9781785046193 by Smakprov Media AB

THE PERIOD BRAIN

The New Science of the Luteal Phase – and

How to Thrive Through It

Dr Sarah E. Hill

Author of How the Pill Changes Everything

The Period Brain

The New Science of the Luteal Phase – and How to Thrive Through It

Dr Sarah E. Hill

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For you

PART I

The Female Mystery: Hormones and “Disordered” Women

1. Why Do We All Have PMS? A neuroendocrinological mystery hidden in plain sight 3

2. Understanding Your Cycle The two halves of your hormonal whole 10

3. How Ignoring Progesterone Created PMS

Estrogen is about sex and progesterone is about . . . disorder? 24

4. The Dawn of Bikini Science

Why everything you thought you knew about “normal” is probably male

PART II

Reclaiming Female: Rethinking PMS and Embracing Hormonal Change

5. Understanding the Luteal Phase

Progesterone creates unique physical and psychological needs 53

6. Is There a Method to Your Madness?

The upside to feeling down 63

7. Sex and Attraction on Progesterone

The great disordering of the female sexual response 85

8. Nutrition, Exercise, Sleep, and Recovery

Advice for the luteal phase 106

9. From PMS to Primary Care

Mystery symptoms, mental health, and premenstrual exacerbation of . . . everything 126

10. When a Good Hormone Goes Bad

Answers and support for those with PMDD 146

PART III

The Future Is Female and Hormonal: Welcome to the Revolution

11. Reclaiming Your Luteal Phase How to promote hormonal balance 177

12. The Future Is Sex Differentiated and Hormonal

The end of bikini science 213

Appendix A 231

Appendix B 234

Appendix C 236

Appendix D 238

Acknowledgments 241 Notes 243 Index 273

Introduction

If you are a woman picking up this book, I’m guessing you have some firsthand experience with feeling terrible premenstrually. Most of us do. The hormonal changes that you experience in the week or two leading up to the start of your period can make you feel overwhelmed and emotional. Gross and unlovable. Hungry and bloated. Irritable and tired. Or maybe you’d describe your premenstrual feelings to me the way they were recently expressed by two women whose responses to my question “How do you feel in the two weeks leading up to the start of your period?” were simply “Inner scream” and “Everyone hates me.” Ouch.

These women are far from being alone in their struggle to feel good in the last two weeks of their cycle. Most women in the world (including me) have had the experience of feeling like they are riding a premenstrual roller coaster every month that makes us feel less than amazing. Although experiences vary from woman to woman and month to month, most women find themselves feeling some distasteful blend of physical, psychological, and behavioral changes that can include things like feeling irritable, anxious, snacky, tired, bloated, and gross. And, interestingly enough, few of us ever question why. We just assume that this is our fate as women. So, why do we all feel like sh*t in the weeks leading up to the start of our periods? And is it actually necessary? This is what I have spent

the last several years of my life trying to figure out. Because biologically, it doesn’t make a whole lot of sense that women should have inherited a tendency to feel bad half the time. And it makes even less sense when you consider that the hormone at the heart of women’s terrible experiences each month, progesterone, is actually supposed to make us feel good. Despite being deeply entrenched in research on the nuances of all things women and hormones, I had no answers for this. And the more time I spent digging into the literature, the clearer it became that no one else had a really good answer for it either. It was a biological mystery hiding in plain sight. Getting to the bottom of this mystery and telling you about the surprising things I discovered is why I wrote this book. What I learned transformed the way I think about myself and has gotten me oﬀ the premenstrual roller coaster. I am hopeful it will have the same eﬀect on you.

First, a bit about me. I am an award-winning research psychologist who studies health and relationships, with a particular focus on women. I have published nearly one hundred scientific research articles on topics in health psychology, evolutionary psychology, and neuroscience. My research is regularly funded by federal grants and discussed in the media, and I am considered a thought leader in the area of women’s hormones and psychology. I am passionate about demystifying the results of scientific research and using it to help women understand themselves. As part of these eﬀorts, I authored How the Pill Changes Everything (a book about the eﬀects of hormonal birth control on women’s brains), I speak and consult on topics related to women’s hormonal health, and I serve on the advisory boards for women’s health companies like Flo Health and 28 Wellness. I tell you these things to orient you to my background but also to help you understand how elusive the issue of PMS actually is. Because if someone like me doesn’t have a clue about why women end up in a socioemotional black hole every month, what hope does it leave for anyone who doesn’t have the luxury of being able to sit around and think about hormones all day?

