I. Introduction

In my last post, I explored what’s often called the standard narrative of human sexual evolution—the familiar story you’ll find in countless evolutionary psychology texts, anthropology lectures, and pop-science documentaries. It’s a narrative with real staying power, partly because it seems to “fit” with both where we are today and where many believe we came from. It’s tidy, reassuring, and it gives monogamy an evolutionary seal of approval.

But what if the story is incomplete? Or worse—what if the evidence points in a very different direction?

That’s where Sex at Dawn by Christopher Ryan and Cacilda Jethá comes in. This book takes direct aim at the standard narrative, arguing that our prehistoric sexual behavior was nothing like the tidy, monogamous picture we’ve been sold. Instead, they propose that early humans lived in egalitarian hunter-gatherer groups where sexual sharing was common, parenting was a group effort, and sexuality itself functioned as a kind of social glue—helping maintain harmony, reduce conflict, and strengthen alliances.

It’s a provocative claim, to be sure, and one that leans heavily on biology, comparative primate behavior, and modern human sexual patterns. Whether you ultimately agree with the authors or not, Sex at Dawn forces us to reconsider some deeply held assumptions—not just about sex, but about cooperation, competition, and the very fabric of early human society.

II. The Standard Narrative Recap

Before we can understand why Sex at Dawn challenges so much conventional wisdom, we need to lay out what that wisdom actually says.

In evolutionary psychology, the standard narrative begins with a few key assumptions:

1. High female parental investment – Because women produce a limited number of large, energetically costly eggs, pregnancy, and nursing, they have a greater biological stake in each offspring.
2. Lower male parental investment – Men produce abundant, inexpensive sperm and can theoretically father many children with minimal biological cost.
3. Adaptive strategy for survival – Given these biological differences, women are thought to have evolved to be choosy—seeking partners who can protect and provide. Men, conversely, are believed to have evolved to seek multiple mating opportunities, spreading their genes as widely as possible while still securing a mate who will bear and raise their children.

From these assumptions flows a tidy evolutionary explanation for monogamy: by pairing up, men gain reasonable certainty of paternity and thus a reason to invest in offspring; women gain steady resources and protection in a world where being a single mother in prehistoric times could be a death sentence.

It’s a neat model—and one that’s been reinforced not by direct prehistoric evidence (since bones, tools, and cave art can tell us precious little about sexual behavior), but by inference. The narrative gains plausibility because it seems to align with our current social norms and because it can be illustrated with examples from the animal kingdom. Evolutionary psychologists will point to monogamous species like gibbons as possible analogs, reasoning that if they do it, perhaps we did too.

The trouble is, this is storytelling dressed in scientific clothing. It’s an elegant story, yes—but one built on assumptions, analogies, and extrapolation rather than hard evidence. And as Sex at Dawn will argue, when you start looking closely at our biology, our closest primate relatives, and patterns of human sexual behavior, the monogamy-at-our-core model begins to look much less inevitable.

III. Sex at Dawn’s Alternative Framework

Ryan and Jethá open Sex at Dawn by taking aim at the assumptions baked into the standard narrative. Their alternative begins with a different picture of our prehistoric past—one they argue is more consistent with our biology, our closest primate relatives, and the realities of small-group living.

In their telling, early humans lived in egalitarian hunter-gatherer bands, small enough for everyone to know everyone else, yet large enough to share labor, resources, and care for children. In this environment, they argue, sexual exclusivity was not the norm. Instead, sex was fluid, multi-partnered, and woven deeply into the fabric of social life.

Rather than being solely a reproductive transaction, sexuality served as social glue:

• It fostered trust and cohesion within the group.
• It helped diffuse conflicts before they could escalate.
• It cemented alliances that could mean the difference between survival and starvation.

In this model, paternity was often ambiguous—and that was a feature, not a flaw. Without certainty about who fathered which child, all men in the group had a vested interest in the wellbeing of all children. This arrangement, known in anthropology as cooperative parenting or alloparenting, ensured a resilient safety net in a world where injury, illness, or scarcity could be deadly.

According to Sex at Dawn, the shift toward monogamy and rigid pair-bonding didn’t arise until the agricultural revolution, when people began to settle, accumulate property, and pass resources down to their offspring. At that point, paternal certainty became critical for inheritance—leading to stricter controls on female sexuality, the rise of patriarchal systems, and the gradual erosion of the more fluid, communal sexual patterns that may have characterized much of our evolutionary history.

It’s a bold reframing—and one that, if true, doesn’t just tweak the standard narrative, but turns it on its head.

