What Makes Men and Women Different?

Article 1 of the series The Game Theory of Sex

Men and women are physically different. Do they also think differently? Yes. And a huge amount of the psychological differences between men and women can be explained by one simple fact.

Women have a uterus.

I’m not trying to be polemic¹. I’m trying to understand human differences from first principles. And I’ve heard people suggest that “aside from some biological differences, men and women are very similar”. What they don’t realize is that these small biological differences—like having a uterus or not—have massive ramifications. If you understand these ramifications, you can understand a lot about women and men, and why humanity is the way it is. So let’s look at them.

A Person with a Uterus

The uterus comes with all the reproductive tools to grow a baby. And that is the root difference between men and women: A woman will receive the sperm and carry the baby for nine months, along with all the discomfort and disability that comes with it. After that, she will give birth². The baby will come out of her, still tied by the umbilical cord. Once in her arms, she can throw that baby away³, but usually she won’t because the body secretes biochemicals⁴ to promote attachment, so that she cares for him or her until the child becomes independent⁵.

What are the ramifications of this simple fact?

Dangerous Liaisons: High Stakes

A pregnancy usually⁶ means a commitment between 9 months and 15 years / forever of the mother’s life. If that’s not high stakes, I don’t know what is.

Compare this to men.

A man who keeps a committed relationship with the mother of his children is also committing 15 years / forever to the child. But if he doesn’t, his commitment is a few minutes⁷.

Think about that. A guy and a girl who have sex have a completely different outlook on the outcome. If they’re not in a relationship, the man can ejaculate and leave. The woman can get pregnant and has now committed between nine months and decades of her life. 

Also, the woman’s commitment to the child will be stronger because the baby emerges into the world from the mother’s uterus. A man might be long gone by then. 

Or he might not know whether the baby is his, which means he might fear that it’s not and decide to leave.

Even if he does think it’s his baby, he might still leave. It’s easier to ignore and abandon a baby you’ve never seen than one that exited your body through traumatic pain and that you hear crying, begging for your attention, comfort, and sustenance.

No matter how you look at this, when a woman commits to sex, her stakes are much higher than a man’s.

Limited Supply

Stakes are high for another reason: Female reproductive capacity is much scarcer than male.

How many children could a man have, theoretically⁸? Let’s assume an average of between two ejaculations a day and one every couple of days. If they can do that from the time they’re 13 years old to their death at 85, that’s a maximum of 72 years, 365 days per year, and twice per day, or over 50k pregnancies! If each of them were triplets, it could be 150k people. Genghis Khan is believed to have impregnated over a thousand women.

Compare that with women: Around 25% of pregnancies miscarry. Those that come to term take nine months. Usually, the body needs at least a few weeks to recover for another pregnancy, and if she breastfeeds, hormones may stop her from getting pregnant again, at least in the short term. But here, we’re going for theoretical maxima, so let’s assume they can get pregnant every 10 months, from age 13 or so to 58 at most. That’s a total of 54 babies. If most of them were twins, she could have up to 100. Indeed, Valentina Vassilyeva birthed 69 children.

However you look at it⁹, men can have orders of magnitude more children than women¹⁰. That means that women’s reproduction is in much lower supply than men’s. A man can have sex with a woman one day and another the next, and have two babies. A woman can’t do the same.

This means there’s a mismatch in supply and demand. From the perspective of evolution, women are more valuable than men¹¹. This has tremendous implications.

The Choosier Sex

In many species, females, but not males, exhibit choosiness. This difference is due in part to females of most species being physiologically obligated to make a greater investment in offspring than males. Where this occurs, females have evolved to be more discriminating, in order to avoid the greater costs associated with mating with those who have low reproductive value.—Haselton & Bus, 2000, via @datepsych

Since a man can biologically get any woman pregnant¹², a reasonable strategy for mating would be to look around for any woman willing to mate with him. 

Of course, every man will try the same thing, so the hotter women will have many candidates to choose from. She will be picky. 

