Celebrating the Birth by Jan Steen depicts a husband who does no realise his wife’s new child is not his Celebrating the Birth by Jan Steen, 1664. © The Wallace Collection, London

In the 19th century, poorer families living in cities in Europe had a higher rate of children who weren’t biologically related to their legal fathers. This is according to a genetic study that looked at how this rate differs for different socio-economic groups.

It is widely assumed many men aren’t the biological fathers of their children. The rate of extra-pair paternity, as this is called, has been claimed to be as high as 30 per cent today. “They look just like the milkman,” goes the popular joke that no parent finds funny.

However, over the past two decades DNA studies in several countries have shown the average rate is low – around 1 per cent. Maarten Larmuseau at KU Leuven in Belgium, who authored one of these studies, wondered whether there was a difference between groups.


He suspected, for example, that the rate was higher among aristocrats in the 17th century, as there was often a large age gap between husband and wife. Extra-pair paternity is depicted in the 1664 painting Celebrating the Birth by Jan Steen, which shows a wealthy Dutch father holding his newborn child. But behind him a man is making the sign of the “cuckold’s horns”, meaning the child was fathered by another.

Cuckold’s horns

Larmuseau’s team identified 500 pairs of men in Belgium and the Netherlands where, according to genealogical records, each pair descended from the same male ancestor through a male lineage. Half of these ancestors were born before 1840 and the oldest was from 1315.

The men in each pair should have inherited their shared ancestor’s Y chromosome, as it comes from the father. When DNA testing revealed a mismatch, the team tested other male descendants to narrow down when a son had been fathered by someone other than the husband. All the men were volunteers and the team didn’t test close relatives to avoid uncovering recent cases.

“What we found was completely the opposite to what we expected,” says Larmuseau.

The rate of extra-pair paternity among farmers and more well-to-do craftsmen and merchants was about 1 per cent, rising to 4 per cent among labourers and weavers and nearing 6 per cent among working class people who lived in densely populated cities in the 19th century. This was in comparison to a rate of around 0.5 per cent among the more well-off.

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What the study cannot reveal is why people were more likely to be in this situation. “We cannot give an explanation,” Larmuseau says. “We cannot interview them.”

One possibility is that poorer women in cities were more vulnerable to male sexual violence and exploitation.

The overall rate was still low, at 1.6 per cent per generation. But that still means a very large number of people alive today may not be aware of their biological parentage. Larmuseau says 30 million people worldwide have done ancestry tests, which suggests up to 500,000 could have made a shocking discovery about their father. Companies offering these tests don’t provide any counselling, he says.

Journal reference: Current Biology, DOI: 10.1016/j.cub.2019.09.075