As I understand it, the working definition of “incest” (http://en.wikipedia.org/wiki/Incest) varies widely among human cultures. E.g., in many Arab countries it is (I have read, no cite) very common, often obligatory, to marry first cousins. Keeps the tribe close-knit, keeps old properties within the family. In the U.S. it is widely assumed that such pairings can lead to “genetic deficiencies” (think Deliverance, “Dueling Banjos,” Jeff Foxworthy jokes, etc.). Yet the Arabs do not, so far as I know, suffer from widespread mental retardation or other forms of genetic ill health. Does our culture, perhaps, exaggerate the genetic hazards of “inbreeding”?
I don’t have any statistics, but I’d say the danger is exaggerated. It’s probably a lot like smoking: not all smokers will get lung cancer, but some will-- yet cancer is what we threaten smokers with. We so abhor incest that we predict dire, inevitable consequences to the offenders.
European royalty interrmarried for hundreds of years before the effects started to show. The Egyptians apparently had hundreds of generations of royal brother-sister marriage with no apparent widespread problems.
The Master has spoken on this: http://www.straightdope.com/columns/041001.html
For one example of problems caused by in-breeding, see http://www.phoenixnewtimes.com/Issues/2005-12-29/news/feature.html (I’m not suggesting that this is a typical consequence, just that it is a consequence).
That reminds me . . . Last summer I was in Chicago and saw a choir of young people, apparently from some religious sect I’d never heard of (they were all wearing old-fashiondy clothes right out of The Village but somehow I don’t think they were Amish) singing on the sidewalk in front of the Art Institute. They did not appear to be retarded, or unhealthy in any way, but I was just a little disturbed by how similar they all looked – all those pale faces and high foreheads and auburn mops and, well, very similar faces . . . Made me wonder. I mean, if it was a “family choir” like the von Trapps, there was no indication on the CD’s they were hawking.