Sunday, June 23, 2013

Cousin Marriages From a Genetic Perspective

A blog about gene expression at Discover Magazine took a look at “the individual & social risks of cousin marriage,” and it even had maps.

In the United States there’s a stereotype of cousin marriage being the practice of backward hillbillies or royalty. For typical middle class folk it’s relatively taboo, with different legal regimes by state. The history of cousin marriage in the West has been one of ups & downs. Marriage between close relatives was not unknown in antiquity. The pagan emperor Claudius married his niece Agrippina the Younger, while the Christian emperor Heraclius married his niece Martina. Marriage between cousins were presumably more common.

How did things change?

With the rise in the West of the Roman Catholic Church marriages between cousins were officially more constrained. Adam Bellow argues in In Praise of Nepotism: A Natural History that there’s a material explanation for this: the Roman church used its power over the sacrament of marriage to control the aristocracy.

That’s interesting.

More precisely the coefficient of kinship between two first cousins is 1/8. That means that at any given locus there’s a 1 out of 8 chance that the two individuals will have alleles which are identical by descent, which means that the genetic variant comes down from the same person in the family line.

If the allele is “good,” that is, totally normal/wild type, not associated with any pathology, then we’re in the clear. That’s why most first cousin marriages don’t produce children who are monsters. What a first cousin marriage does is change the odds. How you present these odds matters a great deal in how scary they sound. If I told you than the chance of first cousins having children with a birth defect is 4-7%, vs. 3-4% for a non-consanguineous couple, it might not sound that bad. But if I told you that the odds of having a birth defect is ~50% greater, then it sounds worse.

He got more technical and moved on to discuss Muslims in Britain.
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  1. Interesting blog. Your idea of how to regulate marriage here is certainly praiseworthy in being consistent and moving to its logical conclusion. However, you are straddling two extremely different groups: on one hand you've got consanguineous and polygynist marital options in Islamic and other Asian cultures, but on the other hand you've got the strongly non-traditionalist option of same-sex marriage. (There is also polyandry, which garners little but opposition. But merging the two previous views might well necessitate advocating this third wheel.) Despite being generally in one camp and not the other, I sincerely wish your blog the best of luck for that camp's sake and because your outlook is delightfully novel.

  2. Thank you for your comment and wishes, anon.

    While I am aware that there are people of varyious identities that want a freedom to marry that will benefit them but not other freedoms to marry, I am hoping more people recognize that full marriage equality under the law would not force anyone into anything. So polygynists could stick to polygyny, same-sex monogamists could stick to their monogamy, and heterosexual cousin couples could stick to their marriages. But everybody should have the the option of marrying the person or persons they want to marry.


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