In Sex at Dawn, Christopher Ryan and Cacilda Jethá argue that everything we think we know about prehistoric human sexuality is wrong. Contrary to popular belief, humans haven’t always formed monogamous pair bonds—instead, the authors say, prehistoric humans lived in foraging societies that encouraged casual sex with multiple mates. In their view, humans only reluctantly embraced monogamy about 10,000 years ago when we stopped foraging for food and started farming.
In this guide, we’ll begin by covering the “standard narrative,” or widely accepted set of beliefs around human sexuality. Then, we’ll examine the logical flaws in the standard narrative, based on evidence from the sociosexual habits of great apes, observations of remote hunter-gatherer societies, and human biology. Finally, we’ll learn the new narrative of pre-agricultural human sexuality that Ryan and Jethá propose. Along the way, we’ll also see what other evolutionary psychologists and anthropologists have to say about Ryan and Jethá’s ideas.
(Shortform note: Sex at Dawn was well-received by general readers, but many evolutionary psychologists and anthropologists were critical of Ryan and Jethá’s interpretation of the evidence. In fact, one scholar wrote an entire book, Sex at Dusk, in which she uses the same scientific evidence as Ryan and Jethá to attempt to disprove their conclusions.)
Before we dive into the authors’ arguments, we need to understand the traditional description of evolutionary human sexuality—the “standard narrative”—that Ryan and Jethá argue against. The standard narrative describes the way men’s and women’s approaches to reproduction evolved over time. (Shortform note: In general, when Ryan and Jethá refer to “men” and “women,” we can infer that they’re specifically referring to cisgender people. Sex at Dawn was published in 2010 and does not explicitly mention transgender or intersex people.)
According to the standard narrative, the human mating system works like this: If a man and a woman find each other desirable, they’ll form a long-term, monogamous bond (from which they’ll periodically escape for flings with other partners). This pairing offers women the security of access to resources and offers men the all-important certainty that they are their children’s biological father.
(Shortform note: In Mating in Captivity, couples therapist Esther Perel argues that the nature of monogamy has continued to evolve beyond this narrative in that modern monogamy is less of a practical exchange of resources and more of an expression of love and commitment to a partner.)
According to Ryan and Jethá, the standard narrative is an example of “Flinstonization,” or the tendency to use modern cultural mores to explain historical human behavior. For example, we frequently hear stories of women “settling” for partners who can provide financial security, even if they’re not a love match. According to the authors, we then mistakenly project the same expectation backward onto prehistoric women because we assume that “settling” must be an innate (rather than culturally-ingrained) behavior. However, prehistoric humans lived in very different social and physical environments than modern humans, so it’s unwise to assume they went through the same thought processes as modern humans in choosing whether or not to commit to a mate.
(Shortform note: Flinstonization is a form of the “narrative fallacy.” In Thinking, Fast and Slow, Daniel Kahneman describes the narrative fallacy as the human tendency to rearrange facts into consistent stories in order to make sense of the world. When scientists Flinstonize the past, they’re essentially rearranging the evidence to fit the current cultural story.)
According to the authors, the standard narrative relies on a set of basic assumptions, which we’ll explore below. Each of these assumptions is based on the underlying idea that passing on one’s own genes is the ultimate motivation for all human beings.
(Shortform note: In The Selfish Gene, Richard Dawkins argues that the motivation to reproduce happens at the level of each individual gene, not the whole-organism level. That’s because, if individual organisms (such as humans) were motivated to reproduce their entire genetic codes, they wouldn’t focus on sexual reproduction, which only passes down half of their genes. In Dawkins’ view, the fact that humans are so focused on sexual reproduction disproves the idea that our objective is genetic reproduction.)
According to the standard narrative, monogamy is the natural result of men’s and women’s differing reproductive imperatives. This logic requires its own set of contributing assumptions:
According to the authors, the standard narrative insists that monogamy is the only system that allows both men and women to meet these reproductive goals. However, the standard narrative also acknowledges that both men and women will be motivated to cheat on their partners if someone with more desirable genetic traits becomes available.
(Shortform note: The motivation to meet reproductive goals is how the standard narrative explains the development of human monogamy. However, other researchers argue that monogamy actually developed as a way for non-dominant males to gain access to females without having to fight dominant males in the group. Instead of fighting, the non-dominant males began sharing their food with females—in return, females granted these males sexual exclusivity. Observation of this behavior led to the idea of monogamy as an exchange of resources. This model of resource-sharing also explains the development of infidelity—if a male came along with access to more or better food, it made sense for a female to abandon her previous mate for the new one.)
According to the authors, the standard narrative states that men naturally get jealous when their female partners are sexually intimate with other men, while women naturally get jealous when their male partners are emotionally intimate with other women. This is also a direct result of their competing agendas: Men need to know their children are their own, so they’re threatened by the idea of another man impregnating their partner. Meanwhile, women need to maintain access to men’s resources, so they’re threatened by the idea of another woman convincing their partner to divert his economic resources to her children.
