Genomic analysis has provided a new way of investigating a tricky topic: the prevalence of extreme inbreeding in humans.

Cultural and religious taboos around inbreeding make its frequency difficult to assess. Rather than relying on surveys, Loic Yengo and his colleagues at the University of Queensland in Brisbane, Australia, examined roughly 450,000 human genomes from a British biomedical database. By looking at thousands of single points in the genome, the researchers were able to identify any long stretches of DNA that were identical on both chromosomes in a pair, which are separately inherited from a person’s father and mother. These stretches indicate that the parents were closely related.

The researchers found that the genomes of one in 3,652 people born in the United Kingdom between 1938 and 1967 show extreme inbreeding, with the two sets of chromosomes sharing more than 10% of their DNA. This indicates unions between full siblings, a parent and a child, a grandparent and a grandchild, or other relatives with similar degrees of relatedness.

People whose genomes showed extreme inbreeding tended to be shorter, less muscular and have weaker cognitive abilities than average.