But not only hot women will be picky. Many women will be, because while men don’t need to discriminate (they can have sex with many women and father many children), women must (they can only have one child at a time). 

The result is that men will compete for the same females. 

These two forces—women being choosy and men competing against each other—will create two types of male selection: inter-sexual selection (between females and males) and intra-sexual selection (between men in this case). Let’s see what each one does, starting with competition between males.

How Male Competition Shapes Men

A man who can physically beat his adversaries will take them out of the competition pool. The bigger, stronger, more aggressive the man is, the more competition he can take out, so men have evolved to have more of these traits than women.

Relative to females, males exhibit greater stature, muscle mass, strength, speed, aerobic capacity, ability to dissipate exercise heat loads, craniofacial robusticity, pain tolerance, risk-taking, behavioral displays of physical prowess and acuity to the formidability of same-sex conspecifics, outgroup discrimination, and a propensity to participate in dyadic and coalitional violence.—Contest Competition for Mates and the Evolution of Human Males, David Puts, David Carrier, Alan R. Rogers.

Men are more attractive with bigger chests and smaller waists. Source.

The usefulness of most of these traits is obvious. Muscle helps prevail over other men. That’s why men have 60% more muscle mass than women, and 75% more in the arms¹³, for a total of 90% upper body strength¹⁴. The average man is stronger than 99.9% of women. This increases sexual success:

Fat-free mass (FFM) and/or limb muscle volume (LMV) are significant predictors of the numbers of total and past-year self-reported sex partners, as well as age at first intercourse. On the cost side, FFM and LMV are strong positive predictors of daily energy intake and strong negative predictors of C-reactive protein and white blood cell count, measures of native immunity.—Costs and benefits of fat-free muscle mass in men: Relationship to mating success, dietary requirements, and native immunity, Lassek & Gaulin, 2009

Same thing for more aerobic capacity and heat dissipation: They allow men to fight (or flee) for longer. 

Thick Skulled

Why a stronger skull¹⁵?
To better withstand blows to the face.

Many small differences better protect the male brain from fist attacks. Source.

Men have stronger brows, jaws, and eye orbits, bigger noses… Many small adjustments to the skull to be stronger in a fight.

Unsurprisingly, many of these are attractive in men. Like a stronger jaw.

Ladies, ladies, overall: Who’s the squarest of them all?

Obviously, it’s better if the enemy backs down without a fight. So men have also developed ways to appear threatening. 

For example, as we’ve just seen, wider jaws are more attractive, but beards have a parallel effect. Why? A beard is not useful for fighting, and doesn’t increase attractiveness, but it increases the apparent size of a jaw and highlights a man’s anger. Not surprisingly, bearded men are perceived to be healthier, more masculine, less appeasing, and have better parenting skills and social status. Note that women don’t have beards, so they must not be vital to life processes. They are mostly useful for sexual selection. 

Pick your player. Source.

The same mechanism is at play with eyebrows: Thicker eyebrows convey size and dominance, and the bigger and more dominant a man appears, the more attractive he is to females¹⁶. 

Meanwhile, the Adam’s apple both protects the trachea and lowers the pitch of the voice, which means it is a benefit in fights and in the perception of masculinity and dominance: The protection to the trachea will protect against frontal punches to the neck, while the lower pitch is seen as more dominant as we’ll see later.

I can’t find papers that discuss how the Adam’s apple evolved as sexual dimorphism to increase both defense and masculinity perception, but it sounds obvious to me, especially since its growth is triggered by testosterone, and also because it happens during puberty, when sexual dimorphism emerges. In any case, take it with a grain of salt since this is my hypothesis. Source.

YOLO

There are other ways to show that you’re scary, aside from cranial differences. For example, by showing how strong and skilled you are. Or if you’re willing to take more risks. Who wants to fight somebody who’s willing to die? So displaying fearlessness and taking crazy risks makes you a scary enemy and useful ally. This explains Jackass.

You thought I was dumb, didn’t you? Well, are you fighting me? And who’s getting the ladies now? Source.

And bullfighting.

Bullfighters are notorious womanizers, and their wives famously attractive. 

