Keeping it Casual

Keeping it Casual

Steve Stewart-Williams
Steve Stewart-Williams
14 min read

Excerpted, with minor changes, from The Ape That Understood the Universe: How the Mind and Culture Evolve, by Steve Stewart-Williams (2018. Cambridge University Press).

Consider the following joke – a favorite of the evolutionary psychologist Donald Symons.

An Irishman, an Italian, and an Iowan are arguing about which bar is the world’s best. “The best bar in the world is Paddy’s Pub in County Cork,” says the Irishman. “After you’ve bought two drinks at Paddy’s, the house stands you to a third.” “That’s a good bar,” says the Italian, “but not as good as Antonio’s in Old Napoli. At Antonio’s, for every drink you buy the bartender buys you another.” “Now, those sound like mighty fine bars,” says the Iowan, “but the best bar in the world is Bob’s Bar and Grill in Des Moines. When you go into Bob’s you get three free drinks and then you get to go in the back room and get laid.” The Irishman and the Italian are astonished to hear this, but they are forced to admit that Bob’s Bar and Grill must indeed be the best bar in the world. Suddenly, however, the Italian gets suspicious. “Wait a minute,” he says to the Iowan. “Did that actually happen to you personally?” “Well, no, not to me personally,” admits the Iowan. “But it actually happened to my sister.”

It’s a straightforward joke, but a gender-neutral alien scientist would struggle to get it. We only do because we’re familiar with certain stereotypes of the sexes: stereotypes about how much they value no-strings attached sex, how easily they can get it if they want it, and the extent to which they perceive it as a favor granted vs. a favor received. The joke therefore raises two questions. First, is there any truth in the stereotypes? And second, if there is, why?

The answer to the first question, in my view, is an unequivocal yes: A mountain of evidence suggests that the sexes do differ – on average – in their interest in casual sex and sexual variety. This evidence, which I explore in some depth in my new book The Ape That Understood the Universe, includes self-report survey responses, analyses of real-world behaviour, and anthropological and historical records.1 In this essay, though, I’d like to focus on the second question: the why question. Why are men more interested than women in uncommitted sex and sexual variety? On one side of the debate are those who argue that it’s all down to social pressure, socialization, and culture. On the other side are those who argue that, even if these factors nudge things around a little, or even a lot, the ultimate roots of the difference lie deep in our evolutionary past.

The Evolutionary Explanation

Let’s kick off with the evolutionary explanation. You’re probably familiar with the basics of the explanation, but a good way to make the logic crystal clear is to imagine that you live in a premodern society – the kind of society in which most of our evolution took place – and that your one and only goal in life is to pass as many of your genes as possible into the next generation. What would be the best way to achieve this goal? The answer, it turns out, depends a lot on whether you’re a man or a woman. If you’re a man, your thinking might go like this:

OK, the best way for me to spread my genes would be to have as many children as I can. Each child will need lots of parental care. So, one option would be to settle down with a fertile female, get her pregnant, and help raise a bunch of kids. On the other hand, if I’m particularly attractive to women, or for some other reason have lots of sexual opportunities, I could try my hand at a different approach: I could mate with as many women as possible. If I were to mate with, say, five women in a year, I could potentially have five kids. I probably wouldn’t be able to help care for them all, so some might not make it to adulthood. But some surely would, and more than likely I’d end up with more kids if I took this approach than if I limited myself to just one woman. And of course, I could always do a bit of both: some fathering and some philandering. At the very least, I shouldn’t be too picky about my casual sexual partners. After all, if I have a fling with a suboptimal partner, it’s no big deal – it costs me little and I’m back on the market again almost immediately. Good times!

Now, with the exception of the deranged fertility doctor who surreptitiously impregnated his clients with his own sperm instead of the husbands’ (true story), no one actually tries to figure out how they can pass on as many of their genes as possible. It’s easy, though, to imagine genes that push around men’s preferences, motivations, and emotions in such a way that they cause men to act as if they were trying to do exactly that: genes that increase or decrease sex drive, for instance, or that increase or decrease the tendency to bond. Any such genes would stand a good chance of being selected.

