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

May 9, 2025

Augustín Fuentes, a Princeton Professor of biological anthropology and author of 'Sex Is a Spectrum', discusses the complexities of human sex and gender. He challenges the binary view of male and female, emphasizing that biological categories are not so clear-cut. Their conversation touches on fascinating animal examples, from hermaphroditic worms to sex-changing fish, and critiques traditional gender roles in caregiving. Fuentes advocates for understanding sex and gender as a spectrum, prompting a deeper look at human nature and relationships.

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INSIGHT

Sex Is More Than Binary

Sexual reproduction involves two gamete types, but animal sex biology rarely fits a strict binary.
Some fish and worms change sex or have both reproductive systems, challenging binary categorization.

ANECDOTE

Earthworms Are Fully Intersex

Earthworms are fascinating because they are fully intersex, having both male and female reproductive organs simultaneously.
They can either mate with another worm or self-fertilize, showcasing flexibility uncommon in many animals.

INSIGHT

Primates Show Diverse Care Roles

Some primates like owl monkeys and tamarins show male-led infant care, sometimes with fathers doing most of the carrying.
This variation in parental care challenges assumptions about strict female caregiving in mammals.

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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