To test this idea, several recent studies have developed new approaches that enable ‘extra-pair paternity’ rates from several centuries ago, before modern contraception, to be estimated.
To be able to reconstruct the sex lives of our ancestors and infer extra-pair paternity rates as they would have been in historical times, one new genetic technique compares family specific Y chromosomal variation between men, with genealogical evidence of paternity.
In this ‘genealogical pair’ method, ‘extra-pair paternity’ events show up in mismatches in the paternally inherited Y chromosomes.
Other indirect approaches have also been developed that provide estimates of past ‘extra-pair paternity’ rates by analyzing the association between Y chromosomal variation and paternally inherited surnames.
Using this first method, historical ‘extra-paternity’ rates were estimated at just 1.8% in Mali. This low estimate was surprising, given that the study got its data from word of mouth, which by itself could have contributed to error.
Subsequently, another study further perfected the genealogical pair method, employing written genealogical evidence, and also introduced the other method to provide an independent estimate, and applied it to a Western population in Flanders, Belgium.
This study arrived at a similarly astonishingly low ‘extra-pair paternity’ estimate of 0.9% per generation over the past 500 years.
Finally, three further studies have now appeared, according to Maarten Larmuseau, Koen Matthijs and Tom Wenseleers, that all confirm these very low occurrences of human ‘extra-pair paternity’ among several other Western populations: 0.9% per generation over the past 300 years in a (Western) Afrikaner population in South Africa, 1.2% per generation over the past 400 years in a north Italian population, and 0.6–1.7% per generation over the past few centuries in Catalonia.
The surprising conclusion from these new studies, according to Larmuseau, Koen Matthijs and Wenseleers, is that human ‘extra-pair paternity’ rates have stayed near constant at around 1% (maximally 2%) across several human societies over the past several hundred years.
If you hypothesize that contraception was more difficult or unreliable in the past, given modern rates of adultery, and if you project these rates back into history, you would have expected much higher rates of ‘extra-pair paternity’ to show up in this genetic testing.
Theoretically high adultery rates would be more likely to be revealed in the genes passed on in past times before modern contraception.
Alternatively, the authors of this investigation argue, it could be that historically, traditional methods to avoid pregnancy, for example, breastfeeding ‘infertility’ or fertility awareness, were much more effective than originally credited. This factor then would allow for higher rates of adultery without it being expressed in high ‘extra-pair paternity’ rates.
All this new data, the authors argue, pose a major challenge to that ‘biological drive’ idea of women ‘shopping around’ for good genes by engaging in extra-pair copulations to obtain genetic benefits for their children.
The authors speculate that there may be several reasons for such a low incidence of ‘extra-pair paternity’ world-wide and across centuries, including perhaps a dread of sexually transmitted infections, risk of spousal violence, divorce or reduced paternal investment by the social partner or his close relatives if the betrayal is discovered.
The fact that women may be a lot more faithful than had previously been imagined may have important and profound psychological impact. We often decide what to do with our lives based on our theories about how everyone else is behaving. If we assume the whole world is sleeping around or that fidelity is rare, it might lower our inhibitions over behaving similarly.
If it’s the case that women were and are more faithful than had been previously assumed, maybe that understanding in itself will influence promiscuity.
All this means that the Archbishop of Canterbury’s experience of his paternity is, in fact, rarer than might be commonly believed, and this fact could in itself have psychological consequences. Beliefs about how common it was for mothers to behave in this manner, could influence his own attitude and coping.
It is possible that the Archbishop’s of Canterbury’s own mother’s statement, as reported by the BBC News site and other media that she recalled sleeping with former colleague Sir Anthony after “a large amount of alcohol on both sides,” reflects some psychology as well.
Fetus image available from Shutterstock