That’s the male half of the equation. Now, what if you were a woman who wanted to pass on as many genes as possible? Well in that case, your thinking might go something like this:

Just like a man, the best way for me to spread my genes would be to have as many kids as possible. But I could never have as many as the most successful men. Whereas a man who mates with five women in a year can potentially have five children, if I mate with five men in a year, I’ll only have as many children as I would if I’d mated with one. Sure, there may be some benefits to having multiple partners. But nine times out of ten I’d be better off if I held out for a super-fit guy who’d give me super-fit kids, or a good provider who’d help me look after the kids – or if possible, a guy who’d do both. At the very least, I should keep well away from any man who clearly doesn’t measure up. After all, whereas a man who hooks up with a suboptimal partner can be back in the game straight away, if I hook up with a suboptimal partner, it could tie up my reproductive resources for at least nine months – and if I decide to keep the kid, for several years after that.

Again, no sane person would ever think like this, but again, they don’t need to for the theory to work. Notice that I’ve tried to word the above in such a way as to avoid a common confusion: the idea that, according to evolutionary psychologists, men are only interested in racking up notches on the bedpost, whereas women are only interested in snagging a lifelong mate. This view is doubly wrong. First, to say that men are more interested than women in casual sex is not to say that men are any less interested in long-term, committed relationships. Most men are just as interested, which fits nicely with the idea that pair-bonding and biparental care are an important part of humans’ evolved reproductive repertoire. Whatever the reason, though, both sexes are capable of falling in love and forming long-term relationships, and this suggests that long-term relationships must generally have been adaptive for both sexes: men as well as women. The desire for casual sex is just one component of men’s mating psychology.2

Second, the fact that men are more interested in casual sex doesn’t imply that women are not interested. Many are, and most evolutionary psychologists argue that casual sex was sometimes adaptive for women in our ancestral past. Casual mates may have provided meat or other resources, they may have helped out with the kids, or they may have had better genes than the guy who was willing to get serious. But racking up sexual conquests didn’t boost women’s fitness in the immediate and powerful way that it did for men. We shouldn’t be surprised, therefore, that on average, women today are less interested than men in racking up sexual conquests. Moreover, given that even a single roll in the hay could potentially have saddled a woman with a nine-month pregnancy and several years of childcare duties, we shouldn’t be surprised that women have higher standards than men for casual sexual partners. Madonna once described herself as “selectively promiscuous.” This is probably a good description of most women’s approach to casual sex (the occasional drunken mistake notwithstanding).

The Sociocultural Explanation

That, at any rate, is the evolutionary psychologists’ story. There’s a rival narrative, however, and that’s the idea that the sex differences come not from evolution but from culture. Here’s how this version of events might go. When it comes to casual sex, men and women are not playing on a level playing field. It’s true, of course, that making a baby entails a greater biological expenditure for women than for men: Women get pregnant; women give birth; women nurse the young. However, unlike other animals, women know this. They know that whenever they have sex just for fun, they’re playing Russian roulette with an unwanted pregnancy. Even if men and women had identical sexual desires, this would surely make casual sex less appetizing for women. Admittedly, pregnancy was a bigger worry before we had reliable contraception. But it’s still going to weigh on women’s minds more than it does on men’s.

And pregnancy isn’t the only concern. Men are larger and stronger than women, and more inclined to violence. This means that a woman places herself at a greater physical risk by going off with a man she doesn’t know than a man does with a woman. She also places herself at a greater social risk. Even today, there’s a sexual double standard in the West: Men who sleep around are viewed as heroes or lovable rogues, whereas women are viewed as sluts and “not marriage material.” As Joan Rivers put it, “A man can sleep around, no questions asked, but if a woman makes 19 or 20 mistakes she’s a tramp.” Again, this is surely going to have an impact on women’s sexual behavior. It’s not evolution; it’s just basic human rationality. People weigh up the costs and benefits of casual sex, and act accordingly.

Besides, even if men and women do have differing sexual appetites, why assume that these come from evolution? Our appetites are powerfully influenced by the culture around us; that’s why people in some cultures think that eating insects is about the most disgusting thing a person could do, whereas people in others think that eating bacon or cheese deserves that honor. Western culture is constantly teaching men that casual sex is bacon but teaching women that it’s a fried grasshopper (or vice versa if you prefer the grasshopper). Girls are taught to keep their legs closed; boys are encouraged to sow their wild oats. Girls are taught that sex without love is a meaningless experience, boys that, as meaningless experiences go, it’s a pretty damn good one. The sexual double standard might initially have appeared because parents worried that, if their daughter got pregnant outside of wedlock, she’d be left holding the baby – or that they would. But however it got off the ground, it soon became an unreasoned social norm: a moral belief that people soaked up from the culture around them and accepted independently of any consideration of the costs or benefits of casual sex.

