Special education data provide misleading trends of changing autism prevalence

If statistics compiled by the U.S. Department of Education are to be believed, in 1992 the state of Illinois had only 322 diagnosed cases of autism among school children. In 2003, according to the same statistical source, Illinois had more than 6,000 children diagnosed as autistic.

National special education statistics, which showed a 657 percent increase in autism over the decade from 1993 to 2003, are routinely used to suggest the country is experiencing an epidemic of autism, a developmental disorder of children characterized by impaired social and communication skills as well as repetitive behaviors and obsessive interests.

But inconsistencies in how the condition is diagnosed throughout the nation's schools, and the fact that the increasing trend for autism coincides with a corresponding slump in the reporting of mental retardation and learning disabilities, challenges the use of special education data to portray a national epidemic of autism, according to a new study published in the current issue of a leading medical journal (April 3, 2006).

Paul Shattuck, a researcher at the University of Wisconsin-Madison's Waisman Center, writing in Pediatrics, says special education data cannot be used to claim there is an autism epidemic because the figures are "hopelessly confounded" by changing and uneven identification and reporting practices among schools and states.

At issue, says Shattuck, is the practice of "diagnostic substitution," where educators, over time, have increasingly applied the autism label to children who, in the past, would have been labeled differently.

"My research indicates that the increase in the number of kids with an autism label in special education is strongly associated with a declining usage of the mental retardation and learning disabilities labels in special education during the same period," Shattuck says. "Many of the children now being counted in the autism category would probably have been counted in the mental retardation or learning disabilities categories if they were being labeled 10 years ago instead of today."

The point, says Shattuck, is that identification and diagnostic practices change over time and can lead to a misperception that a condition is more prevalent than it has been in the past.

"Each year since 1994, the probability of using the autism label has increased while there has been a corresponding decrease in the likelihood of educators using the mental retardation and learning disabilities categories."

In contrast to the dominant pattern, California was found to be one of only a handful of states where there was no decrease in the number of children labeled mentally retarded corresponding with an upward trend in identification of autism. This undermines the use of data coming out of California as a representative indicator of what is happening in the rest of the country, as has been suggested in recent press accounts and official reports, Shattuck argues.