(Shortform note: Research shows that gender isn’t the only important factor determining how people experience jealousy. For example, one study found that people from collectivist cultures experienced higher rates of sexual jealousy, but not emotional jealousy. The same study found that people who had been cheated on by a previous partner experienced higher rates of both sexual and emotional jealousy, regardless of their gender. Therefore, gender is likely just one of a host of factors that predict jealous reactions.)
Is There Really One “Standard” Narrative?
We’ve discussed what the authors claim constitutes the standard narrative, but does a single “standard” narrative even exist? Researcher Emily Nagoski argues that there’s no such thing as a standard scientific narrative. Instead, Nagoski believes that what Ryan and Jethá are refuting is a cultural narrative that misrepresents the science behind it. However, when Nagoski asked Sex at Dawn co-author Christopher Ryan about this, he clarified that the “standard narrative” is meant to be scientific, not cultural.
Nagoski argues that this definition makes Ryan and Jethá’s argument collapse on itself, because if the standard narrative were truly a scientific argument, it would be impossible to refute it using science as the authors do in Sex at Dawn. In other words, the authors are using science both to support and refute their own argument, which creates a contradiction.
However, Nagoski’s argument assumes that evolutionary science is purely objective. In reality, different scientists can, and do, disagree on how to interpret the same data and use these disagreements to drive scientific discovery.
To refute the standard narrative, Ryan and Jethá use evidence from three sources: the sociosexual habits of great apes with close genetic links to humans, observations of remote hunter-gatherer societies, and human biology. In this section, we’ll begin by examining the evidence from the great apes: specifically, chimpanzees and bonobos, which are species with non-monogamous (or multimale-multifemale) mating systems.
Let’s look at those mating systems in detail.
According to the authors, twentieth-century scientists thought chimpanzees were a nearly perfect model of ancient, unrestrained, “primal” humans because while they exhibit very human behaviors, they’re far less inhibited and more openly brutal than modern humans. These scientists also speculated that chimps’ approach to sex must represent ancient humans’ primal reproductive instincts: instincts that we still have, but repress.
(Shortform note: This belief that chimp behavior—sexual or otherwise—is equivalent to primal human behavior is why, in The Chimp Paradox, psychiatrist Steve Peters refers to humans’ most instinctive urges and emotional reactions as the “Inner Chimp” that must be controlled by the rational “Inner Human.”)
The authors note that chimps are quite promiscuous, and female chimpanzees often mate multiple times per day with various males. However, sex for chimpanzees is almost exclusively reproductive—female chimps are only sexually active during the fertile period of their menstrual cycles. (Shortform note: While female chimps may attempt promiscuity while fertile, they’re not always successful. Aggressive male chimpanzees have been known to physically restrain ovulating females to prevent them from mating with other males in the community.)
Ryan and Jethá note that, like chimpanzees, bonobos are famously promiscuous. However, for bonobos, sex serves important purposes beyond reproduction: It’s the social glue that holds the group together. This is evident in the fact that bonobos also engage in other forms of sexual intimacy to cement their bond, like kissing and looking into one another’s eyes while mating. (Shortform note: Bonobos’ non-reproductive approach to sex goes even further: Other researchers have observed bonobos engaging in oral sex and even making sex toys out of fallen tree branches. Bonobos also frequently engage in female-female and male-male sexual contact.)
Crucially, bonobos don’t exhibit the same patterns of male aggression, jealousy, and attempts to control female sexuality that we see in humans. The authors believe this means that these social patterns are rooted in our modern culture, not ancient biology. (Shortform note: Some reviewers have questioned this conclusion because scientists have observed aggression in wild bonobos. According to anthropologist Ryan Ellsworth, Ryan and Jethá gave a purposefully incomplete overview of the evidence from great apes in order to make their point.)
While chimpanzees and bonobos—our two closest evolutionary cousins—have different approaches to sex and reproduction, they both practice the same multimale-multifemale mating system. Thus, the authors conclude that this is the most natural mating system for social primates—including humans.
(Shortform note: In her review of Sex at Dawn, Emily Nagoski takes issue with this conclusion. She argues that primate sexuality is meant to adapt to the social context. In other words, for primates, sex can serve whatever purpose we need or want it to serve (such as reproduction, pleasure, or control) depending on our circumstances and relationships. That means that, while one mating system may be “natural” for bonobos and chimps, that doesn’t mean the same system is “natural” for humans, who have much more complex social systems.)
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.)
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.
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.
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 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.
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.)
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.)
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.)
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:
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.)
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.)
Ryan and Jethá believe that monogamy isn’t “natural” and may not be the best approach for every relationship. Take a moment to reflect on your feelings about monogamy.
Growing up, what kinds of relationships were modeled to you? Did you see any examples of consensual non-monogamy?
How did the evidence in this guide impact your views on monogamy? Were you surprised by anything you read?
Would you be willing to consider ethical non-monogamy in your current or future relationship? Why or why not?