Why women cheat: mate-switching vs. dual-mating

– by Macken Murphy

Socially monogamous birds, like humans, often have “affairs.” In some species, these liaisons seem to serve a dual mating strategy. The females prioritize good looks in extra-pair mates (e.g., more complex ornamentation) and good parenting in primary partners (extra-pair males generally don’t invest in young). Good looks in males are thought to signal genetic benefits, providing the females with more robust—or, at least, more attractive—offspring. And so, many ornithologists take aesthetic differences between extra-pair and primary mates in these species as evidence that the females use extra-pair mating to make a combo deal: good investment at home and “good genes” from outside.

Evolutionary psychologists would have told a similar story about humans two decades ago, just with different evidence. It was popular to argue that women’s infidelity evolved due to the competitive advantage female ancestors gained from conceiving with more attractive affair partners and then raising their affair partner’s child with their more invested primary partners. A flashy series of studies from the previous several years had suggested that women’s behavior and preferences changed around ovulation, prioritizing cues to “good genes” when conception was likely. While the reasoning was less direct than that found in avian species, the initial ratification of this clever prediction lent credence to the underlying dual-mating hypothesis. If women prize different traits at ovulation compared to the rest of the month, perhaps they prize different partners as well, and recruit them towards different ends.

However, the golden age of ovulatory shifts was brief. Failed replications, skepticism about methods, and insinuations of p-hacking cast doubt on the original experiments. Then, newer, more rigorous research often found null results. This anti-climax prompted some scholars to wonder: What if the problem lies deeper, with dual-mating itself?

In 2017, David Buss and his colleagues proposed an alternative primary explanation for women’s infidelity: mate-switching. Drawing on evidence that women who cheat are more likely to be in love with their affair partners and less likely to be satisfied in their relationships than men who cheat, they argued that infidelity primarily helps women assay and seduce replacements. To paraphrase Buss, you wouldn’t quit your job before finding a new one, so why would you dump your mate before getting a better one?

(This hypothesis also has precedent in birds: One small study of cockatiels found that female extra-pair copulation lead to mate-switching in all cases, and these re-pairings were “trade-ups” in that they had higher expected reproductive success.)

However, though it was a persuasive article, its key evidence is open to multiple interpretations, and much of the empirical support for mate-switching is based on data from women who hadn’t had affairs. And, really, it’s perfectly coherent for women’s infidelity to serve a dual-mating strategy without shifting strategies around ovulation. Humans exhibit notable sexual stability across the cycle, so the ovulatory shift sub-hypothesis may not have been the best test of human strategic dualism to begin with.

A more straightforward test of dual-mating than looking for periovulatory changes would be to follow the avian research and test if affair partners are more handsome than primary partners. This pattern would suggest dual-mating since better-looking men are generally accepted to offer more genetic benefits, even if the only benefit they offer is better-looking offspring. On the other side, a similar empirical test of the mate-switching hypothesis—or, at least, its trading-up utility—would be to check whether women prefer their affair partners to their primary partners overall.

Finally, one way to test these hypotheses against each other would be to check if women see their primary partners or affair partners as better dad material. Parental attractiveness ratings are a clean way to pit these hypotheses against each other, as they clearly have dueling predictions. Dual-mating argues the primary partner is the intended father figure for offspring, and mate-switching argues it’s the affair partner. So, if women’s affairs mainly serve a dual-mating strategy, the primary partner should be the better dad. If women’s affairs primarily aid a switch to a new mate, the affair partner should be viewed as the better dad.

Given the utility and similarity of these tests, we decided to conduct them as a package. I, along with primatologist Dr. Caroline Phillips and psychologist Dr. Khandis Blake, recruited a multinational sample of 254 people who had affairs and—in a pre-registered study with open data and materials—had them rate their affair partner and their primary partner in terms of their mate value, their parental attractiveness, and their physical attractiveness.

If mate-switching drove most women’s infidelity, affair partners should have been rated as higher in mate value—but they were not. Women rated their primary partners and affair partners almost exactly equal in overall desirability. Instead, our human results followed the exact structure one would expect from strategic dualists: Affair partners were more physically attractive than primary partners and primary partners were more parentally attractive than affair partners. Our result was the best-case scenario for dual-mating and provides evidence that psychological adaptations to acquiring genetic benefits underlie women’s infidelity.

Figure caption: Interaction plot comparing men’s and women’s ratings of their primary partners and their affair partners, in terms of physical attractiveness and parental attractiveness.

However, dual-mating cannot explain all infidelity. Women in our study reported utilizing extra-pair mating as a means to various ends, including, sometimes, switching mates. Further, the primary motivation for infidelity likely varies based on ecological factors (e.g., women may primarily use infidelity to obtain additional resources in resource-scarce environments). We believe the prevalence of dual-mating in our ancestry likely varied predictably with ecology, and so our results should not be extrapolated across all contexts.

Lastly, it’s conspicuous that men followed the same pattern, cheating up in terms of looks and down in terms of parenting. This gender similarity was a bit surprising to us at first. However, since men’s affairs are broadly accepted to have evolved through producing more offspring, men who cheat, too, are “just conceiving” with affair partners and co-parenting with primary partners. Therefore, it might make sense that they prioritize the conceptive benefits signaled by good looks (e.g., fertility) in affair partners and motherly qualities in primary partners.

While we look forward to further tests of dual-mating’s relevance to human affairs, for now, our results suggest that women—and, surprisingly, men—follow a pattern common in birds: better parenting at home and better looks on the side.

Read the original article here: Murphy, M., Phillips, C.A., Blake, K.R. (2024). Why women cheat: testing evolutionary hypotheses for female infidelity in a multinational sample. Evolution & Human Behavior, 45(5), 106595.