Certainly, times have changed. The arrival of the pill in the 1960s greatly reduced the risks of casual sex for women, and as women started moving into the workplace, they no longer needed to barter their virginity and sexual favors for a slice of a man’s paycheck. And just as we’d expect, premarital and recreational sex have become more common and more acceptable for women since that time. But culture changes only slowly, and the earlier attitudes linger on to some degree. In as much as this drives the sex difference in casual sex, it’s not evolution or rational thinking. It’s just a cultural habit.

Who’s Right?

So, we’ve got two possible explanations for the sex difference: the evolutionary explanation and the sociocultural alternative. It’s time to start sorting the fact from the fiction, the wheat from the chaff, the men from the boys, and the women from the girls. Let’s begin with the question of cultural change. It’s clearly true that, since women got the pill and started earning their own money, casual sex has become more common and more acceptable for women. To a lot of people, this is proof positive that the casual sex gap couldn’t be due to evolution. But the conclusion doesn’t follow. The change shows that the gap couldn’t be due only to evolution; it doesn’t show that it’s due only to culture. And just to be clear about this, no evolutionary psychologist denies that the pill and other environmental factors affect people’s willingness to engage in casual sex. Of course they do! The only reason they wouldn’t is if people’s behavior were completely unaffected by the probable costs and benefits of different courses of action in their immediate environment. But no evolutionary psychologist would make such a ridiculous claim. The claim they do make is that the costs and benefits aren’t the whole story. Environmental variables don’t operate on a blank slate; they operate on minds that are somewhat differentiated by sex. To understand men and women’s behavior, you have to take into account not only the environment but also the evolved sex differences.

With this in mind, men and women’s sexual behavior since the 1960s actually fits better with the evolutionary psychologists’ narrative than with its sociocultural rival. The pill removes one of the main risks of casual sex for women: the risk of getting pregnant and having to raise an unplanned child without the help of the father. Despite this, more than a half century after it first appeared, women are still less willing than men to engage in casual sex. This doesn’t prove that the Nurture Only theory is false; it’s no doubt possible to come up with a non-evolutionary explanation for the tenacity of the sex difference. But the tenacity of the sex difference isn’t what we would have predicted on the basis of the Nurture Only theory. That should make us wary of any post hoc explanation for the persistence of the casual sex gender gap.

Other evidence presents a similar challenge to the Nurture Only view. Consider, for instance, the sexual behavior of lesbians. Unlike straight women, lesbians don’t have to worry about getting pregnant or about being alone with a more physically powerful partner. If these worries were all that kept women from jumping into bed with every good-looking stranger, then lesbians would have more sexual partners than their straight female counterparts. As various studies have demonstrated, however, they don’t; they have fewer partners.3 This removes yet another block from the Jenga tower of the Nurture Only theory. It suggests that the casual sex gender gap couldn’t just be a product of cost–benefit analyses related to pregnancy risk or personal safety.

It doesn’t yet entitle us, though, to conclude that the gender gap is innate. After all, pill-takers and lesbians might simply be following the casual-sex gender norms of their society, or the dictates of their gender-role socialization as children. These are not unreasonable hypotheses; both, however, face formidable – and I would argue fatal – challenges. For one thing, the social pressures don’t all point in the direction presupposed by the Nurture Only theory. Research has generally failed to demonstrate a consistent or pervasive sexual double standard in the modern Western world. Although the belief in the sexual double standard is still widespread, the double standard itself is much less so. Indeed, some people these days hold a reverse double standard, such that they judge men who sleep around more harshly than they judge women.4 Certainly, “slut” is an insulting term. But so is “prude,” and women who don’t engage in casual sex are sometimes called that – by men who want to sleep with them, for a start. And while “slut” is usually reserved for women, there are plenty of pejorative terms for men who sleep around: womanizer, dirty old man, letch, sleaze, “worthless, lazy, good-for-nothing, womanizing asshole” – and so on.5 At the very least, society gives us mixed messages. And yet the sex difference in casual sex persists.

