http://www.scienceblogs.com/insolence/2 ... sm_epi.php
One of the key arguments by advocates claiming a link between mercury in childhood vaccines is that there is an "epidemic" of autism. They'll claim that autism was unknown before the 1930's, when thimerosal was first introduced into vaccines. (Never mind that there are plenty of descriptions of autism-like conditions dating from as far back as the 18th century.) They'll then claim that there is an "epidemic" that accelerated in the 1990's, when additional vaccines were added to the recommended childhood schedule, and that it was the additional mercury from those vaccines that was responsible. It has been argued that a lot of that apparent increase was actually due to a widening of the diagnostic criteria for autism in the early 1990's.
Now there's more evidence to support that view. Just published in the April issue of the journal Pediatrics is a study by Paul Shattuck at the University of Wisconsin, entitled, The Contribution of Diagnostic Substitution to the Growing Administrative Prevalence of Autism in US Special Education. (Hat tip to the "source" who informed me of this article.)
In the paper, Shattuck analyzes special education figures that are being used to bolster claims of an autism "epidemic" and finds them wanting. In essence, diagnostic substitution can explain nearly all of the apparent increase of autism as recorded by the number of children receiving special education services. What that means is that children that would have been diagnosed with something else 15-20 years ago are now being diagnosed with autism. Dr. Shattuck starts with an example from a different condition, mental retardation as one of his reasons for suspecting diagnostic substitution as a cause of the perceived "epidemic":
Second, prior research has established a precedent of diagnostic substitution in special education enrollment. From 1976 to 1992 the number of children in the mental retardation (MR) category decreased by 41%, whereas the number in the learning disabilities (LD) category increased 198%. There is considerable evidence that suggests this was because of a growing likelihood that schools would use the LD label for children with mild MR, presumably because a label of LD was increasingly seen as carrying less stigma than MR. Finally, a recent epidemiological study depicted a downward deflection in the incidence trend of other developmental disorders just as the trend for autism made a sharp upturn in the early 1990s, again suggesting the possibility of diagnostic substitution.