Though many of those cases of autism were on the mild end of the spectrum, he noted, some were more severe.
But Adesman said it seems likely that there are also children in U.S. schools who have an autism spectrum disorder that hasn't been identified.
"There are children who come into my practice at 7 or 9 years old who haven't yet been diagnosed," Adesman said.
The researchers said that replicating the results in other populations of children -- including in the United States and worldwide -- is essential.
The U.S. National Institute of Mental Health has more on autism.
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