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
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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.
