Why Men Cheat

The evolutionary psychology of infidelity

Welcome to the Jungle

I once got an email from a journalist working on an article about why men cheat on their partners. She wrote:

As I don’t really know the reasons that men cheat (I wish I did; that would have saved a few months of crying and overindulging in ice cream), I was wondering if you could provide me with some quotes and pointers. Are men in the spotlight more likely to cheat than other men? Is there anything women can do to stop it?

Some fascinating questions! I approach these issues primarily from an evolutionary perspective (although, of course, learning and culture come into it too). But I immediately felt a bit reluctant about outlining the evolutionary psychologists’ view of infidelity. Why? Because it’s surprisingly tricky to convey this view without people getting the wrong end of the stick.

So what I did in my response, and what I’ll do now in this post, is start by highlighting what I think are the two biggest mistakes people make when they tackle this issue from an evolutionary point of view. I’ll then circle back to the journalist’s questions, and dig deeper into the evolutionary roots of infidelity.

12 Things Everyone Should Know About Evolutionary Psychology

Mistake #1: Exaggerating How Much Men Cheat

Not all men cheat. In fact, most don’t. Studies consistently show that fewer than 50% of men report ever having cheated.

An exception to this rule is research by the famous sexologist Alfred Kinsey, who found that 50% of married men had cheated by the age of 40 (compared to 25% of married women). There are reasons to believe, however, that Kinsey’s sample was unrepresentative, and that rates of infidelity in the general population are considerably lower than this.

To be fair, not all men have the “pulling power” to cheat, and probably more men would stray if they could. Still, let’s not characterize men in general in terms of behavior that most don’t engage in.

Mistake #2: Polarizing the Difference Between the Sexes

It’s not only men who cheat; women sometimes do as well. Plenty of men spend months crying and overindulging in ice cream - or doing other, more manly equivalents - in the wake of partner’s affair.

Admittedly, there is a sex difference in the likelihood of cheating: More men than women do it. But that doesn’t mean that men are the cheating sex and women the faithful sex, any more than the fact that there are more same-sex-attracted males than females means that men are the gay sex and women the straight sex.

Getting Down to Business

With these cautions in mind, let’s tackle the big question: Why do men - and women - sometimes cheat?

One useful way to think about it is to imagine that there’s an average optimal number of sexual partners for each sex. I don’t mean optimal in the sense that this is the number that we should have or that it’s good for us to have; I mean it in the strict technical sense that having this number of partners would produce the greatest number of surviving offspring for us - or would have done through most of our evolutionary history, before we invented reliable birth control.

Why would we care what number of sexual partners would produce the most offspring? Simple: because traits that lead people to produce more offspring are passed on to the next generation at a greater rate, and thus those traits come to occupy a larger and larger fraction of the population as the generations pass. In time, traits that boost one’s offspring headcount are likely to crowd out other variants. This is natural selection in action.

The Evolutionary Tug of War

For both sexes, there were selection pressures pushing the average optimal number of sexual partners up, and selection pressures pushing it down. The final optimal number for each sex is the net result of these countervailing selection pressures operating over vast periods of time in our evolutionary past. We can expect that natural selection will have calibrated our motivations and emotions so that, on average, members of both sexes act as if they’re deliberately aiming at their optimal number.

They’re not actually aiming at any such thing, of course; they’re just acting on their drives and motivations. However, the reason we have these pesky drives and motivations is that people in the past who had them tended to have more offspring than their neighbors.

So, what are the selection pressures that determine the optimal number of partners for each sex? The most important is maximum offspring number. Here we find a fundamental sexual asymmetry: The ceiling number of offspring a man can produce is higher than the ceiling number a woman can. If a man has sex with five women in a year, for instance, he could potentially get all five pregnant; in contrast, if a woman has sex with five men in a year, she’ll only have one pregnancy at the most - same as if she’d had sex with just one man.

This asymmetry pushes the optimal number of partners up for males but not for females, and largely accounts for the fact that, on average, men are keener than women on casual sex and sexual novelty, more interested in having multiple partners - and more likely to cheat on the partners they have. Men in the past who were this way inclined had more offspring than those who weren’t, and as a result, these inclinations became more and more common among the male of the species. In other words, they were selected.

Maximum offspring number is all that people usually think about when they contemplate the evolutionary origins of the infidelity sex difference. As a result, they end up puzzled about why women sometimes cheat and men are often faithful.

To make sense of these data points, we need to look at the selection pressures pushing the optimal number of mates up for females, and selection pressures pushing the optimal number down for both sexes. We also need to look at why pair bonding and male parental care evolved in the human lineage.

