Marrying a first cousin has been legal in Britain since Henry VIII broke with Rome in the 16th century. The practice faded and became taboo as family sizes shrank and understanding of the genetic risks grew. In modern Britain unions between first cousins are rare, and for decades have been a non-issue.
But there is a section of British society where cousin marriages are more common: Muslim communities, particularly Pakistani ones in the north of England. Precise numbers are elusive (no one collects them). But Born in Bradford, a study of about 13,500 families living in the West Yorkshire city between 2007 and 2011, found that 37% of married couples of Pakistani heritage were first cousins, compared with fewer than 1% of white British couples.