Here is a studythat has recently been done in England finding a prevalence rate of 1% of adults with autism, suggesting that there has never been an increase in prevalence of autism as many have suggested. I did not read the whole study in detail but did surface read it, and it seemed that they surveyed a variety of people all over England. There was about a 9:1 ratio of males to females rather than the usual 4:1 ratio and this may have been because of the sampling size, I am not sure. I tried to read it in detail, but my disability made it hard for me to comprehend the entire thing and have all of it sink in. I was debating whether or not to blog about it, but I have decided to anyhow in spite of my concentration problems. This may not be the survey or study to end all survey/studies and I am sure this won't convince the age of autism folks as well as others who believe in an "epidemic" of autism involving huge increases (I don't think anything will ever convince them), but it may give some evidence that there has always been a high and stable rate of autism and disprove that vaccines or something else in the environment caused an autism explosion in the late 1980's, early 1990s.
The environmental hypothesis has appeal among its adherents, however, due to the fact that it provides the possibility of finding a single cause for most ASD's and with the cause, the treatment or possibly even cure can be found.
One of the arguments that those who have this point of view is that there are no comparable numbers of adults with autism. Even though this survey may not prove that conclusively, I think it is the first published survey of its kind that has been done trying to get an assessment of what the prevalence of autism is in older populations. To date, as far as I know, the other side has never done such as study.
Mark Blaxill has given the arguments that a study done by Bard et. al. in a rural part of North Dakota in which they tried to find additional persons with autism diagnoses after the prevalence of a certain sample was obtained in the cohort's childhood, they failed to find any new cases. The problem with this one study was that it was done in a rural area where the prevalence rates for autism are low. If numbers of autistic persons has actually increased it has been in urban areas. For example in the 2007 CDC survey much higher rates were found in New Jersey than in Alabama. In California's regional centers, the autism prevalence is much higher in Los Angeles than in rural parts of Northern California. Some might argue that this is due to more pollutants being found in urban parts of california or a higher rate of pollution in New Jersey than in Alabama. Be that as it may, so far no one has found any pollutant in the environment that has caused this increase. If vaccination rates are much higher in New Jersey than in Alabama and much higher in Los Angeles than in rural parts of Northern California, I have yet to see evidence of this.
Another piece of evidence that Blaxill gave was a study done by Nylander in Sweden where the author searched for persons with autism in psychiatric hospitals and found low prevalences there. Aside from the fact that autistic adults may not be inclined to present in psychiatric hospitals in great numbers, particularly if they are pretty high functioning, we see some inconsistencies in Blaxill's reasoning. In some personal correspondence that he and I engaged in about six years ago he stated that the evidence for a true epidemic of autism was that the increases were geographically specific, only taking place in the USA and UK and not other parts of the world. So, it would seem strange that he would cite a study in an area he had deemed geographically irrelevant.
So, I suggest that the age of autism folks or one of their allies does their own study, refuting the recent British survey showing that the prevalence rate of autism in persons born in 1986 and earlier is in fact lower than in the younger cohorts.
I was skeptical that finding the autistic adults was feasible, since they are not likely to present to special education services and regional centers and such.
I am glad that studies are finally being undertaken to find the so-called hidden horde of autistic adults. I have written about the reasons previously
I apologize to readers of autism's gadfly that I was not able to read and understand the study in more detail. Hopefully more work will be done in this area and we can determine whether or not the prevalence rate is the same in older populations than in younger.