A child that is easily distracted, fidgety and interruptive in school might not have a clinical case of attention-deficit hyperactivity disorder (ADHD), but might rather just be acting his or her age, posit researchers behind two new studies of diagnosis trends.

Kids whose birthdays fall around school enrollment cutoff dates can find themselves as either the youngest or oldest among their peers, depending upon which side of the date they fall on. And that coincidence of the calendar can make a significant difference in a child's chances of getting diagnosed with ADHD, according to the new research.

Two separate studies, both set to publish in a future issue of the Journal of Health Economics, found that students whose birthdays fell just before their school's age enrollment cutoff date—and thus were among the youngest in their class—had a substantially higher rate of ADHD diagnoses than students who were born just a day or two later and were the oldest in the grade below.

"We believe that younger children may be mistakenly diagnosed as having ADHD, when in fact they are simply less mature," Melinda Morrill, an assistant professor of economics at North Carolina Sate University and coauthor of one of the papers, said in a prepared statement. "Similar students have significantly different diagnosis rates depending on when their birthday falls in relation to the school year."

After reviewing U.S. diagnosis and treatment records across a decade, she and her colleagues found that "being young for [a] grade more than doubles the change that a student is diagnosed with or treated for ADHD." And as they pointed out in their paper, "ADHD is an underlying neurological problem and incidence rates should not change dramatically from one birth date to the next."

The other paper, written by Todd Elder, an assistant professor of economics at Michigan State University, found that children who had been among the youngest in their kindergarten class had more than double the chance of regularly taking Ritalin (methylphenidate) by middle school and junior high school than their older peers in the same grade.

"If a child is behaving poorly, if he's inattentive, if he can't sit still, it may simply be because he's 5 and the other kids are 6," Elder said in a prepared statement. "There's a big difference between a 5-year-old and a 6-year-old, and teachers and medical practitioners need to take that into account when evaluating whether children have ADHD…'Symptoms' may merely reflect emotional or intellectual immaturity among the youngest students."

Some 4.5 million U.S. children under the age of 18 have been diagnosed with ADHD. And approximately 2.5 million children take stimulant medication to counteract the symptoms of the disorder. But if the findings in Elder's study hold true nationwide, some 20 percent of those kids might have been misdiagnosed due to their relatively young age in the classroom. And, as Elder noted in his paper: "Inappropriate treatment is particularly worrisome because of the unknown impacts of long-term stimulant usage on children's health." It also adds up to between $320 million and $500 million each year being spent on potentially unnecessary drugs, according to his analysis.

Family doctors are often the ones to make ADHD diagnoses, which are usually based on perceived behavior—especially in comparison with a child's peers (the National Institute of Mental Health's ADHD booklet asks if disruptive and other behaviors "happen more often in this child compared with the child's peers?"). And Elder found that children's school-start age, though a strong predictor of a teacher's perception of ADHD symptoms, for parents, is "only weakly related…perhaps because parents' frames of reference include children of similar ages, rather than children in the same grade."

To be sure, however, age-based development patterns are not to blame for all suspected cases of ADHD. "We are not downplaying the existence or significance of ADHD in children," Morrill said. She and her colleagues acknowledged that the higher rate of diagnoses of younger students might accurately reflecting a higher number of cases in that group—or uncovering a trend of under-diagnosis in the older children. "Even if it is the case that children entering school at younger ages triggers ADHD, this would suggest an important causal mechanism that the medical research should further explore…If being exposed to formal schooling at younger ages is actually causing a rise in ADHD, we must then revisit educational policy," they concluded.

Image courtesy of iStockphoto/NWphotoguy