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Why do so many marriages end in divorce? In Sex at Dawn, Christopher Ryan and Cacilda Jethá (a husband-and-wife team) offer a surprising answer: For most of our evolutionary history, humans lived in societies that encouraged casual sex with multiple partners. According to the authors, humans evolved to be naturally promiscuous and only reluctantly embraced monogamy about 10,000 years ago when we stopped foraging for food and started farming.

In this guide, we’ll learn why everything we think we know about prehistoric human sexuality is wrong. We’ll draw on evidence from great apes (our closest evolutionary cousins), remote hunter-gatherer societies, and modern human biology to show that humans evolved to be promiscuous, not monogamous. Along the way, we’ll cover what other evolutionary psychologists and anthropologists have to say about Ryan and Jethá’s ideas.


Part 3: Evidence From Foraging Societies

In addition to great apes, the authors also use evidence from modern foraging societies to support their arguments against the standard narrative. These societies are geographically isolated from other people and still practice the type of hunter-gatherer lifestyle that our prehistoric ancestors did. The authors present two aspects of modern foraging societies that cast doubt on the standard narrative: partible paternity and non-nuclear families.

(Shortform note: There are about 30 modern foraging societies that we know of. However, as of 2018, there were also roughly 100 uncontacted tribes in the world. Because these tribes have avoided or rejected interactions with outsiders, we don’t know for sure whether they embrace the same hunter-gatherer lifestyle as early humans did.)

Partible Paternity

According to the authors, one way that modern foraging societies refute the standard narrative is through partible paternity, or the idea that more than one man can be the biological father to a child. This argument arises from the fact that people in some remote South American foraging societies understand pregnancy and conception differently than people in most other societies do: They think pregnancy is the result of ongoing deposits of semen rather than a single sex act. Therefore, the authors argue, pregnant women in these cultures will seek out a variety of men with different desirable genetic traits for sex because they believe that each man’s semen will contribute to the baby’s genetic heritage.

(Shortform note: According to some anthropologists, partible paternity beliefs exist along a spectrum for people in remote South American foraging societies. That is, people in some such societies wholeheartedly believe in partible paternity, while people in other, similar societies believe a child can only have one biological father. Researchers note that many people in these societies fall somewhere in the middle: They believe that partible paternity is theoretically possible, but that it’s better for a woman to reproduce with just one male partner.)

As a result of this practice, everyone in the community sees all of these men as biological fathers of the resulting child. According to the authors, each of the men is therefore responsible for providing for that child—which ultimately increases the child’s chances of surviving to adulthood. The authors believe this illustrates an important point: While paternity certainty might be important for individual men in some societies, a lack of paternity certainty may be better for the society as a whole. When men don’t know which children are their own, they have a vested interest in providing for all the children in their social group.

(Shortform note: There is a natural limitation to the idea that men who can’t identify their biological children will be more invested in all the children in their community: It only applies in small, isolated groups, like the bands prehistoric humans lived in. In these groups, it’s likely that at least some of the children in the group are any given man’s biological offspring, giving men motivation to care for these children. However, in larger or more interconnected groups, there’s a much smaller chance of any one child being any particular man’s biological child, giving men less reason to invest in all the children they meet.)

Is Partible Paternity Evidence of “Natural” Promiscuity?

Some reviewers have argued that the mere existence of partible paternity isn’t enough to prove Ryan and Jethá’s point about which mating system is most “natural” for humans. For example, one group of evolutionary anthropologists and psychologists argued that, “the existence of partible paternity in some societies does not prove that humans are naturally promiscuous any more so than the existence of monogamy in some societies proves that humans are naturally monogamous.”

However, if human sexuality develops in response to social context—and if modern foraging societies are socially similar to ancient foraging societies—we can assume that ancient humans probably did have some concept of partible paternity and a similarly non-monogamous approach to sex.

Non-Nuclear Families

According to the authors, partible paternity contributes to another way that foraging societies refute the standard narrative: a lack of traditional nuclear families. In modern foraging societies, children often have both multiple fathers and multiple mother figures. While everyone in the group knows who an infant’s biological mother is, women in the mother’s extended family will also nurse the baby. As they grow, children are free to wander from house to house, where each adult will care for them as they would their own children. The result is a diffuse sense of parental responsibility: Each adult acts as a parent to each child in the community, regardless of any biological relationship between them, because community bonds outweigh individual parent-child bonds.

