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In most of Europe and the Americas, cousin marriage is extremely rare — only a few percent of people are married to their first or second cousins. But in other societies — mostly in the Middle East, North Africa and South Asia — it's much more common. By some estimates, more than 50% of Pakistanis are married to their first or second cousins.

A major problem with cousin marriage is that the children of such unions are more likely to have mental and physical health problems. Why? It has to do with rare recessives.

These are genes that exist at low frequency in the population and whose effects are only observed when you have two copies (recall that you inherit one copy of each gene from your mother and one from your father). They are usually non-functional versions of genes that have some important biological function, so their effects tend to be deleterious.

When two non-relatives mate, each transmits a certain number of rare recessives but, crucially, these are unlikely to be the same rare recessives (you might carry a non-functional version of gene X, while your spouse might carry a non-functional version of gene Y). Hence their children are unlikely to inherit two copies of the same deleterious gene.

Yet when two cousins mate, they are more likely to transmit the same rare recessives — since they could have each inherited a copy from their common grandparents (a non-functional version of gene X, say). It is therefore not so unlikely that their children inherit two copies of the same deleterious gene. And if their grandparents were also cousins (as might be the case in societies that have practised cousin marriage for several generations) the risk becomes even higher.

How much risk are we talking? A major UK study can shed some light on this question. The 'Born in Bradford' study, as it is known, has followed more than 13,000 pregnancies in the city of Bradford from 2007-2010. Approximately 18% involved parents who were first cousins, and an additional 10% involved parents with some other blood relation (usually second cousins), owing to the city's large Pakistani population.

The latest report from the study was published last year. Its findings are shown below.
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Children whose parents were first cousins were more likely to have died. They had higher rates of hospital usage, learning difficulties and speech and language difficulties. And they were less likely to have reached a "good stage of development" by the first year of school. On several of these measures, the difference amounts to a factor of 1.5-2.5, though on other measures it is lower.

Although the rate of cousin marriage among Pakistanis in Bradford has fallen over time, it remains high. A report last year put the rate at 46%. Given the risks, why don't more Pakistanis eschew cousin marriage?

One reason may be that they don't understand the risks. In 2012, Nasreen Ali and her colleagues carried out interviews and focus groups with Pakistanis in Birmingham concerning the practice of cousin marriage. According to Professor Ali, a "majority of participants" attributed still-births and genetic conditions to factors like "the will of God" or "black magic".

Although the rate of cousin marriage in Britain remains low, it is substantially elevated in Pakistanis — and this explains why they have higher rates of still-births and genetic conditions. While Conservative MP Richard Holden has called for cousin marriage to be banned, Labour has said it has "no plans" to do so. For the time being, raising scientific literacy may go some way to curbing the practice.