father and son
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Comment: "A cuckold is the husband of an adulterous wife. In evolutionary biology, the term is also applied to males who are unwittingly investing parental effort in offspring that are not genetically their own." - Wikipedia

Lower-class city-dwellers were many times more likely to become cuckolds than any other class, according to a groundbreaking study that has lifted the lid on 500 years of cheating in Western Europe.

The study authors did this by comparing comprehensive family trees pieced together from church and civil records from Belgium and the Netherlands, stretching back to the 16th century. They also analysed the Y chromosomes — which are passed largely unchanged from father to son — of over 1,000 adult men in from those countries. According to family trees, each of the 500 or so pairs of these men shared a common paternal ancestor, i.e. a great-great-great (etc.) grandfather.

Therefore, each of these men should have shared a Y chromosome with their distant cousin; if they didn't, then somewhere along the line a different baby daddy must have come into the picture. Comparing the DNA with the family trees allowed the researchers to estimate overall cuckoldry rates, and see how they varied between socioeconomic classes.

What proportion of people cheated?

This ingenious historical genetic reconstruction found that while the frequency of extra-pair paternity (cuckoldry) was low overall (around 1% on average), it peaked at close to 6% in people of low socio-economic class living in densely-populated cities in the 19th century.

The study took in periods of great social upheaval, and included the industrial revolution and large-scale urbanization. People from all kinds of different backgrounds and social classes would have been suddenly thrown together into metropolitan melting pots, and according to the results, blood must have been running pretty hot.

It was these new city-dwellers, specifically working class labourers and weavers, who had the highest rates of false paternity (around 4% on average). In contrast, merchants, farmers and craftsmen, especially those living in sparsely-populated small towns, cheated the least, with rates below 1% in those areas.

Urban hot-beds, propriety, or rape?

So what is it about city living that drove Europeans to cheat?

A combination of more opportunities, more anonymity, and reduced social control, most likely. In the countryside, everyone knows everyone else's business — making it harder to sleep around — and the church probably exercised even greater power there than it did in cities.

One unpleasant possibility, but nevertheless an important one, is that rape played a part. Poor women working in urban factories and laundries were very vulnerable, abortion was risky and illegal, while contraception was not yet in the picture.

While many cuckolds may have suspected nothing, some men may have married women knowing that they were not carrying their child, but accepted the offspring as their own, to avoid stigma.

With the advent of commercial home DNA testing kits, such as those offered by 23andMe and AncestryDNA, a lot of skeletons are falling out of closets. It may be that cuckoldry will soon become a thing of the past in a world where a simple mouth swab can tell all, and some men's rights groups demanding mandatory paternity tests at the birth of every child.

But the people in this study, who lived through the industrial revolution, knew nothing of these technologies to come, and must have thought their secrets went with them to the grave.

Peter Andrews is an Irish science journalist and writer, based in London. He has a background in the life sciences, and graduated from the University of Glasgow with a degree in Genetics