As you will see in the pages to come, everything we experience in the last weeks of the cycle—this thing we call PMS—actually makes

biological sense. And it’s the sort of biological sense that doesn’t require us to feel icky two weeks out of every month. Although the way out of this ickiness is a little more complex than take two of these and call me in the morning, it’s still very doable. It just requires wrapping our arms around the fact that our female bodies are diﬀerent from male bodies. And as a part of that diﬀerentness, we have to contend with the fact that (a) we have two primary sex hormones instead of one, (b) these hormones cycle, and (c) as our hormones change across the cycle, our bodies’ needs change too. This means that the way we have all been taught to think about how our bodies work (as a fixed system instead of a cycling process) is wrong. It also means that the one-size-fits-all information that we have been given about what our bodies need doesn’t always fit for us.

Each month, our bodies shift between states dominated by two different and, in many ways, opposing hormones. There is estrogen, the hormone that takes the lead during the first two weeks of the cycle, called the follicular phase. Then there is progesterone, the hormone that takes the lead during the last two weeks of the cycle, called the luteal phase. These hormonal changes matter because they are part of who we are. They change us. They create two biologically distinct versions of what the female body is supposed to look like. Estrogen creates a version of ourselves that is optimized for sex and conception. Progesterone creates a version of ourselves that is optimized for implantation and pregnancy. Each of these hormonal states is characterized by distinct patterns of energy expenditure (how many calories we burn), sexual desire, pain perception, threat sensitivity, and more. This means that our bodies’ response to everything ranging from the tone in our partner’s voice to prescription drugs can—and often does—change over the course of the cycle.

Unfortunately, science has chosen to focus research eﬀorts only on women when we are in the cycle phase dominated by estrogen. This means all of the information we have been given about how to take care of ourselves and what our bodies need was not created with any guidance about what our bodies need or how they behave in the cycle phase dominated by progesterone. And the reason that so many

of us feel terrible during this time is because we’ve been unwittingly fighting against our bodies when they are doing exactly what they are supposed to be doing. It’s no wonder we all have PMS.

I wrote this book to change this for you. To help you understand how your body works and what it needs during the half of the cycle that science and medicine typically ignore. To do this, we are going to take a deep dive into the science of progesterone and the luteal phase. We’re going to talk about the fascinating complexity of what our bodies are trying to do at this time and how we can support them to feel our best.

Although most of us don’t know anything about progesterone, it’s an incredible hormone with a fascinating range of eﬀects on our bodies. It’s known to aﬀect our daily sleep and calorie needs, our moods, who we’re attracted to, the nature of our sexual desire, our ability to read others’ emotional states, our ability to put on muscle mass from exercise, our pain perception, our responses to therapy, our responses to prescription drugs, and the severity of our symptoms of just about everything. We will talk about who we are, what our bodies need, and, importantly, how to support our cycles and smooth out the turbulence that can occur in response to our hormonal changes across the cycle. Although this is a cycle phase when women’s bodies’ needs and vulnerabilities change, there is no reason that most of us should feel as bad as we do.

This book is also going to serve as a rallying cry for all of us to change the way we think about our sex hormones. Many of us have been taught to believe that the most feminist thing we can do when it comes to our sex hormones is pretend they don’t matter, but I hope to convince you that it’s time to revise this position. Downplaying the impact of our hormones doesn’t slow the pace of sexism* or make

* Sexist a**sholes will be sexist a**holes regardless of what science has to say. People will always turn a blind eye to the facts when they are trying to stick to an indefensible position. This is why climate change denial is so common despite the fact that there is a huge body of research demonstrating that this is what is on the horizon.

the world a better place for women. Instead, it provides science with an all-too-convenient excuse to continue to mismanage the way we are handled in research and medicine in the name of convenience and saving costs. Rather than studying women as hormonal, the current gold standard in biomedical research is to systematically exclude women from studies when they are in the luteal phase of the cycle to minimize the impact of their hormones on outcomes. And I am guessing you also didn’t know that in animal research the females are often ovariectomized so that they don’t produce sex hormones at all. Removing sex hormones from what it means to be female totally undermines the spirit behind including female animals in research in the first place. It means women will continue to be in the dark about who they are and what their bodies need when in a high hormonal state, especially in the luteal phase. We’ll talk about how science needs to study women as women, which means studying us as our hormones cycle.

