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Cousin Marriage Laws Outdated, Expert Argues

December 25, 2008
Public Library of Science
Marriage between first cousins is highly stigmatized in the West and, indeed, is illegal in 31 US states. But is the hostility to such marriage scientifically well-grounded?

Laws banning marriage between first cousins are based on outdated assumptions about a high degree of genetic risk for offspring and should be repealed, according to a population genetics expert.

In an opinion article published in the US open-access journal PLoS Biology, University of Otago Department of Zoology Professor Hamish Spencer and Professor Diane Paul, a Research Associate at Harvard's Museum of Comparative Zoology, argue that laws against cousin marriage are ill-advised.

"Neither the scientific nor social assumptions behind such legislation stand up to close scrutiny," says Professor Spencer. For example, a 2002 expert review of studies regarding birth defects in offspring of cousins found that the risk was much smaller than generally assumed, he says.

The US National Society of Genetic Counselors (NSGC) report estimated the average risk as 1.7 – 2 per cent higher than the background population risk of congenital defects and 4.4 per cent higher than general risk for dying in childhood.

"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. People with Huntington's Disease or other autosomal dominant disorders have a 50 per cent risk of transmitting the underlying genes to offspring and they are not barred either," Professor Spencer says.

In the USA, there are 31 state laws that either bar cousin marriage outright, or permit it only where the couple obtains genetic counseling or is beyond reproductive age or if one partner is sterile.

"Such legislation reflects outmoded prejudices about immigrants and the rural poor and relies on oversimplified views of heredity. There is no scientific grounding for it," Spencer adds.

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Journal Reference:

  1. Paul et al. "It's OK, We're Not Cousins by Blood" The Cousin Marriage Controversy in Historical Perspective. PLoS Biology, 2008; 6 (12): e320 DOI: 10.1371/journal.pbio.0060320

Cite This Page:

Public Library of Science. "Cousin Marriage Laws Outdated, Expert Argues." ScienceDaily. ScienceDaily, 25 December 2008. <>.
Public Library of Science. (2008, December 25). Cousin Marriage Laws Outdated, Expert Argues. ScienceDaily. Retrieved November 30, 2023 from
Public Library of Science. "Cousin Marriage Laws Outdated, Expert Argues." ScienceDaily. (accessed November 30, 2023).

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