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One reason for the dramatic increase in children diagnosed with autism in Utah is the expanded definition of the disorder, according to a new study.

University of Utah researchers applied today's criteria for autism-spectrum disorders to children who were considered challenged in the 1980s and who participated in an autism study at that time. The new study found a majority of those kids would now be classified as autistic.

"The modern criteria have expanded the number of folks who are included in this diagnosis," said one of the study's authors, Deborah Bilder, an assistant professor of psychiatry who works in the U.'s autism diagnostic clinic.

It is known that the expanded definition has increased the identification of high-functioning individuals with autism. The new study shows it has also boosted identification of those who have both autism and an intellectual disability.

"It does support the idea that a part of this increase in prevalence is related to the broadening criteria," said Bilder.

She said she is not convinced that the alarming spike in autism prevalence reflects a true increase in the number of cases driven by genetic or environmental causes.

A March study by the Centers for Disease Control and Prevention found Utah has the nation's highest rate of autism at 1 in 47 children, compared to 1 in 88 in the nation.

Bilder believes — speaking only for herself and not the co-authors — that increased awareness and an expanded definition is key.

"I'm not convinced yet this is truly an epidemic," she said. "Rather, we are much more aware of it. We identify it in much more higher-functioning folks than we recognized before."

In the latest study, researchers re-evaluated data from a 1980s Utah autism study. That study was completed "pre-epidemic" — when the autism definition was more restrictive, before autism became a distinct special education category and before campaigns raised the awareness among doctors and parents about the disorder.

That study, which attempted to identify all possible cases of diagnosed or undiagnosed autism in children and young adults between the ages of 3 and 25, found autism affected four in 10,000.

Researchers had solicited candidates through agencies known to treat people with developmental disabilities.

In the new study, researchers applied the current criteria to 108 individuals previously determined not to have autism. (Another 30 who didn't have autism didn't have complete records.)

Sixty-four, or 59 percent, would be classified as autistic today — and most of those would meet the higher threshold of "autism" instead of "autism spectrum disorder not otherwise specified," according to the study.

Most of the newly identified individuals had an intellectual disability, in addition to autism. Their average IQ was 36 — lower than the group originally diagnosed as autistic in the '80s.

The research was published in the Journal of Autism and Developmental Disorders earlier this month.