Christopher Ryan and Cacilda Jethá open Sex at Dawn by directly challenging what they term the "standard narrative" of human sexual evolution — the widely assumed idea that humans evolved as an innately monogamous species, with the nuclear family representing our natural evolutionary state.

Their central challenge:

• The authors argue this standard narrative, while broadly accepted in both popular culture and significant portions of evolutionary psychology, rests on selectively interpreted evidence and unexamined cultural assumptions rather than a genuinely neutral reading of the available anthropological and biological evidence
• They position their book as a corrective, drawing on primatology, anthropology, and human physiology to construct an alternative account of human sexual evolution emphasizing flexibility and social cooperation rather than strict pair-bonding
• This opening explicitly frames the book as entering active scientific and cultural debate, rather than presenting settled consensus

This framing sets up the book's overall approach: marshaling evidence from multiple disciplines to challenge a widely-assumed evolutionary narrative, while acknowledging from the outset that this remains a genuinely contested area of scientific and anthropological inquiry.