Why Have Sex?

This is Chapter 1 from Attraction, Love, Sex: The Inside Story, by Simon LeVay, published 2023 by Columbia University Press

Why have sex? A dumb question, perhaps. Maybe you’ve already come up with an obvious answer. But when we probe a little deeper, it turns out to be one of the trickier questions that science has grappled with.

Let’s start by asking people for their own thoughts on the matter. Or rather, by reviewing a couple of studies that have done this for us. What we’ll learn is that there are two hundred and thirty-seven reasons why people have sex. Or, alternatively, just one big reason.




That figure of 237 reasons comes from a research paper published a few years ago in the Archives of Sexual Behavior.1 The paper was titled, plainly enough, “Why Humans Have Sex.” It was written by Cindy Meston and David Buss of the University of Texas at Austin. Meston is a clinical psychologist and sex researcher, Buss is an evolutionary psychologist. Both are highly regarded in their fields.

To gather their data, Meston and Buss asked their psychology students to “list all the reasons you can think of why you, or someone you have known, has engaged in sexual intercourse in the past.” The most frequent reason given by both men and women was “I was attracted to the person.” The least frequent reason given by women was “I wanted to give someone else a sexually transmitted disease.” The least frequent reason given by men was “The person offered to give me drugs for doing it.”

In between these two extremes the list featured a wide spectrum of reasons, from the mundane to the exotic: “I was curious about sex,” “The person was too hot to resist,” “I wanted to get out of doing something,” “I wanted to change the topic of conversation,” “It was an initiation rite,” “I wanted to get closer to God,” or “It just happened.”

Notably absent from the entire list was any reason connected with procreation. No “I wanted to get pregnant.” No “We wanted to have a baby.” No “It was time to start a family.” The only reason that may have had some connection with procreation was number 152: “I was married and you’re supposed to”—but if that was a reference to childbearing it was an oblique one at best.

Meston and Buss did a good job with this study, in my opinion. Still, it did have some obvious limitations. How well-chosen were the participants, for example, for a study that purported to tell us why “humans” have sex? All University of Texas psychology students are humans, for sure, but not all humans are psychology students. What about those who are not?

Let’s take a 7600-mile hop southeastward from Austin. This brings us to the rainforest of Africa’s Congo Basin, home of the Aka, a Pygmy hunter-gather people. The lives of the Aka have been studied over the course of many years by anthropologists Barry and Bonnie Hewlett of Washington State University.2 Here’s what the Hewletts report regarding the Aka people’s sex lives: Couples engage in sexual intercourse on about three nights of every week, and three to four times during each of those active nights. They continue to have sex at about this rate throughout their fertile years.

The Aka couples might have sex for the same reasons that Texas psychology students do, of course. They might find their partners “too hot to resist,” for example. But that isn’t the reason they gave the Hewletts. Rather, both Aka women and men described having frequent sex as a chore—as “hard work” necessitated by their desire to have as many babies as possible. Some Akas did concede that sex could be pleasurable, but pleasure was not their stated motive for engaging in it.

The reason for the Akas’ desire for many children was twofold: the high rates of infant and child mortality in the Aka population, and the economic and social advantages conferred by having a large family. “I am now doing it five times a night to search for a child,” one young Aka man told the Hewletts. “If I do not do it five times my wife will not be happy because she wants children quickly.” To keep up this level of performance Aka men chew on the bark of a certain tree they call columba. This may be Pausinystalia johimne, a tree in the coffee family whose bark is the source of the sexual stimulant yohimbine.

The Aka believe in a concept called “seminal nurture”—the idea that a pregnant woman’s fetus requires an ongoing supply of semen for it to survive and grow. This belief is incorrect. (As a biologist, rather than a cultural anthropologist, I am permitted to say this.) In fact, if feeding the fetus were the point of the exercise, oral sex would be marginally more useful than vaginal sex, because a man’s ejaculate does offer a few calories.

Although seminal nurture doesn’t help a fetus grow, frequent sex does increase the likelihood that a woman will become pregnant in the first place. That’s for several reasons: Most acts of intercourse are mistimed with respect to the woman’s date of ovulation; those that are well timed may not lead to fertilization; and many fertilized eggs die before they ever implant in the woman’s uterus. Thus the Aka’s belief in seminal nurture, mistaken or not, may be a useful one in the context of a need to have many children.

