Sarah Hill: Most Replayed Moment: Would You Still Love Them If You Came Off The Pill...? Your Hormones Are Controlling You!
Guests: Sarah Hill
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I was reading in your book about how mating and attraction preferences change for a woman throughout her cycle. So, throughout a woman's 28-day cycle, does she always want a "nice guy"?
No. Women's partner preferences and the types of qualities that women tend to prioritize in their choice of partners do change across the menstrual cycle. This changes with our sex hormones.
During the early part of the menstrual cycle, both sets of sex hormones—estrogen and progesterone, which is our second primary sex hormone—are low. Then, as estrogen begins to increase, which is something that happens as eggs start to mature in the ovary, they release estrogen.
As a dominant egg is chosen and begins to fully mature so that it will be released at ovulation—
Which is how many days into the cycle?
Usually, estrogen really starts to surge around day nine of the cycle, but it really hits its high point usually right around day 14, which is when ovulation occurs.
Estrogen begins rising sharply around day nine of the cycle. Again, the first day of the menstrual cycle is the day that your period arrives. So, about a week and a half after your period started, most women will start to have a pretty dramatic increase in estrogen.
Estrogen will climb, climb, climb, climb, climb, and it will peak around day 14, which is when ovulation occurs. This is when an egg is released and pregnancy is possible from sex.
So from day nine until day 15?
Until day 15. Day nine to day 15 are usually when we bookend the fertile window. During this time, not surprisingly, women experience a change in their sexual psychology.
Women become increasingly attuned to all things sex. What we tend to see is that women's sexual desire increases right around that time in the cycle. Women have more sex during that time in the cycle, women masturbate more during that time in the cycle, and women become more exacting about who it is they desire as a romantic partner or a sex partner.
In particular, what we tend to see is that as estrogen is increasing across the cycle, so too does women's interest in cues that historically have been related to high genetic quality or genetic compatibility within their choice of partners.
Two of the traits that we know to be particularly pronounced in terms of women's interest near high fertility in the cycle are cues related to testosterone and masculinity.
The reason for this one is because testosterone is a known immunosuppressant, meaning that it has the effect of suppressing the immune system. People who have relatively high levels of testosterone—this is a cue that their bodies are in such good physical condition, their immune system is so strong and robust, that it can handle being suppressed by testosterone.
People who have higher testosterone are believed to have greater immunocompetence relative to people with lower levels of testosterone. What we tend to see is that, lo and behold, during that peak fertility time in the menstrual cycle when estrogen is high, women exhibit a greater preference for cues related to testosterone in men.
We find, for example, that women desire more masculinized male faces, voices, and behavior at high fertility in the cycle compared to low fertility in the cycle.
How do you test this?
You test this by looking at what it is that women are looking for and you look at it over time. Just to give you an example of one of the most robust studies that's been done looking at this phenomenon, researchers had a group of women bring home a bunch of little plastic tubes and a freezer bag, and they had them spit into these little saliva collection tubes every day of the cycle across two menstrual cycles.
On those same days when women were taking a saliva sample, they were also evaluating the attractiveness of male faces. The thing that the women didn't know is that the researchers knew the testosterone levels of the faces of the men that they had them looking at.
The researchers had men come into the research lab beforehand and took their morning testosterone levels over the course of several days to get an average morning testosterone level for these men.
When they got all the saliva samples from the women, they assayed all of their saliva and they were able to look at women's estrogen changes across the cycle and then how it related to their facial preferences for men.
What they found was first, that of course, estrogen tended to increase and peak during this fertile window—days nine or so to day 15. Women's estrogen levels were high.
During this time, what they also found is that women's preference for testosterone levels in the faces of the men they preferred also increased. There was this really beautiful overlap between women's estrogen levels and their preference for testosterone, going to show that estrogen loves testosterone.
Women during this estrogenic phase in the cycle have a heightened preference for cues related to facial masculinity. They also have a greater preference for vocal masculinity—women at high fertility in the cycle also like the sound of more masculinized male voices. They tend to like more social dominance in terms of behavior relative to what we tend to see when women are at low fertility in the cycle.
So I'm more likely to get laid if I— (Deepens voice)
There you go. I think that voice right there that you did is going to get you all the girls. I would try that one out at the bar tonight.
This is your book, *This Is Your Brain on Birth Control*. Why did you choose to write this book?
I chose to write this book because I was on hormonal birth control for more than a decade of my life. I'm a psychologist who studies women and women's brains, and I had absolutely no idea that any of the things that the birth control pill does to the brain were happening to me.
I went off of hormonal birth control after being on it for more than a decade straight. When I went off it, I felt like I woke up. It was like I had this moment, about three months after I discontinued it, where all of a sudden I realized that over the last three months, I'd been feeling things more deeply.
