Birth dates, school enrollment dates affect ADHD diagnosis rates, study shows

"The question we asked was whether children who are relatively young compared to their classroom peers were more likely to be diagnosed with ADHD," says Dr. Melinda Morrill, a research assistant professor of economics at NC State and co-author of a paper describing the study. "To answer the question, we looked at children born shortly before the kindergarten eligibility cutoff date and children born shortly after the cutoff date and compared the rates of ADHD diagnosis and treatment."

The researchers figured that children born just a few days apart should have the same underlying risk of having ADHD. So finding a significant difference in diagnosis rates between children born only a few days apart is strong evidence of medically inappropriate diagnosis.

Morrill explains that the study shows that children born just after the kindergarten cutoff date were 25 percent less likely to be diagnosed as having ADHD than children born just before the cutoff date. "This indicates that there are children who are diagnosed (or not) because of something other than underlying biological or medical reasons.

"We believe that younger children may be mistakenly diagnosed as having ADHD, when in fact they are simply less mature," Morrill says.

Morrill stresses that "we are not downplaying the existence or significance of ADHD in children. What our research shows is that similar students have significantly different diagnosis rates depending on when their birthday falls in relation to the school year."

For the study, the researchers examined data from two national health surveys and a national private health insurance claims database to evaluate rates of ADHD diagnosis and treatment in children. The data sources covered different time periods ranging from 1996 to 2006.

The paper was co-authored by Morrill, Dr. William N. Evans of the University of Notre Dame, and Stephen T. Parente of the University of Minnesota.