Over the years one issue I’ve revisited over and over is that paternity certainty is quite high in Western societies (and from the spotty evidence we have, in most Asian and Middle Eastern societies as well). The reason this is interesting or of note is that there is an urban myth that 10% or so is the incidence of misattributed paternity. That is when children and fathers believe that they are biologically related, but they are not. The true rate in Western societies seems to be 1-2% (and, from what I have heard about Y chromosomal studies in Arab countries, it is at least as low in those societies).
Some of the confusion arose because early genetic testing was done in paternity determination laboratories. For obvious reasons, this is not an unbiased sampling of the general population. Even here, in only 30% of the cases did it turn out that “you are not the father.”
But is this universally true? There was already evidence that even in a Western modern context the extra-pair paternity rate was not always 1-2%. The frequency increases the lower down on the socioeconomic ladder you go.
A new paper in Science Advances, High rate of extrapair paternity in a human population demonstrates diversity in human reproductive strategies (open access), presents the case of
– almost 50% extra-pair paternity rates
– extremely high paternity certainty, which seemed to match whether the child was the biological offspring of the father
What I’m getting at here is that this paper is pointing to a situation where the extra-pair paternity rates are high due to non-marital liaisons, but the individuals in the community as a whole are quite clear of the situation. There is no great deception.
In the discussion they conclude:
These data provide a stark contrast to the prevailing opinion in the genetics literature that EPP is negligible in humans. While Himba may be at the far end of the range of human variation on this trait, they are not alone in having frequent concurrent partnerships. To assess the true range of variation, more studies from a wider variety of economic and social settings are needed. In addition, we currently know very little about what factors might contribute to variation in EPP among humans. Higher rates of female concurrency have been linked to matrilineal inheritance, reliance on foraging and horticulture, a male-biased adult sex ratio, and prolonged periods of spousal absence (11, 25). However, causal links between these traits and the rate of EPP are opaque at best.
In the specific case of the Himba, their pastoralist economy may play a role here. There is strong gender segregation insofar as men and women play distinct economic roles, and the men spend a fair amount of time away from their wives. Additionally, women and girls are critical economic producers, and they are placed in arranged marriages at a very young age. It does not take much imagination to entertain the possibility that economically semi-independent young women married off against their free choice to strange men, who are often away, may regularly form other relationships when opportunity and inclination arose.
The more general question and phenomenon being highlighted here is the nature of human cultural variation. There have long been debates around evolutionary psychology and “human universals.” There is clearly something here, but the field of cultural evolution has shed light on the reality of a high degree of plasticity of social forms. But these forms are not arbitrary in their distribution. Many societies seem to have moved toward a lower frequency of extra-pair paternity, combined with more “mate guarding.” Why?