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IslamQA: Why does Islam permit cousin marriage?

Salam! Why is permissible in islam to marry one’s cousin/why isn’t it considered incest? Considering the high health risks that could affect the baby?

Alaikumassalam wa rahmatullah,

Islam permits things when prohibiting them would do more harm than good. Sometimes (in an isolated village, for example) the only suitable person for marriage is a cousin. Allowing the marriage despite the risks would be better than requiring the couple to remain celibate. Cousin marriage used to be quite common in Europe until the 20th century. Charles Darwin married his first cousin. Cousin marriage is also far less risky than the Jewish practice of uncle-niece marriages (cousins on average share 12.5% of our genes, while the sharing is 25% between uncles and nieces, and 50% between siblings). 

While cousin marriage can be a burden to the couple due to having children with genetic abnormalities, it can be beneficial to the population as a whole in the long-term. From a scientific article on cousin marriage:

In ‘Consanguinity in Context,’ author and medical geneticist Alan H. Bittles of Murdoch University in Australia examines common misconceptions about cousin marriage from legal, cultural, religious and medical perspectives.

But the risks apply primarily to couples who are carriers of disorders that are normally very, very rare, Bittles explained. “For over 90% of cousin marriages, their risk [of having a child with a genetic abnormality] is the same as it is for the general population,” he said.

One surprising and oft-neglected advantage of marriage between close biological relatives is a phenomenon called purging, in which disease genes are exposed and removed from the gene pool. Thanks to purging, marriage between close relatives in early human populations would have kept the prevalence of genetic disorders low, Bittles explained.

But in the long-term, shrinking family sizes and increased mobility in many parts of the world means that cousin marriage is likely to decline. In the absence of purging, harmful genetic variants could accumulate over time.

“We may be creating a problem for ourselves in future generations,” Bittles said.

And here is a detailed article that shows the risks of cousin marriage are not so clear-cut as is usually thought.

Cousin marriage also has the benefit of creating families that have far more “kin affection” than other families. The children will share more genes with each other. Twins are famous for being especially loving toward their twin siblings. Children born to cousin marriages will be somewhat like that. They will also have far more loving aunts, uncles and grandparents because rather than sharing 25% of their genes with them, they will share something like 35%-40%, so the aunts and uncles and grandparents will treat them almost like their own children. I have seen this in the Middle East where the children of cousin marriages have a very special place in the hearts of their aunts, uncles and grandparents.

So while cousin marriages may not be something to recommend, the risks are not great enough to require prohibiting it if you also consider the benefits.

And God knows best.
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