Re: education: a very good point. It is true that they do not go to school past 8th grade. In PA where we live, however, you must have a minimum of a high school diploma to homeschool, and some Amish do homeschool, though most go to some kind of parochial school. This would be true of the "plainer" Mennonites as well. "Plainer" group such as the Eastern Mennonites and Horning Mennonites are very much against homeschooling but the more "liberal" ones do homeschool (mind you, these are still VERY conservative people! Except they do not by and large vote, though this is changing. Many Amish voted in the last election and voted for Bush.)
Many Amish use alternative medicinal treatments and seem to want to know about health issues so it surprises me they aren't more keen on this issue of inbreeding. But the point comes to this: what are you going to do when your daughter Anna Stolzfus meets her 3rd cousin Amos Stolzfus at a Sunday meeting and they fall in love? It has been done before, what is the big deal?
And actually, in most of people with defects that I personally know, the relationship between the married couples was not very close (not any cousins that they were aware of). My friends who are the 3rd/4th cousins have 6 healthy children although she did just have a miscarriage. (That can happen to anyone).
I just wonder if the genes of the original 1500 Anabaptist people (from whom I am also descended) just keep popping up genetically from time to time across the board with these people.
One of my friends has 10 children but she is VERY aware of the symptoms of MSUD (Maple Syrup Urine Disease) and has looked for signs and symptoms with each child. If you catch it within the first 24 hours, the child will not end up a brain-damaged vegetable. My one friend has 3 cousins in one family with MSUD. Imagine having three vegetable children to raise. THREE.
The real problme is that thes "quaint" people are in violation of the commandments of God by turning their backs on society and creating a world unto themselves, not bringing in new blood, literally and spiritually. They reap the consequences in their bodies.
It's that last phrase that's the kicker. No cousins they were aware of. If you go back into the family tree, you'll probably find multiple cousin marriages further up the line, say 6-7 generations back. Most are unknown to the descendants. My great aunt married her fourth cousin, and they did not know this until after she started researching D.A.R. stuff. They are both descended from the same Revolutionary soldier. They never had children. Probably no connection, because the rest of the family tree is generously out-crossed.
Problem is, if you have multiple cousin marriages even if very far back, the effect is exponential, not just simply cumulative. What you have is not just "double cousins" (where a couple are cousins through two different branches of the family tree) but triple and quadruple cousins. If there are only 1500 ancestors (not all of whom successfully reproduced) back in the 1600s, by now you have people who are, say, not just sixth or seventh cousins but five or six times over. In the genealogy trade, this is known as "pedigree collapse". Or, in more humble jargon, "the family tree don't fork."
We've seen this in cats and in dogs, when a popular bloodline starts cropping up all across the pedigree eight or ten generations later. Even if you don't have any in- or linebreeding in the five-generation pedigree, if there's multiple use of a single bloodline the defects will start to crop up.
I think the problem with the founder effect is that one founder of the original population brings in a recessive gene which in the first generations doesn't show up because all offspring are either normal or carry one copy of the recessive gene. However, as more and more generations build up, more and more of the people in the population carry that recessive gene, and there is a greater chance of two of them marrying and having children. If both carry one copy of the recessive gene, every child born to them has a 25% chance of getting two copies and developing the disease, a 50% chance of getting one copy and being a carrier, and a 25% chance of getting the normal genes.
So at this point the negative recessive genes are spread so far throughout the population that merely marrying more distant relatives isn't going to successfully avoid it.