"This study provides the first clear evidence of a reduction over time in the behavioral severity of individuals diagnosed with Autistic Disorder during a period of stability in diagnostic criteria."
So said the study findings reported by Andrew Whitehouse and colleagues  (a man not afraid to make waves when it comes to thinking about autism or about approaches to intervention) looking at "whether there were changes over time in the qualitative and quantitative phenotype of individuals who received the diagnosis of Autistic Disorder."
Based on prospective registry data of new autism / autism spectrum disorder (ASD) cases (N=1252) in Western Australia (a research favourite) between 2000 and 2006, researchers reported that the severity of the presentation of autism seemed to 'change' between the years. They concluded: "A shift toward diagnosing individuals with less severe behavioral symptoms may have contributed to the increasing prevalence of Autistic Disorder diagnoses." At least in Australia that is...
This is an interesting study. It kinda reiterates that when one talks about the quite phenomenal increase in diagnoses of autism being received, at least one factor contributory to that increase is the inclusion of a wider presentation of the condition. It also feeds into the discussions that have already happened, and will continue to happen, following the introduction of DSM-5 to autism diagnosis and in particular, the future role of the catch-all category that is social (pragmatic) communication disorder (SCD) (see here) when it comes to stiffer ASD diagnostic criteria not being met.
But just before anyone breaks out with the sweeping generalisation that the increasing prevalence of autism over the past two decades is somehow all 'manufactured' (exhibit one) according to the diagnostic procedures used, the available data points to something far more multi-faceted and complex (see here for example)...
 Whitehouse AJ. et al. Evidence of a reduction over time in the behavioral severity of autistic disorder diagnoses. Autism Res. 2017 Jan 19.
Whitehouse AJ, Cooper MN, Bebbington K, Alvares G, Lin A, Wray J, & Glasson EJ (2017). Evidence of a reduction over time in the behavioral severity of autistic disorder diagnoses. Autism research : official journal of the International Society for Autism Research PMID: 28102641