Roughly one in every 3,650 or so people in the UK could have been born of extreme incest, a variety that involves parents that are first- or second-degree relatives, according to a new study published in the journal Nature.
Studying incest is a challenging task for researchers. People are unlikely to be open about the subject because of the cultural taboos and laws against it, which makes self-reporting unreliable. While collecting information on the down-low, without permission, is problematic from an ethical standpoint. And so researchers from the University of Queensland, Australia, have turned to the UK Biobank – a large database containing genetic information from 500,000 people across the UK.
The researchers class "extreme inbreeding" as mating between first- or second-degree relatives; first-degree relatives being parent, sibling or child and second-degree relatives being anyone who shares 25 percent of a person's genes (uncles, aunts, nephews, nieces, grandparents, and double cousins – ie those who share both sets of grandparents).
By identifying long runs of homozygosity (sections of the genome from each parent that are identical) of 10 percent or more, the team was able to determine a high chance of extreme inbreeding in 125 of the approximately 450,000 people of European ancestry born between 1938 and 1967 studied. That equates to one in every 3,650 or so people or 0.03 percent. Of those, 54 had parents who were likely first-degree relatives, they say.
Next, they compared this figure to the number of incest offenses reported in the Crime Survey for England and Wales (CSEW) between April 2002 and March 2017. Based on the number of offenses reported, the researchers worked out the prevalence of incest would exist somewhere between one in 5,247 and one in 4,699 – either way, a substantially smaller proportion than the Biobank data suggests.
After comparing the mean years of birth amongst the incest cases with the other births in the Biobank, they concluded there was no statistically important change in the rate of incest over time, implying their figure might be a more accurate representation of rates of incest in the UK population than that suggested by the CSEW (which includes more recent cases of incest).
The study authors point out there are certain limitations to comparing the CSEW numbers with the Biobank numbers. One being it is likely that many cases of incest are going unreported. Another is that not all cases of incest will lead to "viable" offspring, and a third is that any "viable" offspring with serious cognitive impairment caused by inbreeding would be unlikely to enroll in the Biobank project.
The Biobank project, they say, is not representative of the UK population but "includes healthier and more educated participants than the average population". Therefore, there is a good chance the actual prevalence of extreme incest is higher than the figure reported, they conclude.
Past studies have linked incest to higher childhood mortality and poorer physical and mental function, including lung function and cognitive ability. The study appears to back these up, finding that extreme inbreeding did seem to increase the likelihood of poorer cognitive and muscular function, fertility difficulties, shorter stature, and a higher risk of disease in general.