(CNN) In US states where children must turn 5 by September 1 in order to start kindergarten that year, children born in August, just before the cutoff, are more likely to be diagnosed with and treated for ADHD, according to a study published Wednesday in the New England Journal of Medicine.
The study looked at 407,846 US children born between 2007 and 2009 to see how many diagnoses of ADHD occurred in relation to the child's month of birth.
Researchers found that there were more diagnoses for children born in August and then found it was true only in states that have a September 1 cutoff date to start kindergarten, according to Timothy Layton, lead author of the study. That means the August-born children would be among the youngest in their classrooms, sometimes nearly a year younger than other classmates.
"We saw nothing for any of the pairs of months apart from August to September, where we saw this big difference," said Layton, an assistant professor of health care policy at Harvard Medical School. "We saw that this was only in states that had a September 1 cutoff. There was nothing in states that didn't have that."
According to the Education Commission of the States, which the researchers used as their source, there are 21 states with this cutoff date, including Texas, Arizona and Illinois. The remaining 29 states and the District of Columbia have a range of enrollment cutoff dates, including December 2 in California and as late as January 1 in Vermont.
Layton and his co-authors found that the rate of children with August birthdays who were diagnosed with ADHD was 34% higher than those born in September. Among the children born in August, 85.1 children per 10,000 had ADHD diagnoses, compared with just 63.6 per 10,000 born in September.
Children born in August also had a 32% higher rate of treatment for the condition than those born in September.
ADHD, or attention deficit hyperactivity disorder, is one of the most common neurodevelopmental disorders in children, according to the US Centers for Disease Control and Prevention. Symptoms include trouble paying attention, being overly active and trouble controlling impulsive behaviors.
In 2016, approximately 9.4% US children -- 6.1 million -- between the ages of 2 and 17 had been diagnosed with ADHD at one point in their lives, the CDC said. About 3.3 million of these children were 12 to 17 years old, 2.4 million were 6 to 11, and 388,000 were 2 to 5, the CDC said.
Some symptoms are normal behavior for children, but those with ADHD might not outgrow them. The symptoms can be severe and can cause difficulty at school, at home or with friends, the CDC said.
Children who are younger in their peer group, such as those with an August birthday in a school with a September 1 cutoff date, could be more likely to be diagnosed with ADHD, Layton said, as certain childhood behaviors may be consistent with those that are used for diagnosis.
"Basically, normal childhood behaviors are being diagnosed as ADHD, and they are being treated for them, even though they are just normal kids. They just happen to be younger than their peers, and so they behave a little differently," he said.
Dr. Dimitri Christakis, director of the Center for Child Health, Behavior and Development at Seattle Children's Research Institute, wrote in an email that "This study confirms on a larger scale what other studies have shown on a smaller one. It confirms that there may be developmental confounders in the diagnosis of the condition and that the relative immaturity of young children's brains can make those that are merely younger at school entry demonstrate behaviors consistent with ADHD." Christakis was not involved in the research.
Joel Nigg, a professor of psychiatry and behavioral neuroscience at Oregon Health and Science University, said that tracking the diagnoses through an insurance database allowed researchers to see what clinicians are doing, which can be more accurate than relying on parents' reports about their child's diagnosis.
But it could be a weakness, he said, in that "we don't know how the clinicians are actually doing the diagnosis, how well they are following protocols," which could leave room for "diagnostic bias related to age."
Although it's unlikely to lead to a big change in the results, Nigg and other researchers also noted that the study authors didn't determine whether the children were actually enrolled in school.
Even with these limitations, Nigg believes that the study is valuable when looking at ADHD diagnoses.
"It's really helpful to continue to look at the what we can call sociological drivers of diagnosis. It really helps sort out real cases, where the child actually has a problem that needs to be treated, versus sort of circumstantial cases that may be normal development that only looks bad because of the situation the child is in," he said.