What if 1 out of 10 men were cuckolded?

As I have stated before one of the strangest things to me is the ‘urban myth’ among many biologists that 10% of children exhibit misattributed paternity. In plain English, one out of ten fathers of any given child are not their biological fathers, though that is the social understanding. So this excludes children who were adopted and such. In general these are explicitly cases where the putative father is unaware that he’s been cuckolded. The 10% figure is a nice round number, and I regularly hear it in the public arena, but it is also surprisingly pervasive among academic scientists. A few years ago I was in a seminar where a behavioral ecologist alluded to the figure in passing, and being who I am I had to raise my hand and object that it just wasn’t the true. The researcher, who did not work with humans, was genuinely surprised at my objection, and didn’t seem to be particularly invested in the figure he gave, and was quite open to updating his beliefs about this issue. I experienced this again on Twitter recently, where a biologist casually referred to the 10% value, and I pointed to my 2010 post which leans heavily a on 2006 meta-analysis, which suggests values closer to 1-3%. Instead of being defensive, he simply acceded to the new information.

More recent work seems to have confirmed this finding: Low historical rates of cuckoldry in a Western European human population traced by Y-chromosome and genealogical data. It seems obvious that a 10% rate of confused paternity is going to show up in a discordance between genetic and genealogical paternal lines. The authors of the above paper use two methods:

…based on an unbiased population-wide sample, the Y chromosomes of presumed patrilineally related males were compared with each other. Subsequently, EPP rates were estimated based on the discrepancy between the legal genealogy and the actual genetic relatedness. Second, the historical EPP rate within Flanders was estimated based on the genetic traces of a substantial past migration event from northern France to Flanders.

For the Flemish in Belgium the rates were 1-3%. Perhaps the Flemish in Belgium are unique, but that seems unlikely. There have been a reasonable amount of studies in Western Europe on this topic, and this is in line with other results. There may be cross-cultural differences though. But my bet is that in parts of Asia where there are long-term patrilineages, such as China or Mongolia, you’ll come up with a similar figure. The situation might be different in “small-scale societies,” and in particular those where women are primary economic producers, and not particularly dependent on male resources (these tend to be matrifocal societies, where agricultural labor depends on the hoe and not the plough).

But going back to the 10% figure, consider what it would mean if it was correct. In a random family of two children in about one out of five cases there’d be misattributed paternity in at least one of the families*. Most people have at least five close friends, so misattributed paternity wouldn’t be an abstract issue, it would be something you’d confront in your day to day life. Additionally, it seems unlikely to me that the chance of conception from affairs is going to be higher than within marriages, because many would take precautions in illicit relationships. So the proportion of sexual activity that is “extra-pair” is going to have to be rather high. Is that plausible? The major caveat here is that there are differences between population segments, not just across cultures. The rates for high status individuals seems very low. The rates for low status individuals seem rather higher. The 10% figure is actually not that implausible from samples which are skewed toward the underclass, which is often the case when you are looking at laboratory data uncorrected for background variables (i.e., the men who avail themselves of paternity testing services are not an unbiased sample of the population; they usually have something to worry about at a much higher rate).

The final conclusion is that we should be rather happy that the rates are likely far lower than 10%. A lot of work on extra-pair paternity has been done on birds, and here’s a paper from 2013, Faithful females receive more help: the extent of male parental care during incubation in relation to extra-pair paternity in songbirds:

Parental care provided by males occurs in a diverse array of animals and there are large differences among species in its extent compared with female care. However, social and ecological factors responsible for interspecific differences in male’s share of parental duties remain unclear. Genetic fidelity of females has been long considered important. Theory predicts that females should receive more help from their mates in raising the offspring in species with high genetic fidelity. Using avian incubation behaviour as a model system, we confirmed this prediction. The extent of male’s help during incubation increased with decreasing rate of extra-pair paternity across species (22 species of socially monogamous songbirds from 13 families; male’s share of incubation ranged from 6% to 58%), even after accounting for covariates, biases in species selection and intraspecific variability. Moreover, this result was not sensitive to two different phylogenies and branch length estimates. We suggest that our findings support the notion, backed by theory, that genetic fidelity is an important factor in the evolution of male parental care. We offer several behavioural scenarios for the coevolution between male’s share of parental duties and the genetic mating system.

The basic theoretical logic is pretty obvious. If cheating “pays,” then it will become ubiquitous until everyone becomes a cheater, and the social system will converge upon a new equilibrium. You can have low female fidelity and low male investment in family life, or you can have high female fidelity and high male investment in family life (assuming that the child-mother relationship is the core of a family), but exchanging the traits is probably not a viable long term proposition (see: Hippy communes, which tend to have short half-lives, or societies such as in the highlands of Papua, where males don’t invest much in offspring, but paternity is not a major social issue where sanction is ferocious as in other societies). High fidelity from females toward males who do not provide any resources or care of offspring seems irrational. And conversely, high male investment in offspring that are highly possible to be another male’s seems irrational.**

* Obviously it is unlikely that the events are independent probabilities, so there are more likely to be cases that both children being misattributed than one would expect from assuming that each child has a 10% chance of being from a different biological father.

** From an evolutionary perspective.