Sex in the City: Is your dad really your dad?

In every generation there is about a 1% chance of a child’s father not actually being the mother’s husband...
15 November 2019




In every generation there is about a 1% chance of a child’s father not actually being the mother’s husband, and it's been that way for at least 500 years, a new study shows...

Bringing up another man's child, also known as Extra-pair paternity (EPP) where a married woman mothers a child by a man other than her husband, was most common among poorer families, those living in densely-populated areas, and in the 19th Century in particular, potentially related to the Industrial Revolution

“Extra-pair paternity is an event where a man was unexpectedly not the biological father of his legal child, that was born in wedlock,” explains lead author Maarten Larmuseau. “It’s not only cheating; EPP has a lot of potential different causes,” says Larmuseau, adding that it can also be the result of sexual aggression or adoption, for example.

The study analysed historical patterns in the rate of EPP over the last 500 years in Western Europe. The researchers, from Leuven University in Belgium, compared the legal family trees (based on civil and parish records) with DNA evidence supplied by 513 pairs of live male donors from various parts of Belgium and the Netherlands. Each pair of donors was related, though separated by several generations, to a common ancestor in the male lines.

The analysis checked whether each pair had the same Y-chromosome. Larmuseau explains that “this is the chromosome that each male gets from their father, and so in a male line every man should have the same Y-chromosome variant. If that is not the case, then at least one EPP event happened across the generations.”

By comparing the Y-chromosomes of supposedly-related males, the researchers could even narrow down how far back in their family trees the EPP events occured.

“The overall EPP rate is quite low on average, around 1% per generation,” says Larmuseau. This means that, on average, one in every hundred men in every generation are not biologically related to their father. But in the lower classes in the 19th century, they found ten times the EPP rates than in rural areas. The results were surprising to the researchers, who expected to find higher EPP rates in aristocratic families.

“There are a lot of references to adultery [in the higher classes] in the 17th Century in the literature and in theatre plays, such as Shakespeare’s, and in paintings, like Jan Steen’s and other famous painters.”

Larmuseau suggests that the peak in EPP rate in the 19th century may be related to the Industrial Revolution. Around this time, there were large migrations from rural to urban areas, and the rift between the living conditions of people of different social classes grew.

“In the 19th Century there was a lot of social conflict. People in the lowest social classes lived in very poor conditions. So, we think it was due to the Industrial Revolution that the different social classes developed different behaviours.”


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