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More than 10 percent of U.S. kids have been diagnosed with attention-deficit/hyperactivity disorder (ADHD), up from 6 percent 20 years ago, researchers reported Friday.

It’s not clear why, but it’s startling, said Dr. Wei Bao of the University of Iowa, who helped lead the study.

“It is very common now – one in 10 kids,” Bao told NBC News.

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The team used surveys covering more than 180,000 children aged 4 to 17 between 1997 and 2016. The surveys were in-person with a parent or guardian and asked whether the child had ever been diagnosed with ADHD.

“Over the 20-year period from 1997 to 2016, we found a significant increase in the prevalence of diagnosed ADHD from 1997-1998 to 2015-2016,” they wrote in the Journal of the American Medical Association’s JAMA Open Network.

“We found a consistent upward trend across subgroups by age, sex, race/ethnicity, family income, and geographic regions.”

It’s not clear why. ADHD has a genetic underpinnings. Smoking in pregnancy, low birth weight and other factors also play a role.

“There might be multiple reasons,” Bao said.

“First, we are knowing this condition better, so that we are better at recognizing and diagnosing this condition. Second, the public are more aware of this condition, increasing the possibility of affected kids being screened and diagnosed,” Bao said.

“Third, more infants born early or small (i.e., preterm birth, low birth weight) survive, but they are at higher risk of developing ADHD.”