There’s another reason for this risk-taking. Because women have the uterus, they have much more control over how many children they have. Unless they’re infertile, they will have children if they so choose. They just need to accept the advances of men—or, if they’re not forthcoming, seduce men. Because the cost of sex to men is so much lower, they are much more likely to say yes.

This is not the case for men. Some of them might not be able to get any women to mate with them, which justifies taking more risks in order to get one. As a result, men will take more risks overall than women, be it to impress them directly or to achieve the status needed to win one over.

Here are some other big differences between males and females that are likely due to competition between males:

• Men are 23% more left-handed than women, probably because most people are used to fighting right-handed people, so being left-handed gave you an edge in fights, even if it penalized you elsewhere.

• Men tolerate pain better than females¹⁷.

Body size, speed, strength, jaw size, eyebrow size, risk-taking behavior… So many aspects where men are different from women, all because they must compete for access to uteruses¹⁸.

How Females Choose Men

So far, we’ve seen ways that males have evolved to best other males for access to females. But males are not the only ones with a say in the matter. Females have negotiation power. They possess the prized asset. For whatever characteristics females prefer males, males will have evolved toward having them.

Dominance

The first way in which female choice might have influenced men’s evolution is by intensifying the features selected by male competition. In other words, once males compete over something, females will pick up on it, and use it as a signal to choose their preferred males themselves.

Why? Because a male who prevails over other males is more likely to reproduce. A female who partners with such an “alpha male” will have children who are more likely to reproduce. Evolution will favor females that go for these alpha males¹⁹.

Women will choose the men who are stronger, more threatening to other men, and more dominant. So they’ll pick up on the same signals (height, muscle, voices, beards, eyebrows, jaws…) that help them compete. Males with these characteristics will be more likely to reproduce, and their offspring will carry these genes. Indeed, several studies have shown that the most dominant men are also the most sexually successful men²⁰. Women prefer men who are somewhat more masculine than average in height, body build, voice, and perhaps face.

Why voice?

Voice

Men have lower voices than women, and women prefer deeper voices, so unsurprisingly, men with deeper voices are perceived as more dominant and report more sexual partners. This is interesting. Why would a lower pitch mean you’re more dominant? Consider these facts:

1. The taller you are, the longer your vocal cords, and longer vocal cords create lower frequencies. 

2. More testosterone lowers the pitch.

So a deep voice means you’re taller²¹, but it also means you have more testosterone. And men with more testosterone are seen as more dominant. But testosterone suppresses the immune system, which means men with more testosterone are more likely to get sick. Therefore, only the very strongest and healthiest of males can afford to produce more testosterone. Their immune system must be the GOAT. So when a man displays higher levels of testosterone, he’s displaying more fitness.

But you couldn’t show your blood analysis results to your savanna partner a hundred thousand years ago. You could, however, speak to her in your deeper voice, which she’ll consider a sign of dominance. Other men will, too: Higher status men lower their pitch when they meet lower status men, and vice versa.

Humor

Humor is attractive. Why?

Actually, humor is not similarly attractive to men and women. Women prefer men who can create humor, while men prefer women who can appreciate humor. That there’s a difference in genders here suggests that it’s caused by sexual selection. Also, people try to be funnier with people they find attractive. And people who find each other attractive laugh more together. But why might that be? Why humor and not any other random ability?

It’s hard to create humor, so it’s a sign of intelligence. Indeed, males who were more intelligent tended to be able to make more humor and were more sexually successful. Maybe that explains dad jokes too?

Yes, this is anecdotal evidence. The quant is above.

It also points to a factor that women appreciate: intelligence. Why?

Male Investment

Females don’t just want dominant males. They also want males who can stay with them for a long time, to protect them and the children, and provide resources. So they choose the ones who are more likely to stay with them and provide resources²²:

• Women's preferences for men with resources and willingness to invest appear culturally ubiquitous. 

• There’s a virtuous circle here. Men are likely to provide resources because it’s good for the protection and nourishment of their offspring. But then as females started selecting for that, men started evolving this trait further to attract more females.

