My New Book Is Coming Soon - and You Can Read Half of It Here

I swore I'd never write another book... but then I wrote one anyway

Oct 31, 2025

a close up of a peacock with its feathers open — Photo by Josh Tere on Unsplash

I’ve got some big news. After nearly five years of research and writing, I’ve signed a contract for my next book, A Billion Years of Sex Differences. Not only that, but I’ll be releasing around half of it here on Substack in the coming months. Some excerpts will be for paid subscribers only; others will be free; all will be early access. It’s an experiment in publishing, and I’m excited to finally share what I’ve been working on.

From “Never Again” to Book #3

After finishing my last book, The Ape That Understood the Universe, I decided: “Never again!” But then in 2021, while stuck at home during the pandemic, an idea for a new book popped into my head - and I couldn’t shake it off. So I changed my mind: “Maybe just one more time.”

Originally titled The Peacock’s Predicament, I ultimately settled on A Billion Years of Sex Differences. Here’s the Amazon blurb:

According to an old joke, everyone knows that men and women are different… except social scientists. In A Billion Years of Sex Differences, evolutionary psychologist Steve Stewart-Williams tackles some of science’s most controversial questions: How do men and women differ? Where do the differences come from? And how do they shape modern life?
The result is the most up-to-date, balanced, and engaging account of human sex differences on the market, covering everything from dating and mating to aggression and parenting, from children’s toy preferences and workplace gender gaps to mental health and the politics of equality.
Drawing on a century of research, the book argues that differences between men and women aren’t just social constructs but are legacies of our evolutionary history – and that well-intentioned efforts to erase them can sometimes do more harm than good.
You’ll learn why:
Many sex differences appear despite socialization, not because of it.
Both sexes have their equivalents of the peacock’s tail and the deer’s antlers.
In our mating and parenting patterns, humans are more like the average bird than the average mammal.
Downplaying sex differences can be just as harmful as exaggerating them.
Sex differences are sometimes a sign of social health rather than social injustice.
With more than two decades of research in evolutionary psychology and an international reputation in the field, Steve Stewart-Williams is uniquely positioned to challenge prevailing views in this fraught debate. Avoiding the extremes of either overstating or denying the differences, he argues that, rather than trying to make men and women identical, we should strive for a world where bias and barriers are eliminated, and where people are free to be themselves whether they conform to gender norms or defy them.

How a Substack Post Landed Me a Book Deal

After several years of hard slog, things started falling into place in 2024, and I decided it was time to get a book contract. Instead of approaching agents or publishers, though, I thought I’d try something different: I’d put out the word via Substack.

To my surprise, it actually worked. Within 48 hours, a bunch of literary agents had contacted me, interested in publishing the book. I ultimately signed with the great Christy Fletcher of United Talent Agency (UTA). UTA is one of the biggest talent agencies in the world, representing such luminaries as Timothée Chalamet, Jessica Alba, Harrison Ford, Chris Pratt, Paul Rudd, Guns N’ Roses, and Christina Aguilera. It’s an honor to be on their roster!

Christy shopped my book proposal around, and I met with several big US and UK publishers. In the end, I signed with Mark Richards of Swift Press. Swift is a smaller publishing house that Mark set up a few years ago. It has two main advantages over the larger ones.

1. Creative Control

The big publishers I spoke to wanted to change the book in some fairly substantial ways. With Swift, I have complete creative freedom.

2. Parallel Publishing on Substack

This is the part I’m most excited about. Publishing is changing fast, and before signing with Swift, I contemplated bypassing traditional publishing altogether. Specifically, I considered serializing the book on Substack then self-publishing it via Amazon.

Instead, Mark and I agreed on a hybrid model: publish the book in the traditional way, but also release 35,000 words of it in installments on this newsletter.

So that’s what I’m going to do! Most installments will be for paid subscribers only. But I’ll also make some available for free subscribers, with the aim of creating a buzz around the book.

I’ve been planning this for a while, but had to wait till the book was available for preorder before publishing any of it on Substack. Well, that time has finally come! I plan to start publishing excerpts in the next week or so.

It’s been nearly five years since I began work on this project - and I’m really looking forward to getting it out there.

If you want early access to all excerpts, consider upgrading to a paid subscription. You can check out the other benefits of a paid subscription here.

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If you want to preorder A Billion Years of Sex Differences, click the link below.

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From the Archive

A Billion Years of Sex Differences

Steve Stewart-Williams

November 15, 2024

I recently gave a professorial lecture at my university, focused on some of my theoretical work on the evolution of human sex differences. The following is a lightly edited transcript of the lecture.

Read full story