Why this book
Christopher Ryan and Cacilda Jethá set out to challenge what they call the "standard narrative" of human sexual evolution — the widely popularized idea, associated with figures like Donald Symons, that men evolved to seek sexual variety while women evolved to seek a single, resource-providing partner, producing an inherently monogamous-leaning species merely straining against occasional impulses to stray. Their alternative model draws on comparative primatology, anatomy, and ethnography of remaining foraging societies to argue that pre-agricultural human ancestors lived in relatively egalitarian, cooperative bands where sexual partnering was fluid and shared rather than pair-bonded and possessive, more like our close relatives the bonobos than the more sexually competitive, jealousy-driven chimpanzees.
Their evidence runs through some genuinely striking physical and behavioral details: human testicle size and sperm characteristics consistent, they argue, with evolutionary "sperm competition" between multiple male partners rather than exclusive paternity; concealed ovulation, which they read as evidence against a single male being able to monopolize a female's fertile window; and modern societies practicing "partible paternity" — the belief that a child can have multiple biological fathers through sequential intercourse with different men — which they present as living remnants of a once-widespread cooperative sexual and child-rearing arrangement. They argue monogamy as a strict cultural expectation is a comparatively recent invention, tied to the rise of agriculture, private property, and the paternity certainty that inheritance requires.
Who should read it
Readers interested in a provocative, well-written challenge to assumptions about "natural" monogamy, and anyone curious how much evolutionary psychology's popular narratives rest on selectively chosen evidence, will find this a genuinely engaging, if one-sided, read. It should be read alongside its most serious critics: evolutionary biologist Lynn Saxon's book-length rebuttal, Sex at Dusk, argues Ryan and Jethá cherry-picked and misrepresented evidence on bonobo behavior, partible-paternity societies, and pair-bonding's evolutionary logic, and other anthropologists and evolutionary psychologists have separately challenged the book's central claims as scientifically overreaching — this is a contested, actively disputed thesis, not settled science.
About the author
Christopher Ryan holds a PhD in psychology from Saybrook University, where his dissertation on prehistoric human sexuality became the foundation for this book; he has lived in Barcelona since the 1990s and later wrote Civilized to Death. Cacilda Jethá is a psychiatrist who grew up in Mozambique and completed her medical training in Portugal, later conducting field research on sexual behavior for the World Health Organization before establishing a clinical practice; the two authors are married and live in Barcelona.