Sex in the city: when dad's not dad

And why urban living and low socioeconomic situations make it more likely to happen...
19 November 2019

Interview with 

Maarten Larmuseau, Leuven University




If you think you know who your relatives are, you might want to think again. Because, according to a new study, about one in every hundred of us doesn’t have the dad with think we do. This is called “extra-pair paternity” - it’s where a man ends up bringing up another man’s child - and city living, as well as lower socioeconomic status, make it more likely to happen. Researchers at Leuven University, in Belgium, discovered this by tracking down 513 pairs of men who, according to parish records, shared a male ancestor up to 500 years ago in their family trees. Because men inherit their Y chromosomes exclusively from their fathers, if there’d been no naughtiness in their family trees, the genetic barcodes of the Y chromosomes of each of the pairs of men should match. But, in some cases, they didn’t, with the obvious implications! And by using additional genetic techniques, they were able to work out when in the family history the assignation must have happened. Speaking with Amalia Thomas, Maarten Larmuseau...

Maarten - For the first time we reconstructed the historical patterns of extra pair paternity across the last centuries within our Western populations. What is 'extra pair paternity'. Well it is an event when a man was unexpectedly not the biological father of his legal child. What we found was that the 'extra pair paternity' rates were lower overall around 1 percent, but depending on social context we find that the highest 'extra pair paternity' rates were observed among urban families with low socio economic status, especially in the 19th century.

Amalia - And how did you find out that these women had been cheating on their husbands?

Maarten - But it's not only cheating. It can also be the result of sexual aggression'. Extra pair paternity' has potential different causes. So what we did was find persons who are living today with a common paternal ancestor in direct paternal line; if they have the same Y chromosomes - Y chromosomes are the chromosomes that each male get from his father - in a paternal line, every man should have the same Y chromosome, or variants. If that is not the case then at least one 'extra pair paternity' event happens across the generation.

Amalia - So you studied the genes of over 500 pairs of donors who are alive today and compare that to the family records dating back up to 500 years. How can you be so sure that records that are so old are accurate?

Maarten - We are quite sure about those genealogies, based on civil records and parish records. If we didn't find a biological connection between presumed related persons then we know that there is at least one 'extra pair of paternity In the past.

Amalia - What have you found out from this comparison?

Maarten - Well that the 'extra pair paternity' rate is quite low in average. It's always around 1 percent. But what we saw now is that there were quite differences between socioeconomic classes, especially the low social classes in the 19th Century, they had a 10 fold higher exit per paternity rate than formers in rural areas and this is quite surprising. We didn't expect this! We expected more in aristocratic families, especially because there is a lot of references to adultery in the 17th century in literature and theatre plays. So this looks like based on our results that it's not the case.

Amalia - Could you guess if you don't know exactly, why it's the socioeconomic status population, density and particularly the 19th century where this happened the most?

Maarten - Well we don't have a real explanation because we cannot interview older people any more and say what is the reason. Is it because of adultery is it because of sexual aggression and rape? We cannot ask them again. But what we see is that in the 19th Century there was a lot of social conflict, low social classes lived in very poor conditions. We also had a lot of cholera epidemics, especially in Belgium and the Netherlands where we looked. After the Industrial Revolution there weren't that good conditions and there were much higher differences between the social classes. So after all it was not so surprising that we could see differences.


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