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

In 2010, evolutionary biologist Christopher Ryan and psychiatrist Cacilda Jethá published Sex at Dawn, arguing that human mating systems evolved for non-exclusivity, not monogamy. Their thesis rests on a body of biological evidence: the size of male testes, the timing of female ovulation, female copulatory vocalizations, and patterns of sexual dimorphism across the hominin line. The argument has become foundational to how many scholars and therapists think about human sexual nature. Yet it has also provoked significant pushback from established evolutionary biologists like David Barash, whose objections illuminate what the debate itself reveals about the interpretive premises underlying our models of ancestral sexuality.

Understanding the Ryan-Jethá argument—and its critics—requires us to examine not just the evidence but the story it tells, and to ask what alternative interpretations exist for the same biological phenomena.

The Ryan-Jethá Thesis: Non-Exclusivity as the Ancestral Pattern

Ryan and Jethá argue that human ancestors, particularly across the Pliocene and Pleistocene, lived in mixed groups where sexual access was not exclusively pair-bonded. They propose that females had multiple partners, that offspring paternity was ambiguous, and that group investment in children was the norm. The biological markers supporting this model include the following.

First, testicle size. In species where females mate with multiple males (high sperm competition), males evolve larger testes relative to body size as an adaptation to produce more sperm. Chimpanzees, who live in multi-male, multi-female societies, have testicles that are roughly 4 percent of their body weight. Gorillas, by contrast, live in single-male harems and have testicles at only 0.1 percent of body weight. Human testicles fall at about 0.5 percent of body weight—closer to the monogamous gorilla than to the polyandrous chimp, but notably heavier than would be expected if humans were strictly monogamous with no sperm competition whatsoever. Ryan and Jethá interpret this intermediate position as evidence of moderate, ancestral sperm competition within pair-bonded groups.

Second, hidden ovulation. Unlike most primates, humans have largely concealed their fertile window. Female baboons and chimpanzees display obvious estrous swelling; a human woman in her fertile phase shows no visible sign. Ryan and Jethá argue that concealed ovulation evolved precisely to allow females to mate with multiple males without the pair-bond male knowing when she is fertile, thus obscuring paternity. If paternity is ambiguous, the reasoning goes, males invest more in children universally (since any child could be theirs) and less in paternity certainty behaviors like mate guarding.

Third, female copulatory vocalization. Human females produce vocalizations during sex—moans, gasps—that have no obvious mechanical function and that occur most often at the moment of the male’s orgasm, not during the female’s own climax. Some versions of female orgasm research propose that these vocalizations evolved to signal to other males that copulation is occurring, thus increasing competition and vigor among mating males.

Fourth, sexual dimorphism. Humans show moderate sexual dimorphism (males slightly larger and stronger than females), less than gorillas (extreme) but more than gibbons (monogamous). Ryan and Jethá interpret this as consistent with a mating system that was neither strictly monogamous nor fully promiscuous, but rather characterized by competition within a framework of regular pair bonds with extra-pair sexual contact.

These markers, taken together, form a coherent narrative: human evolution favored non-exclusive mating, and the biology reflects it.

The Critics: Barash and the Alternative Interpretation

David Barash, a prominent evolutionary biologist, has objected to the Ryan-Jethá interpretation on several grounds. His critique centers on the ambiguity of the evidence and the risk of over-interpreting biological markers through the lens of preferred outcomes.

Barash notes that testicle size, while informative, is not a binary indicator. Yes, humans fall between gorillas and chimps, but this could reflect a number of different mating systems—serial monogamy, socially monogamous pairs with occasional extra-pair copulation, or monogamous groups with high levels of within-group infidelity. The intermediate position tells us that human ancestors did not operate under strict monogamy or extreme polyandry, but it does not uniquely specify a model.

On hidden ovulation, Barash argues that the evolutionary origin is unclear. One interpretation is that concealed ovulation allows cuckoldry. Another is that it strengthens pair bonding by increasing the frequency of non-reproductive sex and making females sexually available throughout the cycle, thus binding males more firmly to their mates. A third is that it evolved to prevent infanticide by confusing paternity, not to enable it. The same trait can serve multiple adaptive functions, and retrospective assignment of “why” a trait evolved is notoriously difficult.

Female copulatory vocalization, Barash notes, is also ambiguous. While some research suggests vocalization signals fertility status, other work shows it correlates with male stimulation and male presence, not female fertility directly. And even if it signals anything about reproductive interest to other males, this does not necessarily imply an ancestral system of low paternity certainty.

Sexual dimorphism, too, could reflect several mating systems, none of them uniquely specified by the magnitude of size difference.

Barash’s broader point is this: the Ryan-Jethá argument exhibits confirmation bias. The authors interpret ambiguous evidence in ways that support a non-monogamy thesis, while downplaying or reinterpreting contradictory evidence (such as male jealousy, mate-guarding behaviors, and resource investment in spouses). The biological record, Barash contends, is consistent with a more moderate thesis: humans have been capable of both pair bonding and sexual infidelity, and ancestral populations likely exhibited both, with significant variation across time, geography, and ecological niche.

What the Debate Reveals About Human Mating Flexibility

The argument between Ryan-Jethá and their critics is not ultimately settled by more evidence for testicle size or copulatory vocalization. It is settled by a shift in how we think about ancestral sexuality itself.

Both sides agree on a crucial point: humans are not naturally monogamous in the way that gibbons or swans are. We do not have a genetically hardwired, inflexible pair-bond instinct. Instead, humans appear to have evolved flexible mating systems—systems capable of supporting both pair bonds and extra-pair sexual contact, depending on context.

This flexibility is the real finding. Whether the ancestral norm was 70 percent monogamy with 30 percent infidelity or 40 percent monogamy with 60 percent infidelity is less important than understanding that our biology does not dictate a single path. We have the neural and hormonal architecture for commitment, for attraction to specific partners, for jealousy and mate guarding. We also have the architecture for desire outside the pair bond, for sexual arousal in novel contexts, and for managing non-exclusivity without relationship collapse.

The evidence of this architecture is everywhere: in anthropological data showing vast variation in mating norms across cultures, in the ubiquity of infidelity across societies with strict monogamous ideals, in the capacity of many couples to engage in consensual non-monogamy without relationship dissolution, and in the biological markers that point toward sperm competition as an ancestral reality rather than a modern perversion.

What matters most is this: we cannot appeal to “nature” or “evolutionary design” to justify either monogamy or non-monogamy as the single correct human mating system. Both are expressions of our evolved flexibility. The question is not what humans evolved to do, but rather what humans consciously choose, given the full palette of possibilities.

Synthesis: Moving Beyond Origin Stories

The Ryan-Jethá debate has done something valuable precisely because it refuses easy answers. It has forced us to examine the evidence closely and to question the assumption that monogamy is humanity’s ancestral norm. Even if the specific thesis of ancestral non-exclusivity remains contested, the effect has been to demonstrate that monogamy is not written into our biology in the way we often assume.

For those exploring consensual non-monogamy, this has profound implications. It means that choosing non-monogamy is not a betrayal of human nature. It also means that choosing monogamy is not a surrender to instinct but a conscious cultural and relational choice. Neither is “natural” in an absolute sense; both are expressions of human flexibility.

The real ancestor we should study is not a specific mating system but a capacity: the capacity to form bonds and break them, to commit and to desire outside commitment, to negotiate sexual meaning with partners under varying conditions. That capacity is ancient, and it is ours to exercise.

This article is part of the Evolutionary Biology series at Sacred Displacement. Related reading: The Desire Paradox, The Infidelity Rate