Marriage equality, inbreeding style

Gene ExpressionBy Razib KhanNov 26, 2009 4:28 PM


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The New York Times has an article on cousin marriage that's up. Here's some important bits:

Shane Winters, 37, whom she now playfully refers to as her "cusband," proposed to her at a surprise birthday party in front of family and friends, and the two are now trying to have a baby. They are not concerned about genetic defects, Ms. Spring-Winters said, and their fertility doctor told them he saw no problem with having children. The couple -- she is a second-grade teacher and he builds furniture -- held their wedding last summer on a lake near this tiny town in central Pennsylvania. But their official marriage took place a month earlier in Maryland, at Annapolis City Hall, because marriage between first cousins is illegal in Pennsylvania -- and in 24 other states, according to the National Conference of State Legislatures -- under laws enacted mostly in the 19th century. ... For the most part, scientists studying the phenomenon worldwide are finding evidence that the risk of birth defects and mortality is less significant than previously thought. A widely disseminated study published in The Journal of Genetic Counseling in 2002 said that the risk of serious genetic defects like spina bifida and cystic fibrosis in the children of first cousins indeed exists but that it is rather small, 1.7 to 2.8 percentage points higher than for children of unrelated parents, who face a 3 to 4 percent risk -- or about the equivalent of that in children of women giving birth in their early 40s. The study also said the risk of mortality for children of first cousins was 4.4 percentage points higher. More-recent studies suggest that the risks may be even lower. In September, Alan Bittles, a researcher at the Centre for Comparative Genomics at Murdoch University in Australia and one of the authors of the 2002 study, published a paper in Proceedings of the National Academy of Sciences that reported that the mortality rate was closer to 3.5 percentage points higher. He said he expected ongoing research to find the risk of defects to be lower than previously assumed as well. "It's never as simple as people make it out to be," said Dr. Bittles, noting that very early studies did not account for factors like access to prenatal health care, and did not distinguish between couples like Ms. Spring-Winters and her husband, the first cousins in a family to marry, and those who are part of groups in which the practice is common over generations and has led to high rates of genetic disorders. "But the widely accepted scare stories -- even within academia -- and the belief that cousin marriage is inevitably harmful have declined in the face of some of the data we've been producing," he said. ... Diane B. Paul, a professor emerita of political science at the University of Massachusetts, Boston, and a research associate in zoology at Harvard, was an author of a paper published last year in the journal PLoS Biology that described the difficulty of generalizing about the potential for birth defects or increased mortality in the children of cousins. Each couple's risk depends on the individuals' particular genetic makeup, she said, which means "it's very difficult to determine." And even the small average risk of defects reported in the 2002 study, she added, represents nearly double the risk to children of unrelated parents. ... As a religious Methodist, she said, she also worried that marrying her cousin would be wrong in the eyes of her church. But as it turned out, the Methodist Church has no official position on marriage between cousins, unlike the Roman Catholic Church, which requires cousins to obtain dispensation before marrying. And after talking to a relative who is a Baptist minister, Ms. Spring-Winters said, she discovered that the Bible does not say anything explicitly negative about cousin marriage, although it does list examples of sexual impurity, including relations with "close relatives," like sisters, stepchildren, grandchildren, aunts and stepsisters; and those between mothers and sons, and fathers and daughters. "If the Bible said no, we wouldn't have done it," she said.

A few salient points noted above: * If you want to minimize the perception of risks of cousin marriage, emphasize the small increment in absolute percent increase in risk. For example, a 3% chance of congenital defect vs. a 1.5% chance. * If you want to maximize the perception of the risks of cousin marriage, emphasize the change in relative odds. Instead of a 1.5% incremental increase in the risk, you are doubling the risk! Sounds much scarier. * There is a major distinction between cousin marriages in societies where the practice is rare, and cousin marriages in societies where they are common and customary. In the latter cases the coefficient of relatedness will be far greater than in the former cases. To label them both "cousin marriages" is a function of cultural perception, not genetic reality in terms of equivalence. * Increased risks may make a younger woman equivalent to a 40 year old in the prenatal precautions needed. That is an informative way to present the health and medical consequences, as naturally the costs will be greater. * We do not ban women of the age of 40 from reproducing. Nor do we ban those with genetic diseases from reproducing (well, we all have genetic diseases, so perhaps genetic diseases which are judged to be chronic). The ban on cousin marriage is a function of culture. * The individual above is not a close reader of the Bible, cousin marriage was practiced by the Hebrews. Rebecca, the wife of Isaac, was a cousin (Abraham was Rebecca's great-uncle). Their son, Jacob, married his cousin, Rachael, the niece of Rebecca. Cousin marriage declined in Medieval Europe through bans introduced by the Roman Catholic Church (bans imposed selectively and often opportunistically, since the official degree of relationship banned was so extreme that violation was the norm). The Reformation brought about a collapse of such bans on cousin marriage, ergo, the marriage of Charles Darwin to his cousin Emma Wedgewood. Cousin marriage declined in Europe again with the rise of modern transportation, greater mobility and concomitant expansion of the mate market, and finally the decline of the landed aristocracy which was invested in preserving old wealth rather than producing new wealth (marrying a cousin often kept property "in the family"). * The ban on cousin marriage did have a eugenical aspect to it. Charles Darwin himself was worried about ailments his children suffered which might have been an outcome of his marriage to Emma, as it was consanguineous. There are many classes of illnesses, generally recessive diseases, where the increased risk from cousin marriages is marginal, but most who suffer are the products of cousin marriages. In the age of national healthcare where the decisions of our fellow citizens hit us in the pocketbooks, rather than focus on cousin marriages I think it might behoove us to revisit the constraints that society as a collective may place on the choices of individuals more generally. Happy Thanksgiving!

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