Younger children may be mistakenly diagnosed as having ADHD, when in fact they are simply less mature, a researcher says. (CBC)

As many as one million children in the U.S. may have been misdiagnosed as having an attention deficit disorder because of their age when they started kindergarten, a new study finds.

The youngest children in class were more likely to be diagnosed with attention deficit-hyperactivity disorder, or ADHD, than the oldest children, researchers report in an upcoming issue of the Journal of Health Economics.

"We believe that younger children may be mistakenly diagnosed as having ADHD, when in fact they are simply less mature," Melinda Morrill, a professor of economics at North Carolina State University and a co-author of the paper, said in a release.

In the study, researchers looked at almost 12,000 children to check for differences in ADHD diagnosis and medication rates between the youngest and oldest children in a grade.

Children born just after the kindergarten cut-off date, typically Sept. 1 in the U.S., were 25 per cent less likely to be diagnosed as having ADHD than children born just before the cutoff.

Birth date matters

For example, students who were born on Dec. 1 in Michigan and were the youngest in their class had much higher rates of ADHD than those born Dec. 2 and enrolled a year later as the oldest in their class, the researchers found.

"A child's birth date relative to the eligibility cutoff also strongly influences teachers’ assessments of whether the child exhibits ADHD symptoms but is only weakly associated with similarly measured parental assessments, suggesting that many diagnoses may be driven by teachers’ perceptions of poor behavior among the youngest children in a classroom," the study's authors wrote.

"These perceptions have long-lasting consequences: the youngest children in fifth and eighth grades are nearly twice as likely as their older classmates to regularly use stimulants prescribed to treat ADHD."

Even though the students were just a day apart in age, they seemed to be assessed differently, said study author Todd Elder, a professor of economics at Michigan State University in East Lansing.

If a child is behaving poorly, is inattentive or can't sit still, it may be because he's five and the classmates are six, Elder said.

There is no blood test for ADHD, and experts disagree on its prevalence, Elder said, noting teachers' opinions are often important in deciding to send a child for a mental health exam.

The study used data from two national health surveys and a national private health insurance claims database to examine rates of ADHD diagnosis and treatment.

A second study by researchers at North Carolina State University, Notre Dame and the University of Minnesota came to the same conclusion and appears in the same issue of the journal.