Over the last couple of years, cases of children diagnosed with autism spectrum disorders (ASDs) increased by 30%, according to a reported issued by the  Centers for Disease Control and Prevention. Today, the CDC estimates that one in 150 8-year-olds in the U.S. has an autism spectrum disorder, or ASD.  There’s a whole debate surrounding this topic – where does this slew of new cases come from? Are we dealing with an epidemic-like event? It used to be vaccines that took the hit, but this was long debunked. There’s another, maybe more plausible explanation: it’s all a statistical mishap as far as diagnosis goes. In effect, if this is true, ASDs prevalence is stable, it’s in the way we count the cases that the problem might lie.

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In 2003, for instance, a large study conducted in Atlanta found that one in 166 to one in 250 children had autism, according to a report published in the Journal of the American Medical Association. Another study conducted by the CDC in 14 states found an overall prevalence of one in 152, which is general accepted figure for autism prevalence. But one study published in 2005, found the same rates of prevalence among children born in the same area of England from 1992 to 1995 and then from 1996 to 1998. Paul Shattuck, PhD, assistant professor of social work at Washington University in St. Louis and an autism researcher argues that the increase in reported cases is because of “diagnostic substitution”.

“A kid labeled autistic today could have been labeled mentally retarded 10 years ago in the same school system,” Shattuck says.

Indeed, it wasn’t until 1992 that schools began to include autism as a special education classification. One recent study published by Swedish researchers yet again found no direct evidence of an “autism epidemic.” The team used two different data sets, both from Sweden. In the first set, 20,000 children were included, all twins, born in that country over a span of 10 years. The second data set was compiled using a national patient register, based on diagnosis codes typically associated with ASDs (which includes autism,  Asperger’s syndrome and a condition known as pervasive developmental disorder).

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When parents were contacted directly and asked about symptoms associated with autism, the number of children who met the cutoff for an ASD remained stable—for the 10-year period under investigation. Over that same period, the number of diagnoses of ASDs in the register (data based on an alphanumeric code given to your provider) increased steadily.

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In the first data set, parents were contacted directly and asked about their children’s symptoms. For the ten years period, the number of children who met the cutoff for ASD, based on these criteria, stayed stable. Over the same time frame, however, the number of diagnoses of ASDs in the register increased year our. Based on this, the authors conclude that something else than an actual increase in the number of cases is at play. Earlier this year, another study, this time authored by Danish researchers, reached similar conclusions.

Going back to the 2014 CDC report cited at the beginning of this article, the authored reported a wide racial disparity in those diagnosed. Does this mean that ASDs affects children preferentially by race? That’s highly unlikely. There was also a significant variation of ASDs prevalence from state to state. Moreover, the criteria used to diagnose children with autism has been changed, and some have reason to believe fewer cases will be diagnosed as such.  All of these seem to point to the direction that we’re not actually seeing an autism epidemic; instead, it’s highly likely that the extra wave of diagnoses we’re seeing come from previously misdiagnosed cases or previously unreported ones in the first place.