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Originally Posted by Tiki
the diagnosis itself didn't exist until the 1940's
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The diagnosis didn't exist--or the disease itself basically didn't exist? When did widespread vaccination use begin, again? Oh yes, the 1940s.
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Originally Posted by Tiki
A lot of disorders, such as ADHD, OCD, and depression, are sharply on the rise, and it's possible (perhaps even likely) that there are environmental factors, but I also suspect that diagnostic tools are simply getting a lot more refined and more people who would formerly not have been diagnosed at all, now are.
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Many people suspect that. It's called the "Hidden Horde" theory. Because what is undeniable is the vast majority of these autistic individuals are not capable of caring for themselves into adulthood (please recall again that in this case I am talking about diagnoses of pure autism--not PDD, not Asperger's, not ADHD. All of those are rising too, as you say, but those individuals are often capable of caring for themselves and confound the data, so we look at the data without them to get a clearer picture.) So if these people were formerly not diagnosed at all, there must be hundreds of thousands of adults, aged anywhere from 30 to 70, either in institutions with a different diagnosis, homeless on the streets, or being cared for by relatives completely removed from the social services system.
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Burd et. al. did a prevalence study of all autistic children born in North Dakota between the years 1967 and 1983 and found a rate of 3.4 per 10,000. A follow up study on the same cohort done twelve years later showed that the original study detected 98% of the cases of autism (they missed only one individual.)
Nylander and Gillberg screened adults at outpatient psychiatric faclities in Sweden, looking for undiagnosed cases of autism. They presumed they'd find people who had never been evaluated for autism in the past, at least using modern criteria. They did find nineteen people who met autism criteria who'd previously been undiagnosed as such, but that brought their prevalence rate to only 2.7 per ten thousand, similar to the other population rates quoted before 1980.
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I can type out the footnote references to the actual studies if you really need me to, but there are dozens of them.
ETA: Just to make sure the reference is all in one post: the current prevalence rate as of 2009 is 67 per 10,000.