Everything You Wanted to Know About Sex Differences but Were Too PC to Ask

The first excerpt from my book, A Billion Years of Sex Differences

Nov 05, 2025

Opening a Can of Worms

In 1998, I fell in love. Awkwardly, I was on my honeymoon at the time. You see, I’d fallen in love in 1997 as well, with the woman who was now my wife. We’d married the week before, and were now travelling around New Zealand, my home country, seeing the sights. But we also had some spare time, and I spent a lot of it reading a book I’d picked up on a whim: The Moral Animal by Robert Wright.

People often talk about how a book changed their life, and sometimes they just mean “I really liked it.” But The Moral Animal really did change my life. It was my introduction to the field of evolutionary psychology, and reading it set the course of the rest of my career. It was the ideas contained between the covers of this book that I fell in love with on my honeymoon.

More precisely, it was the evolutionary explanation for sex differences. From the moment I grasped this simple but powerful idea, I found it hugely intellectually satisfying. I got the same feeling I did as a kid when I first understood how the phases of the moon work: Everything just clicked into place. But the click was even more satisfying this time, because the a-ha moment was about something we all encounter in everyday life, and which matters a lot to most people: differences between men and women.

So, I fell in love: with sex differences, with the logic of the evolutionary explanation for sex differences, and with the sense that evolutionary psychologists had made genuine progress in explaining a central feature of human life. As everyone knows, though, the course of love is never smooth, and over the following years and decades, my relationship status with evolutionary psychology switched to “it’s complicated.” There are various reasons for this.

The first is that love can sometimes blind you to the imperfections of the object of your affection. As I immersed myself in the field, I slowly but surely came to the view that its approach to sex differences, though definitely on the right track, was in need of a serious makeover. Much of that makeover is now complete. In many ways, the field as it exists today is very different from the one I met in the 1990s. It’s grown and matured – and in the process, my early infatuation with the area has given way to a more realistic, deeper love.

A second complication is that not everyone reacted to the evolutionary psychology of sex differences in the way I had. Whereas for me it was love at first sight, many took an instant dislike to the whole project. In their view, sex differences – if they exist at all – are largely a product of discrimination and socialization. As such, abolishing the differences is the only path to gender salvation: Trucks and dolls should be gender-neutral toys; engineering and teaching should be gender-neutral jobs – and evolutionary psychology, which downplays the unjust origins of society’s gender gaps, should be banished to scientific purgatory. With attitudes like these running in the background of many people’s thinking, the field was pelted with criticisms and widely demonized. To be sure, some of the criticisms were fair, and helped us refine our ideas. But many were not, and partly as a result of the unfair criticisms, many misunderstandings about evolutionary psychology now have wide circulation in the culture. You probably have some yourself.

A third complication is the mirror image of the second. Sex differences minimizers are always in the foreground of evolutionary psychologists’ attention, because the minimizers are their main academic foes. But minimizing sex differences isn’t the only way to get sex differences wrong. Some people – including, to be fair, some evolutionary psychologists – make the equal-but-opposite mistake: They massively exaggerate the differences, and the extent to which they’re irreversibly locked into human nature. More than that, some people moralize the differences, arguing not only that the sexes differ, but that they should differ. Although this view has little institutional power in the West today, it was the dominant trend historically and still has cultural momentum. Indeed, the attempt to minimize or deny the differences is largely a reaction to the traditional view. As we’ll soon see, however, both the sex differences minimizers and the sex differences exaggerators fall foul of the current science.

A fourth and final complication is that the minimizers and the exaggerators both create serious problems for individuals and society as a whole. With these positions dominating the public discourse, almost everyone gets sex differences wrong: laypeople, policymakers – even many scientists. But getting sex differences right really matters. It matters for our dating lives, our parenting lives, and our work lives. It matters for solving the social problems that plague us, from domestic violence to sexual harassment to disparities in the workplace and home. It matters for reducing discrimination, not just against women but against men. And it matters for our physical health and our mental well-being. By taking the science of sex differences seriously, we can set a new course to a better world.

It’s for all these reasons that I decided to write this book. I see the book as at once an ode to a loved one, a progress report from the frontlines of a rapidly evolving science, the case for the defence of that science against its academic prosecutors, and a deep dive into the personal and cultural implications of the knowledge that that science has produced.

I’ve got an aim, a hope, and an expectation for the book. My aim is to upload everything I’ve learned about sex differences over the past quarter-century from my brain into these pages, so you can download it into your own brain. My hope is that the book will do for you what The Moral Animal did for me in the 1990s, while taking into account what we’ve learned in the interim. And my expectation is that having read this book, you’ll never see men, women, or society the same way again – not because I’m so great, but because the science is. If you’re not ready for that, then you might want to consider pausing the download and putting the book aside, at least for a while. Don’t say I didn’t warn you!