Along the same lines, the Aka seem not to practice—or even be aware of—non-procreative sexual activities such as masturbation or sex between two men or two women. With regard to masturbation, the Hewletts relate the experience of a medical anthropologist, Robert Bailey of the University of Illinois at Chicago, who wanted to obtain semen samples from men of another Pygmy people. Bailey had to give the men detailed instructions on how to masturbate, and although the men did come back with semen samples, all but one of the samples contained vaginal secretions mixed in with the semen. All in all, the peoples of the Congo rainforest have sex to make babies, and they don’t seem to know of any other reason.

How can two different groups of people—American psychology students and African hunter-foragers—come up with such different motives for having sex? Part of it, of course, has to do with the fact that the Americans were college students. Making babies doesn’t rank high on most students’ to-do lists. It probably ranks higher on their not-to-do lists. Yet they are not alone in this: According to an analysis by the Guttmacher Institute, the average American woman spends 31 years of her life avoiding pregnancy, something that would be unthinkable to Aka women.3

Furthermore, a question starting with “Please list all the reasons you can think of …” as Meston and Buss’s question did, may be interpreted more as a challenge to creativity than as a probe of the participants’ actual motivations. It reads like one of those open-ended IQ questions, like “List all the uses you can think of for a copy of Psychology Today,” where you get credit for “swatting flies,” “starting campfires,” and so on. That’s very different from how the Hewletts probed the sexual motivation of the Aka men and women.

Meston and Buss’s study was not really about why humans have sex; it was about what people say when they are asked why they have sex. But by and large, people don’t know why they do the things they do, in which case they can’t tell you. That doesn’t stop them from giving you some answer, though, because the human mind is a wonderful confabulator.

We need to dig deeper if we want to understand why humans have sex. One direction to dig is deep into the brain, so that we can inspect the inner mechanisms that underlie sexual desire and behavior. That’s something I’ll attempt in later chapters. For now, I want to dig in a different direction—deep into the past. Into the question, Why has sex evolved? Or, given that it has evolved, why doesn’t it disappear?




It’s natural for us humans to think that sex is necessary, because if everyone gave up on sex our species would quickly go extinct. But why shouldn’t we reproduce by splitting in two, like amoebas? If that’s too far-fetched a notion, why shouldn’t we all be female and reproduce by virgin birth—by parthenogenesis,[1] as it’s called—as some lizards do? Why do we reproduce by sex, a messy, time-wasting, and disease-spreading activity that, among other problems, requires the invention of a superfluous gender—males?

         It’s easier to think of reasons why we shouldn’t reproduce sexually than reasons why we should do so. Some simple arithmetic makes this clear. Think about a female who reproduces sexually. Let’s say she’s capable of having 4 offspring—2 daughters and 2 sons. The next generation will consist of the offspring of the two daughters, i.e., 8 individuals in total. The next generation will consist of 16 individuals, the next 32, and so on. The numbers will double with each generation, until their multiplication is limited by external factors such as a limited food supply.

That sounds quite efficient, until you compare it with the situation for a female who reproduces asexually, like those lizards. This female’s four offspring will be like herself, so they will all be asexually reproducing females. The next generation will consist of 16 females, the next 64, and so on—the numbers quadruple with each generation. Thus, if the two populations are sharing the same environment, the asexually reproducing individuals will rapidly outnumber those that reproduce sexually. Other things being equal, they will monopolize the available resources, causing the sexually reproducing lineage to die out. This twofold cost of sexual reproduction, caused by the necessity to produce males, was initially pointed out by the British evolutionary biologist John Maynard Smith in the 1970s.4

         This isn’t just a hypothetical exercise; it’s something that has been observed in real life. Not in humans, of course, but in a species of tiny snail that lives in the lakes and streams of New Zealand. It’s called the New Zealand mud snail, or Potamopyrgus antipodarum. What’s interesting about these snails is that the females come in two kinds. One kind reproduces sexually, that is, it mates with males; the other dispenses with males and reproduces asexually. Over the last several decades the snails have been studied by a group led by evolutionary biologist Curtis Lively of Indiana University. They studied the snails in Lake Alexandrina, which is on the South Island near—near nowhere in particular, which makes it the perfect site for field research.