• But before agriculture, humans were mostly nomadic, and as a result they couldn’t accumulate physical wealth easily. Women, therefore, were forced to look for proxies that suggest the man will be able to gather resources, such as foraging ability, intelligence, or creativity.

• Another way to achieve the same goal is to prefer men with high status (however they achieved that status): It’s a signal that they have some skill or asset that is very valuable.

Here’s something interesting: The interest of women in male intelligence seems to disappear near ovulation²³. Why?

Long-Term vs Short-Term Mating

Research suggests that females prefer some male features during ovulation, when they are maximally fertile, and other features at other times. Intelligence is one of them, but not the only one.

If you stop to think about it, this is crazy, but intuitive, and logical.

When they’re not fertile, women prefer men who are faithful, warm, good parents, financially successful, and intelligent. These are all features conducive to long-term relationships.

When women are fertile, they prefer men who are strong, arrogant, attractive, muscular, and influential²⁴.

This suggests that women evolved to have a two-pronged approach to sex, one for long-term mates and another one for short-term mates. Long-term mates are selected for features that help raise a family, and short-term mates are selected for dominance over other men in sexual competition for females.

If you ever needed a 10-second illustration of this:

Instagram, the never-ending source of people’s inner truth. We will talk much more in the next article about the differences that women do in men for long-term vs short-term relationships.

Additionally, two studies²⁵ found that, when women are fertile, they are more attracted to men other than their partners, but not more attracted to their partners. These effects are more pronounced in women who don’t see their partners as attractive short-term mates. 

That doesn’t mean that women cheat all the time. Those partnered with a man who appear to have good genes will not be as interested in looking for those genes elsewhere. But even for the women who might be interested in other men (especially during ovulation), there are lots of risks to cheating! But it looks like when these risks are low and the upside might be worth it, these changes in attraction might tip the scale. 

Also, it doesn’t mean that women only want to have sex with men who exhibit short-term assets. They can also have evolved genes to want sex with long-term mates, as a way to have their children but also to keep their mate around²⁶. 

Finally, it could also mean that all these instincts evolved a very long time ago, and that society has evolved past that. But we still carry these genes with us…

Male–Female Differences

Let’s stop here for now. We’ve barely scratched the surface, and here’s a list of things we’ve already discovered that are different in males and females, just because females have uteruses:

Men have:

• More muscle mass

• More strength

• Greater aerobic capacity

• Faster heat dissipation

• More speed

• More height

• More humor creation

• More discrimination against strangers

• More likelihood to band together for violence

• Stronger skulls

• Bigger jaws

• Bigger noses

• Adam’s apples

• Beards

• Bigger eyebrows

• Lower voices

• More left-handedness

• Higher pain tolerance²⁷

• And they take more risks

Men will also try to develop and display:

• Dominance

• Aggressiveness

• Muscles

• Intelligence

• Creativity

• Humor

• Money

• Arrogance

• Faithfulness

• Warmth

And this is just the beginning.

How do we know these differences appeared due to sexual selection, and not, say, to hunt better?
How do men adapt to the long-term vs short-term interest of women?
How does all this affect men’s strategies to attract women?
How do women attract men?
What are other ways in which males compete against each other?
Men are bigger than women, but why not even bigger? Why not twice their size? Why not seven times, like elephant seals?
What does this all tell us about the art of seduction?
About kinks?
What does this all tell us about current debates on sex, gender, equality…?

This is what we’re going to explore in the upcoming articles in this series, starting with the premium one this week: What are other ways men have evolved simply because they don’t have a uterus.

The elephant seal male is 5-7x bigger than the female. Source.

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I hope you enjoyed this article. The objective is not to ruffle feathers. Quite the opposite. This is a topic that is full of very arrogant debate on every side, and I want to do the exact opposite. I want to understand what we know on the topic, and from there, use first principles to form an opinion on the debates of today. As such, I might have missed some of the science, I probably got some of my own conclusions wrong, and that will continue being the case over the next few articles. I hope you’ll join me in this learning process. Please feel free to comment, correct me, add to the topic, and share your opinions to improve our shared perspective on the topic.

