Shah et al (1982) screened mentally retarded adults and found that 38% of the individuals in this population had social impairments consistent with those of autism.
Consider that about 2.5% of the general population scores in the mentally retarded range (IQ below 70). Granted, only 0.9% can be assumed to be truly retarded according to Baroff (1991). Still, if the 38% proportion holds (confirmed below) then the prevalence of autism, from this population alone, would have to be at least 34.2 in 10,000. This prevalence would essentially be the prevalence of autism with mental retardation in adults, back in 1982. This is not "high functioning" autism, if you buy the IQ score criteria for dividing functioning levels.
Shah et al (1982) refers back to an earlier study by Wing & Gould (1979) which also addresses the issue of identification of autism in the populations with and without mental retardation. It would appear that Lorna Wing, who co-authored the 1982 study, should have been aware that increasing recognition of autism in the population with mental retardation could easily lead to the appearance of an "autism epidemic". The abstract of a recent paper, Wing & Potter (2002), does not mention this specifically, but says the following:
The evidence suggests that the majority, if not all, of the reported rise in incidence and prevalence is due to changes in diagnostic criteria and increasing awareness and recognition of autistic spectrum disorders. Whether there is also a genuine rise in incidence remains an open question.
Shah's proportion is confirmed by a very recent study. La Malfa et al (2004) screened an Italian population with Intellectual Disabilities (ID) for PDD-MRS (PDD with Mental Retardation). This raised the prevalence of PDD in this population from 7.8% to 39.2%.
Shah et al (1982) and La Malfa et al (2004) provide substantial support for my assertion that a contributing factor in administrative autism caseload growth is increasing recognition of autism in the population that tests in the mentally retarded range. (This is basically the same as diagnostic substitution from mental retardation to autism). That is, time-dependent differences and regional differences in the proportion of identified autism in this population are simply administrative, not real.
As I have noted previously, recognition of autism in the mentally retarded population in the CDDS was about 3.5% in 1992. It stands at about 7% currently. As of 2006, the recognized proportion is 2.48% in Central Valley (Fresno), and 10.73% in Westside (West LA). Clearly, there still is a lot of room for prevalence growth in this population.
And there is, of course, a lot of room for prevalence growth in the population without mental retardation (IQ of 70 or above). Consider Posserud et al (2006) who found that 2.7% of the total population of 7-9 year olds were high scorers in the ASSQ (Autism Spectrum Screening Questionnaire). This is the prevalence of "geek" if you will. A generation from now we might be talking about an ASD prevalence of 290 in 10,000. Or maybe not.