Ryan and Jethá believe that this evidence—coupled with the fact that, historically, nuclear families have needed significant help from tax breaks, religious edicts, and marriage laws to survive—undermines the idea of “natural” nuclear families. In human prehistory, it would have been unthinkable for a couple and their children to try to fend for themselves without the help of the community.

(Shortform note: The authors argue that prehistoric humans wouldn’t have relied on nuclear family arrangements because the task of raising and providing for children was too difficult for two parents to manage without outside help. However, this argument fails to take into account the fact that families can both be nuclear and receive outside help. For example, it’s possible for nuclear families to live in close communities with their extended families. In that case, parents get extra help taking care of their children, but they’re still recognized as the parents—the nuclear family still exists. For all we know, our ancestors could have had similar arrangements.)

Foraging Societies Raise Happier, Healthier Children

As part of their argument, the authors cite evidence from a number of foraging societies, including the Ye’kwana (sometimes spelled Ye’kuana) people of modern-day Venezuela and Brazil. Ryan and Jethá only focused on the aspects of the Ye’kwana child-rearing methods beneficial to basic survival, but other researchers have noted that these methods also carry important psychological benefits.

In the mid-twentieth century, American writer Jean Liedloff lived with the Ye’kwana for several years. Liedloff found that, in addition to a diffuse sense of caregiving responsibility, the Ye’kwana people employed a number of unique parenting practices, including cosleeping and breastfeeding on demand. Liedloff observed that the Ye’kwana children were happier and more self-confident than their American counterparts.

Liedloff concluded that these benefits were the result of raising children in alignment with their bodies’ evolved expectations. For example, before car seats and strollers existed, babies spent most of their time in the arms of a loving adult. Over time, infants evolved to depend on this contact to help regulate their internal temperature and heart rate. Liedloff believed that when evolutionary expectations like this aren’t met—for example, when babies spend most of their time in strollers, bouncers, or cribs instead of being held—children aren’t able to thrive.

Furthermore, the lack of emphasis on nuclear families in societies like the Ye’kwana enables these evolutionarily sound parenting practices—when the whole community takes responsibility for childcare, there’s always someone available to hold the baby, even if the child’s mother and father are busy.

Part 4: Evidence From Modern Human Biology

Lastly, the authors use several aspects of modern human biology as evidence for their argument that prehistoric humans lived in promiscuous societies. These include body-size dimorphism, the size and shape of the human testicles and penis, and female copulatory vocalization. Let’s explore each in more detail.

Body-Size Dimorphism

Body-size dimorphism is the average difference in body size between males and females of the same species. According to the authors, the more males in a species have to compete over females, the bigger the body-size dimorphism. That’s because bigger males tend to win competitions for females and pass on their genes, so each generation of males gets a little bit bigger. On the other hand, when there’s no need to compete, the genes for body size remain equally distributed in the population, so each generation of males stays about the same size.

In humans, adult men tend to be, on average, 10% to 20% bigger than adult women. The authors believe that’s a relatively small difference compared to other species like gorillas, in which males can be up to 100% larger than females. They argue that the relatively small body-size dimorphism in humans is evidence that, for most of human history, there was little need for males to compete for females—otherwise, modern men would be much larger.

According to the authors, this lack of male social competition is indicative of a multimale-multifemale mating system, in which no one had exclusive sexual access to anyone else—no one was “taken”—so there were more potential partners available for sex at any given time. (To contrast, monogamous societies have slightly more male social competition, because there are a finite number of female partners available.)

A Different Explanation for Body-Size Dimorphism

While most scientists now agree with Ryan and Jethá’s interpretation of body size dimorphism, that was not always the case. For example, in 1986, paleontologist Martin Pickford proposed that body-size dimorphism arose because men and women have different energy demands.

In Pickford’s view, pregnancy and lactation are so energy-intensive that women gradually evolved a smaller body size (smaller bodies require less energy to maintain basic functioning, so they can afford to divert more energy to having children). Men, on the other hand, don’t have to devote energy to pregnancy, so they can afford to spend all their energy maintaining a larger body. To support this argument, Pickford cites the fact that the combined weight of an average woman and her infant child is close to the weight of an average man, which implies that the combined energy needs of mother and child would be about equal to a man.

