60 per cent more likely to be diagnosed with ADHD than the oldest children, a study from Michigan State University has found.

A second study, by researchers at North Carolina State University and elsewhere, came to similar conclusions. Both are scheduled for publication in the Journal of Health Economics.

Misdiagnosing children can have long-lasting effects, said Todd Elder, an assistant professor of economics and author of the Michigan State study. In fifth and eighth grades, the youngest children in a class were more than twice as likely to use Ritalin, a stimulant commonly prescribed for ADHD, compared with the oldest students, his study says.

While many parents say Ritalin has helped their children, it also can have significant side effects, causing headaches, dizziness and even high blood pressure, the North Carolina State paper says.