Are the incidents of autism spectrum disorder on the rise, or is there an increase due to better definition of diagnostic criteria?
Sociologists King and Bearman have been looking into this (see King &amp; Bearman, 2009a; King &amp; Bearman, 2009b; King &amp; Bearman, 2011; King et al., 2009). They estimate that some of the increases in ASD identification are due to the following:


25% attributed to diagnostic accretion - particularly children with mental retardation now being concurrently diagnosed with ASD.
15% attributed to improved awareness of ASD.
10% attributed to the increase in the age of mothers choosing to have children.
4% to geographic clustering - that is, families might move to an area where services are available, leading to increased diagnoses in specific areas.


This leaves a significant proportion of the increase in ASD identification unexplained, and in need of further investigations.