Overall, this explanation for body-size dimorphism has a unique consequence. While scientists like Ryan and Jethá believe that male competition for mates causes body-size dimorphism, Pickford believes such competition is the result of the body-size dimorphism that arose from men and women’s differing energy needs: in other words, that males only started to fight for females shortly after and because they evolved to be bigger, rather than evolving to be bigger because they needed to fight.

If Pickford’s argument is correct, it would mean that male social competition for mates (and the associated monogamous mating system) began at least four million years ago, shortly after men and women first evolved to have different average body sizes. This would undermine Ryan and Jethá’s argument that humans have only been competing for mates since the advent of agriculture (around 10,000 years ago) and lived in more peaceful polyamory until then.

Testicle Size

According to the authors, in highly sexually competitive species (such as gorillas), males tend to have smaller testicles—since only the dominant male has sexual access to a given female, his sperm is guaranteed to be the one that reaches her egg, so his body needn’t worry about producing large amounts of it. However, in more promiscuous species where each female might have many mates, males have evolved over time to have larger testicles, resulting in more sperm production and a higher chance of being the one to impregnate a given female and pass on their genes. According to the authors, this “sperm competition” replaces social competition among males; if males don’t need to compete for sexual access to a given female (because she’s mating with all of them), producing higher concentrations of sperm increases an individual male’s chances of being the one to impregnate that female.

(Shortform note: In Sex at Dusk, Lynn Saxon argues that the emphasis on sperm competition is part of Ryan and Jethá’s agenda to remove active choice from the narrative of female mate selection. In Saxon’s view, the authors of Sex at Dawn believe that women’s mate selection process is entirely unconscious (because when she gives sexual access to multiple partners, she can’t choose whose sperm ultimately fertilizes her egg) and should remain that way. In other words, Saxon believes Ryan and Jethá are arguing that women never did—nor should they now—consciously choose which men to mate with.)

Human testicles, on average, are larger than those of polygynous (one male mating with multiple females) gorillas and smaller than those of promiscuous chimpanzees and bonobos. The authors believe this means that, until about 10,000 years ago, humans were a notably promiscuous species. However, with the advent of agriculture came monogamy and one-to-one sexual matches, which meant that even males with smaller testicles (who produced less sperm) had a chance of impregnating someone. Thus, the genes for reduced fertility were passed on and allowed to spread in the general population. The authors argue that this led to an overall reduced testicle size in the last 10,000 years. Therefore, they conclude that, until 10,000 years ago, humans lived in a non-competitive, multimale-multifemale mating system.

(Shortform note: Ryan and Jethá’s argument is not universally accepted among evolutionary anthropologists. One reviewer, William Buckner, pointed out that human testicles (which weigh in at 34 grams on average) are much closer in size to gorillas’ (23 grams) than to chimps’ (149 grams) or bonobos’ (168 grams). Buckner argues that it’s unlikely that human testicles reduced from a size comparable to chimpanzees’ and bonobos’ to their current size in just 10,000 years—therefore, it’s more likely that humans evolved in polygynous groups, similar to gorillas.)

Penis Shape and Size

According to the authors, the human penis is much larger than that of any great ape. It’s also uniquely shaped: Whereas animal penises typically have a tapered or curled end, the human penis has a flared glans. Ryan and Jethá believe this unique anatomy is the evolutionary result of thousands of years of humans living in promiscuous mating systems. Their argument is that this shape, combined with the thrusting motion of sex, is designed to create suction inside the vagina, which pulls the sperm of other males away from the female’s cervix. This suction only happens before ejaculation (at which point the penis changes shape), ensuring that a man’s own sperm isn’t similarly suctioned out. The authors claim this evolutionary adaptation only makes sense if humans evolved in a mating system where sperm competition was important—such as a multimale-multifemale mating system.

(Shortform note: Other anthropologists have questioned Ryan and Jethá’s interpretation of the evidence. They argue not only that the shape of the human penis isn’t particularly unique in the animal kingdom, but also that its shape most closely resembles those of polygynous primates rather than primates who live in multimale-multifemale mating systems.)

