Social media could be costing your tween some of their brain power, a new study says.

Children between 9 and 13 years of age who spent more time on social media performed worse on tests of reading, memory and language two years later, researchers reported today in the Journal of the American Medical Association.

"This study found that even low levels of social media use were associated with poorer cognitive outcomes," said lead researcher Dr. Jason Nagata, an associate professor of pediatrics at the University of California-San Francisco.

"Our results suggest that the early adolescent brain may be particularly sensitive to social media exposure, underscoring the importance of age-appropriate introduction to these platforms and careful monitoring," he said in a news release.

For the study, researchers analyzed data from more than 6,500 kids participating in the nationwide Adolescent Brain Cognitive Development (ABCD) Study, the largest long-term study of brain development in the U.S.

About 58% of the kids spent virtually no time on social media; 37% were spending an additional hour a day on social media by 13; and 6% as many as 3 additional hours by 13, results showed.

The ABCD study examined kids' brain power using a battery of tests designed by the National Institutes of Health, researchers said.

The kids with low use of around an hour a day scored an average 1 to 2 points lower on memory and reading tests, researchers found

Those who spent up to three hours a day scored up to 4 points lower on tests, the study said.

"These differences are subtle but consistent," said Nagata.

"Because cognitive skills such as reading and memory are fundamental to learning, even small declines across a large population could have meaningful educational implications."

Researchers suspect some kids might be ignoring homework to instead scroll social media, affecting their education and development.

"Social media is highly interactive and can displace time spent on reading or schoolwork," Nagata said.

"Building healthy screen habits early could help protect learning and cognitive growth."

The results support recent efforts by schools to limit phone use during the day, as well as more stringent measures like stricter age restrictions on social media, researchers said.

However, because the study is observational, it could not draw a direct cause-and-effect link between social media use and children's brain power, researchers noted.