Why Women Cheat

Let’s start with the selection pressures pushing up the optimal number of partners for females. They include:

1. Genetic Diversity. Having kids with more than one partner increases offspring diversity, which can increase the chances that at least some will survive and thrive. (This selection pressure applies to males as well as females.)

2. Fertility Backup. Seeking additional mates can safeguard against a partner’s infertility.

3. Resource Extraction. Throughout the animal kingdom, females sometimes leverage sex to secure resources.

4. The Best-of-Both-Worlds Strategy. Some argue that females occasionally seek superior genes from one partner and reliable investment from another.

You can read more about these ideas in any good evolutionary psychology textbook; I won’t say too much about them now, except that some are more plausible than others.

Why Men and Women Don’t Cheat

If infidelity has evolutionary benefits for both sexes, why isn’t everyone constantly cheating?

The selection pressures pushing the optimal number of mates down for females and males include the following:

1. Disease Risk. Cheating increases the risk of contracting a life-threatening STD.

2. Reputation Damage. Cheating can earn people a bad reputation, thus lowering their chances of obtaining a decent long-term mate - generally a bigger deal for women than men.

3. The Green-Eyed Monster. Cheating can provoke violence from jealous spouses and partners, or from their relatives and friends.

On top of all this, there was a strong selection pressure in our species for males to help care for their young (see here and here). Among chimps and bonobos (our closest living relatives), males don’t invest in their young at all, and males and females don’t form pair bonds. In our own species, however, men and women fall in love and form long-lasting partnerships, and men usually invest in their young - albeit less than women.

Why are we so different from our cousins the chimps and bonobos? The reason is that our young are much more dependent than theirs, particularly during the early years. Pair bonding and male parental care are much more common in species with highly dependent young.

The fact that, for most of our evolutionary history, our offspring were less likely to survive without paternal care made it less profitable, evolutionarily speaking, for our male ancestors to spend all their time pursuing new sexual conquests. In other words, it lowered the average optimal number of mates for men.

These competing selection pressures have molded us into a species in which men and women both sometimes cheat (albeit men more than women), but in which both sexes commonly form relatively durable and largely monogamous pair bonds.

A Billion Years of Sex Differences

Vive la Différence

This isn’t the end of the story, however; there are also individual and situational differences that predict cheating. Here’s a brief sampling:

1. High Status = More Opportunity. High-status men may cheat more because their status makes them more attractive to women, and they therefore have more opportunities to cheat.

2. Attractiveness Mismatches. People are more likely to cheat if they’re more attractive than their partners, and less likely to cheat if their partners are more attractive than them.

3. New Temptations. Many modern societies create evolutionarily novel conditions that make cheating more likely. Alcohol is one example; long-distance relationships are another; larger pools of potential partners are a third.

4. Social Sanctions. In some societies, cheating is relatively rare because of social sanctions designed to prevent it. In certain countries, for instance, the penalty for adultery - if you’re a woman, at least - is death. Not surprisingly, cheating is less common in these countries. That doesn’t mean, however, that there’s no human nature and that everything’s down to culture; it just means that people are generally smart enough and self-controlled enough to refrain from acting on aspects of human nature that are likely to get them killed.

Can You Stop Your Partner From Straying?

One last question: How can you prevent your partner from cheating? Well, there’s nothing you can do to guarantee a faithful mate. But one thing you might want to remember is that the most reliable way to predict what someone’s going to do in the future is to look at what they’ve done in the past. So, if you don’t want a partner who’ll cheat on you, don’t give a cheat a second chance!

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Thanks!

Steve

Follow Steve Stewart-Williams on Twitter/X for more psychology, evolution, and science.

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Related Reading From the Archive

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Further Reading

Many of my academic publications deal with the nature, origins, and magnitude of human sex differences. They’re all available for free on ResearchGate. These two are particularly relevant to the topic of this post:

• Stewart-Williams, S., & Thomas, A. G. (2013). The ape that thought it was a peacock: Does evolutionary psychology exaggerate human sex differences? Psychological Inquiry, 24, 137-168. https://doi.org/10.1080/1047840x.2013.804899

• Stewart-Williams, S., & Thomas, A. G. (2013). The ape that kicked the hornet's nest: Response to commentaries on “The Ape That Thought It Was a Peacock”. Psychological Inquiry, 24, 248-271. https://doi.org/10.1080/1047840x.2013.823831

Also, Chapter 3 of my book The Ape That Understood the Universe tackles the evolution of sex differences in sexuality - and for what it’s worth, is probably the piece of writing I’m most proud of.

• Stewart-Williams, S. (2018). The ape that understood the universe: How the mind and culture evolve. Cambridge University Press. https://doi.org/10.1017/9781108348140

From the Archive