Sex at Dawn and the Case Against Monogamy's Origin Story

We inherit a story about human mating that feels inevitable: the nuclear family, the pair bond, lifelong commitment to a single partner. This narrative reaches so far back in our consciousness that it reads as biological destiny. Yet in 2010, Christopher Ryan and Cacilda Jethá challenged this assumption head-on. In Sex at Dawn, they argued that humans evolved in small, egalitarian groups where sexual exclusivity was not the norm, where childcare was distributed, where food was shared, and where partners — particularly women — moved between sexual relationships without the shame or violence that later patriarchal systems would impose. Their thesis did not simply propose an alternative origin story. It suggested that monogamy as we know it is not the expression of human nature but the suppression of it.

The Ancestral Sharing Hypothesis

The case Ryan and Jethá present rests on a reinterpretation of how ancestral human societies organized mating and reproduction. They propose that human beings, like our bonobo cousins, evolved in environments where multiple partnerships and shared sexual access were not aberrations but baseline arrangements. In this model, the pair bond existed but was neither exclusive nor central to how groups sustained themselves. Instead, multiple overlapping partnerships allowed resources and childcare to be distributed across networks rather than concentrated in nuclear units. This would have meant that children grew up with multiple caregivers, that women’s sexual autonomy was not contingent on paternity certainty, and that jealousy and possessiveness, while potentially present, operated within a social framework that normalized and accommodated variety rather than punishing it. The evolutionary pressure would have favored individuals and groups that could maintain cooperative networks across sexual difference — not isolation within pairs.

Male Investment and Comparative Anatomy

The evidence Ryan and Jethá marshal is comparative. They point to our closest living relatives — bonobos and chimpanzees — and note a pattern that sexual dimorphism scholars have long tracked. Gorillas, which live in harems with one dominant male and multiple females, show extreme sexual dimorphism: males are nearly twice the size of females, with correspondingly large testicles and pronounced canine teeth. Chimps, which live in multi-male groups where females mate with multiple partners and males compete through sperm competition rather than direct physical dominance, show moderate sexual dimorphism and testicles proportionally larger than gorillas relative to body weight. Humans fall somewhere between: we show less dimorphism than gorillas but more than gibbons (which are monogamous), and our testicle-to-body ratio is moderate — not small like pair-bonding species, but not massive like chimps. This anatomical position in the continuum suggests humans evolved in an environment where some mating competition occurred but was not total or unchecked. The reduction in our sexual dimorphism relative to gorillas implies that male-to-male aggression over female access was not the dominant selective pressure. Yet our moderate testicle size, relative to chimps, suggests that multiple male partners for a single female remained a relevant scenario — otherwise, sperm competition would have been relaxed and testicle size would have shrunk further.

Female Reproductive Anatomy and Hidden Ovulation

Ryan and Jethá add further observations from female anatomy that challenge conventional readings. Women experience hidden ovulation — we cannot easily tell when we are fertile — unlike chimps or baboons, where females display obvious swelling and behavioral changes during peak fertility. This feature is commonly framed as evidence of monogamy: hidden fertility keeps her mate guessing and thus keeps him nearby, ensuring paternity certainty and his investment. Yet Ryan and Jethá propose an alternative reading that reorients the entire argument. Hidden ovulation, they argue, may have evolved precisely to confuse paternity — to make it impossible for any male to be certain the child is his, thus making infanticide pointless and distributing investment across the group. If a man cannot know whether a child is biologically his, he has evolutionary incentive to invest in children more broadly within his community. Females’ copulatory vocalizations, which they document as linked to ovulation and sperm-retaining behaviors, may function to signal fertility to multiple partners, not to mask it. In this framing, female reproductive anatomy is not a lock designed to secure monogamy but a strategy for ensuring male investment across a wider network. The very features that monogamy theorists cite as proof of pair-bonding become, under this lens, proof of the opposite — evolved mechanisms for maintaining multiple partnerships and preventing male certainty.

Flintstonization and the Ethnographic Record

They further argue that we have systematically misread ancestral human societies by projecting modern family structures backward in time — a phenomenon they term “Flintstonization,” the mistake of seeing Fred and Wilma as our ancestors when in fact they are ourselves in animal skins. This projection error shapes how anthropologists and evolutionary theorists interpret both the archaeological record and contemporary ethnographic data. Anthropological evidence from contemporary hunter-gatherer societies, which are often treated as models of ancestral human life, shows remarkable sexual fluidity. Many contemporary egalitarian groups practice what anthropologists call “socially recognized non-monogamy” — partnerships where sex outside the bond is acknowledged and normalized rather than concealed or prosecuted. The Mosuo of China, the Na of Southwest China, several Inuit populations, and various African pastoralist societies have documented histories of flexible sexual arrangements that coexist with stable child-rearing and resource distribution. Ryan and Jethá argue that this flexibility reflects something closer to the baseline human condition than the enforced exclusivity we treat as natural. If exclusivity were deeply rooted in biology, these societies would face greater instability and reproductive failure. Instead, many have persisted for centuries with lower rates of intimate partner violence than societies built around monogamous constraint.