Female Copulatory Vocalization (FCV)

Women tend to make more noise during sex than men—a phenomenon that scientists call “female copulatory vocalization,” or FCV. According to the authors, FCV is common in many species. Primatologists have observed that the more promiscuous a species is, the more likely women are to vocalize during sex.

What does FCV have to do with promiscuity? The authors believe that these vocalizations are designed to attract more men, thereby increasing sperm competition and ensuring they pass on the best possible genes to their offspring. Thus, they argue, the presence of FCV in humans is more evidence that humans evolved as a highly promiscuous species.

(Shortform note: A 2011 study found that, in humans, FCV might not be a completely unconscious response to orgasm. While heterosexual women are most likely to orgasm during foreplay, they were most likely to vocalize intensely during vaginal penetration. This intensity reached a peak at the point of male orgasm. Thus, researchers concluded that FCV might have evolved as a partly conscious strategy to advertise the male orgasm to surrounding males. This might help to attract more male mates and therefore increase sperm competition, ensuring that only the male with the strongest sperm (and thus, presumably, the best genes) actually succeeds in getting the female pregnant.)

Part 5: The New Narrative

Ultimately, Ryan and Jethá argue for a new narrative of human sexual evolution to replace the standard narrative. Their new narrative is based on the assumption that, before the advent of agriculture, humans lived in multimale-multifemale mating systems, not polygynous harems or monogamous pair bonds.

(Shortform note: It’s important to note that this is just one possible theory of many. As we’ve explored, there isn’t a universally agreed-upon narrative—other researchers have come to different conclusions despite using the same evidence as Ryan and Jethá. For example, recall that the authors use average human testicle size as evidence that humans evolved in a multimale-multifemale mating system, but William Buckner used the same data as evidence that humans evolved in a polygynous mating system.)

This underlying assumption gives rise to a set of new conclusions about the nature of human sexuality:

Conclusion 1: Monogamy Isn’t “Natural”

According to the authors, monogamy is extremely rare in the natural world, occurring in just 3% of all mammals. Furthermore, among humans, adultery is a common occurrence in every culture around the world, even in spite of the brutal punishments that some societies inflict on adulterers. In Ryan and Jethá’s view, the only reason anyone would risk such punishment is to satisfy a deep evolutionary urge. The authors conclude that monogamy is thus not “natural” at all, contrary to the standard narrative.

(Shortform note: The authors use the ubiquity of adultery as evidence against the standard narrative. However, the standard narrative does account for adultery—it acknowledges that it makes sense for both men and women to stray when a genetically superior mate comes along. Thus, the fact that adultery is common may not actually be an effective argument against the standard narrative.)

What should we do with this information? The authors assert that they’re not out to promote any particular lifestyle or destroy monogamy as we know it—they merely aim to present the facts and let the reader decide how to respond to them. As a starting point, they recommend questioning the cultural rules around monogamy and potentially exploring whether some version of consensual non-monogamy might be a better fit for your relationship.

(Shortform note: Ryan and Jethá don’t go into detail about the practice of consensual non-monogamy, or what many people call “ethical non-monogamy.” According to experts, ethical non-monogamy is different from cheating because everyone in the relationship must consent to the arrangement in advance. These relationships are “ethical” because they are rooted in trust and consent, not secrecy and betrayal.)

Conclusion 2: Jealousy Is Socially Constructed

According to the authors, some scientists argue that humans are naturally, universally jealous creatures and therefore could never have lived in non-monogamous societies.

Ryan and Jethá believe that this reasoning is fundamentally illogical. They argue that jealousy stems from insecurity, and insecurity stems from the fear that there is not enough of something (like love or material resources) to go around. Therefore, in non-monogamous societies where no one is expected to rely on just one other person for love, sex, and resources, jealousy is reduced because there are always others who can provide what we need. Jealousy is thus a result of our cultural emphasis on monogamy rather than a cause of it.

(Shortform note: The authors’ logic implies that jealousy should be less of an issue for people in polyamorous relationships because there is more than one person available to meet their needs. However, many people in polyamorous relationships report that jealousy is still very present—it’s just something they actively manage with intentional communication. It’s difficult to tell whether jealousy arises in these cases because it’s a human universal or because people in modern polyamorous relationships were raised in cultures that normalized jealousy.)

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