This book is one part science, one part PMS self-help, and one part exposé on how science has mishandled women. You’re going to learn new things about your hormones, about yourself, and about how science works. Learning everything that I have learned in the process of researching and writing this book has changed the way I see myself and has been a total game changer in how I feel. I am confident it will have the same eﬀect on you.

As one final note before we dive in, I want to address the fact that, although much of the research that will be presented in this book focuses on cisgender, heterosexual women, this choice isn’t because I believe this is the only—or most important—experience of being a woman. It’s because most of the research we have on sex hormones, behavior, and the menstrual cycle overwhelmingly focuses on this population.

It’s my hope that this book contributes to a broader conversation about understanding bodies, biology, and lived experiences in all their complexity. The science is a starting point, not the end point. So, while the data I’ll present will often focus on the experiences of the cis-het majority, the ultimate goal is to expand what we know and

who we listen to, ensuring that all voices are included in shaping the future of this science. You are welcome here, and your curiosity and perspective matter, no matter how closely—or loosely—you fit the mold of the studies I’ll be discussing. As I always encourage those who feel like science isn’t speaking to their experiences: Be part of the change you’d like to see. Push for better science that is able to tell your story. Better yet, become a scientist. We need your perspective to ensure that science serves all people’s needs.

I have spent the last several years of my life uncovering the fascinating science of progesterone and the luteal phase, probing into what it means to be a biological female during a time in the cycle when so many of us feel awful. As you’ll see, progesterone—this quiet little hormone that’s been sitting at the back of the class and never gets called on—is at the heart of a lot of the things we wrestle with as women. But the problem isn’t what progesterone does or who it makes us become. The problem is that we haven’t been paying any attention to it. This book is going to be your first step toward changing all of that.

So, let’s get started. It’s time to discover who we all become in the two weeks of the cycle that the world decided to ignore.

PART I

The Female Mystery

Hormones and “Disordered” Women

Why Do We All Have PMS?

A neuroendocrinological mystery hidden in plain sight

Have you ever thought about how weird it is that most women feel like sh*t in the week or two before their periods start? And how much weirder it is that we all just assume it’s inevitable? I literally can’t think of anything else in the human experience that is so ubiquitously unpleasant yet assumed to be normal. I mean, childbirth and pain from injury are both universally unpleasant, but they make logical sense. Childbirth hurts because in the evolutionary-size war of babies versus vaginas, babies always win. And pain from injury feels bad to keep us from injuring ourselves further or doing it again. But PMS? Where is the sense in that? If men felt the way we do two weeks out of every month, science and medicine would be working around the clock to direct all available resources toward finding a solution to their suﬀering. But for us? It’s just assumed to be part of the female experience.

So is it? Are women really supposed to be feel some version of awful roughly 50 percent of their adult lives?

Well, if we define normal as something that happens to most people, then PMS is normal. Research finds that 75–90 percent of reproductive-age women have it. They report feeling physically and psychologically less well in the week or two leading up to their periods than they do outside that window.

And even though we might think PMS is a privileged-white-ladywith-too-much-time-on-her-hands sort of problem, research finds it’s not just white Western women who feel terrible premenstrually. Research finds that women on all six habitable continents around the world show a consistent pattern of feeling worse in the last two weeks of their cycle than they do in the first two.

For example, in one recent study, researchers looked at data from 3.3 million women living in 109 countries around the world to evaluate mood changes across the cycle. Below, you can see the mood map they created based on women’s responses. Regions in white show the areas where women reported more happiness and better mood premenstrually. Regions in dark colors show areas where women reported less happiness and worse mood premenstrually. Regions with black dots are areas where there were no mood diﬀerences premenstrually. And striped regions are those for which there wasn’t enough data.

PREMENSTRUAL MOOD E FFECT S

Even though you are likely as unsurprised at these results as I was, we should all be appalled by our collective complacency in accepting

this depressing mood map as our fate. Because the idea that this is just how things are supposed to be feels defeatist. How is it that we live in a world so rich with possibility that we have self-driving cars and food delivery robots, yet no one has ever bothered to ask whether it is necessary for women to feel bad in the week(s) leading up to the start of their periods?

