Kids who are young for their grade level are unusually likely to be diagnosed with attention-deficit hyperactivity disorder (ADHD) — a worrying sign that, for many kids, plain old immaturity has been misdiagnosed as a clinical disorder.

In two separate studies — both appearing in a forthcoming issue of the Journal of Health Economics — researchers at Michigan State University and at North Carolina State University analyzed the relationship between date of birth and ADHD diagnosis. Both studies found that, for kids with birthdays just before the kindergarten-eligibility cut-off date in their state, the odds of an ADHD diagnosis were much higher than for kids who start school almost a full year older, born just after the eligibility cut-off.

In the Michigan State study, for example, the youngest kids were found to have 60% greater risk of ADHD diagnosis, according to USA Today, which first reported the study results this morning. That newspaper reports that — based on these findings — nearly 1 million children may be misdiagnosed, simply because they are less mature than their classmates.