Who’s your daddy?

Sexual infidelity may be as old as marriage, but it is not an equal opportunity pursuit. For once, it seems the poor get more.

New research has quantified for the first time the occurrence of extramarital sex over the past 500 years in the Netherlands and Belgium, identifying stark differences between locations and socioeconomic groups.

Researchers from the university KU Leuven in Belgium found that through the centuries 1.6% of children of married couples were the result of extramarital affairs.

But this figure rises sharply among lower income and status communities in densely populated areas and falls among the well-to-do and farmers. It seems that fresh air and money help to keep marriages on track.

Lead researcher Maarten Larmuseau says the team’s work is the first large-scale study of how social context affects the incidence of what is politely called extra-pair paternity (EPP), a term which simply means dad isn’t the man married to mum.

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

To shed light on the topic, the researchers checked the Y chromosomes of 513 pairs of men who, according to family trees, should be related genetically. In particular, they searched thousands of genealogical records including birth, marriage and occupation details and looked for genetic proof of common male ancestors in the participants’ DNA.

The team found the overall rate of mismatches – where the DNA did not match the genealogical record – was low enough to debunk the notion that EPP rates in Western society are generally high. But the results provided some unexpected insights.

For example, the rate of EPPs in Flanders and the Netherlands was almost identical, despite Flanders being predominantly Catholic and the Netherlands being mostly Protestant. So religion appears to play no part in infidelity.

The most important finding, however, was how EPP rates varied among occupations and living conditions.

In places of average population density, the EPP rate among farming families was 1.1%, dropping to 0.5% in sparsely populated regions.

Well-to-do families, such as merchants, recorded an EPP rate of around 1% in average areas, dropping to 0.4% out in the countryside.

But in the densely populated towns with more than 10,000 people per square kilometre, EPP rates in lower socioeconomic groups such as the families of labourers and weavers were nearly 15 times higher at 5.9%, peaking in the late Nineteenth Century as towns filled with low-status workers.

“Our research shows that the chance of having extra-pair paternity events in your family history really depends on the social circumstances of your ancestors,” says 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.”

The researchers write in the journal Current Biology that the association between EPP rates and population density is also observed in the animal kingdom. Put simply, there’s just more opportunity for EPCs (that’s extra-pair copulation, or science talk for having a fling).

For humans, it’s also easier to keep things secret in the anonymity of a big city where there is less social control.

On a darker side, the higher incidence of EPPs in poor, crowded communities is likely to reflect women’s need to obtain material or social benefits in adverse conditions, and also the occurrence of forced sex and workplace exploitation.

“In human populations, the incidence and correlates of extra-pair paternity remain highly contentious,” the researchers note.

“These findings show how contemporary genetic data combined with in-depth genealogies open up a new window on the sexual behaviour of our ancestors.”