I'd been interested in sex in a way that I hadn't been in a long time. I was exercising and going to the gym again. I was downloading new music onto what was then an iPod for the first time—I know, it's like an archaic iPhone, people.
I was downloading new music onto my phone, my iPod. I was cooking again. I was just interested in pleasure. I had more energy. I was feeling things more deeply, and I thought, "What is this?"
During the time that I was on birth control, I never had any issues with it. I wasn't one of those people who had negative mood-related side effects, and I didn't know that I was experiencing any side effects at all.
It was only after I went off of it and started to realize how much more alive I felt that I started to dig into the research behind the way that hormonal birth control changes women.
It was then that I learned that people had been studying this, in some cases for 30 years—the way that hormonal birth control changes the way that women think, feel, and experience the world.
It was then that I knew that I had to share this information with other women because here I was as somebody who is on the pill, studies women, studies women's brains, I'd even published papers on the effects of women's changing hormones over the cycle and the way that they influence women, and it never even occurred to me that my birth control was changing me.
I knew that if I didn't know that the birth control pill was changing me, that nobody else knew it either. That led me to dig into the science behind hormones, hormonal birth control, and how that changes women, and I put together that book to tell everybody the things that I learned.
Pretty dangerous subject matter to take on.
It can seem like a dangerous subject matter to take on, and for good reason. Hormonal birth control and having a reliable way to prevent pregnancy has been by far the biggest game changer that women have ever experienced in history.
There's been nothing that has been more instrumental to women's ability to be able to achieve political and economic independence from men than the birth control pill. Because of this, saying anything that is critical of the birth control pill can be seen as very dangerous. It's like, "Oh no, don't say that too loudly or else they're going to take our birth control away," or "Oh no, don't say that too loudly, what else are we going to do?"
But this book was really important to me to be able to present to women all of the different ways that hormones influence us, and the surprising ways that hormonal birth control can change who we are and the way that we experience the world.
Then I give that information to women so that way when they're making decisions about whether to be on or off of hormonal birth control, they have all of the information about the tradeoffs that they're making. This will allow them then to make decisions about whether or not to be on it depending on what's going on in their own lives.
Everything that I've learned about the way that the birth control pill changes women means that the answer that you come to when you're asking yourself the question, "Do I go on this or do I not go on this?" is going to be very woman-specific. For some women, the decision is still going to be, "Yes, I'm willing to make these tradeoffs," and for other women, the answer's going to be, "No, I am not willing to make these tradeoffs."
Where we land on that is going to differ depending on where we are in our life and so on and so forth. It was really important to me to present this information as a way to really empower women to be able to make informed decisions about who they want to be and how they want to regulate their fertility.
What do you wish you were told? On that day when you were first given that little packet of pills, what do you think they—if they were being completely honest with you and they knew what you know now—what is the list of things you would have said to a young Sarah about this decision?
To me, the one thing—and then we can kind of dive down into what this all means—but the thing that would have really made a difference to me is that your hormones make you who you are, and when you change your hormones, you change who you are.
The birth control pill is going to change you. It changes the version of yourself that your brain creates. So if you're going to be going on this, you need to understand what that does.
This, of course, begs the question: what does it do? There are several things that the birth control pill does, and there are at least five different things that the birth control pill does to change who we are.
It changes our sexual desire and who we're attracted to. It changes our emotional states—it can affect our moods. It can affect our ability to regulate and to manage stress. It influences sexual desire and sexual function. It influences our ability to be able to put on muscle mass and affects our nutrition and fitness goals. It affects all of us.
Not knowing all of that, and not knowing the potential risks of having side effects related to mental health and libido, all the way to who a person is attracted to—I wish I would have known that. That probably would have impacted decisions that I would have made about staying on it when I didn't need to, because there's no reason to change who you are if you don't need the contraceptive benefits.
So on that first point then, it changes who you're attracted to, your sexual desire. How?
Mechanistically, the way that this happens has everything to do with the hormonal changes that are initiated by the birth control pill. Just to talk about the mechanics of this first and then talking about the downstream effects of it next—mechanically, what goes on when you take hormonal birth control is you are suppressing ovulation.
The way that birth control does this is it does this by giving you a combination of hormones or a single hormone that tells your brain not to stimulate the ovaries and not to ovulate. It does this by way of a synthetic progesterone called a progestin.
Progestins, which are in all forms of hormonal birth control, when they get picked up in the hypothalamus in the brain, that sends a signal that the brain should not stimulate the ovaries and should not lead to egg maturation and egg development.
When this happens, when the brain is not stimulating the ovaries, this means the body is not producing estrogen. Estrogen levels are kept really low, and instead, you get this daily dose of a synthetic progesterone or progestin that is supplanting that.