Between 2012 and 2016 Lively, along with his wife Lynda Delph—who is also a biology professor at Indiana University—and graduate student Amanda Gibson, took the long trip to Lake Alexandrina every southern summer, thus skipping a big chunk of the Indiana winter. (P. antipodarum is a globally invasive species now found, for example, in Lake Michigan, just a few hours’ drive from Indiana University. Presumably it’s more than just the weather that necessitates the trip to New Zealand.)

Gibson, who is now an assistant professor at the University of Virginia, told me that the protocol was as follows: Lively donned a wetsuit and swam around with a fine-mesh net, sweeping it through the shallow-water vegetation. (It’s somewhat unusual, and laudable, that a senior professor had time and inclination to do this.) Gibson stood in the shallows and received the full nets, whose contents she rinsed to get rid of the large debris. She then handed the snails to Delph who labeled and packaged them for conveyance to the Edward Percival Field Station north of Christchurch. Once there, all three researchers separated out the tiny juvenile snails—a mix of sexual and asexual individuals—and put them in outdoor tanks, about 800 snails to a tank. They then went home to Indiana, leaving the snails to fend for themselves.

By the time Lively’s group returned to New Zealand in the following summer, the previous year’s juvenile snails had matured into adults and produced offspring of their own. Gibson took samples of the adult females and their offspring back to Lively’s lab at Indiana University. To determine the proportions of sexual versus asexual animals, she took advantage of the fact that the asexual snails possess three sets of chromosomes in each cell nucleus, whereas the sexual snails possess only two sets—the normal number in most animals—so the asexual snails have more DNA in each cell nucleus than the sexual snails. She ran hundreds of cell nuclei from each animal through a device called a flow cytometer, which measures the amount of DNA in each nucleus as it is carried past a sensor in a fast-running stream of fluid.

Gibson found that the asexual females had indeed produced relatively more offspring than the sexual females, and this was true in all four years that she repeated the experiment.5 In fact, the relative numbers of sexually and asexually reproducing snails corresponded closely to what one would expect based on Maynard Smith’s theoretical argument. This study showed for the first time that the cost of sex—more specifically, the cost of producing males—is real and not just the product of a mathematical exercise. In other words, asexual females do have a major advantage over sexual females, at least in P. antipodarum.




Yet sexual snails haven’t disappeared, nor have sexual humans or any other of the multitude of species—over 99 percent of all animals and plants—in which sex is either the necessary means of reproduction, or an optional one. There must therefore be something about sex that offers an advantage over virgin birth—an advantage great enough to compensate for Maynard Smith’s twofold cost of producing males. Not just that twofold cost, in fact, but also the substantial costs of finding mates, persuading them to have sex with you, actually getting it on, and contracting whatever diseases your partners may have picked up in the course of their earlier conjugations.

         The key feature of sexual reproduction is that individuals inherit a mixture of genes from their two parents, half from one and half from the other. Which genes come from which parent is a matter of chance—each sibling inherits a different mix, unless they are identical twins.

         What’s the advantage of mixing genes? Many theories have been put forward, but there are two main ideas, both of which are likely to be part of the answer. The first idea is that when genes are mixed they sometimes end up in a combination that provides a defense against a novel external threat. The threat could be of any kind, but the one that has been most widely discussed is that of infection by microorganisms or parasites. These organisms are capable of rapid change. When they do change, combinations of genes that have previously protected animals against infection may suddenly lose their efficacy. But the shuffling of genes during the process of sexual reproduction means that some individuals in the next generation may have combinations that restore protection. It will only be a few individuals that get these useful combinations, certainly, but those are the ones that survive and reproduce.

This scenario is a never-ending process: The pathogens change their attacks, the animals change their defenses, the pathogens change again, and so on—it’s a running battle just to remain as successful as you were before. For that reason, this explanation for sexual reproduction has been called the Red Queen model, after the Red Queen in Lewis Carroll’s Through the Looking-Glass, who told Alice that “it takes all the running you can do, to stay in the same place.” 6, 7

The Red Queen model may explain why the sexual snails studied by Amanda Gibson are not crowded out of existence by the asexual, parthenogenetic snails, even though the latter can multiply much more rapidly. In Lake Alexandrina, snails risk being parasitized by a certain flatworm, whose eggs the snails may inadvertently ingest. The eggs hatch inside the snail, and the resulting larvae, like aliens in a horror movie, devour every part of the snail that can be eaten without actually killing it. These non-vital parts include the genitals, so the infected snails lose the ability to reproduce. In her experiments Gibson took care to use only uninfected snails, and in their absence the cost of sex—slower propagation—was evident. In the natural situation, however, this cost may be balanced out by the advantage of resistance against the flatworms.

