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It is estimated that at least 55% of British Pakistanis are married to first cousins and the tradition is also common among some other South Asian communities and in some Middle Eastern countries.
But there is a problem: marrying someone who is themselves a close family member carries a risk for children - a risk that lies within the code of life; within our genes.
Communities that practice cousin marriage experience higher levels of some very rare but very serious illnesses - illnesses known as recessive genetic disorders.
Now, one Labour MP is calling for an end to the practice. "We have to stop this tradition of first cousin marriages," Keighley MP Ann Cryer tells Newsnight.
Mrs Cryer believes an open debate on the subject is needed because - despite the risks - cousin marriage remains very popular.
Mrs Cryer's constituency is in the Bradford area, where the rates of cousin marriage are well above the national average. It is estimated that three out of four marriages within Bradford's Pakistani community are between first cousins.
The practice remains so popular because the community believes there are real benefits to marrying in the family. Many British Pakistanis celebrate cousin marriage because it is thought to generate more stable relationships.
Such unions are seen as strong, building as they do on already tight family networks.
"You have an understanding," explains Neila Butt, who married her first cousin, Farooq, nine years ago.
"Family events are really nice because my in-laws and his are related," she says.
"You have the same family history and when you talk about the old times either here or in Pakistan you know who you are talking about. It's just a nicer emotional feel."
But the statistics for recessive genetic illness in cousin marriages make sobering reading.
British Pakistanis are 13 times more likely to have children with genetic disorders than the general population - they account for just over 3% of all births but have just under a third of all British children with such illnesses.
Indeed, Birmingham Primary Care Trust estimates that one in ten of all children born to first cousins in the city either dies in infancy or goes on to develop serious disability as a result of a recessive genetic disorder.
Recessive genetic disorders are caused by variant genes. There are hundreds of different recessive genetic disorders, many associated with severe disability and sometimes early death, and each caused by a different variant gene.
We all have two copies of every gene. If you inherit one variant gene you will not fall ill.
If, however, a child inherits a copy of the same variant gene from each of its parents it will develop one of these illnesses.
The variant genes that cause genetic illness tend to be very rare. In the general population the likelihood of a couple having the same variant gene is a hundred to one.
In cousin marriages, if one partner has a variant gene the risk that the other has it too is far higher - more like one in eight.
Myra Ali has a very rare recessive genetic condition, known as Epidermolisis Bulosa.
Her parents were first cousins. So were her grandparents.
"My skin is really fragile, and can blister very easily with a slight knock or tear," she says.
Myra has strong views about the practice of cousin marriage as a result. "I'm against it, because there's a high risk of illness occurring", she says.
According to Ann Cryer MP, whose Keighley constituency has a large Pakistani population, much of the Pakistani community is in denial about the problem.
She tells Newsnight that she believes it is time for an open debate on the subject: "As we address problems of smoking, drinking, obesity, we say it's a public health issue, and therefore we all have to get involved with it in persuading people to adopt a different lifestyle", she says.
"I think the same should be applied to this problem in the Asian community. They must adopt a different lifestyle. They must look outside the family for husbands and wives for their young people."