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.
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.
No wonder why so many Muslims from the Middle East are so attracted to jihad: their family trees don't fork.
Centuries of inbreeding and polygamy have made their dispossessed young men insane with sexual frustration.
Blowing yourself up for seventy-two virgins in the afterlife is an attractive prospect if your other options are your cousins, your left hand, or your goat.