Some people think that homosexual couples should be allowed to marry. I say so myself. Some of those who say so argue that we shouldn't impose public morality on private relationships, but they don't really believe that. We impose a spectrum of public moral principles through marriage laws, and those limitations fall equally on everyone. No man can marry his mother or his sister under any set of laws anywhere that I know of. That universally accepted rule is an imposition on private relationships. To say that it is not a moral imposition badly misreads the facts. Whenever you tell people not to do what they are tempted to do, you are moving in the sphere of morality.
The grounds for the prohibition of incest have not changed at all over the centuries, but the interpretation of those grounds have changed. Incest was once universally prohibited because it was thought to be taboo. The evidence of this was the increased likelihood of birth defects. Today, this bit of public morality is reinforced by biology. There are two reasons why incest is medically contraindicated. The offspring of such a union, especially where incest were common, would be more likely to inherit two copies of a recessive gene coding for some birth defect. Second, dangerous pathogens will be more likely to find their way through the genetic defenses of a population that is inbred.
An interesting example of this kind of analysis concerns first cousin marriages, which are sometimes, but not always, illegal. William Saletan's Human Nature blog, posted at Slate, considers the biological calculations that might underlie such laws.
Many cousin couples can't pass on genetic diseases, since they're infertile. Are you going to ban them from marrying? If not, maybe the 24 states that ban cousin marriage should follow the lead of the five states that allow it if either party is sterile. And if procreation between first cousins is too dangerous, why stop there? Six states ban marriage between first cousins once removed, i.e., marrying the son or daughter of your first cousin. Theoretically, that's half as risky as marrying your first cousin, in terms of increasing the probability of passing on a genetic disease to your kids. How about marriage between second cousins? Theoretically, that's one-fourth as risky. No state bans such marriages. Should we change that?
In other words, one could come up with a pretty well-founded genetic calculation, since the number of genes two relatives share follows well established rules. A son is genetically related to his mother by a factor of 50%, since he gets half his genes from each parent. He is also 50% related to his sister, since he and she have a 50% chance of inheriting any given gene. He is 25% related to his aunt, but only 12.5% related to a first cousin. I note that is apparently possible in some places to marry your aunt, as Mario Vargas Llosa's marvelous novel, Aunt Julia and the Script Writer, is apparently semi-autobiographical.
So first cousin marriages look much less risky than coupling between brother and sister, based on genetic calculations alone. But not so fast. Saletan has a piece on some recent research that is thought provoking. One study found that the genetic risk involved in first cousin marriages was pretty small.
Their report concluded that the risks of a first-cousin union were generally much smaller than assumed—about 1.7%-2% above the background risk for congenital defects and 4.4% for pre-reproductive mortality—and did not warrant any special preconception testing. In the authors' view, neither the stigma that attaches to such unions in North America nor the laws that bar them were scientifically well-grounded.
But those calculations were based on North America, where it is generally assumed that, as you go up the family escalator, the grandparents and great grandparents were largely unrelated to one another. It's a small world, but I think it very unlikely that the Blanchards, Martins, Shooks, and Daughertys had many close relatives in common.
The same is not true of other societies and subpopulations.
In North America, marriages between consanguineal kin are strongly discouraged. But such an assumption is unwarranted in the case of UK Pakistanis, who have emigrated from a country where such marriage is traditional and for whom it is estimated that roughly 55%-59% of marriages continue to be between first cousins. Thus, the usual risk estimates are misleading: data from the English West Midlands suggest that British Pakistanis account for only ~4.1% of births, but about 33% of the autosomal recessive metabolic errors recorded at birth.
So if your grandparents were marrying their first cousins, it becomes a much riskier proposition for you to marry your first cousin, at least if you intend to have kids. There are a couple of lessons in this. One is that culture matters precisely because genetics matters. It's not a matter of one or the other: each influences the other.
Another is that first cousin marriages are a bad idea. They are probably pretty safe in North America just now. But that is only because they have been relatively rare in the past. They will probably continue to be safe, even where first cousin marriages are legal, provided that there is no culture pressure towards inbreeding. In populations where there is such pressure, it is not safe at all. The bottom line is that first cousin marriages ought to be prohibited by law. Cultures that encourage them are, in that respect, defective. That is the heart of moral argument.
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