Nearly 1 million children may have been misdiagnosed with attention deficit hyperactivity disorder, or ADHD, not because they have real behavior problems, but because they're the youngest kids in their kindergarten class, researchers say.

Kids who are the youngest in their grades are 60% more likely to be diagnosed with ADHD than the oldest children, according to a study out today from Michigan State University, given exclusively to USA TODAY. 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.

About 4.5 million children have been diagnosed with ADHD, according to the studies.

Misdiagnosing children can have long-lasting effects, says assistant professor of economics Todd Elder, author of the Michigan State study. In fifth and eighth grade, the youngest kids 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 kids, it also can have significant side effects, causing headaches, dizziness and even high blood pressure, according to the paper from North Carolina State.

The findings could influence the way that teachers evaluate children with ADHD symptoms — as well as complicate parents' decisions about when to start children in kindergarten, Elder says.

Although cutoff dates for kindergarten vary by state, the most common date is Sept. 1, Elder says. In states using that date, those who turn 5 by Sept. 1 could start kindergarten, while those with Sept. 2 birthdays would have to wait a year.

Regardless of the date chosen, some kids in a class are always going to be a year older than others, Elder says. Teachers and pediatricians — who actually make the diagnoses and prescribe treatments — should evaluate kids based on their age, not their grade, he says.

Experts note that the new studies have limitations. Although the studies clearly show that younger kindergartners have higher ADHD rates, the studies don't explain why, says John Ratey, an associate clinical professor of psychiatry at Harvard Medical School.

Instead of younger children being overdiagnosed, Elder says, it's possible that older children are underdiagnosed, possibly because they seem more mature than their classmates.

And Ratey notes that teachers may contribute to children's developing ADHD by treating them differently, perhaps because they perceive some younger kids as being more disruptive.

Parents don't necessarily need to hold back all kids with birthdays before the September cutoff date, Elder says. Parents need to consider each child's needs.

In some cases, he says, parents can't afford to hold kids back a year. While public school kindergarten is free, an extra year of day care or preschool can cost tens of thousands of dollars.