We are accustomed to hearing that men are eagerly trying to propagate their genes into future generations via casual sex. If that is true, then one might expect that husbands would be much more likely to have affairs than wives. Are women truly more faithful than men?

Men’s adaptations for polygyny

Masculine sexual anatomy and physiology suggests that natural selection designed men for mating with multiple women. The fact that men are taller and heavier than women is indicative of mild polygyny. The idea is that body size is an advantage in fighting over sexual access to females.

Moderately large human testicles also produce a larger volume of semen than is required in a strictly monogamous species, such as gibbons (1). Men also produce a larger ejaculate upon reunion with their mates following a separation (2). This suggests sperm competition, or a race by sperms from different men to fertilize an egg.

Men’s penchant for casual sex is a commonplace of evolutionary psychology and it creates the market for pornography and prostitution, industries that have few female customers. In addition to these psychological propensities for sex with different women, men report many partners when questioned in surveys.

Women’s adaptations for polyandry

If men are adapted for sperm competition, this implies that women must have sometimes had sex with different men during the five-day window that sperm remains viable in the female reproductive tract.

More than any other female mammal, women are equipped to experience sexual pleasure (3). While the whole-body changes that take place during sexual interactions might strengthen a monogamous bond, they could also motivate women to seek out different sex partners.

Women are said to have continuous sexual receptivity. Unlike other mammals that restrict mating to a few days of the year when they are most likely to conceive, women can have sex at any time (4). Interestingly, they are more attracted to he-man physiques during the most fertile days of the cycle. This implies that their sexuality is on a dual track. When most fertile, they seek men whose appearance indicates good genes. The rest of the time, they are less selective, and scholars speculate that sexuality is used to extract resources from men.

In theory, women might have affairs to obtain better genes for their offspring. Yet, this happens rarely and husbands have a high confidence of paternity in most societies (3). The problem is that infidelity is extremely risky because it can destroy marriages, to the detriment of children, and expose women to jealous violence, not to mention severe legal penalties.

Wives may well be interested in sexual pleasure for its own sake but casual sex is more costly for them in terms of unwanted pregnancy, reputational damage, and fear of sexual assault. Their opportunities may also be restricted by spending more time caring for children and also by the fact that many women avoid sex while menstruating. Statistically, this means that wives would have fewer affairs than their husbands.

Just the facts

So husbands evidently have both a higher motivation and more opportunities for casual sex than wives do. Consistent with these notions, women characteristically report about half the lifetime sex partners reported by men. Researchers generally do not believe the difference as it takes two to tango (5).

If married men are interested in casual sex for its own sake, most do not act upon such impulses possibly because of the threat to their marriage. Yet, they are twice as likely to cheat as their wives are, if survey responses are to be trusted (6).

Whereas men who cheat are motivated primarily by sexual pleasure, women are most likely to have affairs if they are dissatisfied with their marriages (3). Infidelity is thus a way of moving on from a bad marriage to one that is more satisfying.

So men may indeed be more likely to cheat because they are motivated by sexual pleasure. This sometimes makes them appear shallow and irresponsible. On the other hand, by being more in control of their sexual impulses and using an affair to transition out of their marriage, women can seem manipulative. No one is at their best while cheating.

1. Birkhead, T. (2000). Promiscuity: An evolutionary history of sperm competition. Cambridge, MA: Harvard University Press.

2. Baker, R. S., and Bellis, M. A. (1995). Human sperm competition. London: Chapman & Hall.

3. Barber, N. (2002). The science of romance. Buffalo, NY: Prometheus.

4. Thornhill, R., & Gangestad, S. W. (2010). The evolutionary biology of human female sexuality. New York: Oxford University Press.

5. Einon, D. (1994). Are men more promiscuous than women? Ethology and Sociobiology, 15, 131-143.

6. Wiederman, M. W. (1997). Extramarital sex: Prevalence and correlates in a national survey. Journal of Sex Research, 34, 167-174.

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