A rise in autism rates may be the result, at least in part, of a change in diagnoses that has seen many individuals previously diagnosed with other intellectual disability disorders reclassified as autistic, researchers say.

A study conducted by Penn State researchers suggests a tripling of the number of U.S. school children, who were placed in special education programs between 2000 and 2010 and labeled as autistic, was the result of educators "swapping" diagnoses.

As an example, children who used to be identified as having non-specific conditions labeled "intellectual disability" have now been identified as autistic, they say.

This change has created an increase in the prevalence of autism that is likely at last a statistical phenomenon more that an actual increase in the number of children with the condition, researchers explain.

"If you asked me, 'Is there a real increase in the prevalence of autism?' maybe there is, but probably much lower than the reported magnitude," said Penn State research leader Santhosh Girirajan.

Girirajan and colleagues analyzed data collected on approximately 6.2 million U.S. school children diagnosed with disabilities that have resulted in their being placed in special education programs.

Based on a diagnosis, each child was assigned to one of 13 categories of disability, one of which was autism.

Between 2000 and 2010 the number of children in the autism category more than tripled from 93,624 to 419,647, the researchers say, yet almost two-thirds of that increase was offset by a decline in the rate at which children were labeled as having an "intellectual disability."

That strongly suggests the autism rise is partly down to students being reclassified from one category to another, Girirajan says.

"For quite some time, researchers have been struggling to sort disorders into categories based on observable clinical features, but it gets complicated with autism because every individual can show a different combination of features," he explained.

Figures released by the U.S. Centers for Disease Control and Prevention suggest autism has increased in prevalence from 1 in 5,000 in 1975 to 1 in 68 in 2012.

While much of that rise has been linked to an increased awareness of autism and a broadening of the diagnostic criteria for the condition, the Penn State study is evidence that a significant portion of that increase may simply be a result of the reclassification of individuals with related or similar neurological disorders rather than an actual increase in the rate of new cases of autism.