IV. Biological & Behavioral Evidence for Non-Monogamy

If the standard narrative rests on inference, Sex at Dawn tries to anchor its case in something more concrete—comparative biology and observable behavior. The authors argue that when we look at our bodies, our sexual habits, and our closest evolutionary cousins, the evidence points toward a history of multi-partner mating rather than strict monogamy.

1. Comparative Primatology

When evolutionary psychologists search for animal analogs to explain human mating systems, they often point to monogamous species like gibbons. But Ryan and Jethá note a problem: gibbons are evolutionarily distant from us. Our closest relatives are bonobos and chimpanzees—neither of which is monogamous. Bonobos in particular are famously sexual, using intimacy to cement bonds and resolve conflicts. The patterns we share with these apes suggest that our lineage may have been far more sexually fluid than the standard model allows.

2. Sperm Competition Indicators

One of the strongest clues comes from sperm competition—the evolutionary arms race that occurs when multiple males mate with the same female during her fertile period.

• Testicle size: In species where one male monopolizes females (like gorillas), testicles are small because sperm competition is minimal. In highly promiscuous species (like chimps and bonobos), testicles are large, producing more sperm to outcompete rivals. Humans fall in the middle, suggesting a mixed mating system with significant competition.
• Penis morphology: Human penises have a pronounced coronal ridge and greater thickness than those of our primate cousins. According to the authors, these features may function like a suction device—removing the semen of previous mates before depositing new sperm.
• Ejaculate composition: Studies suggest the chemical makeup of human semen changes during intercourse. Early and late fractions may contain substances that hinder rival sperm, an adaptation unnecessary in a strictly monogamous species.

3. Sexual Duration & Female Orgasm

Humans also last longer during intercourse than most other great apes, which often mate in under a minute. Longer sessions may help displace rival sperm. Yet women frequently report not reaching orgasm with a single partner—something that could be resolved more easily if multiple partners were involved, as seen in bonobo mating systems.

4. Female Vocalization

In several primate species, females vocalize loudly during sex—calls that attract other males to mate in quick succession, increasing sperm competition. Human females are generally more vocally expressive than males during sex, a pattern that may echo this ancestral signaling function.

5. Modern Echoes

The authors even point to certain modern sexual preferences as possible remnants of this past. They note that genres such as gangbang or cuckold pornography, involving one woman and multiple men, rank among the most-searched categories online. While the accuracy of using porn trends as evolutionary evidence is debatable (and methods for measuring it are questionable), it’s a provocative observation.

Taken together, these anatomical and behavioral clues don’t fit comfortably with a species “designed” for monogamy. Instead, they hint at an evolutionary past where multi-partner mating, not exclusive pair-bonding, may have been the norm.

V. Cooperative Parenting Model

One of Sex at Dawn’s most compelling points is that our evolutionary past may have favored collaboration over competition when it came to raising children. If paternity was uncertain—because women had multiple partners—then the whole male population in a band had a stake in the survival of every child. This concept, often called alloparenting or cooperative breeding, turns the logic of parental investment theory on its head.

Shared Parenting

In small hunter-gatherer bands, survival was precarious. Illness, injury, or a bad hunting season could be catastrophic for a family unit. But in a system where “it takes a village” wasn’t just a saying but a biological and social reality, no child was left with only one adult’s resources. All adults—men and women—were invested in the group’s young, spreading the burden and increasing the odds that the next generation would survive.

Paternity Confusion as a Feature

From the standard narrative’s point of view, ambiguous paternity is a problem—it undermines male investment. But Ryan and Jethá argue the opposite: in a cooperative system, paternity confusion was adaptive. If each man believed he might be the father of a given child, each was more willing to protect, feed, and teach that child. Instead of fueling jealousy, sexual sharing created a safety net.

Bonobo Parallels

Among bonobos, sex is as much about maintaining peace as it is about reproduction. Sexual contact diffuses tension, forges alliances, and reinforces group cohesion. Sex at Dawn suggests our ancestors may have used sex in similar ways—making intimacy an all-purpose social tool, not just a reproductive act.

Survival Advantage

In the harsh conditions of prehistory, group cooperation could make the difference between life and death. Injured hunters still had others to bring food to their children. Widows weren’t left destitute. Orphans were absorbed into the group. This shared responsibility model would have been especially effective in bands where mobility and mutual dependence were critical.

The Purpose of Sex Revisited

In a previous post, I wrote about the importance of determining the purpose of sex—especially when wrestling with the moral questions surrounding it. If, as the bonobos illustrate, sex serves a social function beyond the strictly reproductive one, then we have some serious thinking to do. Our moral frameworks often hinge on reproduction as the “point” of sex, but if intimacy historically also bonded communities, eased tensions, and promoted cooperation, then our ethical conversations may need to account for more than just fertility.

