This 2006 article - after making conjectures about autism and vaccinations suggests that autism rates might be lower for (unvaccinated) Amish children, and offers some anecdotal evidence:
"I have not seen autism with the Amish," said Dr. Frank Noonan, a family practitioner in Lancaster County, Pa., who has treated thousands of Amish for a quarter-century.
"You'll find all the other stuff, but we don't find the autism. We're right in the heart of Amish country and seeing none, and that's just the way it is."
The article points out the conclusions of a study wouldn't prove a link between vaccination and autism, but calls for such a study.
Is there a relevant study showing that the autism is low among Amish?