This paper would appear to be a good place to start answering the question:

Autism

Prof Susan E Levy MD David S Mandell ScD Prof Robert T Schultz PhD

The Lancet, Volume 374, Issue 9701, Pages 1627 - 1638, 7 November 2009 doi:10.1016/S0140-6736(09)61376-3

Autism spectrum disorders are highly genetic and multifactorial, with many risk factors acting together. Genes that affect synaptic maturation are implicated, resulting in neurobiological theories focusing on connectivity and neural effects of gene expression.

they also state that:

Increases over time in the frequency of these disorders (to present rates of about 60 cases per 10 000 children) might be attributable to factors such as new administrative classifications, policy and practice changes, and increased awareness.