First cousins who marry increase the risk of giving birth to a baby with birth defects including defects affecting the heart or lungs, a British study suggests.
Researchers looked at birth anomalies in Bradford in the north of England where there is a large Pakistani community. In that community there's a high level of consanguineous marriage — matrimony between blood relatives. ...
The risk of birth defects was double that in those of white British origin. ...
Of the more than a total of 11,000 babies in the study, 386 or 3 per cent had a congenital anomaly, such as heart and lung defects, cleft palates and Down syndrome. The children were born between 2007 and 2011.
These are just major birth defects. These poor kids probably have lots of minor defects, too, and learning disabilities that aren't obvious until school age.
Less than one per cent of babies of white British origin were born to first cousins compared with 38 per cent in the Pakistani group.
Births to second cousins are probably not insignificant either, although the fraction of "shared alleles" is only 1/4th as high in offspring of second cousin marriages. (This is assuming de novo cousin marriages -- if your ancestors have been cousin marrying for a long time, your mileage may vary.)
Keep in mind that first cousin marriage is not just a cultural ideal in large parts of the world, it's a key engine of immigration fraud. An arranged marriage of an English-born girl to her cousin in the Old Country creates a visa for a member of the extended family, opening doors to more visas under "family reunification."
A dozen years or so ago, the horrible anti-immigration rightwing Danish government passed a law not giving a visa to a foreigner if his Danish-born bride is under age 24.