Genetics 101: "The Amish Anomaly"
Expecting to find 1 in 166 Amish people to be autistic (probably from US Department of Education data, but no source cited), he could only find one. This child, a girl adopted from China, had been (gasp!) vaccinated. This, apparently, was enough of a smoking gun for Mr. Olmsted to indict the mercury in vaccines as the cause of autism (see also), since the Amish generally don't vaccinate their children.
The obvious first question is how accurate Mr. Olmsted's "epidemiological survey" might have been. The Amish are not known to be an extroverted group who welcome intrusive investigative reporters into their homes and tell them the intimate details of their families. It is entirely possible that Mr. Olmsted may have missed a few autistic people. But leaving aside the question of accuracy, is it possible that this "anomaly" of apparent low autism rates among the Amish might have another explanation?
There is another equally plausible (some might even say more plausible) explanation - one that Mr. Olmsted has apparently dismissed, since he had it right in his hands. The Amish, and their neighbors the Mennonites, have been studied by geneticists for some time because they are a genetically isolated community. Although they accept converts, they don't get very many and so they don't get much "new blood" (genes). In addition, they don't move around much and their members tend to marry within the community - those who don't often leave and join their "English" (as they call people outside their communities, regardless of ethnicity) spouse outside of the community.
In genetic terms, what is happening is called "inbreeding" - and it has some very predictable results. One of these is that some variations of genes are lost and other variations become more common. There is a tendency to genetic uniformity, with only a few of the possible variations remaining. Among the Amish and Mennonites, the result has been a dramatic rise in genetic disorders that are more rare in the general population (see here, here and here) as a result of this rise in frequency of the "abnormal" variations of several genes.
Although inbreeding has a generally bad reputation (think Romanov's) because of the rise in genetic disorders, it can also lead (with equal likelihood) to the loss of genes for "bad" things, like autism. So, it seems only fair that, with their increased prevalence of so many genetic disorders, the Amish might have gotten a bit of luck and lost the gene for autism. Of course, this sort of thing doesn't make good UPI articles, and so apparently got flushed with the rest of the rational science that Mr. Olmsted managed to dig up.
Now, I have made this argument before and gotten the response, "Do you really think that the Amish have different genes than we do?" No, I don't, but that question exposes the ignorance of the speaker. All humans, with a few exceptions due to chromosomal deletions or duplications, have the same genes. It is the different variations of these genes - called alleles - that make us all unique individuals (except, of course, for indentical twins, who have the same alleles - sorry, folks).
It is quite reasonable to assume that the Amish have different alleles (more precisely, different allele frequencies) than the general population because they are pretty much genetically isolated from the rest of us. The genetic work done on the Amish and Mennonites confirms this - the only stretch is assuming that this difference will have no effect on the prevalence of a certain disorder, namely autism.