The prestigious science journal, Nature, has an article on the rising prevalence of autism. It says ...
Shifting diagnoses and heightened awareness explain only part of the apparent rise in autism. Scientists are struggling to explain the rest.
... and much more (see more here).
The article suggests more research is needed. Recently, the Australian government squandered its opportunity to create a Cooperative Research Centre (CRC) for Living with Autism Spectrum Disorders so autism will not be a focus of research in Australia (see https://www.crc.gov.au/Information/ShowInformation.aspx?Doc=14th_Selecti...).