Why do we feel so universally bad in the week or two before our periods? And is it necessary for us to feel this way?

These are the questions that have been bugging me for years. And not just as a person who has hated feeling like sh*t half the month because I had PMS. They bugged me as a scientist. Because in the world of evolutionary biology, there is nothing about the experience of PMS that makes any sense whatsoever. The idea that reproductiveage women are supposed to go through half of their adult lives feeling awful is evolutionarily bizarre and, frankly, nonsensical. Traits that make people feel awful in their peak reproductive years usually get eliminated from the population because they impinge on our ability to survive and reproduce. This makes them less likely to be inherited than traits that don’t cause problems. That is how natural selection works. So, the idea that most of us inherited PMS—in addition to being a total bullsh*t deal for women—is something that the evolutionary biologist in me has never known what to do with.

One possibility I considered is that PMS might be an inevitable side eﬀect of having cycles. And that the reason it persists is because it isn’t costly enough for natural selection to remove from the population. Sort of like a belly button—it serves no purpose but also causes no real harm. Maybe PMS is just a meaningless by-product of the hormonal changes we experience each month that hasn’t caused enough problems for evolution to decouple the terrible feelings we have premenstrually from the hormonal changes it’s associated with.*

* Note that for the process of evolution by selection to be able to decouple the tendency to feel terrible from the hormonal changes that happen at the end of the cycle would require there to be suﬃcient variability in how women feel in response to these hormonal changes. If there were no women who felt just fine in response

As appealing as this argument sounded at first, the more time I spent with it, the clearer it became that it wasn’t very plausible. While PMS might not pose the same type of survival threat as something like cancer or cardiovascular disease, it’s hardly uncostly. PMS and period-related symptoms are responsible for an average of 8.9 days of lost work productivity per woman per year. Women who suffer from PMS are more likely to experience low self-esteem and relationship difficulties than nonsufferers. All in, researchers estimate that the impact of PMS on women’s quality of life and economic functioning is comparable to major depressive disorder. And for women with PMDD (premenstrual dysphoric disorder), these impacts can even cost them their lives. The risk of suicidality in these women is seven times higher than what it is for nonsufferers.

So, PMS isn’t just costly. It’s super costly. And even though our ancestors cycled a lot less frequently than we do now (they were too busy being pregnant and lactating to have as many cycles as we do), there is simply no way that the overwhelming majority of contemporary women would have inherited this costly predisposition if it wasn’t providing us with some sort of benefit that could outweigh these enormous costs.

Another explanation I considered is the possibility that PMS isn’t something that our female ancestors had to deal with at all. Maybe all the terrible feelings we saw splayed across the premenstrual mood map are the result of all the environmental changes we’ve made in the last hundred years that mess with our hormones. The quality of the human diet has gone down and exposure to toxic chemicals has gone up since the time of our ancestors. So, if these changes correspond to changing rates of PMS, it could be that it is a disease of civilization, like type 2 diabetes and heart disease.

to end-of-cycle hormonal changes, evolution could never select against the tendency to feel terrible. However, such variability does exist. There are some women who experience no premenstrual yuckiness, some who experience only marginal levels, and some who experience hugely terrible levels. There is enough variability for selection to work on this trait.

Although there was a lot to like about this explanation, it also failed to connect all the dots. While it is true that the modern environment and all its exposures don’t do us any favors premenstrually (more on this in chapter 11), this isn’t the only reason we feel bad. PMS isn’t just something that happens to women fortunate enough to live in countries with terrible diets and an overreliance on plastics. It happens to all of us. And research on nonhuman animals finds evidence of PMSlike symptoms in species ranging from rats to baboons. For example, female yellow baboons (Papio cynocephalus) are less social and more combative with peers premenstrually than at other times in the cycle. Others find that creating a hormonal state that mimics PMS causes rats and mice to exhibit increases in anxiety-like behavior. Premenstrual changes in anxiety and aggression are also observed in vervet monkeys and even dairy cows, suggesting that PMS may have less to do with where we live and what we’re exposed to than it does to the fact that we’re female and experience hormonal changes each month. This brings us back to square one. Why do females great and small routinely feel bad in the weeks leading up to their periods? And why, despite the growing public awareness of menstrual cycles and hormonal changes, hasn’t anyone spent time trying to figure out what our bodies need at this time to help us feel better? There are many books and other resources talking about the fertile window and how wonderful and magical estrogen makes us feel. And while this is great and all, most of us don’t need a survival guide to help navigate the experience of feeling awesome. We need one that tells us what to do when we’re feeling bad. And currently there is almost no conversation around what most of us experience in the second half of the cycle or the sex hormone progesterone, which is at the heart of everything we feel at this time.