Many forms of hormonal birth control, in addition to having that synthetic progesterone or progestin that turns off the brain's communication with the ovaries, also has a relatively low level of estrogen that is also added. Combination hormonal birth control pills contain progestin and a little bit of estrogen, but the estrogen levels are kept really low and the progesterone levels or the progestin levels are the dominant hormonal message you get, that same hormonal message every day.
When you do that, what happens when you flatline a woman's own production of hormones and replace them with a daily dose of synthetic progesterone—this does a few things to women's sexual psychology.
The first thing that it does is it turns off that estrogen surge that you get right prior to ovulation that's related to a heightened preference for sex. That estrogen surge is related to an increased preference for testosterone markers in mates. It's related to women's interest in sex; it's related to sexual function.
All of those things that increase right prior to ovulation that help to promote reproduction, sexual reproduction, all of those things are turned off on women on hormonal birth control because instead, they're getting the same daily message in which progestin is the dominant hormonal message and estrogen levels are really low.
But they're still horny, right? They still—
Yes, I mean they still will have sex and they still want sex, but many women report experiencing a real decrease in their libido when they're on hormonal birth control. That's a very common response.
The reason for this is twofold. One, one of the things that's a big factor in promoting women's sexual desire and libido is estrogen. As estrogen is rising in the cycle, because when estrogen is rising this is a time in the cycle when sex can lead to conception, this is something that's known to fuel the female sexual response.
So women's sexual response and their sexual desire all increases with estrogen levels across the cycle. When you take hormonal birth control and that's flatlined, which is what it does, this means that sexual desire is kept more constant across the cycle. So you do tend to find that women who are using hormonal birth control tend to have fewer peaks and valleys in sexual desire than naturally cycling women do, but on the whole, it tends to be lower.
The other thing that happens on hormonal birth control that can lead to a decrease in libido is that all of those synthetic hormones that are in hormonal birth control, they tend to lead to an increase in what's known as Sex Hormone Binding Globulin, which is a real mouthful.
But what it does is that this is something that's released by the liver and it binds up free testosterone. It binds up testosterone and makes it inactive in the body. Testosterone, even though we tend to think of it as a guy thing and this is a male hormone, it's actually really important in terms of promoting women's sexual desire.
Women who are on hormonal birth control have levels of free testosterone that are about 60% lower than that of their naturally cycling peers. What this means again is that you have another blow to women's sexual desire when they're on hormonal birth control. Those low levels of estrogen and then really low levels of free testosterone, those two things work together to suppress sexual desire in women.
It can also change attraction because as we noted, when women are approaching ovulation, their estrogen levels increase. This is something that research finds to be related to women's interest in testosterone markers in men.
Researchers more recently said, if that's true, what happens when you put women on hormonal birth control and estrogen levels are kept really low? What they find is that, lo and behold, women who are using hormonal birth control do seem to have a dampened preference for cues related to masculinity and testosterone in partners relative to what's observed in naturally cycling women.
Which specifically means that a woman on birth control is less likely to be interested in a guy who is what?
A guy who has really masculinized features. For example, if you see a face that has high levels of testosterone, generally men will have more deep-set eyes, they tend to have a wider jaw. In the body, they tend to have broader shoulders and a more narrow waist. Men with more deep voices, men who have behavioral cues to social dominance—that's also something related to testosterone.
What the research finds is that women who are using hormonal birth control do prefer a less masculinized male face relative to what is observed in naturally cycling women.
I mean this is slightly concerning if you're a man, right? Because if my partner's on the pill and then she comes off the pill, she might not like me.
Right, yeah, no, and that's a real important question. It's very provocative because what does that mean? There have been some research studies that have looked at that exact question. One of the best studies that's looked at this question is one that was a longitudinal study of married couples.
They followed women who were using hormonal birth control at the time that they met their partner, and then they just followed them over time to see what would happen when women discontinued the pill. What happened when they discontinued was that there were real significant changes in how attracted women were to their partner depending on whether they chose them using hormonal birth control.
But what was interesting about it is whether the women became more or less attracted to their partner depends on how attractive their partner was. What the researchers found is that women who were partnered to attractive men, when they went off hormonal birth control, they were more attracted to their partner and were more sexually attracted to their partner and more sexually satisfied within their relationship relative to where they were beforehand.
But for women who were partnered with less attractive partners, they had the opposite effect. So when they went off hormonal birth control, they became less attracted to their partners and reported being less sexually satisfied with their partners than where they were beforehand.
This suggests that, yes, it is a very real possibility that if you have a partner who chose you when you're using hormonal birth control, that this can change attraction within the context of the relationship. But whether that's good or bad probably depends on a bunch of other dimensions that you need to take into effect, including partner's attractiveness.
So if you're not a very good-looking guy, you probably want her to stay on it.