The Critique and Its Legitimacy

Yet the thesis has not gone unchallenged. David Barash, an evolutionary biologist and author of Mismatch, has publicly critiqued Ryan’s use of anthropological data, arguing that they cherry-pick societies that fit their narrative while overlooking the prevalence of pair-bonding across human cultures. Barash contends that sexual jealousy, female mate choice, and male investment in offspring are sufficiently robust across cultures to suggest that pair-bonding is not merely cultural but deeply rooted. He further questions whether contemporary hunter-gatherer societies represent ancestral conditions accurately or have themselves been shaped by historical contingency — exposure to agriculture, warfare, trade, and other factors that may have altered their sexual practices from what ancestral humans actually did. The problem, Barash argues, is that we cannot simply treat living foragers as time machines. The debate is not settled, and responsible engagement requires acknowledging both the evidence Ryan and Jethá present and the legitimate critiques of their interpretation. The truth may be more granular than either position allows: humans may have evolved flexible capacities that respond to local conditions, meaning both pair-bonding and non-monogamy could have been adaptive under different circumstances.

What matters for the reader navigating this question is not which side is definitively correct — that answer may not be knowable — but what each position implies if true. If Ryan and Jethá are right, and human sexuality evolved in egalitarian groups with fluid partner networks, then monogamy is not a biological ideal but a culturally constructed restraint. This does not mean it is bad or should be abandoned wholesale. But it does mean that the sense of monogamy as the “natural” or “normal” baseline against which all other arrangements are deviations is inverted. The deviation, in this view, is the enforced exclusivity itself. The norm is the capacity to move between partners, share desire, and construct arrangements that serve community and individual wellbeing rather than property accumulation or patriarchal control.

This reframing has practical implications. If we are not fighting against nature when we move beyond traditional monogamy, but rather working with inherited capacities that culture has suppressed, then the shame, guilt, and sense of transgression that often accompany non-monogamous exploration become artifacts of cultural training rather than authentic biopsychological responses. This does not eliminate jealousy or insecurity — those arise in any human relationship because we are social creatures with attachment needs. But it relocates the source. Jealousy and insecurity are not evidence that non-monogamy is wrong; they are evidence that we were trained to feel them, and that training runs deep.

The Limits of Origin Stories

The challenge with origin stories — whether monogamous or non-monogamous — is that they reach too far into a past we cannot directly observe. We have no recordings of ancestral human mating arrangements. We have comparative anatomy, archaeological fragments, and extrapolations from contemporary societies, but none of these provide certain evidence of what humans did or felt 50,000 years ago. Ryan and Jethá’s thesis is compelling partly because it challenges a dominant narrative, but it remains a hypothesis, not a proven fact. The same is true of counter-arguments: the emphasis on pair-bonding and jealousy in human evolution is likewise an inference from incomplete data.

What we can say with confidence is that humans show remarkable behavioral flexibility. We have produced societies ranging from strict monogamy with severe penalties for infidelity, to practices of partner-sharing, to complex kinship systems where sexual and reproductive roles are distributed in ways that defy contemporary Western categories. We have produced individuals and couples who thrive in exclusive partnerships and others who thrive in networks. This flexibility is itself evolutionary evidence. If monogamy were the only natural state, this range of actual human behavior would be inexplicable. If non-monogamy were the only natural state, the same problem would apply in reverse.

The honest reading is that humans evolved the capacity for multiple mating strategies. We possess the neurochemistry for pair-bonding and for sexual novelty, for attachment to one partner and for attraction to others. We have the social apparatus to construct both exclusive and inclusive arrangements. Evolution did not lock us into one mode. Culture, circumstance, individual temperament, and choice determine which capacities we activate and which we suppress.

What This Means

Ryan and Jethá’s contribution is not that they have proven monogamy is wrong or that non-monogamy is right. It is that they have articulated a plausible case that the enforcement of monogamy is not a biological necessity but a cultural choice — one that may serve certain institutions and individuals while constraining others. If we accept this frame, we can ask different questions. Instead of “Is monogamy natural,” we ask: “What are the costs and benefits of different arrangements for the people in them? What social conditions allow different partnership models to function?” Instead of treating monogamy as the default against which all else must justify itself, we treat it as one option among several, each with its own logic, challenges, and possibilities.

This shift has consequences for how we understand ourselves. It means that desire for other partners, curiosity about other bodies, attraction that arises despite commitment — these are not signs of deficiency or relationship failure. They are signals that we are human, that we have not somehow transcended our nature through sufficient love or commitment. It means that couples and groups designing relationships around their actual desires and capacities, rather than around the monogamous template, are not fighting nature. They are working with it.

This article is part of the Evolutionary Biology and the Shared Mate series.