1. I’m also not trying to get into the gender and sex orientation debate. In fact, I steer away from it. It’s a hard topic, there’s a mismatch in the debate between knowledge and confidence, and I don’t know more than others who write more intelligently on the topic. Instead, I want to cancel out the noise and think about all of this from first principles. Form my own opinion along with you. In this article, I am considering only heterosexual, cisgender people. If, in the future, I have something to add to the conversation on other topics like homosexuality or gender, I will do it then.

2. Often traumatizing (especially the first time around), and nearly always very painful.

3. And in history, many times she did, usually forced by the circumstances.

4. AFAIK oxytocin plays the primary role.

5. Usually during adolescence.

6. Provided that the child is born healthy.

7. Or a few hours. Or, let’s not kid ourselves, a few se…

8. All of this is based in the past, at the time when we evolved. So no division of sperm for in-vitro fertilization.

9. Whether you take the theoretical or empirical maximum.

10. Through this article, we’re going to see how deep all these ramifications get, and as a result, how little science knows about it, despite the vast amount of research in human reproductive processes. As an example: Scientists tend to look at the Operational Sex Ratio, the ratio of fertilizable men to women. They assume that this ratio accurately describes the imbalance between sexes. But they’re having a hard time connecting the OSR with these imbalances in real life. One reason for this, I think, is that the OSR doesn’t say much. A woman might be able to have kids right now, but that doesn’t change the fact that she knows if she gets pregnant, she will commit a long time to the baby. So the OSR might not show big differences, but the reality is quite different. Scientists might not be looking at this properly. 

11. Or at least their reproductive system; however, from a genetic perspective, that detail doesn’t matter, since the point is reproduction.

12. Rule of thumb, for fertile males and females.

13. This is similar to gorillas!

14. There’s also a big difference in lower body strength, but not as big: Men have 50% more muscle mass in the legs, and are on average 65% stronger there.

15.  “Craniofacial robusticity”

16. Which is why I assume men tend not to remove their eyebrow hair, but women do. Women’s eyebrows do change a bit with fashion, but I’d argue that trends that make them appear bigger will be less appreciated by men than those that make them appear smaller. Note that there’s an equilibrium here, because eyebrows too big in women might suggest masculinity, but if they’re too thin they might suggest disease. It’s hard to be human.

17. Both of your female editors, Heidi and Shoni, think this is a contentious topic. The paper I linked is a meta-analysis. I just read the abstract and am no expert. I am open to corrections. Healthy debates and playful trashing in the comments are welcome.

18. Here, I’m talking about evolution and economics. From an evolutionary perspective, it’s the reproductive ability of females that is the scarce resource, and from an economics angle, scarce supply is more valuable.

19. I don’t like the term, but it’s a shorthand for “more dominant males that have sexual success”.

20. Usually, dominance means attractiveness. But when a trait appeared attractive but not dominant, it did not have much impact on sexual success.

21.  I couldn’t find a paper on this, but I’ll assume that there’s another factor here: Bigger men have a deeper voice because their body acts as a resonance chamber. You can’t fake that easily aside from just being bigger.

22. All the points on male investment are outlined in this paper, which quotes other papers that support each point.

23. Gangestad et al., 2007; Prokosch et al., 2009, but see Haselton & Miller, 2006.

24. Your editor Shoni adds: Maybe this is just animal instinct kicking in when your body just wants sex, and when you have brain space to consider what is actually better, you can make those wiser choices. To which I reacted: Depends on what "wiser" means… From the point of view of a brain that wants to optimize long-term happiness, probably. From the point of view of an existing gene that wants to spread, not so much.

25. Gangestad, Thornhill, & Garver, 2002; Gangestad, Thornhill, & Garver-Apgar, 2005; cf. Pillsworth, Haselton, & Buss, 2004.

26. Here I’m talking about it from an evolutionary perspective. 

27. As discussed, the ultimate scissor statement in the internal UT team