 Some further observations on the mud snails support the Red Queen model. Curt Lively’s group found that the sexually reproducing females were most common in parts of Lake Alexandrina where the snails were most likely to ingest flatworm eggs. In other parts of the lake, where there were few eggs, asexual females were more common. This suggested that ever-changing attacks by the parasites promoted the form of reproduction in which new, protective gene combinations could be rapidly assembled. What’s more, followed over many seasons, the proportions of asexual and sexual snails, and the proportions of infected and uninfected snails in these two populations, rose and fell periodically in a manner predicted by the Red Queen model.8




The other idea why sex is so popular has to do with mutations—changes to an organism’s genome that may affect its ability to survive and reproduce. This model proposes that sexual reproduction exists because it helps get rid of harmful mutations and preserve beneficial ones.

A newborn human child inherits an average of about 70 new mutations—mutations that its parents did not themselves inherit. This figure was obtained in an Icelandic study that compared the entire genomes of parents and their children.9 The mutations crop up in the parents’ germ lines—the lineages of cells that develop into sperm or ova. Most mutations occur in the process of cell division. It takes far more rounds of cell division to produce sperm than to produce ova, because a man must generate billions of sperm over his lifetime, whereas a woman creates only a million or so ova, and these are already present (in an immature state) when a girl is born. Thus the great majority of new mutations are inherited from a child’s father, and the older the father is, the more new mutations he is likely to pass on.

Many mutations have little or no effect, so the child of an older father may not be impaired in any way. Of those mutations that do have an effect, however, the great majority are harmful, and only a small number are beneficial. That’s hardly surprising: You seldom improve computer software by introducing random changes, and the same is true for the genome.

When a species reproduces asexually, natural selection can preserve beneficial mutations, but harmful mutations inevitably come along for the ride—they are unwelcome hitchhikers on the road to the future. And when beneficial mutations arise, it’s not likely that they will all arise in the same line of animals, so they can’t be combined. Instead, they may end up competing with other, which often causes one of them to be lost.

When a species reproduces sexually, on the other hand, the random mixing of genes causes the genomes of the offspring to differ from each other. Thus the relative numbers of harmful and beneficial mutations vary from individual to individual—some have more than their share of harmful mutations and some have more than their share of beneficial ones. This potential for sexual gene-mixing to separate good from bad mutations increases the power of natural selection to work its magic—individuals with mostly good mutations reproduce more, while those with mostly bad mutations reproduce less, so the genomes with good mutations come to predominate in the population.

According to this model, then, the important difference between asexual and sexual reproduction is this: With asexual reproduction, natural selection “sees” the entire genome as a unit; with sexual reproduction, it “sees” individual mutations, preserving the good ones and removing the bad ones. This advantage for sexual reproduction has been called the “Ruby in the Rubbish” model—the phrase was invented by the British evolutionary biologist Joel Peck.




An incisive test of the Rubies in the Rubbish model was published in 2016 by a team at Harvard University led by evolutionary biologist Michael McDonald.10 (McDonald is now at Monash University in Melbourne, Australia.) This group studied brewer’s yeast—a unicellular organism named Saccharomyces cerevisiae that has become a workhorse not only of brewers but also of molecular biologists. The yeast can be induced to reproduce either sexually or asexually by varying the conditions in which it is grown. The researchers maintained some samples of yeast cells in conditions in which they reproduced asexually, and others in which they reproduced sexually. Each sample consisted initially of a clone—a set of genetically identical cells. After the cells had been allowed to reproduce for about 1000 generations, the researchers tested the “fitness” of the sexual and asexual cells. This they did by running the cells in head-to-head competition with the ancestral strain from which they were derived. They found that both kinds of cells had increased in fitness (they outpropagated their ancestors), but the sexual cells had done so to a much greater degree than the asexual cells.

