Cousin Marriages May Be Taboo, but They're Not Genetic Disasters

By Eliza Strickland | December 29, 2008 8:52 am

family treeIn the western world, marriage between first cousins is labeled incest or inbreeding, and in the United States the practice is banned or restricted in 31 states. But a new essay argues that such laws are based on an outdated notion of the genetic risks involved in cousins marrying and reproducing. [T]hose laws “seem ill-advised” and “should be repealed,” a geneticist and medical historian write…. “Neither the scientific nor social assumptions that informed them are any longer defensible” [Scientific American].

First cousins share about an eighth of their genes, and are therefore more likely to receive two copies of some recessive gene that poses health problems. Scientists had assumed that the children of first cousins would therefore be more likely to be born with birth defects. But coauthor Hamish Spencer writes that the risk of congenital defects is about 2 per cent higher than average for babies born to first-cousin marriages – with the infant mortality about 4.4 per cent higher – which is on a par with the risk to babies born to women over 40. “Women over the age of 40 have a similar risk of having children with birth defects and no one is suggesting they should be prevented from reproducing,” said Professor Spencer [The Independent].

The editorial, published in the journal Public Library of Science Biology, focuses on the laws that restrict cousin marriages (China, Taiwan, and North and South Korea also ban the practice) and the stigma that is attached to such unions, especially in the United States. “Unlike the situation in Britain and much of Europe, cousin marriage in the U.S. was associated not with the aristocracy and upper middle class but with much easier targets: immigrants and the rural poor,” they write [Wired News].

However, the authors note that the low rate that they cite for birth defects from cousin marriages is based on populations where the practice is rare; if cousins in such places marry, their offspring are likely to look outside the family for mates and will therefore bring new genes into the bloodline. Birth defects may be more likely in places where cousin marriages are commonplace, but the studies of this topic remain difficult and controversial. In February 2008, British Environment Minister Phil Woolas sparked a row in the United Kingdom when he attributed the high rate of birth defects in the Pakistani community to the practice of marriage between first cousins, the commentators said. Earlier studies estimated that between 55 per cent [and] 59 per cent of marriages continue to be between first cousins in Pakistan [CBC News].

Related Content:
DISCOVER: Go Ahead, Kiss Your Cousin

Image: flickr / m-louis

CATEGORIZED UNDER: Health & Medicine
  • Cronan

    2% (in the first generation) sounds pretty high to me, and a risk not worth taking.

    Anyway, my cousins are ugly.

  • Jumblepudding

    Didn’t Charles Darwin, himself married to his cousin, attribute the high rate of defects in his family to this practice?

  • Eliza Strickland

    You’re exactly right — Darwin married his cousin Emma Wedgwood, and they had 10 children together. And you’re right that in his later years Darwin wondered if their consanguinity had an impact on the health of their children, three of whom died in infancy or childhood.

    But it’s not at all clear whether the family’s inbreeding played a role in those three deaths — I believe scarlet fever and tuberculosis were the culprits, which anyone could fall prey to. There is speculation that the last child, who died when he was one, may have suffered from Down syndrome, but I haven’t read anything more than speculation. If anybody has more concrete info I’d be happy to hear it.

  • chilton

    I read somewher saying that the unions in families like the rockefelers and such used a techniqe (to keep the money in the family as the saying goes) that although never ditched the 2% risk had some benifits such as resistance to certine benign diseases that only ocurred because of a hidden gene that occured and when two of those hidden genes were paired to provide a newborn with a benifit instead of a disability, although the chances are just as low that the benefit would come out, that when it did, it was permanently passed down to further generations, as opposed to a dissabled person being less likely to mate and pass along their “bad” genes

    pretty touchy issue though, it’ll probably never affect me but for those who think that nothin says lovin like &$@%ing thier cousin, I’m sure they’ll feel pretty good to hear that not everyone is on their side

    by the way who are we, the products of mutated genes to say who can and can’t partake in possable evolution within our families???? it where we all came from, and nobody can take that away nor should they try

  • AfroLove

    I don’t see anything morally wrong with cousin marriages (or sibling marriages, parent-child marriages etc. for that matter) but I am often irritated when people downplay the genetic similarity between first and second cousins. Not because I’m against cousin marriages but because I consider my cousins to be brothers and sisters that I wasn’t raised with. I’m proud that my cousins are my cousins and I value the fact that I’m genetically more close to them then I am a stranger.

    Third and fourth cousins I don’t consider to be family members.

  • tj

    I think people are forgetting that non related couples have a two percent risk of passing on a genetic disorder anyways, It is the same increase in risk as a woman of 35 having children. by the numbers if the claims are true that 20% of the worlds population are in cousin couples then that is 1,400,000,000 couples. supposing every child at risk gets a genetic disorder that is 28,000,000. just because there is an increased risk for a disorder that doesn’t mean that they are going to get one. It also doesn’t specify the type of disorder. on the other hand rh incompatablity effects 15% of the population (as high as 34% in some places) and is deadly in 16% and at times as high as 30% of the cases. so that is 168,000,000. an interesting fact to consider is that rh disorders are almost nonexistant in pregnancies resulting from cousin marriages. by the numbers if risk factors are ALL your looking for if you have children by a cousin you are increasing your childs chances of survival by 14%. The numbers are all thereyou just have to do the math.


    he he
    The Quran allowed this 1400 years ago. LOL!

  • Aaaa

    My grandmother and my grandfather were first cousins and they had 6 children. My mom was the first child and ALL of them are smart. None are mentally defected as you western people claim.

  • http://yahoo goldy malik

    please find the solution of this problem as early as soon as possible in north india it is not allowed to getting married with first cousin and it is true that 3% chance of defected baby with non cousin couple and with cousin couple ti is 1.7 to 2 % so why it is not allowed in whole world openly. please do some thing if the scientist will say openly that it is not harmful so every thing will be all right .


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