On Humans9 Mai 2025

An Essential Difference? Males, Females, and the Spaces In Between ~ Augustín Fuentes

Binary thinking is out of fashion. But what about biological sex?

Whatever we might say about diversity and fluidity, the ideas of “male” and “female” seem essential in biology. I’ve taught the subject. I’ve drawn bees and flowers, with arrows from anthers to ovaries. I’ve used the terms “dad cell” and “mother cell” while doing so. I don’t know how I could have done it any differently.

And maybe that’s just fine. Human sex cells are binary: sperm and egg. But here’s the twist: humans aren’t sex cells. Humans are animals. And animals aren’t so easily grouped. XY chromosomes don’t always produce a penis. And when we turn to hormones, brains, and behaviours, the picture gets even messier.

Or so argues Princeton Professor Agustín Fuentes, a leading expert on human biology. His new book, Sex Is a Spectrum, came out this week. It makes the case that biology no longer supports a sharp separation between the male and the female.

I approached this topic with a mix of curiosity and scepticism. And while I'm still unsure of what to make of the conclusion, I do not regret the conversation one bit. We had a great chat, ranging from hermaphroditic worms to sex-changing fish, and from gender stereotypes to intersex humans. At times, I pushed back. But I learned a lot throughout the conversation. I hope you do, too.

Thoughts about Fuentes’s argument? Or my takes? Share them at onhumans.substack.com. You'll also find more links to dig deeper.

Support the show: patreon.com/onhumans

MENTIONS

Agustín Fuentes: Sex is a Spectrum; Creative Spark; Race, Monogamy, and Other Lies They Told You

Yuval Noah Harari: Sapiens, A Brief History of Humankind

Jared Diamond: Guns, Germs, and Steel

Steven Pinker: How the Mind Works

Sara Blaffer Hrdy: Father Time (see episode in April 2023)

KEYWORDS

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