Marrying your cousin might help keep your family healthy, a new Dutch study found.

This might come as a surprise, because it also means your children are more likely to have birth defects. But cousin marriages can have a useful effect, say scientists from the University of Groningen. They keep life-saving genetic resistance to disease within a family.

Bad copies
It’s common to marry your cousin in some parts of the world, including India, Egypt, and Saudi Arabia. These are also regions that historically have a lot of disease. It could be that there the health advantages of inbreeding outweigh the extra risk of birth defects.

A genetic resistance to the HIV virus is a recent example. HIV, the virus that causes AIDS, kills roughly two million people a year. But people who’ve inherited two bad copies of a gene called CKR-5, one from their mother and one from their father, seem to be immune to HIV infection.

It’s pretty rare to inherit both copies of the gene. But in some Ashkenazi Jewish communities, where marrying a close relatively is common, as many as 20 percent of people have a double copy. It’s the cousin marriages that keep the life-saving genes in the family.

Risk
The only problem is that being resistant to one disease might make you more likely to get another. People with two bad copies of the CKR-5 gene may be more at risk of getting West Nile virus illness, if they’re bitten by an infectious mosquito.

Children of first cousins are about two to three percent more likely to have birth defects than the rest of the population. To put this into perspective, this is the same risk as for children born to women who are 41 years old.

Royal inbreeding
But when marrying your cousin goes wrong, it can go really wrong. Take the 17th century Spanish King Charles II. Because of generations of royal inbreeding, he suffered all sorts of mental and physical problems. Unfortunately one of them was infertility. When he died with no children in 1700, that was the end of the Spanish Habsburg dynasty.

To prevent these kinds of inbreeding disasters, doctors now encourage genetic screening in communities where marriage between cousins is common.
 

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