By Sarah E. Hill, PhD

Can the use of birth control influence a woman’s sexual behavior?

Research consistently demonstrates that women are less likely than men to engage in casual sexual behavior, a finding reaffirmed by a vast body of studies. For example, in one of the most-talked-about experiments of its kind, researchers had attractive male and female actors take turns standing in a quad on a college campus in Florida.

The actors were instructed by the researchers to approach random members of the opposite sex and say in a casual tone, “I’ve been noticing you around campus. I find you very attractive.” They would then follow this with one of the following three randomly assigned requests: “Would you go out with me tonight?” “Would you come over to my apartment tonight?” or the not very subtle, “Would you go to bed with me tonight?”

According to the results, fifty percent of both men and women agreed to the date. After that, things differed fairly dramatically depending on whether the request was being made to someone who had a male or female brain. Specifically, for women, fewer than ten percent agreed to go back to the man’s apartment. And none of the women agreed to sex. Not one.

At this point, you might be thinking to yourself, “Of course no one said yes to sex. Who would do that? Only a total lunatic would request sex from a stranger, so, of course, no one would say yes to such a bizarre, apropos-of-nothing sexual invitation.” Except, eighty percent of the men did.

Most men are more sexually opportunistic than most women because sex has historically been very costly for women and far less costly for men. In fact, for men, it has come with the possibility of being able to transmit genes to the next generation without too many follow-up costs.

Being a female, on the other hand, means that sex is costly, in large part due to the rare, expensive gametes and the minimum nine-month investment, followed by lactation. Moreover, since sex can result in pregnancy, children of single mothers were historically more likely to die from all recorded causes of mortality than those whose fathers remain to provide food, care, and protection.

Nowadays, thanks to breakthroughs such as the birth control pill, women can pretty much have it all—careers, relationships, casual sex without pregnancy—quite unlike their female ancestors who weren’t quite as privileged.

That said, it’s important to understand both the advantages and disadvantages of the pill (which fundamentally changes the hormonal profile) so that women can make more informed choices about their health, relationships, and opportunities.

Hormones and Birth Control

Sex hormones such as estrogen and progesterone influence how you think, feel, perceive the world, behave, look, and smell; they affect the excitability of your brain cells, the function of your immune system, how much you eat, and nearly every other aspect of your body and mind.

In short, your sex hormones play a role in creating the version of yourself that you have come to know as you. In the context of birch control, that’s a big deal. Most women who take the birth control pill do so for the targeted reason of preventing pregnancy. A small handful of other targeted reasons include having clearer skin or knowing the exact day their next period will begin.

Targeted effects just aren’t possible when taking a hormone. Especially a sex hormone. So, although you’ll get the desired effect, it’s not strictly targeted. The hormones in the birth control pill are picked up by all the cells in the body that have sex hormone receptors.

This means that they simultaneously influence the activities of billions of cells in your body at once, from head to toe, and particularly in the brain. It’s sort of like
dropping an atomic bomb on your house to blow out a candle. Dropping a bomb on a house will blow out the candle. It’s just that its effects are sufficiently…nonspecific…to make this a fairly unpopular way to deal with one’s candle-extinguishing needs.

In the context of pregnancy prevention, one of the key ways the birth control pill works is by disrupting the hormonal cascade that triggers ovulation. However, in addition to preventing the maturation and release of an egg (the intended, targeted effect of the birth control pill), it also prevents women’s bodies and brains from doing all the other things that they’re supposed to do at times in the cycle when an egg is ready to be released.

For some women, this might be a good thing, for others, maybe not. There’s really no way to know what the birth control pill means for you without knowing who you are without it.

It turns out the hormones in the birth control pill have downstream effects on women’s thoughts, emotions, stress responses, and romantic relationships, including partner choice and sexual satisfaction. Although the research is still emerging, enough is known to make informed decisions—not just about health, but about identity and partner selection.

Preliminary research suggests that the pill influences nearly every aspect of love and sex, from attraction to relationship satisfaction. This is provocative stuff, as it suggests the pill may be reshaping modern women’s relationships in ways we are only beginning to understand.

