Over the past twenty years, the rate of attention deficit/hyperactivity disorder (ADHD) diagnoses among children has risen dramatically. No one is really sure whether this is due to a true increase in the incidence of AHDH, a greater ability to recognize the condition, or simply inaccurate diagnosis. Whatever the cause, researchers publishing in The New England Journal of Medicine on Wednesday have found another strange factor at play: a child’s birthday.

“There’s a kind of battle being waged between people who think ADHD has been historically under-diagnosed and people who think it is now being over-diagnosed,” explains study lead author Timothy Layton, Ph.D., to Inverse. Layton is an assistant professor of health care policy at Harvard Medical School. Through a somewhat unorthodox research route, he and his team determined that the month that a child is born plays a role in the likelihood that a child with be diagnosed with ADHD.

Specifically, they found that children who are born in August and attend schools that adhere to a September 1 cutoff enrollment date are 30 percent more likely to receive an ADHD diagnosis, compared to their older peers enrolled in the same grade.

A new study suggests that a child's birth month influences whether or not they are diagnosed with ADHD. Unsplash / NeONBRAND

The relationship between birth month and ADHD diagnosis suggests the possibility that kids are being diagnosed with ADHD because they are simply immature compared to their older, elementary school classmates. A child fidgeting in class doesn’t necessarily have ADHD; they may simply be younger and thus have a harder time sitting still.

This isn’t to say that all children diagnosed with ADHD who have late birthdays don’t have ADHD, says Layton, but it does suggest something is up.

Somewhat peculiarly, the team discovered this trend in large insurance databases, which they used to compare the differences in AHDH diagnoses among 407,000 children who have either August or September birthdays and attended schools with a September 1 enrollment. The kids with September birthdays would be the oldest in their class.

On average, 86 out of 100,000 students born in August were either diagnosed or treated for AHDH, compared with 64 students per 100,000 born in September.

“The ironic thing, however, is that we can’t say definitively whether the August kids are being over or under diagnosed,” explains Layton. “But what we can say is that we learned something about the diagnosis process — which is that, however these diagnoses are occurring, it’s also seemingly not as a scientific as a process that people might hope it would be.”

Adderall is a prescription drug for ADHD. Wikimedia / Guinnog

At the very least, says Layton, the findings support the need for a more objective way to diagnose the condition. The birthday trend suggests that diagnoses are reliant on the subjective observations of teachers within the classroom, which then inform the doctor’s diagnosis.

That said, even if diagnoses become more accurate, there’s still the problem of how to treat these kids. Prescription drugs, like Ritalin and Adderall, can be extremely beneficial to people with ADHD by helping them become more focused. These drugs affect people without AHDH differently: Instead of having a calming effect, the drugs flood the brain with dopamine and cause both euphoria and, potentially, increased blood pressure, decreased appetite, and feelings of paranoia. Because ADHD medications are prescribed for long-term treatment, some scientists argue that doctors must get better at evaluating their patients before prescribing these drugs.

In subsequent studies, the team plans to investigate other drivers of ADHD diagnoses in classrooms in subsequent studies and to examine what happens to kids with August birthdays but September diagnosis rates. In the meantime, many debates about the rising ADHD diagnosis rate remain. Are more kids really getting misdiagnosed? Or did fading stigmas and the Affordable Care Act simply make it easier for parents to help their kids?

“While we can definitely show that being young for your class leads to more diagnoses and more treatment, what we can’t say about it is whether or not kids actually benefit from that additional attention because we didn’t observe educational outcomes in our study,” says Layton.

“All we can really say is that there is definitely an effect on the context on whether or not you get diagnosed.”