Human romantic relationships are often described as "serial". That is, they happen one after the other. Just like lights in the serial (or series) circuit, you may have learned about in school. Like in serial relationships, damage to one of these lights in a series circuit can really hamper the others. You can solve this problem in electronics by creating redundancy (such as a parallel circuit). A new hypothesis claims humans developed a similar solution: we evolved to create redundancy by having affairs.
The mate switching hypothesis
The "mate switching hypothesis" is the technical term for this new idea. And it isn't restricted to the idea that humans evolved to have affairs. It posits many other adaptations to deal with the costs of ending a relationship and starting another. Like cultivating backup partners. Affairs might also help with this.
Well, that doesn't really clear things up. Perhaps I should go back a bit and try and give a less click-bait description of this idea.
The mate switching hypothesis observes that mate switching (that is, moving from one serial relationship to another) can be costly. You have to spend money on tissues for all that crying you'll do. Plus, you need to invest time in developing a new relationship. You have to buy extra superlikes on Tinder. Or worry about revenge porn. The latter of which is evidence for another hypothesis: the people-are-a-bunch-of-bastards theorem.
Some of these problems are new. Many of them, the hypothesis argues, aren't. People in the Palaeolithic probably weren't able to just hop from one relationship to the next. This created an evolutionary pressure for adaptations that reduce these costs. The mate switching hypothesis posits some adaptations that may have developed in women. They speculate men may have evolved similar strategies, but don't really build on that.
Such adaptations include keeping an eye on mate value (and your own personal value) to determine when to walk away with an advantage. Women might also cultivate backup mates so they can easily switch to a new partner after the breakup. They might also test how committed these backups are to them by having affairs with them.
Affairs in the Upper Palaeolithic
Evolution is a surprisingly simple process. If something leads to more babies it will become more common in the population, as those babies mature and have even more kids. For a trait to evolve, it must increase reproductive success. This then makes it quite easy to begin investigating the mate switching hypothesis. For example, they claim one evolved trait is the drive to keep their backups single. So there should be a nice correlation between the effort women spend in keeping backups single and the overall number of kids they have. All else being equal, of course.
And this creates something of a contradiction. When you start looking at these sorts of correlations you find something interesting. Serial monogamy is, in general, bad for women (from a reproductive sense. No doubt divorcing an asshole is great from many other perspectives). Women who have multiple, sequential partners have no extra children (but invest a lot of extra effort) compared to their more monogamous contemporaries. Multiple studies have found this. So if evolution has been influenced by serial monogamy, it should be to try and avoid it.
Affairs are the opposite way to do this. In fact, an extra-marital action is the primary reason for divorce. I'd even add that's true across cultures, which is more than you can say for the evidence cited in support of the hypothesis. Only three of their examples come from non-Western groups, raising questions as to how representative their sample is. Can we really extrapolate backwards from American couples to study how many affairs people in the Upper Palaeolithic had?
FYI, if you did want to try and avoid serial monogamy, the best way to do that is with children. 2 or more kids greatly decreases your risk of divorce.
Conclusion
The mate switching hypothesis claims to offer a "compelling" case that many aspects of women's behaviour evolved to mitigate the risks of serial monogamy. Like having affairs to cultivate backups that can be easily switched to after a breakup.
But before anyone can claim an evolutionary justification for infidelity, their data needs a lot of work. It's mostly tangentially related studies from Western cultures. And they seem to have been cherry picked. After all, there's no discussion over the best way to mitigate the risks of mate switching: having a bunch of children.
As an interesting aside there may be a link between reproductive success and affairs (perhaps implying evolution is involved). But mate switching costs sure as hell don't seem to be the underlying cause. There was no reported link between such successful affairs and successful serial monogamy.
References
Betzig, L., 1989. Causes of conjugal dissolution: A cross-cultural study. Current Anthropology, 30(5), pp.654-676.
Buss, D.M., Goetz, C., Duntley, J.D., Asao, K. and Conroy-Beam, D., 2017. The mate switching hypothesis. Personality and Individual Differences, 104, pp.143-149.
Fisher, H.E., 1989. Evolution of human serial pairbonding. American Journal of Physical Anthropology, 78(3), pp.331-354.
Forsberg, A.J.L. and Tullberg, B.S., 1995. The relationship between cumulative number of cohabiting partners and number of children for men and women in modern Sweden. Ethology and Sociobiology, 16(3), pp.221-232.
Jokela, M., Rotkirch, A., Rickard, I.J., Pettay, J. and Lummaa, V., 2010. Serial monogamy increases reproductive success in men but not in women. Behavioral Ecology, 21(5), pp.906-912.
Scelza, B.A., 2011. Female choice and extra-pair paternity in a traditional human population. Biology Letters, p.rsbl20110478.