This much was expected based on previous work. But McDonald’s group added a new level of analysis to the experiment—an analysis made possible by the precipitous drop in the cost of DNA sequencing in recent years. They sequenced the entire genomes of both the sexual and asexual populations at intervals of 90 generations over the entire 1000-generation experiment. By this method they identified the mutations as they occurred and followed them over time. They then assayed the harmfulness or benefit of each mutation; this they did by transferring the mutation into the ancestral line of yeast and competing this modified line against the unmodified ancestral line. If the new line grew better than the ancestral line then the mutation was a beneficial one; if grew less well, the mutation was a harmful one. By these means the researchers were able to reconstruct the history of all the mutations as they appeared and survived or disappeared over the course of the experiment. It was rather like charting the life histories of all the characters in an especially convoluted film noir.

What were the findings? In the asexual population many harmful mutations persisted up to the end of the experiment, by virtue of their unbreakable connections to beneficial mutations; meanwhile, some beneficial mutations died out in the face of competition from different beneficial mutations in other lines within the population. In the sexual population, by contrast, the mixing of genes allowed the good and bad mutations to separate into different lines; the cell lines with bad mutations were reduced in number or eliminated, while those with good mutations persisted. In fact, many of the beneficial mutations vanquished all their competitors, meaning that every surviving cell in the population carried the mutation.

McDonald’s findings certainly don’t disprove the Red Queen model. What they do show is that the Ruby in the Rubbish model works as theory had predicted—in yeast, anyway. The Red Queen model probably also works in some situations, especially in those where novel environmental threats are frequently encountered.

What about more complex creatures, such as humans? The relevance of the Ruby in the Rubbish model is made clear by looking at the human genome—more specifically, at the Y chromosome. This chromosome is unique in the human genome in that it is not paired with a “homologous” partner. Most chromosomes come in pairs, one being inherited from the mother and the other from the father, and gene exchange between the members of each pair is what makes the separation of harmful and beneficial mutations possible. But the Y chromosome exists as a singleton, and then only in males.

Because of the Y chromosome’s unpartnered status, the Ruby in the Rubbish mechanism doesn’t operate, or at least not in the usual fashion. As a result, most of the Y chromosome’s genes have accumulated harmful mutations to the point that they are no longer functional. Many others have disappeared completely, so that the Y chromosome is tiny in comparison with the X chromosome and most other chromosomes. Only a few genes remain fully intact—these include an important gene that causes its owner to develop as a male, as well as some other genes involved in male development. For the most part, though, the Y chromosome is a scrapyard of derelict genes, along with massive amounts of “junk DNA,” much of which may have originated in ancient viral infections. Most of the rubies have been lost, and the rubbish has spread like an invasive weed. This illustrates the importance of gene exchange over evolutionary time.

The human Y chromosome might have disappeared completely—as has actually happened in some rodents—except that it has invented a devious-seeming trick. As reported by a team led by David Page of MIT, the Y chromosome contains some long palindromic DNA sequences—the sequence runs in one direction at one location on the chromosome and in the reverse direction at another location.11, 12 Thus the genes located within these sequences, which include all the genes that are still functional, exist in two copies that are mirror images of each other. This allows the chromosome, by folding back on itself, to exchange copies of the same genes that are located within the palindromic sequences. It’s as if you corrected the typo in “Maram, I’m Adam” by folding the phrase back on itself and using the “d” to correct the faulty “r.” This trick could almost be considered a form of auto-erotic sex. It allows the Ruby in the Rubbish mechanism to preserve the few Y chromosome genes that are still functional.




What about those species, like the parthenogenetic lizards, that never reproduce sexually? Does their existence show that sex isn’t necessary at all?

The best-known parthenogenetic lizard, the desert grassland whiptail or Cnemidophorus uniparens, is a whiptail lizard that inhabits dry scrublands of the American Southwest. (It’s also known Aspidoscelis uniparens.) The species arose by hybridization between two species of sexually reproducing lizards, which still exist in the same areas. The asexual lizards retain some behavioral memory of their sexual ancestors, in that they still go through the motions of sex. This behavior—a coupling between two parthenogenetic females—has been given the unromantic name of pseudocopulation.13 Although no sperm or ova are exchanged, pseudocopulation does have a useful function: It promotes the maturation of eggs through a hormonal mechanism.

