Certain birth months are linked to higher rates of ADHD

(CNN) -- Children born in August in states where the cutoff to start kindergarten is a September 1st birthday are more likely than children in other states to be diagnosed and treated for ADHD. That's according to a study published in the New England Journal of Medicine.

There are 21 states with this cutoff date, according to the Education Commission of the States, meaning kids born in August in these states are usually the youngest in their classrooms--sometimes nearly a year younger than some classmates. Researchers say that could be part of the problem.

Health experts say this study confirms on a larger scale what some other studies have shown on smaller ones, but it doesn't account for how kids were diagnosed or if their physicians were following the right protocols to properly diagnose them.