The Science of Human Sex Differences

What sex differences are we talking about, are these differences real, and where, if they are, did they come from? These are difficult questions: questions that have stumped some of the greatest minds of the ages. This is partly because there are no easy answers, and partly because the discussions around these issues have often been so fraught. But although the questions are difficult, they’re not beyond our scientific reach, and in the last few decades in particular, we’ve made substantial progress in answering them. Let’s begin our exploration of the answers with what I call the standard-issue sex differences: a collection of female-male differences that profoundly shape our lives and societies, and which form the backbone of the rest of the book.

Standard-Issue Sex Differences
Men are larger and stronger than women; women can perform miracles (they can make new life). ● Girls develop faster than boys and hit puberty earlier; boys, after taking a more leisurely path to puberty, make a more rapid beeline to the grave: They age faster and die younger. ● Most men are primarily attracted to women; most women are primarily attracted to men. ● Men are often more interested than women in casual, no-strings-attached sex; women are often choosier about their sexual partners. ● Men often place more weight than women on a romantic partner’s looks and youthfulness; women often place more weight than men on a partner’s social status and ability to accrue resources. ● Men corner the market on violence, risk-taking, and other antisocial antics; women are more involved in childcare. ● Men tend to be more interested than women in things and gadgets; women tend to be more interested than men in people. ● Men tend to have somewhat better spatial skills; women tend to have somewhat better verbal and social skills. ● Boys engage in more play-fighting, and usually prefer toy weapons and vehicles to dolls; girls engage in more play-parenting, and usually prefer dolls to weapons and vehicles. ● Males are more prone to ADHD, autism, and substance abuse; females are more prone to depression, anxiety, and most eating disorders.

Now, it’s fair to say, I think, that a lot of people would bristle at this list. Many would argue that, leaving aside the undeniable physical and developmental differences, it’s basically a compilation of antiquated stereotypes of the sexes: the kind of stereotypes that your drunken grandfather might spout at a family gathering while everyone cringes and stares at their shoes.

It is true that many of the claims are stereotypes, in the sense that they’re widely held beliefs. It’s also true that most of these stereotypes have been around for a long, long time. However, we can’t just assume without checking that people’s stereotypes are false. On the contrary, given how often most people interact with members of both sexes, we might expect that sex stereotypes will tend to be accurate. More than that, we might argue that the mere existence and persistence of the stereotypes constitutes suggestive evidence that the differences they depict are real – especially given that they’re remarkably similar from culture to culture.

Fortunately, we don’t need to rely on intuition here. We now have more than a century’s worth of research on sex differences, and we’ve mapped out the territory quite well – much better, in fact, than most people realize. And contrary to the view that the unwashed masses are simply hallucinating the differences, many of your granddad’s stereotypes turn out to be surprisingly accurate. This is an example of a common phenomenon that the social psychologist Lee Jussim dubbed stereotype accuracy.

Sure, plenty of people exaggerate the size of the differences. But at least as many do the reverse: They underestimate how much men and women differ. And the average stereotype of both sexes tends to be close to the mark.

The Nature and Nurture of Sex Differences

Of course, to say that the sexes differ in certain ways is to say nothing about the causes of the differences. It’s certainly not to say they’re innate. Where, then, do the standard-issue sex differences come from?

This question turns out to be two questions rolled into one. Question number one is: To what extent are the differences due to nature, and to what extent to nurture? And the first thing to say about this is that learning, socialization, and culture clearly play a role in shaping the standard-issue sex differences. If they didn’t – if the differences were entirely due to nature – they’d presumably be invariant across cultures and times, which they’re not.

So, the issue isn’t whether nurture is involved in shaping the differences; it’s whether nurture is the whole story. Spoiler alert: For most of the sex differences we’ll be discussing, the answer is a resounding no. As a complete explanation for the differences, the sociocultural approach has holes in it big enough to drive a gender-neutral truck through. Among other things, the differences often appear in an embryonic form early in the lifespan; they’re found across cultures even when the culture pushes against them; they’ve been linked to prenatal hormonal exposure; and they’re comparable to differences found in other species – species with a similar evolutionary backstory to our own. The convergence of these very different lines of evidence adds up to a near-watertight case that, when it comes to explaining most sex differences, nurture alone can’t cut it. Nature matters as well.