On the face of it, the existence of C. uniparens challenges both explanations for sexual reproduction that I described above. These lizards can’t use sex to assemble beneficial combinations of genes, nor can they use it to get rid of harmful mutations. So why don’t they die out?

The truth is that they may die out over long periods of time, because of their inability to reproduce sexually. Even so, they are replaced by new lines created in new hybridization events. In fact, researchers associated with the Howard Hughes Medical Institute have succeeded in replicating this process in the lab: By mating males and females of the two ancestral species, they produced asexual females that continued to reproduce parthenogenetically.14

No one has observed the appearance of new lines of whiptail lizards outside the laboratory. At least one such natural event has been observed in another animal, however—the marbled crayfish. This is a new species of asexual crayfish that arose in Germany in the early 1990s, as a result of a single mutation in a sexual species of crayfish.15 The marbled crayfish now number in the billions; they threaten to displace other crayfish species in many parts of the world, thanks in part to their asexuality. They are genetically identical to each other, however, so that when the right parasite appears it may wipe out every last one of them, putting an abrupt end to the species’ meteoric career.

 In short, the asexual lizards and other asexual animals, fascinating though they are, fail to invalidate existing hypotheses about sexual reproduction. They can do without sex, but probably not forever.




Before we conclude that sex is necessary, however, I have to acknowledge that there is one kind of animal that can do without sex forever. These are microscopic invertebrates called bdelloid rotifers. That’s DELL-oid, which means leechlike—they look and act like tiny versions of leeches—and ROTE-ifer, which means wheel-bearing: A bdelloid rotifer sports a circlet of ever-whirring bristles around its mouth, which doubles as its anus. Swiss microbiologist Bernard Jenni has posted an entertaining video of them on YouTube.16

Bdelloid rotifers are strange animals in other ways than their looks. One of their many talents is that they are more resistant to ionizing radiation than any other animals. They also survive complete desiccation. Once dry, they wait out the drought, for years if necessary, until they encounter water again, whereupon they reassume their normal appearance and carry on as if nothing had happened. A bdelloid rotifer might survive a trip from Earth to Mars on the outside of a spacecraft.

Like parthenogenetic lizards, bdelloids are all-female. They haven’t had sex for millions of years, according to molecular-genetic studies by a team led by Matt Meselson of Harvard University.17 Like the lizards, they are often cited as a challenge to the idea that sex is necessary.

It turns out, though, that bdelloid rotifers have a trick up their tiny sleeves: They can take up DNA from the environment and patch it into their own genomes. This happens during the episodes of desiccation, when the cell membranes become leaky (allowing the foreign DNA to enter) and the rotifers’ own DNA is subject to random breakage (allowing the foreign DNA to be spliced into the breaks). The foreign DNA can come from any source—other rotifers, bacteria, fungi, or even plants. Although the DNA can come from anywhere, the bdelloids preferentially retain DNA from species related to themselves—species whose genes might be useful to them.18, 19

It is this “horizontal gene transfer” that likely takes the place of sexual reproduction. This process harks back to the bacterial world; it is one method by which bacteria acquire genes conferring resistance to antibiotics. In a sense, bdelloid rotifers do have sex, but in a random, promiscuous fashion that doesn’t involve direct contact between organisms.




But there’s one aspect of sexual reproduction that has opened the door to all kinds of strange-seeming deviations—and I don’t mean that in a bad way.

The aspect I’m referring to is this: Sexual reproduction requires complicated behaviors before and after pregnancy, especially in mammals. Before pregnancy, there’s courtship and sexual intercourse; after pregnancy, there’s nursing and rearing, which may continue for years. But sexual reproduction doesn’t require any behaviors during pregnancy. Some women have not even known they were pregnant until the day they gave birth,20 and there’s at least one documented instance of a woman being in a coma for the entire duration of her pregnancy.21, 22

Nature has to provide the motivation for sexual intercourse, but she doesn’t have to provide the motivation for pregnancy, because pregnancy looks after itself. Once a baby is born, a new set of motivations, falling under the general title of parent-child bonding, comes into play.

