Study Shows Birth Dates, School Enrollment Dates Affect ADHD Diagnosis Rates

Rising rates of attention deficit hyperactivity disorder (ADHD) and large differences in diagnosis rates have led to fears that the condition is often being misdiagnosed. A new study from North Carolina State University demonstrates that these concerns are justified. The researchers found large discrepancies in diagnosis and treatment rates based on small differences in children’s dates of birth.

“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, “Measuring Inappropriate Medical Diagnosis and Treatment in Survey Data: The Case of ADHD among School-Age Children,” was co-authored by Morrill, Dr. William N. Evans of the University of Notre Dame, and Stephen T. Parente of the University of Minnesota. The paper is being published in a forthcoming issue of the Journal of Health Economics, in conjunction with a related paper from Michigan State University that arrives at similar conclusions as the result of a separate study.

NC State’s Department of Economics is part of the university’s College of Management.

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Note to editors: The study abstract follows.

“Measuring Inappropriate Medical Diagnosis and Treatment in Survey Data: The Case of ADHD among School-Age Children”

Authors: Melinda S. Morrill, North Carolina State University; William N. Evans, University of Notre Dame; Stephen T. Parente, University of Minnesota

Published: Forthcoming, Journal of Health Economics

Abstract: We exploit the discontinuity in age when children start kindergarten generated by state eligibility laws to examine whether relative age is a significant determinant of ADHD diagnosis and treatment. Using a regression discontinuity model and exact dates of birth, we find that children born just after the cutoff, who are relatively old-for-grade, have a significantly lower incidence of ADHD diagnosis and treatment compared with similar children born just before the cutoff date, who are relatively young-for-grade. Since ADHD is an underlying neurological problem where incidence rates should not change dramatically from one birth date to the next, these results suggest that age relative to peers in class, and the resulting differences in behavior, directly affects a child’s probability of being diagnosed with and treated for ADHD.