Early brain overgrowth is often a feature of autism, but the cellular defects that cause abnormal overgrowth are not known. To investigate whether increased numbers of neurons are responsible, the authors examined postmortem brain tissue from children aged 2 to 16 years of age. Autistic children had both greater numbers of prefrontal cortex neurons and greater brain weight compared to control children. Brain weight was 17.6% greater (on average) in autistic children. Children with autism also had 67% more neurons in the prefrontal cortex, 79% more neurons in the dorsolateral prefrontal cortex and 29% more neurons in the medial prefrontal cortex.

Courchesne E et al.: Neuron number and size in prefrontal cortex of children with autism. JAMA 306(18):2001-2010 (November 2011).

http://www.ncbi.nlm.nih.gov/pubmed/22068992