A new study suggests that changes to autism diagnosis criteria may be more to blame for rising rates of the developmental disorder than anything else.

Since the 1960s, autism prevalence rates have skyrocketed from 4 in 10,000 children to a current reported rate of 1 in 88. The reason behind the rise, however, has remained unclear.

Now researchers are shedding new light on the trend by applying current diagnostic criteria to data from a 1980s study on autism prevalence in what’s believed to be a first-of-its-kind analysis.

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The original study, published in 1989, looked at hundreds of Utah residents ages 3 to 25 who were suspected to have autism. Clinicians used DSM-III criteria to assess individuals as “diagnosed autistic” or “diagnosed not autistic” and ultimately found an autism prevalence rate of 4 in 10,000 in Utah at that time.

But when a research team from the University of Utah applied current diagnostic criteria from the DSM-IV-TR to records from participants in the two-decades-old study, they found that most who were deemed to be autism-free at that time would receive the label today.

What’s more, the study authors indicate that the vast majority of those who went overlooked in the original study had low IQ’s and would now be diagnosed with both autism and intellectual disability.

“Thus, while it is well known that current DSM-IV-TR criteria increased the identification of high functioning individuals, our results indicate that they also increase identification of ASD among individuals with autism and intellectual impairment,” wrote researchers in the study published online in the Journal of Autism and Developmental Disorders this month.

The analysis found that 59 percent of those who were “diagnosed not autistic” in the 1980s would qualify as having autism today, while an additional 38 percent of people in this group showed some characteristics of autism.

Meanwhile, those who were found to have autism in the 1980s study continued to qualify for the diagnosis using the current criteria, the study found.

“The results of this study demonstrate a significant effect on ASD case status attributable to changing ASD criteria, particularly with regard to individuals with intellectual impairment,” the researchers said. “An important caveat, however, is that we were unable to determine whether it was the broadening of the criteria themselves, or the interpretation of the criteria, which lead to this effect.”