Dear Cecil: What is the deal with cousins marrying each other? In most states it's against the law. Yet where I am working, in a West African francophone country, there is a saying, "Cousins are made for cousins." Is this practice really genetically unsound, or is that just an American old wives' tale? Jay Davidson, Peace Corps volunteer, Mauritania

Cecil replies:

Among the many things Americans just know, without ever having thought about it, is that if first cousins marry, their children will be drooling half-wits. The handful who wonder if there’s any logic to this belief probably think: Royal inbreeding. Prince Charles. Case closed.

As recent events have shown, however, a lot of things we Yanks just know aren’t so. The supposed evils of cousin marriage may not be the first one that comes to mind, but it’s definitely on the list. In his dissection of the issue, Forbidden Relatives: The American Myth of Cousin Marriage (1996), anthropologist Martin Ottenheimer points out the following little-known facts — little-known, that is, here in the U.S.:

The U.S. is virtually alone among developed nations in outlawing marriage among first cousins. European countries have no such prohibition. In some cultures, particularly Islamic ones, first-cousin marriage is encouraged. Even in the U.S. laws forbidding the practice are far from universal. First-cousin marriage is currently illegal or restricted in 31 states. (Some states allow it if there’s no chance of procreation — interesting in light of conservative opposition to gay marriage on the grounds that the institution’s function is to produce children.) It’s legal in the rest — and no, Kentucky and West Virginia aren’t among the permissive ones. Try California and New York.

First-cousin marriage isn’t a surefire recipe for congenital defects. True, marriage among close kin can increase the chance of pathological recessive genes meeting up in some unlucky individual, with dire consequences. The problem isn’t cousin marriage per se, however, but rather how many such genes are floating around in the family pool. If the pool’s pretty clean, the likelihood of genetic defects resulting from cousin marriage is low. A recent review (Bennett et al, Journal of Genetic Counseling, 2002) says that, on average, offspring of first-cousin unions have a 2 to 3 percent greater risk of birth defects than the general population, and a little over 4 percent greater risk of early death. While those margins aren’t trivial, genetic testing and counseling can minimize the danger. An argument can be made that marriages of first cousins descended from strong stock can produce exceptional children. Charles Darwin, for example, married his first cousin Emma, which wasn’t at all unusual in their prominent and successful family — their common grandparents were cousins too. Three of Charles and Emma’s ten kids died in childhood, it’s true, but that was standard for Victorian England; the others went on to productive and in some cases distinguished careers.

All kidding aside, the formerly high incidence of congenital defects, specifically hemophilia, among European royal families isn’t the classic demonstration of the perils of inbreeding that everybody thinks it is. The short explanation is that hemophilia is an X-chromosome-related characteristic, transmitted only through the female line. The children of royal female carriers would have been at risk no matter whom their mothers had married.

Why are Americans and their legal system so phobic about first-cousin marriage while Europeans aren’t? Ottenheimer blames several factors. First, bad research in the 19th century greatly exaggerated the dangers of imbecility, blindness, etc, among children of close kin. This research was eventually discredited in Europe, but Americans and their state legislators never got the word. Second, cousin marriage in the U.S. was considered a sign of barbarism (probable translation: hillbillies did it). In Europe, on the other hand, particularly in Mediterranean cultures, cousin marriage had a long and reasonably respectable history, although it’s rare today. Finally, European deep thinkers contended that certain forms of cousin marriage increased social cohesion. No such positive arguments were advanced in the States.

Let me emphasize we’re talking strictly about cousin marriage here. The incest taboo regarding parent-child and sibling unions is still strong in Europe and most other places. Setting aside the issue of exploitation where minor children are concerned, such unions have a much higher risk of “adverse medical outcome” — 7 to 31 percent, according to Bennett et al.

As for cousin marriage — admit it, you admire me for keeping the word “kissin'” out of this discussion — Ottenheimer thinks U.S. laws against it ought to be repealed. I’m not seeing it: Jerry Lee Lewis got a buttload of flak for marrying his first cousin once removed in 1957, and the uproar over gay marriage suggests that rewriting the rules about whom one may properly wed is likely to be a tough sell now. Still, the issue reminds us of the importance of asking, when confronted with some instance of conventional wisdom: Says who?

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.