Run for the hills! No, I mean, honestly, it's one of these things where you can really quickly become alarmed with this. The good news is this, and that is: most women who choose their partners when they are on hormonal birth control go off of hormonal birth control and then there's really not a huge shakeup in their relationship.
The reason for this is the way that hormones influence us is they kind of nudge us a little bit one way or the other. They kind of nudge us toward preferring this type of mate or that type of mate. It's generally not these big sweeping changes where all of a sudden it's like the wool comes off your eyes and you're like, "What in the world was I thinking?"
These are little nudges. It's just that for some people who are teetering on the edge of not necessarily being attracted to their partner, going off hormonal birth control can tilt them out of attraction. The same is true for—I've talked to people who've had the experience of feeling like they were lesbian and all of a sudden they feel like they're bisexual, or women who were bisexual and then they go off of the pill and all of a sudden they realize that they're not interested in women anymore.
I mean it's like sexual orientation, attraction of all sorts—whether it's who you're attracted to in terms of the sex you're attracted to or who you're attracted to in terms of the specific partners that you're attracted to—all of those things are influenced by sex hormones.
When sex hormones kind of nudge you this way or the other way, depending on where you fall on that distribution of where you are in terms of sexual orientation or "Am I attracted to this mate or that mate?", those people who are on the tails can get nudged into a place that puts them into the uncomfortable situation where they realize that they're no longer attracted to the person they chose on the pill.
As a man, is my attraction going to change if my partner comes off the pill?
So there is evidence that suggests that women are more attractive to men when they're naturally cycling relative to when they're on hormonal birth control. Chances are if you have a partner that you were attracted to when she's on hormonal birth control, when she goes off of it, it's actually going to lead to an increase in attraction.
There's a ton of research that finds that men find women sexiest—they think that they smell better, they think that they look more attractive—men will tip women more at strip clubs when the women are at high fertility across the cycle relative to low fertility across the cycle, meaning that men are keyed into estrogen levels.
How?
Scent, visual appearance, and even just the way that women act and move. They've done studies where they have men looking at women moving, just seeing their silhouettes move behind a sheet. They'll see women walking or dancing at high or low fertility across the cycle, and men find the women more attractive when they just see their outlines moving at high fertility compared to low fertility.
Men's brains are wired to pick up on estrogen cues, and this makes perfect sense when we consider the evolutionary process that designed us. Because over the course of evolutionary history, men who were really dialed in to women's estrogen levels, they would have passed down more genes than men who didn't really pay attention to those cues.
Estrogen across the lifespan is linked with fertility, and estrogen across the cycle is linked with fertility. It's linked with fertility no matter which way you look at it. Men who are really keyed into estrogen levels, they would have done really well.
So if you are a man, your partner chose you when she was using hormonal birth control and now all of a sudden she starts cycling again, my guess is that most men will experience an increase in attraction to their partner, particularly at high fertility in the cycle, because there's so much research that suggests that this is exactly what should go on.
Here's one where I put a few dots together. If my partner is on the birth control pill, are my testosterone levels going to be lower?
That's a fascinating question and it's one I'm super interested in. I actually just had this conversation not that long ago with my graduate students in my lab. There is research that suggests that men's testosterone levels increase in response to the scent of fertile women.
When women are at high fertility, if men sniff t-shirts that they were wearing—or in one study they actually had men sniffing the air that was being piped through a nebulizer that had a woman's panty liner in it that was either worn at high or low fertility. Smelling women's vaginal secretions, both of those things have been shown in research to be related to an increase in testosterone levels in men.
So it increases men's testosterone levels to be around these cues to high fertility. What happens then when men are spending their time or are partnered with somebody who's on hormonal birth control? Given that estrogen levels can increase testosterone, does a lack of estrogen presence lead to lower levels of testosterone on average?
This is a question that we do not have a research answer to, but it's one that's incredibly fascinating because here we are, 2024, almost 2025, and we have a testosterone crisis on our hands. Men's testosterone levels are much lower than what they used to be even 50 years ago.
It's possible that one contributor to this is the fact that so many women are using hormonal birth control. When you have, on average, lower levels of estrogen in the population, might this then also predict lower levels of testosterone in men? The answer to that is we don't know.
Another possibility with that, by the way, and one that I think is so interesting—we talked about kids and we talked about the fact that men's testosterone levels will decrease when men are in long-term relationships and then they'll decrease a little bit more when men have children that they're caring for.
Another possibility for men's lowered levels of testosterone, in addition to all the trash that's in the water and all the chemicals that we're exposed to, is the fact that men are now responsible for more caregiving than they ever have been in history.
With many women being in the workforce, men are having to also do more in the home than they ever did before. It's also possible that some of the differences that we see in testosterone levels that have changed over time may be the result of men's bodies actually decreasing testosterone production in response to their changing roles in the home, which is also sort of a fascinating possibility.
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