There is fairly extensive data (not published in the peer reviewed literature) that groups which are unvaccinated have far lower autism rates than the general public.

UPI Reporter Dan Olmsted went looking for the autistic Amish. In a community where he should have found 50 profound autistics, he found 3. The first was an adopted Chinese girl who'd had vaccinations rushed before she was shipped from China and more here on the way to the adoptive parents. The second had been normal until developing classic autism symptoms within hours of being vaccinated. The third there was no information about. http://www.putchildrenfirst.org/media/e.4.pdf

Olmsted continued his search for unvaccinated Amish with autism beyond that community, finding none for a long time, but eventually found a Doctor in Virginia who had treated 6 unvaccinated Amish people from various places with autism. 4 of them had very elevated levels of mercury.

A telephone survey commissioned by the nonprofit group Generation Rescue compared vaccinated with unvaccinated boys in nine counties of Oregon and California [15]. The survey included nearly 12,000 households with children ranging in ages from 4 to 17 years, including more than 17,000 boys among whom 991 were described as completely unvaccinated. In the 4 to 11 year bracket, the survey found that, compared with unvaccinated boys, vaccinated boys were 155% more likely to have a neurological disorder, 224% more likely to have ADHD, and 61% more likely to have autism. For the older boys in the 11-17 year bracket, the results were even more pronounced with 158 % more likely to have a neurological disorder, 317% more likely to have ADHD, and with 112% more likely to have autism. [15]

In addition to the Generation Rescue Survey, there are three autism-free oases in the United States. Most publicized are Amish communities, mainly studied in Ohio and Pennsylvania [16].The Amish are unique in their living styles in largely self-sustaining communities. They grow their own food. Although they have no specific prohibitions against medical care, very rarely do they vaccinate their children. In local medical centers available to the Amish, most centers reported that they had never seen an Amish autistic child. The only Amish children that were seen as a rule were those with congenital disorders such as fragile X. The one autistic Amish child that was discovered during the surveys was taken to a medical office for an ear infection where the child was incidentally vaccinated, probably without the mother’s consent.

The second is the Florida-based medical practice of Dr. Jeff Bradstreet. While treating several thousand autistic children in his practice, Bradstreet has observed that “there is virtually no autism in home-schooling families who decline to vaccinate for religious reasons” [17]

The third, the “Homefirst Health Services” located in Chicago, has a virtual absence of autism among the several thousand patients that were delivered at home by the medical practice, and remained non-vaccinated according to the wishes of the parents [18].

Clusters of autistic children have also been found among parents with occupational exposures to chemicals prior to conception [19], and in children exposed prenatally to organochlorine pesticides [20].

excerpted from:

http://vactruth.com/2012/03/13/vaccines-human-animal-dna/

Reportedly the CDC has been surveying the vaccination status of the Amish for years, attempting to induce them to vaccinate (with some success I believe), and has consistently refused requests to include an autism question with their survey to gather data.

Its probably worth noting that Seneff et al, http://www.mdpi.com/1099-4300/14/11/2265 who have identified one pathway by which vaccines might be causing autism, have also in other work argued that glyphosate may invoke the same pathway, and the same groups may also be avoiding glyphosate. http://people.csail.mit.edu/seneff/WAPF_Slides_2012/Offsite_Seneff_Handout.pdf