Autism rates could be leveling off at just above one percent of children, Wisconsin researchers suggest.
Between 2002 and 2008, they found the number of kids in the state's special education autism category nearly doubled. But the increase was only seen in those schools that started out with very few autistic kids, hinting that the statewide rates may be stabilizing.
Autism spectrum disorders, which range from mild Asperger's Syndrome to severe mental retardation and social disability, affect about one in 110 children in the U.S., according to the Centers for Disease Control and Prevention.
Nationwide, the number has been rising inexplicably over recent decades, and experts have argued about the reasons.
One possibility is that kids who used to be classified as mentally retarded, or just plain eccentric, are now getting an autism-spectrum diagnosis. Another, more worrisome suggestion, is that some environmental factor could be impacting children's brain development.
The new study hints that at least some of the increase could be due to schools putting more and more kids in the autism category, said Matthew Maenner, a PhD student at the University of Wisconsin-Madison, who worked on the new study.
"The prevalence of autism in special education doesn't seem to be the same everywhere, and it doesn't seem to be increasing at the same rate everywhere," he said.
The findings, published in the journal Pediatrics, show that the statewide rate of children receiving special education for autism went from five to nine cases per 1,000 over seven years.
Not all of these kids may have a medical diagnosis of autism, but Maenner said data on enrollment in special education programs are often used as a proxy for the prevalence of disabilities.
Schools varied widely in the number of autistic children they had. But over time, the gap narrowed from a spread of more than 24 times to less than two.
The schools with more than one percent autistic kids at the beginning of the study period saw little or no change.
Dr. John Harrington, an autism expert who was not involved in the research, said it looked as if the autism rate could be stabilizing.
"As you get better at identifying something, your numbers get less varied," he told Reuters Health. "Finally we can look at these kids and say, they are not just odd, they have a diagnosis."
Harrington, who wrote a commentary on the new findings, said he believed the same patterns could be found in other parts of the country.
But he stressed they don't necessarily mean that the national increase in autism is solely driven by better diagnoses.
"Most of the experts don't want to hang their hat on it and say we're just calling a chicken a rooster," said Harrington, of the Eastern Virginia Medical School in Norfolk.
Last year, the CDC estimated that autism rates had gone up by 60 percent for boys since 2002 and 48 percent for girls.
"There's a multi-pronged approach going on because we know that there is no single cause for autism. We're not going to find the one answer," the CDC's Catherine Rice told Reuters at the time.