It has long been known that incest is not as bad as you think. Anti-cousin marriage laws are like prohibition laws and blue laws. They arise from a Christian conservative movement that swept Western Civilization from the late 18th century through the 19th century, up to about the time of the repeal of Prohibition.
Sure, marrying, or just plain having sex with, your sibling is disgusting. I mean, think about it. No, wait, don’t even think about it. But cousin marriage? That depends. Your cousin may be kinda cute, you never know.
But seriously, anthropologists have long known of … and have had two distinctly different explanations for … patterns of marriage that involve linking up various kinds of cousins. At this point, let’s just say that cousin marriage tends to benefit … in terms of coalition formation, power management, and with respect to inclusive fitness …. the ascending (older, in charge) generation, even if it is no necessarily ideal for the marrying generation. This may well explain the pattern that we see: Prescribed cousin marriage is common in many scieties, but the degree to which it happens is at least somewhat correlated, it would seem, with he level of patriarchy. The more control older typically male power brokers have over things, the more people stick with cousin marriage. The less such power, the more rule breaking we see.
Forbidding laws are foreboding things. If you make a law that says that some behavior should never, ever happen, then people may become more fearful of the outcome of such actions. Cousin marriage laws instituted mainly during the last half of the 19th century have led to the general understanding that if cousin have a baby, it will have two heads. But in fact, and this has been known scientifically for decades, the increased rate of revelation of hidden recessive mutations in marrying cousins is small. It is about the same as a woman over 35 or so having a child.
A paper just out in PLoS Biology reviews the history of cousin marriage, its prohibition and the related controersy, in the West.
The conclusion is the following interesting conundrum:
…we note that laws barring cousin marriage use coercive means to achieve a public purpose and thus would seem to qualify as eugenics even by the most restrictive of definitions. That they were a form of eugenics would once have been taken for granted. Thus J.B.S. Haldane argued that discouraging or prohibiting cousin marriage would appreciably reduce the incidence of a number of serious recessive conditions, and he explicitly characterized measures to do so as acceptable forms of eugenics …. But Haldane wrote before eugenics itself became stigmatized. Today, the term is generally reserved for practices we intend to disparage. That laws against cousin marriage are generally approved when they are thought about at all helps explain why they are seemingly exempt from that derogatory label.
It is obviously illogical to condemn eugenics and at the same time favor laws that prevent cousins from marrying. …
Interesting, that link between religious belief and eugenics.
As a paper published in an Open Access journal, you can review it yourself. Do read this paper, it is well done and quite accessible.
Diane B. Paul, Hamish G. Spencer (2008). “It’s Ok, We’re Not Cousins by Blood”: The Cousin Marriage Controversy in Historical Perspective PLoS Biology, 6 (12) DOI: 10.1371/journal.pbio.0060320