Field of Science

Go on, marry your cousin

ResearchBlogging.orgNot that I was ever thinking about it, but should I marry my cousin? Should anyone? Is it such a bad idea that there should be laws against it? You may not know that there are laws prohibiting first cousins from marrying in most US states. In this picture the white colored states are the ones that do not.

The map is from “It's Ok, We're Not Cousins by Blood”: The Cousin Marriage Controversy in Historical Perspective, an article published today in PLoS Biology, which argues that there is no sense in having laws against first-cousin marriage.

The reason for the animosity is of course the fear that the offspring of first-cousins has an increased risk of developmental anomalies, resulting from inbreeding. Genetic disorders can result when deleterious recessives are inherited by both parents, meaning that some bad allele (think of it as a version of a gene) is inherited not from just one parent (which would be fine, since one good copy is enough), but from both (which is not good, because now the child lacks a good copy of the gene). Cystic fibrosis and Tay-Sachs disease are examples.

But, the authors argue, there isn't really that much of an increased risk at all. Previously there was much disagreement about the risk, so some experts convened to review existing data.
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.
If you think that increased risk is bad enough to warrant laws against first-cousin marriage, consider that women above the age of 40 are not prevented from having children, nor are people with autosomal dominant diseases. A woman over 40 has an elevated risk of giving birth to a child with defects. People who with autosomal dominant disorders have a 50% chance of having children with the same disorder. If we allow these people to give birth, why not allow the same for first-cousins (or, indeed, for them to marry).

Additionally, there might even be a bias in assessment of what the risk is, which might render some of the arguments against first-cousin marriage moot.
Inbred populations, including British Pakistanis, are often poor. The mother may be malnourished to begin with, and families may not seek or have access to good prenatal care, which may be unavailable in their native language [20]. Hence it is difficult to separate out genetic from socio-economic and other environmental factors.
Lastly, it can be argued that the ill will against first-cousin marriage stems from the eugenics movement ("prevent the ugly from breeding"), and I would think most people nowadays would shun it for that reason alone, at least once they realize what the conclusion of eugenics was.
It is obviously illogical to condemn eugenics and at the same time favor laws that prevent cousins from marrying. But we do not aim to indict these laws on the grounds that they constitute eugenics. That would assume what needs to be proved – that all forms of eugenics are necessarily bad. In our view, cousin marriage laws should be judged on their merits. But from that standpoint as well, they seem ill-advised. These laws reflect once-prevailing prejudices about immigrants and the rural poor and oversimplified views of heredity, and they are inconsistent with our acceptance of reproductive behaviors that are much riskier to offspring. They should be repealed, not because their intent was eugenic, but because neither the scientific nor social assumptions that informed them are any longer defensible.
So sometimes a careful look at the evidence shatters old myths. Unless we are somehow differently invested in these myths, and the practices that go along with them, then we should change our views on them accordingly.

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


  1. Moot, not mute. Mute means "silent" while moot means "debatable". From "moot court" from the Scots, I think.

  2. I can deal with this in several ways:

    1) 'Mute'? What do you mean? It says 'moot' up there! Always have.
    2) No, actually, I meant 'mute', see. It was, if I may say so myself, a rather clever way of mixing 'mutation' and 'silent', meaning that the arguments are changing, becoming silent, meaning unsaid, see.
    3) As you may have noticed this blog does not have a lot of reader comments, so I was deliberately trying to provoke the readers by making an error - on purpose - that would make the cleverest of them notice. It worked.
    4) Thanks!

  3. I didn't read the paper, but from what you wrote, it sounds like they only dealt with first generation first cousin marriages. What about the inbreeding of a population over time with successive generations of first-cousin marriages? Could preventing that be a logical role for the laws?

  4. Actually, I commented on the issue of inbreeding every generation over on Greg Laden's blog:

    The laws are about discouraging something that's bad if it was done a lot, like every generation. Then how about spitting. If we all spat all the time, then city-living would be horrible. Thus, no one should be allowed to do it?

    This is what they say about it in the paper:

    Moreover, the consequences of prolonged inbreeding are not always obvious. The uniting of deleterious recessives by inbreeding may also lead to these alleles being purged from the population. The frequency of such deleterious alleles, then, may be decreased, which (as shown above) means the relative risk is greater, even as the absolute risk decreases.

    I am not a hundred percent sure what they mean by absolute and relative risk here, but I do think they forget that "alleles being purged from the population" has the unwanted effect of either many spontaneous abortions and/or children born that never manage to reproduce themselves. Not exactly a happy outcome.

  5. It should be noted that over evolutionary time the average parent only had two children survive and reproduce. If the number was smaller than 2, humans would have gone extinct. If it was larger than 2, the population would have reached levels that we know did not happen.

    In the absence of birth control, women would get pregnant many more than 2 times in their lifetime. The ongoing purging of deleterious alleles has been an ongoing aspect of human reproduction until very recently.

  6. There is still a lot of purifying selection going on all the time. Many spontaneous abortions, for example, could be due to deleterious mutations. We have definitely no way not reached a level where deleterious mutations are tolerated, which you seem to imply.

    People often forget that being selected against is exactly what people (i.e. parents) fear so much. Having a child with a genetic or developmental disorder is horrible, so we cannot easily argue that the purging of alleles is a good thing when considering individuals.


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