The more I began to dig into these issues to try to unwind the neuroendocrinological and evolutionary principles behind them, the clearer it became that PMS is an inevitable consequence of experiencing hormonal changes in a world that was not created with female bodies in mind. It’s biology and sexism in equal measure. PMS is one part functional biological changes that happen on purpose and

make us feel diﬀerent in the last two weeks of the cycle than we do in the first. And it’s one part dysfunctional changes resulting from the fact that most of us are unknowingly eating, sleeping, exercising, and socializing according to advice that works in the first half of our cycle, but not the second. No one has taught us that our bodies’ needs change throughout the month. And no one has taught us that when we try to care for ourselves following guidance that only works for our bodies half the time, it’s going to make us feel awful.

Here are five things about PMS that have become crystal clear to me over the last several years:

• The luteal phase has a purpose. Many of the physical, psychological, and behavioral changes that occur in the last two weeks of our cycles reﬂect changes that happen on purpose. And we have inherited them because they provided important benefits to our female ancestors. Seriously. There is a method to at least some of the madness.

• Progesterone is the hormone at the heart of these feelings. These on-purpose changes are prompted by the rise and fall of the sex hormone progesterone, which ﬂoods our brains and bodies during the second two weeks of our cycle before our periods arrive.

• Women aren’t disordered; we have just been ignored by science. Ignoring our cyclicity has created disorder. PMS is the tip of a much larger iceberg encompassing ways that women have suﬀered because our changing needs across the cycle—specifically those occurring in the two weeks leading up to our periods—have been ignored by science and medicine.

• Our environment plays a role. Environmental changes that we have made in the last hundred years or so have both decreased our exposure to progesterone and made us more sensitive to hormonal changes, both of which can contribute to physical and psychological turbulence during this time.

• You can feel better: Giving women the information they need to support their bodies during the luteal phase and other periods of hormonal transition can ease distress and leave them feeling a lot better.

This is what we are going to be spending our time together talking about in the pages of this book. And it’s about time. Half the world’s population has no business feeling terrible half the time. When you change the way you think about your cycle and begin to embrace each of the distinct halves to the female hormonal whole, it can put you on a path to feeling so much better.

Although most of us know about our periods, our hormonal changes don’t just affect us from the waist down. They affect everything. This includes our moods, our energy levels, our calorie needs, our sexual interest and responsiveness, our taste in partners, and how our bodies respond to physical pain, exercise, immunological threats, and even psychotherapy. They. Affect. Everything. And this includes our very sense of who we are. When our hormones go through changes, it changes the way we experience the world.

Let’s dive into the science of our cycles and sex hormones. As you’ll see, although our hormones change across the cycle, these changes don’t need to be the source of our suﬀering. And if we nurture our cycles instead of trying to fight them, ignore them, or medicate them out of existence, most of us can feel a whole lot better than we do right now.

CHAPTER 2 Understanding Your Cycle

The two halves of your hormonal whole

If you’re anything like me, I’m guessing that you didn’t get much of a hormonal education growing up. I mean, I’m sure we all saw the same awful puberty video in fifth grade. And maybe your mom told you that your hormones can make you feel more emotional during your period. But for most women I know (including me), the extent to which we were taught about our hormones and the important role they play in our mental and physical health never went much further than that.

