Twenty percent of ADHD children may have been misdiagnosed because of their age

Michigan State University economist Todd Elder, using a sample of nearly 12,000 children taken from Early Childhood Longitudinal Study Kindergarten Cohort funded by the National Center for Education Statistics, examined the difference in ADHD diagnosis and medication rates between the oldest and youngest children in a class. He found that the youngest kindergartners were 60% more likely to be diagnosed with ADHD than the oldest children in the same class and that when a similar group of reached the fifth and eighth grades, the youngest were more than twice as likely to be prescribed ADHD drugs such as Ritalin. This suggests that  around 20% of the 4.5 million children currently diagnosed in the US  as having ADHD may have been misdiagnosed.

Elder used the children’s birth dates and the kindergartens’ eligibility dates to fix on the youngest and oldest children in a class, the most usual date being September 1st with most schools requiring that the children have turned five by that date to be enrolled.

The results of the study were definitive: in Michigan, where the kindergarten cutoff date is December 1st, children born on December 1st (the youngest in the class) had much higher rates of ADHD than children born on December 2nd (the oldest in the class). Similarly, August-born kindergartners in Illinois were much more likely to be diagnosed with ADHD than Michigan kindergartners born in August of the same year because Illinois’ kindergarten cutoff date is September 1st so the August-born children were the youngest in their grade, whereas the Michigan children were not.

These children are significantly more likely than their older classmates to be prescribed behaviour-modifying stimulant drugs such as Ritalin. Such inappropriate treatment is particularly worrying because of the unknown impacts of long-term stimulant use on children’s health, while it wastes an estimated $320 million-$500 million a year on unnecessary medication.

If a child is behaving poorly, Todd Elder suggests, is inattentive or cannot sit still, it may simply be because he is five and the other children are six. There is a big difference between the behaviour of a five-year-old and a six-year-old which needs to be taken into account when evaluating for ADHD, especially since there are not neurological markers such as a blood test for ADHD. Many ADHD diagnoses may influenced by teachers’ perceptions of poor behaviour among the youngest children in a classroom but this behaviour may merely reflect emotional or intellectual immaturity rather than ADHD.

Journal of Health Economics

First published in June 2010

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