Being human entails an astonishingly complex interplay of biology and culture, and while there are important differences between women and men, there is a lot more variation and overlap than we may realize. Sex Is a Spectrum offers a bold new paradigm for understanding the biology of sex, drawing on the latest science to explain why the binary view of the sexes is fundamentally flawed—and why having XX or XY chromosomes isn’t as conclusive as some would have us believe.

In this lively and provocative book, leading biological anthropologist Agustín Fuentes begins by tracing the origin and evolution of sex, describing the many ways in the animal kingdom of being female, male, or both. Turning to humans, he presents compelling evidence from the fossil and archaeological record that attests to the diversity of our ancestors’ sexual bonds, gender roles, and family and community structures, and shows how the same holds true in the lived experiences of people today. Fuentes tackles hot-button debates around sports and medicine, explaining why we can acknowledge that females and males are not the same while also embracing a biocultural reality where none of us fits neatly into only one of two categories.

Bringing clarity and reason to a contentious issue, Sex Is a Spectrum: The Biological Limits of the Binary (Princeton University Press, 2025) shares a scientist’s perspective on why a binary view of sex and gender is not only misguided but harmful, and why there are multitudes of ways of being human.

Agustín Fuentes is professor of anthropology at Princeton University.

Caleb Zakarin is editor at the New Books Network.