Time magazine article, "Is the autism epidemic a myth?"
The changing prevalence of autism in California
Croen LA, Grether JK, Hoogstrate J, Selvin S.; J Autism Dev Disord 2002 Jun; 32(2):207-15
We conducted a population-based study of eight successive California births cohorts to examine the degree to which improvements in detection and changes in diagnosis contribute to the observed increase in autism prevalence. Children born in 1987-1994 who had autism were identified from the statewide agency responsible for coordinating services for individuals with developmental disabilities. To evaluate the role of diagnostic sustitution, trends in prevalence of mental retardation without autism were also investigated. A total of 5038 children with full syndrome autism were identified from 4,590,333 California births, a prevalence of 11.0 per 10,000. During the study period, prevalence increased from 5.8 to 14.9 per 10,000, for an absolute change of 9.1 per 10,000. The pattern of increase was not influenced by maternal age, race/ethnicity, education, child gender, or plurality. During the same period, the prevalence of mental retardation without autism decreased from 28.8 to 19.5 per 10,000 for an absolute change of 9.3 per 10,000. These data suggest that improvements in detection and changes in diagnosis account for the observed increase in autism; whether there has also been a true increase of incidence is not known.
The epidemiology of autistic spectrum disorders: is the prevalence rising?
Wing, L, Potter D.; Ment Retard Dev Disabil Res Rev 2002;8(3):151-61
For decades after Kanner's original paper on the subject was published in 1943, autism was generally considered to be a rare condition with a prevalence of around 2-4 per 10,000 children. Then, studies carried out in the late 1990s and the present century reported annual rises in incidence of autism in pre-school children, based on age of diagnosis, and increases in the age-specific prevalence rates in children. Prevalence rates of up to 60 per 10,000 for autism and even more for the whole autistic spectrum were reported. Reasons for these increases include changes in diagnostic criteria, development of the concept of a wide autistic spectrum, different methods used in studies, growing awareness and knowledge among parents and professional workers and the development of specialist services, as well as the possibility of a true increase in numbers.
Remember, please, that it is not possible to prove a negative. The research above pretty conclusively shows where the numbers are coming from, though. I'm not trying to spur debate, just provide the information which was requested. Just call me your friendly neighborhood medical librarian.