VI. Agricultural Shift and the Rise of Monogamy

According to Sex at Dawn, the world of small, mobile hunter-gatherer bands began to change dramatically with the agricultural revolution. The shift from foraging to farming altered not only how humans fed themselves, but also how they related to one another.

For the first time, land could be owned, resources could be stored, and wealth could be accumulated—and passed down to future generations. This new reality created a powerful incentive for paternal certainty. If a man was going to leave his land, livestock, or harvest to his children, he wanted to be sure they were actually his children.

This drive for certainty sparked a cultural transformation:

• Control over female sexuality became a social priority, enforced through marriage contracts, taboos, and in many cases, legal systems.
• Patriarchal frameworks solidified, aligning inheritance rights with male dominance over reproduction.
• The open, cooperative sexual networks of hunter-gatherer life gave way to exclusive pair bonds—or at least the social expectation of them.

While cooperation still existed in agricultural communities, it was increasingly overshadowed by competition—over land, water, resources, and lineage. The new economy of property and inheritance recast sexuality in terms of possession and exclusivity, replacing communal parenting with nuclear family units.

From this perspective, monogamy isn’t an ancient biological inheritance but a relatively recent cultural adaptation—a response to the social and economic demands of settled life. It may have served those demands well, but it also marked a sharp departure from the patterns that Sex at Dawn argues shaped most of our evolutionary history.

I can’t help but wonder if this agricultural turning point is also where the biblical story of Adam and Eve comes into the picture. In Genesis, there is a decisive moment when they “knew good from evil”—a moral and relational awakening that forever changed their world. I’m not sure how neatly that moment aligns with either the standard evolutionary narrative or Sex at Dawn’s reconstruction of prehistoric sexuality, but it seems to sit right at the same kind of cultural inflection point. There is fertile ground here for exploration, especially when considering how shifts in subsistence, property, and morality might have intersected in our distant past.

VII. Criticism and Counterarguments

Unsurprisingly, Sex at Dawn has faced strong pushback from within evolutionary psychology and anthropology. Critics—most notably Lynn Saxon—have accused the authors of selectively interpreting evidence, oversimplifying primate comparisons, and allowing their argument to lean too heavily toward endorsing polyamory. Saxon’s own work often reads like a deliberate defense of monogamy, countering Ryan and Jethá point-by-point.

Supporters of the standard narrative also argue that even if prehistoric sexuality was more fluid, the stability and predictability of monogamy may have been crucial for building complex societies. In this view, the agricultural revolution didn’t corrupt some idyllic sexual past—it created the necessary conditions for population growth, infrastructure, and technological progress.

That said, I think Ryan and Jethá’s narrative has better explanatory power for our anatomy than the standard model. The evidence from sperm competition, genital morphology, sexual duration, and female vocalization fits more comfortably within their framework than within the tidy assumptions of lifelong prehistoric monogamy.

At the same time, I recognize a complication: some readers have embraced Sex at Dawn as a kind of evolutionary permission slip for modern lifestyles. That association, fair or not, risks undermining the seriousness with which other scholars might engage with the work. For my part, I think Ryan and Jethá have put forward a provocative narrative—one that doesn’t have to be taken as moral prescription to still be taken seriously as a hypothesis. It is worth further study, if only because it challenges us to rethink deeply ingrained assumptions about human nature and the purposes of sex.

VIII. Implications for Today

If Sex at Dawn is correct about our prehistoric past, what does that mean for modern life? The answer, I think, is both simple and easily misunderstood: it doesn’t tell us what we should do—it only suggests what we may have once done.

The book’s framework explains aspects of our anatomy and certain patterns of behavior—why we have mid-sized testicles, why the human penis is shaped the way it is, why sexual duration is comparatively long, why women’s sexual vocalizations may have an ancestral signaling function. It also offers a plausible account of why jealousy and exclusivity might have evolved later, alongside agriculture and property.

But none of this tells us whether those ancient behaviors are right or wrong. Understanding that our bodies may have been shaped for multi-partner mating doesn’t answer the moral question of whether such arrangements are good, wise, or beneficial in today’s world. Evolutionary history, fascinating as it is, is not a moral compass.

From a Christian perspective, this distinction is critical. The Genesis account—whatever one’s interpretation of its historicity—frames moral boundaries for sexuality not on what humans can do or have done, but on what God calls them to. Even if our deep past contained patterns that don’t match those boundaries, our moral vision is not dictated by biology alone.