So, before we can get to the bottom of PMS, the first thing we have to do is get acquainted with how our hormones work.*

Hormones are chemical messengers in your body whose job it is to get all your cells marching in the same direction toward goals set by the brain. In short, when the brain says “[insert name of some goal here],” your hormones are part of the marching orders that are

* If you’re already well versed in the ins and outs of the endocrine system (or have read my first book), you can probably skip to the end of this chapter. What we are going to go over here is pretty entry-level, Hormones 101 type of stuﬀ. For those of you who haven’t received any sort of hormonal self-education, though, these pieces will be helpful to you as we continue to march our way through the remaining chapters of this book.

sent throughout the body to help accomplish that goal. For example, if you haven’t eaten in a while, your brain will call for the release of hunger hormones to get your body’s wheels in motion for finding and consuming food. Or if we get a hug from a friend, our brain will release hormones like oxytocin and dopamine, which help ﬂag our friend as being one of our people and help reinforce that they are someone who should be treated with more love, generosity, and forgiveness than we oﬀer most others in our life.

As master coordinators of all our body’s parts, hormones are a part of the physiological software programs that create the experience of being you. Whether it’s growth and development, eating, drinking, stressing, having sex, maintaining a pregnancy, or bonding with our loved ones, hormones get all your bits and pieces working together to do whatever it is that the brain has in mind.

The communication between the brain and the parts of the body that create and release hormones goes on through a three-part communication pathway that looks a little like this:

Hypothalamus (in your brain) ➝ Pituitary ➝ Endocrine Organ

It starts with the hypothalamus, which is a part of your brain. One of its big jobs is to release special hormones to the pituitary gland, which is a little pea-shaped organ that’s right beneath your brain, near the brain stem. These releasing hormones tell the pituitary to stimulate whichever endocrine organ produces the hormone the brain would like released. The pituitary then communicates directly to whichever endocrine organ is being called to duty, ultimately leading to hormone release.

When hormones are released, they go into the bloodstream and go everywhere that blood travels, from head to toe. Everywhere.* This sort

* This is why, contrary to what some doctors tell their female patients, the hormonal IUD—which many women use for birth control—does not act locally. You cannot have a locally acting hormone because hormones go everywhere blood goes, which means that the hormones go—you guessed it—everywhere.

of diﬀ use release is what allows hormonal messages to reach multiple cells, tissues, and organs quickly, eﬃciently, and all at once. Once in the bloodstream, hormones get picked up by all cells in the body that have receptors for them. This will then cause the cell to change what it is doing based on the messaging the hormone provides.

For example, if a cell is being stimulated by sex hormones, it will do whatever it is supposed to do to help the body prepare for sex and reproduction. If it is stimulated by stress hormones, it will do whatever it is supposed to do to help the body prepare for stress. And if it receives both types of hormones at the same time? Well, it will do whatever it is supposed to do to help the body prepare for stressful sex (or sexy stress . . . whatever that might look like). As master coordinators of physiological processes and behavior, hormones change the activities of lots of diﬀerent systems within the body at once. Whether you’re eating, drinking, fighting, ﬂeeing, stressing, sleeping, sexing, or doing whatever else it is your body might be up to at any given moment, your hormones are part of your body’s invisible machinery that makes all this possible. They are part of what makes you you. We are our brains. We are our neurotransmitters. And we are our hormones.

The Thing About Sex Hormones

Although we have a lot of important hormones that do a lot of important things in our bodies (I’m looking at you, cortisol and thyroxine), there’s a strong case to be made that none of them are quite as important or impactful as our sex hormones. This is because—as a species that’s inherited every single trait that we have from sexual reproduction—our brains and bodies are literally wired for sex and pregnancy. And this is true even for those of us who don’t want sex or babies. Every trait you have is something you inherited because it was possessed by a female ancestor who (a) had sex and (b) had babies. If even one of your direct ancestors failed to do either of these things, you wouldn’t be here. Ancient humans who had sex and babies passed

their traits down to descendants. And ancient humans who failed to do these things didn’t pass their traits down to anyone.

This process of inheriting traits that promote sex and reproduction (and not inheriting traits that are antagonistic to either pursuit) is called natural selection. And when this process of diﬀerential trait inheritance causes changes in gene frequencies over time, it is called evolution. It’s important to understand how this process works because it can help us understand why our sex hormones are so important when it comes to calling the shots within our bodies. Without our sex hormones, there’d be no reproduction. And without reproduction, we’d fail to persist as a species. Although most of us think of sex as being nothing more than a pleasurable pastime to be shared with the person (or people) we love (or lust after), it’s the engine that has powered our continuity as a species for hundreds of thousands of years. This makes the hormones that coordinate these activities some of the most powerful and inﬂ uential pieces of molecular machinery our bodies have.

