This is a question that researchers have been struggling with for decades, as developed nations such as the United States have seen an alarming rise in the number of children diagnosed as autistic.
Now, a new study suggests that this increasing prevalence of autism may, at least in part, be caused by a gradual broadening of the definition of the disorder.
In a re-evaluation of 38 patients ages 15 to 31 who had been diagnosed with developmental language disorder when they were children, eight met the current criteria for autism and five had characteristics of milder autism spectrum disorders, according to Dorothy Bishop, D.Phil., of the University of Oxford, and colleagues.
"It would be rash to conclude that the increasing prevalence of autism is entirely explicable in terms of broadening diagnostic criteria," they said, "but the data reported here illustrate how...changes in diagnostic concepts and clinical awareness have led to diagnostic reassignment from language disorder to autistic disorder."
There are two main hypotheses for the apparent increase in autism in recent years, according to the researchers.
One — the "autism epidemic" hypothesis — is that the prevalence of the disorder is actually increasing. The other — the "diagnostic substitution" hypothesis — is that changes in diagnostic criteria have expanded the number of children labeled as "autistic."
It is very plausible that the same child might be diagnosed at one time with developmental language disorder and at another time with autism, the researchers said, because communication problems are fundamental to both disorders.
To explore the issue further, the Oxford researchers re-evaluated 38 patients (31 males and seven females) who had participated in studies of developmental language disorder as children from 1986 through 2003 and had never been diagnosed with autism.
Most of the participants had been evaluated as children using two earlier versions of the standard medical reference, the Diagnostic and Statistical Manual of Mental Disorders — DSM-III and DSM-III-R.
"Both of these schemes, especially DSM-III, adopted more stringent criteria [for a diagnosis of autism] than are currently used," the researchers said, "and milder forms of autism, currently referred to as [autism spectrum disorders], were not well recognized."
The earlier DSM editions described autistic children as completely closed off, with little interest in interacting with other people, a description, the researchers said, that did not fit the participants in this study.
The authors acknowledged that their study was limited by the small sample size and added that caution should be used to interpret the results. Nevertheless, they concluded, "this study provides direct evidence of diagnostic substitution, indicating that many children who were diagnosed with severe language disorders in the 1980s and 1990s displayed behaviors that would be regarded as meriting a diagnosis of [autism spectrum disorder] according to contemporary criteria."
This study is reported online in the May 2008 edition of Developmental Medicine & Child Neurology.