If anything, the most important takeaway here is the reminder that is does not equal ought. Sex at Dawn may offer a provocative explanation for our sexual anatomy and social tendencies, but it doesn’t—and can’t—tell us how we should live now. That is a different conversation, one rooted in values, ethics, and the purposes we believe sex is meant to serve.

IX. Conclusion

The standard narrative of human sexual evolution paints a picture of prehistoric monogamy shaped by parental investment and survival needs. Sex at Dawn offers a very different image—one of sexual sharing, cooperative parenting, and anatomy shaped by sperm competition rather than exclusivity.

Whether one accepts the authors’ full case or not, their framework raises important questions. It challenges the neat assumptions of the standard model, offers a plausible explanation for features of our anatomy, and invites us to reconsider how much of our sexual behavior is cultural adaptation versus biological inheritance.

But here’s the caution I keep returning to: while Sex at Dawn may explain how we came to be the way we are, it does not tell us how we should live. Evolutionary history is not moral guidance. Knowing what may have been “natural” in the past does not decide what is right, good, or wise in the present.

That’s a conversation for ethics, theology, and philosophy—and one I intend to continue. In fact, there’s much more to explore: the “purpose of sex” beyond reproduction, the intersection of biology and moral law, and how competing narratives of our sexual past influence the way we think about relationships today.

So stay tuned. This is just one step in a larger journey, and there are more blog posts to come.

Final Reflection & Questions

Understanding our possible sexual past is fascinating—but it also stirs questions that go far beyond biology. I’d like to leave you with three to ponder, and I invite you to share your thoughts in the comments below:

• If our anatomy and behavior evolved for multi-partner mating, what does that mean—if anything—for how we think about relationships today?
• Should morality be shaped by what is “natural,” or should it be grounded in something beyond our biology?
• If the agricultural revolution reshaped human sexuality for social and economic reasons, could our culture reshape it again—and should it?

Your perspective matters, and the conversation is richer when we wrestle with these questions together. Comment below and let’s keep the discussion going.

Resources

• The Evolution of Human Sexuality 1st Edition by Donald Symons
• Evolution and Human Sexual Behavior by Peter B. Gray and Justin R. Garcia
• The Evolution of Desire: Strategies of Human Mating by David M. Buss
• The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature by Geoffrey Miller
• The Red Queen: Sex and the Evolution of Human Nature, by Matt Ridley
• The Psychology of Human Sexuality 3rd Edition, by Justin J. Lehmiller
• Sex at Dawn: How We Mate, Why We Stray, and What It Means for Modern Relationships, by Christopher Ryan and Cacilda Jetha
• Sex at Dusk: Lifting the Shiny Wrapping from Sex at Dawn by Lynn Saxon
• Gray, P. B. Evolution and human sexuality. American Journal of Physical Anthropology, 152, 94-118. https://doi.org/10.1002/ajpa.22394
• Kramer KL, Schacht R. Human Sexuality: The Evolutionary Legacy of Mating, Parenting, and Family Formation. In: Wiesner-Hanks ME, Kuefler M, eds. The Cambridge World History of Sexualities. The Cambridge World History of Sexualities. Cambridge University Press; 2024:1-21. https://www.cambridge.org/core/books/abs/cambridge-world-history-of-sexualities/human-sexuality-the-evolutionary-legacy-of-mating-parenting-and-family-formation/96A87C50C1F8AC210E619A60859CB309
• Pazhoohi F. Parental Investment Theory. In: Shackelford TK, ed. The Cambridge Handbook of Evolutionary Perspectives on Sexual Psychology. Cambridge Handbooks in Psychology. Cambridge University Press; 2022:137-159. https://www.cambridge.org/core/books/abs/cambridge-handbook-of-evolutionary-perspectives-on-sexual-psychology/parental-investment-theory/3FF01CF8AF3B165BF1F09B4A25246F2F
• Zeveloff, Samuel I., and Mark S. Boyce. “Parental Investment and Mating Systems in Mammals.” Evolution, vol. 34, no. 5, 1980, pp. 973–82. JSTOR, https://doi.org/10.2307/2408002. Accessed 20 July 2025.
• Garcia, S., Lopez, S.A., Longo, K. (2024). Sex Differences: Parental Investment. In: Shackelford, T.K. (eds) Encyclopedia of Sexual Psychology and Behavior. Springer, Cham. https://doi.org/10.1007/978-3-031-08956-5_2109-1

Excerpt

Sex at Dawn challenges the familiar evolutionary story of monogamy, arguing our anatomy and behavior point to a far more communal sexual past. But evolutionary history isn’t moral guidance. This post explores what the book explains well—and why knowing our past doesn’t decide how we should live today.