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Genetic, Genealogical Study Examines Historical Rates of Extramarital Paternity

NEW YORK – While the rate of men raising a child who was not biologically related to them was fairly low among 19th century couples in Europe, a new genetic study finds that the frequency of such non-biological fatherhood events varied with people's social circumstances.

Behavioral ecology studies have indicated that both males and females from species with long-term pair bonds can increase their fitness by mating with individuals other than their partners, and theories have suggested that environmental factors, such as population density and resource availability, might influence how often this occurs. But estimates of so-called extra-pair paternity events in humans have been controversial.

Researchers from KU Leuven in Belgium adopted a genetic genealogy approach to study extra-pair paternity among the ancestors of 513 pairs of men who appeared paternally related based on genealogical data from Belgium and the Netherlands. As they reported in Current Biology today, the researchers found that the rate of extra-pair paternity was low, though it varied between less than 0.5 percent and nearly 6 percent. In particular, extra-pair paternity was more common among people of lower socioeconomic status and among people who lived in more densely populated regions.

"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," first author Maarten Larmuseau from KU Leuven said in a statement. "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 identified 513 pairs of men who appeared to be distant paternal relatives based on genealogical data from civil and church records and examined how similar their Y chromosomes were. If there were no extra-pair paternity events in their family tree, then their Y chromosomes should be the same, they noted. On average, the men in these pairs shared a common ancestor who was born in about 1840, a time of social change in Western Europe due to the Industrial Revolution.

They genotyped the men using a panel of 191 Y chromosome SNPs and 38 Y chromosome STR loci and used mismatches between putatively paternally related men to identify instances of extra-pair paternity.

Based on this, they estimated a low average historical rate of extra-pair paternity in the Netherlands and Belgium of 1.6 percent.

But the researchers also found a range in that rate of extra-pair paternity that varied with socioeconomic status and population density. Using archival records, they determined the place and date of birth for all 6,818 male ancestors in the genealogies they were studying and, for the men born between 1750 and 1950, their occupations. They used that data to infer the ancestors' socioeconomic status and the historical population size and density of the places they lived

They reported that extra-pair paternity was higher, about 4.1 percent, among families of lower socioeconomic status, such as laborers, as compared to families of middle and high socioeconomic status, such as skilled craftsmen and merchants, where it was about 1.0 percent. It was also low, 1.1 percent, among farmers.

Similarly, extra-pair paternity was higher, about 2.3 percent, among city dwellers, as compared to those living in small rural villages, where it was about 0.6 percent.

When combined, the researchers noted that the estimated extra-pair paternity rates ranged from about 0.5 percent among farmers in sparsely populated areas to 5.9 percent among people of low socioeconomic status living in a city. Social context appears, as evolutionary theory has suggested, to influence extra-pair paternity, the researchers said.

"Indeed, by zooming in on specific social strata, our study uniquely shows that there is much variation to be uncovered in the degree of extra-pair paternity within human societies," they wrote in their paper. 

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