Laws banning first cousins from marrying are based on outdated assumptions about higher risks for offspring, population genetic experts say.

In a commentary appearing in Monday's issue of PloS Biology, zoology Prof. Hamish Spencer of the University of Otago in New Zealand and Diane Paul of Harvard's Museum of Comparative Zoology argue the laws should be repealed.

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 59 per cent of marriages continue to be between first cousins in Pakistan.

Cousin marriage — and marriage between a niece or nephew and their uncle or aunt — is legal in Canada, according to the federal Marriage (Prohibited Degrees) Act of 1990.

On the other hand, in the United States, 31 state laws either bar the practice or allow it only when the couple has genetic counselling, is beyond reproductive age or if a partner is sterile. The authors of the paper ask whether the laws are grounded in scientific fact, such as the commonly cited three per cent additional risk of birth defects in those born to cousins.

"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," the pair conclude in a journal  forum where historians and philosophers reflect on topical issues in biology.

"They should be repealed, not because their intent was eugenic, but because neither the scientific nor social assumptions that informed them are any longer defensible."

The researchers give the example of a 2002 expert review on birth defects. The panel concluded the risk to those born to cousins is smaller than generally assumed at 1.7 to two per cent higher than the population-wide risk of birth defects, which is between two and three per cent. The review panel thought such a rate did not warrant any special testing before conception.

Risk portrayal

"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," the article said. 

"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."

On the other hand, those who portray the risk as large tend to describe it in relative terms or to risks that are generally considered unacceptable, such as the rhetorical question: Would anyone knowingly take a medication that has double the risk of causing permanent brain damage?

The expert panel noted that it is difficult to calculate the increased frequency of birth defects in children of parents who are cousins since socio-economic factors such as a lack of good prenatal care for British Pakistanis in their native language or malnourished mothers are difficult to tease out from other environmental and genetic factors.