Perhaps Nature should have thought this through more carefully, because by offering all kinds of carrots before pregnancy—sexual attraction, arousal, the pleasure of sexual contact, and the reward of orgasm—while offering no carrots at all, and perhaps even some sticks, for the duration of pregnancy itself, she has invited animals with any degree of creativity to grab the carrots and dodge the sticks.

We thus see the emergence of numerous non-reproductive sexual behaviors of the kind that St. Thomas Aquinas, the thirteenth-century arbiter of sexual ethics, condemned so fervently. Non-coital sex acts (e.g. oral or anal sex), masturbation, same-sex behaviors, contraception, sex during menstruation, and sexual kinks—if Aquinas knew about them—were all sinful in his eyes. Those 237 reasons to have sex listed by Meston and Buss’s students? The only one that Aquinas might have let pass was the one mentioned earlier: number 152: “I was married and you’re supposed to,” and then only if contraception wasn’t used. The sexual motivations of the Aka, on the other hand—sexual pleasure subordinated to the rational purpose of procreation—would have earned Aquinas’s unqualified praise, if he had known of the Aka, or the Aka of him.23

In terms of their evolutionary origins, the motivations for non-reproductive sexual behaviors are accidental byproducts of the motivations for reproductive behaviors. That doesn’t mean that they are maladaptive, however—that is, they don’t necessarily hinder the perpetuation of the genes that generate these motivations. In fact, they can be adaptive.

That becomes clear if we consider our oversexed primate relatives, the bonobos.24 A female bonobo is sexually active not just when she is ovulating, as many other mammals are, but for her entire 60-day menstrual cycle, with the exception of a few days around the time of menstruation. She is also sexually active while breastfeeding an infant, even though breast feeding suppresses ovulation. In other words female bonobos, like women, may initiate or respond to sexual advances even when they are incapable of becoming pregnant.

In addition, bonobos engage in sexual behaviors that are by their nature non-reproductive, such as same-sex contacts and contacts between adults and sexually immature juveniles. Regarding same-sex contacts, two males may rub their penises together while hanging upside down from the branch of a tree. Female pairs engage in a behavior called genito-genital rubbing—another unromantic technical term—in which they sweep their vulvas against each other with a rhythmic side-to-side motion. There are no exclusively “lesbian” bonobos, so far as we know, but all females have sexual contacts with other females more frequently than they do with males.

Bonobos evidently engage in these non-reproductive behaviors because they are pleasurable. But in doing so, they are not simply cheating sex of its reproductive goal. Rather, they have found an adaptive value for the behaviors. According to primatologist Frans de Waal of Emory University, the behaviors help resolve conflicts and promote social cooperation. Sex between females strengthens female alliances, and these alliances help females achieve a measure of social dominance over the physically stronger males.24

These behavioral adaptions must have a biological basis. For females to be sexually active at times when they are not ovulating has required evolutionary changes in their neuroendocrine control systems. Also, it is an anatomical trait that makes genito-genital rubbing possible—the forward-facing position of bonobos’ vulvas. There must in addition be changes in brain circuitry that make same-sex individuals into attractive sex partners, although these changes have not been studied. In other words, even though non-reproductive sex wasn’t what Nature originally intended, she has acknowledged the inevitable and made the best of it.




I’ve explained why the evolution and persistence of sexual reproduction is paradoxical: Asexual reproduction should be twice as efficient at producing offspring. I’ve discussed two main ideas for why, in spite of this disadvantage, the capacity for sexual reproduction remains universal, or nearly so. One idea is that the central feature of sexual reproduction—mixing genomes—helps protect against environmental threats such as parasites (the Red Queen model). The other is that mixing genomes helps get rid of harmful mutations and combine beneficial ones (the Rubies in the Rubbish model).

Given that there’s evidence supporting both models, it’s reasonable to conclude that each represents part of the reason why sexual reproduction has been so successful. Yet the aesthete in me rebels from that conclusion: To my mind, there should be a single, beautiful reason for something as paradoxical, and yet so central to life on Earth, as sex.

If I had to choose between the competing models, I’d have no hesitation: The Rubies in the Rubbish model is more beautiful. Parasites come and go, but mutations lie at the core of why sustaining life across the generations is so challenging. It may be that the Red Queen model can be thought of a feature or subroutine of the Rubies in the Rubbish model: Without her Rubies, after all, the Red Queen would eventually stop running and be swept away.

