The cause of autism is unknown, but studies show that people with autism have larger brains than normal.
There are different kinds of autism. Low functioning, middle functioning and high functioning.
Some of the lower functioning people with autism have the intelligence of a baby, in which they can only scream, grab at your shirt, drool, and do other baby type stuff.
However, there are people that are autistic that are very smart, such as Bill Gates.
To people that generalize autistic kids as retarded, be quiet, because you're probably using Windows.