updated 8/17/2010 1:18:02 PM ET 2010-08-17T17:18:02

How mature a child is compared to his peers may partly determine how likely he is to be diagnosed with attention deficit hyperactivity disorder (ADHD), according to a new study. This finding validates concerns that the condition is misdiagnosed, researchers say.

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North Carolina State University researchers found that children born just after the kindergarten eligibilty cutoff date were 25 percent less likely to be diagnosed with ADHD than children born just before the cutoff date. Children born just after the cutoff date are among the oldest in their class, and those born just before the cutoff date are among the youngest in their class.

For example, in schools where the cutoff date is Sept. 1, children born on Aug. 31 make the cutoff and are the youngest in their class; children born on Sept. 2 will wait an additional year to enter school and be among the oldest in their class.

Children born just a few days apart should have the same underlying risk of having ADHD, according to the researchers, so this significant difference in diagnosis rates is strong evidence that medically inappropriate diagnoses are being given.

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"We believe that younger children may be mistakenly diagnosed as having ADHD, when in fact they are simply less mature," study researcher Melinda Morrill said in a statement.

The characteristic behaviors associated with ADHD are inattention, hyperactivity and impulsivity, according to the National Institutes of Health. Rising rates of ADHDand large differences in diagnosis rates have led to fears that the condition is not accurately diagnosed.

Accurate diagnosing is crucial, the researchers say, because stimulant medicationsare often prescribed as a treatment for ADHD. The stimulants have side effects, including insomnia, stomachache, headache, dizziness and decreased appetite, and they have been shown to increase heart rates and blood pressure. Little is known about their long-term effects.

Youngest are 50 percent more likely to get ADHD diagnosis
The new findings are consistent with those of another forthcoming study that suggests children born just before the kindergarten cutoff date are 50-percent more likely to be diagnosed with ADHD than those born just after, Morrill and her colleagues write in a forthcoming issue of the Journal of Health Economics.

The findings indicate that there are children who are diagnosed, or who are not diagnosed, because of something other than underlying biological or medical reasons, according to Morrill.

"We are not downplaying the existence or significance of ADHD in children," Morrill said. "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."

In 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 findings will be published in a forthcoming issue of the Journal of Health Economics.

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