The remainder of this book focuses on sex in our own species. Humans, or the westernized humans who have been the focus of most research, have developed the non-reproductive functions of sex to an extraordinary degree—so much so that Meston and Buss were able to list 237 reasons for having sex without once mentioning reproduction. Yet we can’t escape our evolutionary history: Sexual attraction, the topic of the next chapter, is all about making babies, even if we never have any.


1.        Meston, C.M. and D.M. Buss (2007): Why humans have sex. Archives of Sexual Behavior, 36: 477-507.

2.        Hewlett, B.S. and B.L. Hewlett (2010): Sex and searching for children among Aka foragers and Ngandu farmers of Central Africa. African Study Monographs, 31: 107-125.

3.        Guttmacher Institute (2014): Moving forward: Family planning in the era of health reform.  http://tinyurl.com/yd3mxfsu.

4.        Maynard Smith, J. (1978): The evolution of sex. Cambridge University Press.

5.        Gibson, A.K., L.F. Delph, and C.M. Lively (2017): The two-fold cost of sex: Experimental evidence from a natural system. Evolution Letters, 1: 6-15.

6.        Liow, L.H., L. Van Valen, and N.C. Stenseth (2011): Red Queen: from populations to taxa and communities. Trends in Ecology and Evolution, 26: 349-58.

7.        Ridley, M. (2003): The red queen: Sex and the evolution of human nature. Harper Perennial.

8.        Gibson, A.K., et al. (2018): Periodic, parasite-mediated selection for and against sex. American Naturalist, 192: 537-551.

9.        Kong, A., et al. (2012): Rate of de novo mutations and the importance of father's age to disease risk. Nature, 488: 471-5.

10.     McDonald, M.J., D.P. Rice, and M.M. Desai (2016): Sex speeds adaptation by altering the dynamics of molecular evolution. Nature, 531: 233-6.

11.     Kuroda-Kawaguchi, T., et al. (2001): The AZFc region of the Y chromosome features massive palindromes and uniform recurrent deletions in infertile men. Nature Genetics, 29: 279-86.

12.     Rozen, S., et al. (2003): Abundant gene conversion between arms of palindromes in human and ape Y chromosomes. Nature, 423: 873-6.

13.     Crews, D., M. Grassman, and J. Lindzey (1986): Behavioral facilitation of reproduction in sexual and unisexual whiptail lizards. PNAS, 83: 9547-50.

14.     Lutes, A.A., et al. (2011): Laboratory synthesis of an independently reproducing vertebrate species. PNAS, 108: 9910-5.

15.     Gutekunst, J., et al. (2018): Clonal genome evolution and rapid invasive spread of the marbled crayfish. Nat Ecol Evol, 2: 567-573.

16.     Jenni, B. (2016): Bdelloid rotifers: So common yet so weird!  https://tinyurl.com/2p8jf4n7.

17.     Mark Welch, D. and M. Meselson (2000): Evidence for the evolution of bdelloid rotifers without sexual reproduction or genetic exchange. Science, 288: 1211-5.

18.     Gladyshev, E.A., M. Meselson, and I.R. Arkhipova (2008): Massive horizontal gene transfer in bdelloid rotifers. Science, 320: 1210-3.

19.     Debortoli, N., et al. (2016): Genetic exchange among bdelloid rotifers is more likely due to horizontal gene transfer than to meiotic sex. Current Biology, 26: 723-32.

20.     Ponting, B. (2017): SoCal woman, 45, unexpectedly gives birth, didn’t know she was pregnant.  https://tinyurl.com/ppdspvkh.

21.     Bruni, F. (1996): Woman, 29, still in 10-year coma, is pregnant by a rapist. New York Times, January 25.

22.     Yglesias, L. (1996): Sad and silent b'day for mom in coma. New York Daily News, April 28.

23.     Milhaven, J.G. (1977): Thomas Aquinas on sexual pleasure. Journal of Religious Ethics, 5: 157-181.

24.     de Waal, F.B.M. (1995): Bonobo sex and society. Scientific American, 272 (March): 82-88.

Attraction, Love, Sex: The Inside Story