“Victorian” might be a by-word for prim morality, but the era's cities were, relatively speaking, dens of sexual impropriety, a new study has found. Many suspected this all along, but now we have the DNA receipts to prove it.

When zoologists started DNA testing animals for paternity they discovered a lot of species that appear monogamous get up to far more extra-pair activity than anyone had realized.

Privacy issues make this harder to test in humans. A lot of exaggerated conspiracy theories have filled the gap, but now science is finding a way. A series of studies of fairly isolated populations have found that, however often people might be having sex outside marriage, cases of children being born as a result of adultery are historically relatively rare. Now, Dr Maarten Larmuseau of Belgium's KU Leuven has expanded the scope, building a picture of 500 years of cuckoldry, or its absence, in Belgium and the Netherlands.

Larmuseau identified 513 pairs of men who could trace their family trees back to a common male ancestor who lived around the year 1500. If none of their ancestors were born as the result of adultery (or adopted with this fact being lost) these men should have the same Y-chromosomes. Where they didn't Larmuseau investigated other relatives to identify the time when divergence occurred.

"Of course, extra-pair paternity, especially due to adultery, is a popular topic in gossip, jokes, TV series, and literature," Larmuseau said in a statement. "But scientific knowledge on this phenomenon is still highly limited, especially regarding the past.”

Larmuseau's findings, published in Current Biology, broadly support those of previous studies comparing genealogy with genetics. Extra-pair paternity averaged just 1.6 percent, rather than the inflated figures drawn from highly unrepresentative samples, but varied from 0.4-6 percent with circumstances.

“Our research shows that the chance of having extra-pair paternity (EPP) events in your family history really depends on the social circumstances of your ancestors," said Larmuseau. "If they lived in cities and were of the lower socioeconomic classes, the chances that there were EPP events in your family history are much higher than if they were farmers."

Besides farmers, merchants and other middle-to-upper class families had little EPP, even if living in cities. On the other hand, Larmuseau and co-authors found no significant differences across the countries' religious and cultural divides.

Given how dirty and unpleasant cities were during the Industrial Revolution, it seems possible the chance of diversifying their sex lives was a major reason so many people flocked to them.

What the paper cannot explain is why humans have much lower rates of EPP than most pair-bonded animals, although in a previous paper Larmuseau discussed the possibility that even pre-1800 contraception was more effective than had been thought.