More than that, the difference persists even when society pushes against it. As Donald Symons points out, men’s stronger interest in casual sex and sexual novelty has survived society’s best efforts to eradicate it.6 It has survived the efforts of parents, partners, and moralists to inculcate men with a healthy respect for monogamy. It has survived Christian moral teachings and threats of eternal damnation. It has survived cultural and legal institutions that endorse and incentivize lifelong monogamous marriage. It has survived worries that one might lose one’s marriage, one’s children, or even one’s livelihood over an adulterous affair that won’t stay hidden. And it has survived pop psychological attempts to stigmatize men’s desire for casual sex by blaming it on psychosocial immaturity, psychological maladjustment, repressed homosexuality, low self-esteem, fear of commitment, a Peter Pan syndrome, misogyny, male entitlement, toxic masculinity, and rape culture. Meanwhile, women’s greater reticence about casual sex has survived the efforts of some feminists and other thought leaders to persuade women to cast off the shackles of patriarchy and match men in the casual sex arena. This is all rather awkward for the Nurture Only theory. It suggests that, rather than being a product of culture, the sex difference in attitudes to casual sex often emerges in spite of culture.

Arguably, though, the most persuasive argument against the Nurture Only view is that sex differences in sexual inclinations and choosiness can be found in many individuals who have no gender norms, no socialization, and little in the way of culture: that rather sizeable group, so often overlooked by psychologists, known as other animals. The differences aren’t found in all other species, but they are found in many, including most birds, mammals, and reptiles. And when we find the differences in other animals, evolution is the only reasonable explanation. Why should humans be different?

It’s logically possible, of course, that the differences are products of evolution in squirrels, turkeys, and frogs, but of learning and culture in Homo sapiens. But it hardly seems likely. In other species, the differences appear when the ceiling number of offspring for males is higher than that for females. Humans meet this condition, and our species presumably evolved from earlier species that displayed the normal sex differences. As such, what the Nurture Only theory asks us to believe is that, in our lineage and ours alone, natural selection eliminated the normal sex differences, despite the fact that the selection pressure that initially created them was still operative. Why would it do that? It’s particularly perplexing given that, when we look around the world, we still find the sex differences that selection supposedly eliminated. Thus, the Nurture Only theory asks us to believe not only that selection eliminated the differences for reasons unknown, but that learning and culture then coincidentally reproduced exactly the same differences in every culture on record. This is not a compelling thesis.

Cultural forces clearly influence people’s willingness to engage in casual sex, and to some extent their desire to do so as well. But the idea that culture creates these sex differences out of nothing not only clashes with the available evidence, it clashes with everything we know about how evolution works.


1 See, e.g., Baumeister, Catanese, and Vohs (2002); Betzig (1986); Buss and Schmitt (1993); Schmitt and 118 Members of the International Sexuality Description Project (2003)
2 Buss and Schmitt (1993); Stewart-Williams and Thomas (2013).
3 See, e.g., Bell and Weinberg (1978).
4 Allison and Risman (2013); Stewart-Williams, Butler, and Thomas (2017).
5 The quote is from Ken Wishnia’s novel Soft Money (2015), p. 187.
6 Symons (1979).


Allison, R., & Risman, B. J. (2013). A double standard for “hooking up”: How far have we come toward gender equality? Social Science Research, 42, 1191-1206.

Baumeister, R. F., Catanese, K. R., & Vohs, K. D. (2002). Is there a gender difference in strength of sex drive? Theoretical views, conceptual distinctions, and a review of relevant evidence. Personality and Social Psychology Review, 5, 242-273.

Bell, A. P., & Weinberg, M. S. (1978). Homosexualities: A study of diversity among men and women. New York, NY: Simon & Schuster.

Betzig, L. L. (1986). Despotism and differential reproduction: A Darwinian view of history. Hawthorne, NY: Aldine.

Buss, D. M., & Schmitt, D. P. (1993). Sexual strategies theory: An evolutionary perspective on human mating. Psychological Review, 100, 204-232.

Schmitt, D. P., & 118 Members of the International Sexuality Description Project. (2003). Universal sex differences in the desire for sexual variety: Tests from 52 nations, 6 continents, and 13 islands. Journal of Personality and Social Psychology, 85, 85-104.

Stewart-Williams, S. (2018). The ape that understood the universe: How the mind and culture evolve. Cambridge, UK: Cambridge University Press.

Stewart-Williams, S., Butler, C. A., & Thomas, A. G. (2017). Sexual history and present attractiveness: People want a mate with a bit of a past, but not too much. Journal of Sex Research, 54, 1097-1105.

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.

Symons, D. (1979). The evolution of human sexuality. Oxford, UK: Oxford University Press.

Wishnia, K. (2015). Soft money: A Filomena Buscarsela mystery. Oakland CA: PM Press.

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Steve Stewart-Williams

Steve Stewart-Williams is an Associate Professor at the University of Nottingham Malaysia Campus, and author of the book The Ape That Understood the Universe: How the Mind and Culture Evolve.