Reposted from Freedom Democrats, 2009.
A perspective from PLoS Biology by Diane Paul and Hamish Spencer argues that there is no scientific basis for social stigma and laws against marriages of first cousins. The basic argument is that the risk of birth defects from first-cousin mating is negligible, and fear of such defects is the only basis for these attitudes and laws. This issue is interesting to libertarians as an illustration of how lawmakers often pretend that they are implementing the scientific management of society, when in fact their acts are based on nothing more than ignorant prejudice.
This issue is also interesting in how it connects with the evolution of our views towards government:
The laws must also be viewed in the context of a new, post–Civil War acceptance of the need for state oversight of education, commerce, and health and safety, including marriage and the family. Beginning in the 1860s, many states passed anti-miscegenation laws, increased the statutory age of marriage, and adopted or expanded medical and mental-capacity restrictions in marriage law. Thus, laws prohibiting cousin marriage were but one aspect of a more general trend to broaden state authority in areas previously considered private. And unlike the situation in Britain and much of Europe, cousin marriage in the US was associated not with the aristocracy and upper middle class but with much easier targets: immigrants and the rural poor.
One flaw in the argument from Paul and Spencer is that they emphasize the low costs of mating among cousins, but they ignore the low costs of prohibiting cousins from marrying--laws against cousin incest produce a negligible reduction in the pool of prospective spouses (as opposed to laws against same-sex marriage or inter-racial marriage, for example). I'm sure that this issue is terribly important for those few people who are romantically in love with their cousins, but it will never get onto the radar screen of anyone who is considering the total welfare of humanity, or the overall injustice in the world.
Paul and Spencer also largely ignore the social issues around cousin marriages. From modern American perspective, cousin marriages may disrupt the stability of the extended family. Conversely, in some societies, cousin-marriages seem to increase family stability and support--providing the child with benefits that may outweigh the risks of genetic problems (I'll link to the report if I can find it). We could also speculate that frequent cousin marriages may result in a more fragmented society with insular families: at its most extreme, it may facilitate the formation of cult-like social structures. Ultimately, I suspect that day-to-day social concerns play a much more direct role in policy formation than expert-mediated scientific knowledge.
A Google search reveals a fair amount of commentary on this issue. At Slate.com, William Salaten notes that we will soon be able to genotype everyone to look for couples who carry identical alleles: if we embrace the logic of the prohibition on marriage of cousins, does that mean that genetic testing (a la GATTACA) should be a mandatory part of a marriage application, and the application be rejected if both individuals carry a potentially harmful recessive allele? Of course, as GATTACA illustrates, genetic selection on in vitro embryos is a solution to that problem.