One of the things that gets bandied about with regular frequency is the proportion of severely impaired individuals on the spectrum and the accompanying rates of ID. I've run across another study that looks at both of these things. Chakrabarti and Fombonne (2001) found in a sample of 97 individuals with a pervasive developmental disorder: "A total of 97 children (79.4% male) were confirmed to have a PDD. The prevalence of PDDs was estimated to be 62.6 (95% confidence interval, 50.8-76.3) per 10 000 children. Prevalences were 16.8 per 10 000 for autistic disorder and 45.8 per 10 000 for other PDDs. The mean age at diagnosis was 41 months, and 81% were originally referred by health visitors (nurse specialists). Of the 97 children with a PDD, 25.8% had some degree of mental retardation and 9.3% had an associated medical condition."
I'll include interesting snippets from the study:
"The sample included 77 boys (79.4%) with no significant difference (22 = 0.33; P = .85) in the proportion of boys in the AD (76.9%), Asperger syndrome (84.6%), and PDD-NOS (80.4%) groups. Of the 97 children, 29 (29.9%) had no functional use of language defined as the daily spontaneous use of 3-word phrases. The proportion of children without functional language was however strongly associated with diagnostic subtype (AD, 69.2%; Asperger syndrome, 0%; PDD-NOS, 16.1%; 22 = 30.6; P<.001)."
Only a third of the sample had "no functional use of language."
A fourth of the sample had "some degree of mental retardation."
Not surprisingly, a "significant difference was found for the presence or absence of mental retardation between the 3 PDD subtypes (22 = 40.6; P<.001), the AD group having more frequent and severe cognitive delays than the Asperger syndrome and PDD-NOS groups."
There are folks in the autism community that insist there's an epidemic and we're fixing to be hit with a tidal wave of nonverbal, diapered adults, but this does not seem to be the case:
"In our survey, AD accounted for only 27% of the cases with these children showing much greater cognitive and language impairments. By contrast, the majority of cases was found at the mild end of the autistic spectrum, with the PDD-NOS and Asperger syndrome groups accounting for 71.1% of the cases. High proportions of PDDs were also found in recent surveys (46.8%18 and 40%19). Prior surveys focused on a narrow definition, which led to the exclusion of these milder forms although it has been recognized for some time that they represented a group as sizable if not bigger than that of autism.5 The inclusion of these milder variants certainly may account for a substantial part of the increase in prevalence rates."
Contrary to this fantastical and fanatical assumption that most individuals with autism are severely disabled, incapable of speech, etc., the reality appears to be that a loosening in diagnostic criteria that allows for the diagnosing of many more individuals with less severe impairments had occurred. Now, this study's been out there for nine years. I've said before, and I'll say it again here, research is poorly disseminated to the public. I'll go a step further, here, and say that is almost as poorly disseminated to the clinicians and practitioners who diagnose and treat autism. These discrepancies between what researchers know and what practitioners, clinicians, parents, and the public know are widening, not lessening. This is frightening, and undoubtedly accounts for all manner of misinformation and woo to abound.
Chakrabarti S, Fombonne E (2001). "Pervasive developmental disorders in preschool children". JAMA 285 (24): 3093–9. doi:10.1001/jama.285.24.3093. PMID 11427137. http://jama.ama-assn.org/cgi/content/full/285/24/3093.