A study shows the rate of autism among adults is about the same as children, except most adults don't know about it:
University of Leicester researchers present further evidence from first ever general population survey of autism in adulthood (…) There was no evidence of an 'autism epidemic' of marked increase in people with the condition. [Dr Traolach Brugha, Professor of Psychiatry at the University] says "Overall our findings suggest that prevalence is neither rising nor falling significantly over time".
(lifted from http://healthnewsdigest.com/news/Asthma_Issues_670/Most_Adults_With_Autism_Go_Undiagnosed.shtml).
This is a bit of a fly in the ointment for anyone who asks the question: "What is causing this recent explosion of cases of autism?" Answer: There is no explosion. Diagnosis got better. Instead of dismissing autistic children as "eccentric" (at one end of the spectrum) or "mentally retarded" (at the other end of the spectrum), the medical community got better at pinpointing the exact problem, and parents and teachers are more likely to give the diagnosed the help they need.
The article goes on to note that most of those diagnosed, being at the higher-functioning end of the spectrum, weren't even aware that they were autistic. This lends credit to my own theory that, due to misdiagnoses, higher functioning autistic people tend to suffer more than the lower functioning ones (a fate, I'm glad to said, probably happens a lot less often in the 21st century).
There are still lots of questions around this mysterious thing called autism. As an example, a recently released University of California study suggests children conceived in winter had a significantly higher risk of being diagnosed with autism than those conceived in summer: http://www.abc.net.au/news/stories/2011/05/06/3209926.htm. To which I say: "What the ****? It's a seasonal thing?"