Marriage is a contract. All contracts imply potential conflicts of interest. Men cheat on their wives (1). Women deceive their husbands — more often than is generally appreciated. Perhaps a tenth of the children in a marriage is conceived extramaritally (according to genetic tests 2), although some estimates are as low as 1-4 percent.
Such evidence is shocking to our notions of monogamous marriage. The trust of husbands is often cruelly misplaced. Still, women have a great deal to gain by mating with an extra-pair partner (i.e., lover) who provides them with better genes for their offspring. From that natural-science perspective, cheating is predictable.
Among socially monogamous barn swallows, about a quarter of the offspring results from extra-pair trysts, for instance. The sexiest males, those having the longest tails, are preferred as extra-pair partners. Women are also more swayed by physical attractiveness in choosing a lover than they are in selecting a husband. A cheating wife shops for good genes for her love children.
If cheating provides such obvious genetic benefits, why don't females do it all the time? Evidently, marriage contracts would break down if paternity confidence got too low and children would miss out on paternal support and protection. Most of the offspring are sired by resident males for monogamous birds, as well as for human husbands. This implies that female cheating is somehow kept in check.
Moralists might be disappointed that humans are not much more trustworthy than swallows. Some might argue that cheating reflects the moral depravity of modern times. They would be mistaken. Stone-Age infidelity was a recurrent problem. Indeed our sexual anatomy, physiology, and behavior were explicitly designed to confront this issue.
Male private parts hint at the sexual antics of ancestral women. Comparatively large testicles produce fairly large volumes of semen. This served to dislodge the seminal deposits left by previous lovers in a woman's vagina. Upon reuniting after a separation, men also produce a larger emission. Even the shape of the human penis with its large glans and prominent coronal ridge seems designed to scoop out rival sperm (3). If men are adapted for sperm competition, this implies that ancestral women were fooling around. To be specific, they must frequently have had sex with more than one partner within the five-day time frame that sperm remains viable in the reproductive tract.
Some peculiarities of the female orgasm also suggest that it may allow promiscuous women to unconsciously favor the sperm of one man over another. To begin with, orgasm occurs more often among women who have many sexual partners and a stronger sex drive. Orgasm draws semen up into the reproductive tract thereby favoring the chances that the man who has recently ejaculated will win the race to fertilize the egg (4).
Women prefer physically attractive men as extramarital lovers. Because attractive men are more sexually exciting, cheating women are more likely to reach orgasm during their extramarital coitus than they are during marital intercourse. Translation: They are shopping for good genes and bias the chances of being impregnated by their extramarital partner rather than the husband.
Wives who conduct extramarital relationships today continue a very ancient practice. We do not know exactly how many cheated in the Stone Age. Yet, the number and complexity of adaptations for sperm competition in men and women suggest it was possibly as common for humans as for barn swallows.
Ethnographic descriptions of hunter-gatherer societies point to the same conclusion. Some hunter-gatherer women entertained dozens of sexual partners in the course of their lives (5). They did so despite a great deal of sexual violence and personal tragedy.
Stone Age infidelity happened! We should care because we have inherited sexual propensities from our ancestors. Suspected infidelity remains the single biggest cause of severe violence against women everywhere and it often precipitates divorce. Interest in having numerous sexual partners varies greatly around the world. The reasons may surprise you, as detailed in a future post.
1. Barber, N. (2002). The science of romance: Secrets of the sexual brain. Amherst, NY: Prometheus.
2. Birkhead, T. (2002). Promiscuity: An evolutionary history of sperm competition. Cambridge, MA: Harvard University Press.
3. Gallup, G. G. (2004). Semen displacement as a sperm competition strategy in humans. Evolutionary Psychology, 2, 12-23.
4. Baker, R. S., & Bellis, M. A. (1995). Human sperm competition. London: Chapman and Hall.
5. Shostak, M (1981). Nisa: The life of a !Kung woman. Cambridge, MA; Harvard University Press.