Love & War: Does the Mate-Switching Hypothesis Explain Female Infidelity?

Evolutionary theory says men stray to increase offspring, but what motivates women? Enter the mate-switching hypothesis.

David Buss is professor of psychology at University of Texas, Austin. His work has been published widely in journals and he is the author of several books, including The Evolution of Desire: Strategies in Human Mating (new edition, 2016). Edited by Pam Weintraub

David Buss writes: Scientists now know much about human mating. The menu includes at a minimum: brief sexual flings, long-term pair-bonding, some infidelity, some polygyny (one man, multiple wives), rare polyandry (one woman, multiple husbands), occasional polyamory, some divorce, and frequent serial mating. These strategies are not well-captured by single labels such as ‘monogamous’ or ‘polygamous’. And we know with reasonable certainty that lifelong monogamy does not describe the primary pattern.

Divorce rates in the United States have hovered just below 50 per cent, and are variable but comparable across cultures around the globe. Among married couples, infidelity is far from a trivial occurrence. In 1952, the sexologist Alfred Kinsey estimated it at 26 per cent for women and 50 per cent for men, although other studies put rates lower or higher. We know that infidelity is the leading cause of divorce worldwide, from the Inuit in Alaska to the !Kung San of Botswana. And we know that most adults in the modern world, including roughly 85 per cent in the US, have experienced at least one romantic break-up.

But there has always been one missing piece of the puzzle when it comes to understanding mating strategies, especially among women. Why do women have so many affairs when these do not increase the number of offspring they can produce?

From an evolutionary perspective, male infidelity is fairly straightforward. Men have evolved a strong desire for sexual variety, stronger than women’s on average, due to the large asymmetries in parental investment. Men can reproduce with as little effort as it takes to inseminate a fertile woman. Women require a metabolically costly nine-month pregnancy to produce a single child. Stated differently, an ancestral married man with two children could have increased his reproductive output by 50 per cent by a single successful reproduction with an affair partner. Adding additional sex partners for women who already have one generally does not, and never could have, dramatically increased their reproductive success.

Yet women do have affairs, a phenomenon that, up until now, has been explained by the ‘good genes hypothesis’: the concept that women have evolved a dual mating strategy – securing investment from one man while mating on the side with men who have better genes than their regular partners.

But the good genes hypothesis fails to explain why, in the wake of infidelity, so many women literally stray, throwing over a current mate for the affair partner. My team’s new concept – the mate-switching hypothesis – fills the gap in scientific understanding, explaining what we observe in the real world. The mate-switching hypothesis posits that women have affairs to extricate themselves from a poor mateship and trade up to a better partner.

For both sexes, the hypothesis explains what we commonly observe: a year after publicly declaring her marriage vows, a woman finds herself sexually attracted to her co-worker. After changing his child’s fifth diaper of the day, a man wonders whether he made a terrible mistake and fantasises about his high-school sweetheart that got away. After six years of marriage, a woman finds that she’s the primary breadwinner and her husband’s laziness has eroded her confidence in their union; she notices that her co-worker lingers longer in the doorway of her office than strictly needed. After years of living a life of quiet desperation, a man starts a passionate affair with his next-door neighbour. A woman confesses to her best friend that she’s in love with another man and surreptitiously lays the groundwork for leaving her husband – a separate bank account and a deposit on an apartment.

These diverse scenarios stem from a common cause – humans have evolved strategic adaptations for mate-switching, a phenomenon that is widespread across species. The simplest such adaptation is the ‘walk-away’ strategy, in which organisms simply physically separate themselves from costly cooperative partners. The mate-switching hypothesis proposes a version of the walk-away strategy underpinned by human psychological adaptations designed to detect and abandon costly mates in favour of more beneficial ones.

Many in modern cultures grow up believing a myth about lifelong love. We are told about falling for the one and only. We learn that the path to fulfilment is paved with a single glorious union. But the plots of fictional love stories often come to a close upon the discovery of that one and only, and rarely examine the aftermath. The story of Cinderella ends with her getting the prince. After overcoming countless obstacles, a union is finally consummated. Few romantic fantasies follow the storyline of committed mating – the gradual inattentiveness to each other’s needs, the steady decline in sexual satisfaction, the exciting lure of infidelity, the wonder about whether the humdrum greyness of married life is really all life has to offer.

In fact, we come from a long and unbroken line of ancestors who went through mating crises – ancestors who monitored mate value, tracked satisfaction with their current unions, cultivated back-ups, appraised alternatives, and switched mates when conditions proved propitious. To understand why, we must turn our gaze to those ancestors and uncover the mating challenges that they confronted. Read the rest of this entry »