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PHILADELPHIA (CBS) — Children who enter elementary school a year younger than their peers are 30 percent more likely to be diagnosed with attention deficit disorder.

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That’s according to a study at Harvard Medical School that looked at children in states with a September enrollment cutoff.

add study children Children Who Enter School Year Younger Than Peers More Likely To Be Diagnosed With ADD, Study Finds

Credit: CBS3

Researchers found children with August birthdays were often a full year younger than classmates and had a harder time sitting still.

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They say factoring in a child’s age when assessing behavior is key.