The study looked at 407,846 American children born between 2007 and 2009 to see how much the diagnosis of ADHD occurred in relation to the child's birthday.
"We saw nothing in a few months from August to September, when we saw this big difference," said Layton, a health policy professor at Harvard Medical School. "We saw that it was only in countries that had had their absence on September 1, in countries that did not have anything."
Layton and his co-authors found that the rate of children with birthdays in August diagnosed with ADHD was 34% higher than those born in September. Among children born in August, 85.1 children per 10,000 had ADHD diagnoses, compared to just 63.6 per 10 000 births in September.
Children born in August also had a 32% higher rate of treatment than those born in September.
Some symptoms are normal behavior for children, but those with ADHD may not overcome. Symptoms can be serious and can cause difficulties at school, at home or with friends, said the CDC.
Children who are younger in their peers, such as those with a birthday in August at a school on September 1, could be diagnosed with ADHD, said Layton, since certain childhood behaviors may be in line with to those used for diagnosis.
"In essence, normal childhood behaviors are diagnosed as ADHD and they are treated for it, although they are just normal children. They are only younger than their peers, so they behave a little differently," he said.
But that can be a weakness, he said, in that "we do not know how clinicians actually do the diagnosis, how well they follow the protocols," which could leave room for "diagnostic bias against age".
Although it is unlikely to lead to a major change in results, Nigg and other researchers also noted that the authors of the study did not determine whether the children were actually enrolled in school.
Even with these limitations, Nigg believes that the study is valuable when considering ADHD diagnoses.
"It is really useful to continue to observe what we can call sociological drivers of diagnosis. It really helps to solve real cases where a child actually has a problem to be treated, in relation to some kind of indirect cases that can be normal development that just looks bad because of the situation in to whom the child is, "he said.