This paper [0] is really interesting from the perspective of looking at Williams syndrome as the opposite of autism. The authors found that an individual with a duplication of the Williams syndrome deletion region that was autistic. Basically they found that not enough of the genes in this region (i.e deletion) then you end up with Williams syndrome, too much and you get autism.

Autism is genetically more complex than Williams syndrome as there are many other genes that can cause autism.

[0]http://link.springer.com/article/10.1007/s10803-011-1389-4