Relationships, Mate Preferences, and Birth Control

Decades of research suggest that as estrogen increases across the cycle, so, too, does sexual motivation and attunement to cues of good genetic qualities in men. In particular, this research finds that at times in the cycle when estrogen is high—which naturally happens mid-cycle—women have a stronger preference for men whose faces, voices, and behaviors have testosterone markers (e.g, deep voices, swagger, square jaws) than when estrogen is low. Remember, estrogen loves testosterone.

But what happens, then, to women’s mate preferences when they’re on birth control? Since (a) pill-taking women don’t ovulate and (b) the artificial hormones in the pill make the brain think that it’s in the progesterone-dominant luteal phase of the cycle, this raises the possibility that birth control pills might have the ability to influence mate preferences.

In light of what the artificial hormones in the pill do (make every day the same, hormonally), it’s probably not terribly surprising to learn that pill-taking women don’t experience any cyclicity in their mate preferences. Instead of experiencing an increased preference for masculine men at high fertility, like naturally cycling women do, pill-taking women are more likely to exhibit an unwavering preference for men with less masculine faces and voices—similar to naturally cycling women during the second half of their cycles, when progesterone is high.

Additionally, research in neuroscience and psychophysics suggests that the lack of preference for genetic-quality markers on the part of pill-taking women may actually run far deeper than simply a preference for more overt masculine features—and potentially blunt sensory acuity.

For example, in one study, researchers measured women’s sensitivity to six different scents. Three of the scents—peppermint, rose, and lemon—didn’t have a whole lot to do with sex or mate choices. The other three were scents believed to play a key role in women’s ability to discriminate between high- and low-quality partners. These were a musk odorant (similar to the scent of men’s naturally occurring body odors) and two testosterone metabolites: androstenone and androsterone.

Researchers measured sensory acuity to each of these types of scents in three groups of women: pill-taking women, naturally cycling women during the periovulatory (estrogen-dominant) phase of the cycle, and naturally cycling women during the luteal (progesterone-dominant) phase of the cycle.

When it came to the non-sex-related scents, there were no differences among the groups of women. Pill-taking women were just as sensitive to the scents of peppermint, rose, and lemon as both groups of naturally cycling women were.

When it came to the sexy scents, though, clear differences emerged. The naturally cycling women were significantly more sensitive to the scent of musk and the two testosterone metabolites than women who were on birth control.

Follow-up analyses found that the differences were most pronounced when the pill-takers were being compared with the naturally cycling women during the periovulatory phase of the cycle (when conception risk was high). Women were
less sensitive to each of these scents during the luteal phase of their cycles, with their sensitivity thresholds becoming more similar to pill-taking women’s (although never reaching the same nadir).

While it may seem a little surprising that your sense of smell is influenced by sex hormones, it makes good evolutionary sense for the brain to throw all available resources into maximizing ability to discriminate between mate options when estrogen is high and conception is possible. Experiencing increased sensitivity to sensory cues at high fertility is the sort of thing that would have offered women a distinct mating advantage.

This idea that women’s brains should go full tilt at high fertility is supported by research in neuroscience, which shows that estrogen acts like fertilizer on multiple regions of women’s brains. Although we tend to think of the brain as being a stable entity that doesn’t change a whole lot over the course of weeks or months, it’s actually a very dynamic system that changes continuously. And one of the things known to be a significant player in this dynamic is the presence of estrogen.

For many key populations of brain cells—including those involved in olfaction, learning, and memory—estrogen works like Miracle-Gro, causing brain cells to sprout new connections and making them more excitable and responsive to their environment.

So, for the brain, estrogen ushers in springtime, when everything blossoms into its most exquisite form, increasing sensitivity to the environment during times when conception is possible.

When estrogen levels fall, these connections retract to their dormant state, ushering in a period of a peaceful, quiet hormonal winter. That is the beauty of hormones; to be a woman is an evolutionary success story.

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A leading researcher in the dynamic and rapidly expanding field of evolutionary psychology, Sarah E. Hill completed her PhD at UT Austin and is now a professor at TCU. With more than fifty scientific publications and multiple prestigious research grants to her credit, Hill has become an authority on evolutionary approaches to psychology and health. She has been quoted in The New York Times, The Washington Post, Scientific American, and The Economist.

Well Being Journal adapted the above excerpt from This Is Your Brain on Birth Control: How the Pill Changes Everything by Sarah E. Hill. Copyright 2019 by Sarah E. Hill. Reprinted with permission from Avery, an imprint of Penguin Random House.