Cousins have no more risk of producing a child with birth defects than women over the age of 40, according to new research.
Otago University professor of molecular ecology and evolution, Hamish Spencer, and US researchers led by Diane Paul, of Harvard University, said laws in some parts of the world banning marriage between first cousins are based on outdated assumptions of the degree of genetic risk.
While marriage between cousins is legal in New Zealand and Britain, but not generally recommended, such a union is banned or restricted in 31 states in America.
Some states permit the union if the couple have genetic counselling, if one partner is too old to produce children or is sterile.
The average risk of birth defects in children born to cousins is estimated at 1.7% to 2% higher than the risk to the general population. The risk of the baby dying in childhood is estimated to be 4.4% higher.
"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," Spencer says.
Outdated legislation in some countries reflected prejudices about immigrants and the rural poor, and relied on oversimplified views of heredity, he says.
"There is no scientific grounding for it."
Socio-economic and environmental factors were often responsible for clouding the birth defect statistics, the research team found.
"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 pre-natal care."
This resulted in fewer defects being picked up through pre-natal screening and fewer terminations as a result, skewing the overall statistics.
Women over the age of 40 were not prevented from childbearing, nor were people who had a hereditary disease, such as Huntington's, which carries a 50% risk of being passed on, researchers say.
Laws banning cousins from marrying should be repealed because "neither the scientific nor social assumptions that informed them are any longer defensible", they say.
The research was published in PLoS Biology - a public access, peer-reviewed journal.