Since there are definitely genetic links in autism, as it defintely does run in families.
I have a special needs kid (brain damage from an illness), so I am on lots of online boards with parents of autistic kids. Some of them had one autistic kid who was vacinated, so they didn't vacinate their next child. Well, the 2nd child still got autism.
I'm not saying that vacinations don't contribute, but we don't know yet what does.
My thoughts about the Amish is that they are all closely linked genetically (related), so that is probably why they don't get autism as high as other people.
Also, autism tends to run in highly educated families.
I have a friend whose daughter was labeled as autistic. Her mother just rolled her eyes over it and said it was the new designer disability. She disagreed with it. Her daughter didn't have social skills that matched what is considered normal for her age but she talked to people, made eye contact, and made friends. Just a typical geeky kind of kid; which is why her mother disagreed with it.