Now, as you might expect from their name, the job of sex hormones is to get all the parts of our body working together to help coordinate sex (and reproduction). And this is true for all of us. But what sex and reproduction actually end up looking like and demand of the body is something that differs in pretty dramatic ways depending on whether you are male or female. Male bodies do one thing; female bodies do another. Although biological sex can seem trivial and unnecessary in an age during which there is growing awareness of gender fl uidity, it is at the heart of understanding why women are so hormonally different from men.

Let’s start by talking about what biological males must do to reproduce.* And this won’t take us very long because the only thing that

* From here on out, I am going to drop the “biological” before I talk about biological males and biological females, calling them male and females instead. This is being done in the service of speech economy and does not imply that I assume that all biological females will be female gendered or that biological sex is somehow more important than gender. Neither of these things are true. We are just going to

men need to do to reproduce is have sex.* I mean, of course they have to find a partner first. Then they have to do all the things necessary to get their partner interested in sex. But that’s pretty much the extent of what’s required of men to get their genes into the next generation. It’s one job with three steps: Find a partner, mutually conclude that sex is a fine idea, and then do the deed.

Now, because men only need to jump through one set of hoops to get their genes into the next generation, they only need one primary sex hormone. This sex hormone is called testosterone. And as the hormone charged with coordinating the one job that men’s brains and bodies need to do to get their genes into the next generation (sex, sex, and more sex), testosterone is known as a potent motivator of sex (duh), as well as all its various antecedents.

Research finds, for example, that men’s testosterone levels are related to just about everything having to do with sex and attraction, including sexual desire, sexual function, their desired number of lifetime sexual partners, their interest in—and success with—attracting serious dating partners, and their interest in—and success with— attracting extra-pair or casual sexual partners. We see these patterns when looking at diﬀerences in testosterone levels among men. We see these patterns when looking at changes in testosterone levels occurring within men (for example, in response to changes in the time of day). And we see these patterns in response to testosterone being administered or blocked for clinical or experimental purposes. For example, clinicians regularly report that male-gendered biological females experience strong increases in sexual desire soon after beginning testosterone therapy to aﬃrm their male gender identity. The opposite is found in their female-gendered biological male counterparts taking testosterone-blocking drugs (in combination with es-

be talking about biological diﬀerences that owe themselves to the sex-diﬀerentiated nature of reproduction, which means talking about biological sex.

* Parenting/fathering is a diﬀerent story. Human men are actually one of the standouts in the animal kingdom in terms of just how much they typically invest in the care and rearing of children.

trogen) to help aﬃrm their female identity. Research conducted on nonhuman animals—such as on songbirds and rats—finds that you can turn sexual behavior on and oﬀ like a light switch simply by administering and blocking testosterone.

So, men’s primary sex hormone, testosterone, motivates all things sex, including sex (intercourse and all things intercourse-adjacent), as well as behaviors like competing for status, power, and other resources that will make them more competitive in the mating market. For example, testosterone levels predict men’s willingness to take risks (whether physical, financial, or business related), how fiercely they compete to win, their preference for wealth-signaling luxury products, and the magnitude of their displays of generosity (particularly toward partners). So, testosterone is all about sex and attraction, which is the one set of activities men’s bodies are responsible for to get their genes into the next generation.

Things aren’t quite as simple for those of us who are female. Instead of having one job our bodies need to do to get our genes into the next generation, we have two. Like men, our brains and bodies have to invest eﬀort in sex and attraction. So, we need a hormone to help coordinate all of that. But we also have to invest in embryo implantation and pregnancy. Which means we need a hormone to coordinate all of that too. Two jobs. Two hormones.

For women, the sex hormone that’s in charge of coordinating all the activities related to sex and attraction is estrogen.* Estrogen, in addition to being the hormone charged with coordinating the development and maintenance of our secondary sexual characteristics (things like breasts, hips, and pubic hair), is also the driver of how sexy we feel, how sexy we appear to others, and our motivation for sex. Estrogen is highest in the cycle during times in which sex can lead

* Although testosterone also plays a role in women’s sexual desire and functioning (like in men, it promotes sexual desire, sexual function, and the desire to win at competitions), its role is more secondary. Women’s testosterone is about twenty times lower than men’s. The big driver of all things female sex, sexiness, and attraction is estrogen.