by Greg Mayer

A lot of work in behavioral and evolutionary biology concerns the evolution of mating systems—polyandry, polygyny, monogamy, promiscuity, and the like—and elucidating the factors that lead to the evolution of one or another. Mating systems can be variable within a species, and human societies exhibit a range of mating systems, with monogamy and polygyny being perhaps the most common. But these culturally defined mating systems may exist more as aspirational norms than as universal practices. From a biological point of view, it is actual paternity and maternity that count, not what the culture deems most appropriate.

In a piece in the New York Times, Carl Zimmer takes a look at this in human societies, asking how often is it the case that the father of a family’s children is not the mother’s spouse, but rather some other male who has fathered the children through adultery.  This is a popular theme of “reality” TV, but Carl concludes it’s an old wives’ tale, citing a review by Maarten Larmuseau and colleagues in press in Trends in Ecology and Evolution. Using a variety of approaches that have become possible only since the development of DNA sequencing, including ordinary paternity testing and the clever use of Y chromosomes in concert with family genealogies and historical migration events, they show that “extra pair paternity” (EPP, i.e. cuckoldry) is actually rather rare—only 1-2%.

Carl notes that this is not as unexpected as the myth would have you believe. In species with females who frequently have multiple mates, males evolve a number of adaptations to insure the precedence of their sperm over other males—and men have none of these sorts of adaptations.

But, as luck would have it, on the very day Carl’s article appeared, a case of human EPP is also in the headlines: of all people, it turns out that the Archbishop of Canterbury’s father was not his mother’s husband! As the Independent put it “DNA test reveals Archbishop of Canterbury Justin Welby is illegitimate son of Sir Winston Churchill’s private secretary“.

My doctoral mentor, E.E. Williams, always said that observations refuting your favorite hypothesis don’t come early in your hypothesizing; a Malevolent Nature ensures that they come only after you’re really convinced you’re right. And in the journalism version of this, the contrary example doesn’t come till you get your piece on the front page of the New York Times. In fairness to Carl, though, one example does not refute his general conclusion, and it isn’t even quite cuckoldry: the Archbishop’s mother had sex with the private secretary just before her marriage, so the birth of the Archbishop-to-be nine months after the wedding had been wrongly assumed to be the fruits of the wedding night.

Larmuseau, M.H.D., K. Matthijs, and T. Wenseleers. 2015. Cuckolded fathers rare in human populations. Trends in Ecology and Evolution in press. pdf