MAHA SAAD ZAKI, a professor of clinical genetics, ushers Ahmed, Fatima and their family into her room at Egypt’s National Research Centre. At least three of their six children have a rare neurological illness that manifests itself around age four, causing mental retardation, loss of the use of their limbs and, later, death. The couple’s nine-year-old daughter slumps, twitching. This congenital illness has appeared because, as well as being husband and wife, Ahmed and Fatima are also first cousins (their names have been changed).

Cases like these are all too common in the Middle East and north Africa. Marrying a close relative markedly increases the chance that both parents are carriers of dangerous recessive genes, which can then cause disease when a child inherits a copy of the gene from both parents, as will happen in 25% of cases. The gamut of such illnesses runs from known ones such as microcephaly (in which children have unusually small heads) cystic fibrosis and thalassaemia, a blood disorder, to wholly new disorders. “Ninety percent of the cases I see are caused by consanguineous marriages,” says Ms Zaki.

Statistics on the prevalence of marriages between close relatives today are scarce. Once common practice in Western societies, estimates suggest the Middle East, along with Africa, continue to have the highest levels in the world. In Egypt, around 40% of the population marry a cousin; the last survey in Jordan, admittedly way back in 1992, found that 32% were married to a first cousin; a further 17.3% were married to more distant relatives. Rates are thought to be even higher in tribal countries such as Iraq and the Gulf states of Saudi Arabia, Yemen and Kuwait.

Today the first reason men and women look to wed within the family is because they know a lot about their relatives: who they are, what they earn, any past blunders. And large families mean they have lots of them. “People are looking for ethics and manners,” says Atef al-Shitany of Egypt’s health ministry. Tying the knot within also ensures property remains in the family. In Upper Egypt, a rural farming area, rates are the highest in Egypt.

Unlike in the West, there is no social stigma; quite the opposite. A 38-year-old Egyptian woman, who has two sons with micro-syndrome (which causes cataracts, small genitalia and learning difficulties) due to her marriage to a cousin, says relatives nonetheless criticise her for allowing her 18-year-old daughter to get engaged to a “stranger”—the fiancé is not a relation.

Many wrongly think that maternal cousins are not blood relatives. Prevailing Islamic belief reinforces intermarriage, although it happens among Christians too. The Koran allows marriage to anyone but parents, siblings, aunts and uncles, nephews and nieces. Indeed, Fatima, the daughter of Muhammad, married her cousin, Ali Ibn Abi Talib, notes Ahmed Mamdouh of Al Azhar, an Egyptian university.

Many in the region, though, simply do not know of the risks of marrying a family member. A small survey in Saudi Arabia found that participants were 50% less likely to view marrying a cousin positively when warned of the problems. “We wouldn’t have married if we had known,” says Ahmed.

To reduce genetic disorders, some countries have made blood tests mandatory for fiancés, which have helped reduce incidence of diseases such as sickle cell anaemia. In Tunisia, the government mandates premarital counselling for all those betrothed to a relative.

In Egypt, where education is often rudimentary, there is much more work to be done. Couples that do know of the risks often believe—sometimes because their doctor tells them so—that basic blood tests rule out the risk of any genetic illness. “The only way to avoid the suffering is not to marry relatives,” says Ms Zaki. “But that will be impossible to achieve here.”