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Children who are youngest in their classroom are more likely to be diagnosed with ADHD, study finds

photo of teacher in classroom
REUTERS/Jim Young
The rate of ADHD diagnoses among children in the United States has risen dramatically during the past two decades.

The youngest children in a classroom are more likely to be diagnosed with attention deficit hyperactivity disorder (ADHD) than their older classmates, according to a study published Wednesday in the New England Journal of Medicine (NEJM).

That finding is troubling, for it means that the excessive activity, impulsiveness and other behaviors attributed to ADHD may, in many cases, have more to do with the age a child enters school than with any underlying neurological disorder.

“Our findings suggest the possibility that large numbers of kids are being over-diagnosed and overtreated for ADHD because they happen to be relatively immature compared to their older classmates in the early years of elementary school,” said Timothy Layton, the study’s lead author and an assistant professor of health care policy at Harvard University, in a released statement.

Rising numbers, growing concerns

As background information in the study points out, the rate of ADHD diagnoses among children in the United States has risen dramatically during the past two decades. That’s been particularly true for young children. Between 2007 and 2012, the rate of diagnosis among children aged 2 to 5 increased by more than 50 percent.

In 2016, more than 5 percent of all children under the age of 18 were taking medication to treat ADHD.

The increasing rate of ADHD diagnoses, coupled with the fact that the rate varies greatly from one geographical area to another, has raised concerns — and generated controversy — about how the diagnoses are being made.

“The main factor that may contribute to the probability that a child will receive a diagnosis of ADHD is the child’s behavior, as observed by teachers or parents, relative to the behavior of other children in the same school grade,” write Layton and his colleagues in their NEJM paper. “Some evidence suggests that teachers or school personnel are more likely than parents or physicians to first suggest a diagnosis of ADHD.”

Yet, any classroom in which the children’s ages span 12 months is likely to have a wide range of normal behavior, the researchers stress.

Several earlier studies have suggested that the youngest children in a particular grade are more likely to be diagnosed with ADHD.  Those studies are now more than a decade old, however, and relied on information provided by surveys, which is not always accurate. Layton and his colleagues decided to take a fresh and more objective approach to investigating the topic.

Study details

For their study, the researchers used information from a large insurance database in the United States. They compared the difference in ADHD diagnosis by birth month — specifically August versus September — for more than 407,000 elementary school children born between 2007 and 2009. They followed the children through the end of 2015, which meant that all the children had completed at least one year of school.

The researchers focused on the birth months of August and September because many states — including Minnesota — use Sept. 1 as a cutoff data for enrollment in kindergarten. That means that children who turn 5 after Sept. 1 must wait until the next year to start school. August-born children, therefore, are usually the youngest in a classroom, and September-born ones are usually the oldest.

The study found that in states with a Sept. 1 cutoff date, rates of ADHD diagnosis and treatment were 34 percent higher among children with August birthdays than among those born in September. The association was particularly strong among boys.

In states without a Sept. 1 cutoff date, however, the children’s birth month made no difference in the rates of ADHD diagnosis and treatment.

Raising reasonable questions

In an opinion piece in the New York Times, Layton and two of his co-authors say their study’s findings underscore just how subjective the diagnosis of ADHD can be.

“Despite growing awareness that A.D.H.D. may be overdiagnosed and the fact that the medications used to treat it have serious side effects, something as arbitrary as the month a child is born still has a meaningful impact on the likelihood that the child is determined to have the condition,” they write.

“At a minimum, physicians who frequently diagnose A.D.H.D. in children should be aware of these findings,” the researchers stress. “A simple mental ‘adjustment’ for whether a child is born in August may be sufficient to help physicians reduce overdiagnosis.”

“School personnel and parents should also be aware of how simple cognitive biases can creep into how important clinical decisions are made,” they add. “Both our and previous findings suggest that parents of children who are young for their grade could reasonably question whether the initiation of medical treatment for A.D.H.D. should be delayed.”

FMI:  You’ll find an abstract of the new study on NEJM’s website. You can read the New York Times opinion piece by the study’s authors on that newspaper’s website.

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