The Evolution of Sex Differences

So much for the first question. The second is: Given that there’s an innate component, why? Where did it come from? This brings us to the heart of what I fell in love with all those years ago: the evolutionary explanation for differences between the sexes. Here’s the gist. Although both sexes evolve to pass on their genes, the best way to do this sometimes differs for each sex. The main reason it differs is that, in most species, males can potentially produce more offspring than females. A male that has, say, three sexual partners in a breeding season could end up with three litters of offspring. A female that has three sexual partners in a breeding season, on the other hand, will probably end up with no more litters than she would if she’d only had one. This is largely because, in most species, females invest more time and resources into each new offspring they produce. In mammals, for instance, females get pregnant, females give birth, and females manufacture milk for the young. In contrast, males’ contribution to the baby-making enterprise is often just a sperm donation.

The asymmetry in maximum offspring number turns out to be the key to unlocking the mystery of sex differences. The logic is easiest to see in highly sexually dimorphic species like peacocks and red deer. (Sexual dimorphism is biologist-speak for sex differences.) In most dimorphic species, a minority of males sire the bulk of the offspring in any given mating season, whereas the majority sire few or even none. For males in this situation, any trait that increases their chances of being one of the lucky few that hits the reproductive jackpot is likely to be selected – in other words, the genes underlying the trait will tend to be passed on to more offspring than competing genes, causing the trait to become increasingly common over the generations. What kind of traits are we talking about? For peacocks, they include a huge, flashy tail to seduce the peahens; for red deer stags, they include a big, strong body and massive antlers to fight off rival males; and for many species, they include an unquenchable thirst for mates, a desire to show off to impress potential mates, and an aggressive disposition to motivate male-male combat over territory, status, and ultimately (you guessed it) mates.

For females, on the other hand, traits such as these are generally less useful, because females can’t greatly increase the number of offspring they produce by mating with multiple partners. A better policy for females in most species is to be ultra-choosy about which males they mate with. Choosy females – females that hold out for males with the best genes or the healthiest tail feathers or the largest territories – are likely to have more surviving offspring than their less discerning sisters. As such, the genes underpinning their choosiness have a good chance of going viral in the gene pool.

To be clear, males and females aren’t thinking about how to get their genes to go viral; they’re just acting on their built-in desires. But why do they have those built-in desires in the first place? That’s the question that the evolutionary approach aims to answer. The answer it gives is that it’s because genes that happened to nudge development toward those outcomes typically found themselves copied into more new bodies, and eventually crowded out competing variants.

The Ape That Thought It Was a Peacock

In short, many common sex differences trace back to the maximum number of offspring that each sex can produce. This is the idea that really clicked for me on my honeymoon: the idea that I fell in love with. If you’re anything like I was when first encountering it, you’ve already started wondering about how this framework might apply to one particular species we both have a special connection to: our own. This turns out, however, to be a trickier question than it first appears.

Humans, you see, are oddities of the animal kingdom. In most species, females alone are choosy about their mates, while males alone compete for as many mates as possible. As a result, males evolve sexual ornaments like the peacock’s tail, and weaponry like the deer’s antlers. Humans aren’t like that. We have what’s called mutual mate choice: In our species, both sexes are choosy about their long-term mates, and both compete among themselves for the best mates on offer. For that reason, both sexes have their equivalents of the peacock’s tail and the deer’s antlers. Indeed, when it comes to physical attractiveness, females in our species have the larger “peacock’s tail”: a reversal of the usual pattern found in nature. It’s all very weird.

Another important difference between us and most of our animal cousins is that sex differences in our species tend to be less pronounced and profound. This is an area where evolutionary psychologists sometimes go off the rails: They talk as if human sex differences are huge, when in general, they’re relatively modest (emphasis on the “relatively”). As I and my colleague Andrew Thomas have argued, this is because humans evolved to fall in love and form pair bonds, and because both sexes, rather than just the females, evolved to invest in their young. This reproductive arrangement lowered the ceiling number of offspring that most men could produce, bringing it closer to the ceiling for women. And as that happened, it ushered in a corresponding reduction in the magnitude of sex differences in our species. Pair-bonding and biparental care are rare in the animal kingdom, and especially among mammals. But they’re not unique. In fact, in birds, they’re the norm. This means that, in our primary approach to making babies, humans are more like the average bird than the average mammal.Again: all very weird.

Now, don’t get me wrong; men and women are clearly not carbon copies of each other. Some sex differences are genuinely large, including – unsurprisingly – the sex difference in whether one is primarily attracted to men or women. Many sex differences are modest throughout most of life, but large in early adulthood. And many sex differences are larger among the minority at the extremes than the majority in the middle: the shortest and tallest, for instance; the most aggressive and the least. Still, when it comes to sexual dimorphism, humans are swimming in the shallow end of the pool compared to most of our fellow mammals. And rather than contradicting an evolutionary explanation for the differences, that’s exactly what an evolutionary explanation predicts for us, given our penchant for pair-bonding and biparental care.

That’s it for this time. Stay tuned for the next installment: The War on Sex Differences!

You can pick up a copy of A Billion Years of Sex Differences here.

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