More than one in 10 schoolchildren in the U.S., including one in five high school boys, has been diagnosed with attention deficit hyperactivity disorder, or ADHD, according to the U.S. Centers for Disease Control and Prevention.

The 6.4 million children between ages 4  and 17 diagnosed with ADHD is a 16 percent increase since 2007 and a 53 percent increase since 2003, according to the New York Times.

“Those are astronomical numbers. I’m floored,” Yale School of Medicine pediatric neurologist William Graf told the Times. “Mild symptoms are being diagnosed so readily, which goes well beyond the disorder and beyond the zone of ambiguity to pure enhancement of children who are otherwise healthy.”

The CDC also found that around two-thirds of the kids diagnosed with ADHD also receive medication like Adderall or Ritalin. While ADHD medication can improve a child’s attention and concentration, such stimulants can come with side effects like nervousness, insomnia, weight loss and lower blood pressure. Adderall and Ritalin have also been tied to psychotic episodes and can potentially be abused -- both medications affect the same dopamine pathways in the brain as methamphetamines and cocaine (though Ritalin’s effects on the brain are much more measured than Adderall).

Are children becoming more and more restless and inattentive? Or is the rise in ADHD diagnoses the result of diagnostic practices that seem to catch more children these days. Are doctors catching more kids that need treatment, or are we pathologizing childhood behavior that deviates from the “neurotypical” norm? (Similar questions are posed by the increasing rate of autism diagnoses in children.)

Since there’s no blood test or other firm marker for ADHD, diagnosing a child requires him or her to fit several criteria. The most commonly used guidelines now, outlined in the American Psychiatric Association’s Diagnostic and Statistical Manual-IV, have a five-point checklist for diagnosing ADHD. Primarily, the child must have been exhibiting six or more symptoms of inattention or hyperactivity for at least six months. But it can be hard to distinguish between these diagnostic criteria (“often does not seem to listen when spoken to directly”; “often gets up from seat when remaining in seat is expected”) and the restlessness of childhood.

“There’s a tremendous push where if the kid’s behavior is thought to be quote-unquote abnormal -- if they’re not sitting quietly at their desk -- that’s pathological, instead of just childhood,” Harvard Medical School professor Jerome Groopman told the Times.

The new edition of the DSM, coming out this year, is also changing the diagnostic criteria in a way that some critics have said makes it easier for kids to be labeled ADHD.

But by either diagnostic standard, there is scientific evidence showing that other factors besides a child’s behavior can play a role in whether he or she is deemed hyperactive or inattentive.

Numerous studies have shown that kids that are the youngest in their grade are more likely to be diagnosed with ADHD – something called the “relative age effect.” A younger child may seem more hyperactive or inattentive compared to the other kids in his grade because he’s at a different developmental stage.

One recent paper from University of British Columbia researchers published in the Canadian Medical Association Journal examined the medical records of more than 930,000 children, looking for evidence of a relative age effect in ADHD diagnoses. In Canada, the cutoff date for entry into kindergarten or first grade is Dec. 31, so children born in December are usually the youngest in their grade.

The findings were striking -- boys born in December were 30 percent more likely to be diagnosed than boys in January; girls born in December were 70 percent more likely to be given an ADHD diagnosis than girls born in January.

Also, December-born boys and girls were 41 percent and 77 percent more likely, respectively, to have a prescription for ADHD medication than their January-born peers.

“Our data underscore the dimensional and developmental nature of the symptoms of ADHD and the impact of contextual expectations on the likelihood of the diagnosis being made,” the authors wrote. “It is possible that closer consideration of a child’s behavior in multiple contexts, including those outside of school, may lessen the risk of unnecessary diagnosis when assessing children for ADHD.”