Modern Wisdom #992 – Dr. Sarah Hill: The Period Brain – How Hormones Change Women’s Behaviour
Host: Chris Williamson
Guest: Dr. Sarah Hill
Date: September 11, 2025
Overview
In this thought-provoking episode, Chris Williamson sits down with Dr. Sarah Hill, evolutionary psychologist and author of The Period Brain, to unravel how female hormones affect not just reproductive health, but the entirety of women’s behavior, mood, relationships, and even society at large. The conversation explores the cyclical nature of female hormones, evolutionary rationale, implications of birth control, misunderstandings perpetuated by science and modern feminism, and practical advice for women wishing to better understand their “period brain.”
Key Discussion Points & Insights
1. Why Study the "Period Brain"?
[00:10]
- Dr. Hill’s previous book focused on birth control's impact on women. In this work, she digs into a deeper evolutionary puzzle:
- Why do many women feel “terrible” during the second half of the menstrual cycle?
- Why would evolution, a process that generally weeds out dysfunction, lead to such ubiquitous discomfort?
- Her driving question: "What is the deep evolutionary wisdom behind self-loathing that tends to go on during the second half of the cycle?" (Dr. Hill, [00:32])
2. The Female Cycle: Two Jobs, Two Brains
[01:54], [05:39]
- Biological Mandate: For women, reproduction involves two discrete jobs:
- Attract/choose a mate and have sex (estrogen-dominated, first half)
- Support implantation and potential pregnancy (progesterone-dominated, second half)
- Cycle Phases:
- Follicular Phase (Day 1–14): Estrogen rises → increased energy, sexual desire, sociability, goal pursuit.
- Luteal Phase (Day 15–28): Progesterone rises → increased hunger, sleepiness, caution, and emotional sensitivity.
- Progesterone: The "forgotten hormone"—coordinates pregnancy preparation, modulates brain activity ("taps the brakes" on motivation and risk-taking).
"Women are a somewhat different version of themselves during the last two weeks of the cycle than they are during the first two weeks."
— Dr. Hill [04:50]
3. Adaptive Reasons for Hormonal Shifts
[12:27], [30:53]
- Estrogenic State: Orients women toward finding high-quality partners—heightened energy, discernment, sexual attraction.
- Progesterone State: Shifts toward energy conservation, threat sensitivity, and social vigilance to optimize for possible pregnancy.
- Increased motivation to "nest," maintain close bonds, and avoid unnecessary risks.
- Women’s heightened threat detection, both for external dangers and relationship stability, is evolutionarily functional.
- Food & Metabolism: Basal metabolic rate increases (by 8–11%) during the luteal phase, leading to real, biologically-driven calorie cravings.
4. Heightened Sensory and Social Abilities During High Fertility
[13:36], [17:16], [21:23], [25:22]
- Mate Choice:
- Women are hyper-attuned to subtle cues of male genetic quality (e.g., testosterone-markers) during the fertile window.
- Fascinating studies:
- Scent discrimination: Men find women’s odor more attractive during high fertility; women can discern more about men's testosterone levels by smell and by facial subtlety.
- Stripper earnings: Dancers earned more during peak fertility, less on the pill (functioning as a “free-of-cost hotness surge”) [20:20].
- Movement perception: Even silhouettes of women during high-fertility are rated more attractive.
- Sexual Motivation: Increases near ovulation, confirmed in longitudinal within-women studies.
5. The Luteal Phase: Why “Misery” Might Be a Mismatch
[34:05], [36:21]
- Shift in Social Energy:
- Outward-facing, gregarious drive gives way to introspection, threat vigilance, and a desire for nurturing relationships.
- Even in baboons, females tend to withdraw socially post-ovulation: "They sit up in a tree...I identify with that baboon."
- Pair Bonding:
- Sex, though less frequent, is more about maintaining intimacy than raw desire.
- Non-conceptive sex during the luteal phase fosters emotional bonding; akin to prairie voles and other monogamous mammals.
“Luteal phase sex is probably different...Being able to explain to your partner, like, yeah, you know, last week a really hot quickie right before my afternoon meeting was exactly what I needed. That would hit me the wrong way right now.”
— Dr. Hill [40:08]
6. How Modern Medicine and Science Have Misunderstood Women
[04:32], [46:31], [90:51]
- One-Size-Fits-Men Model: Historically, research subjects were male, and when women were included, it was only during the hormone-low window.
- Consequences:
- Mismanagement of women’s health
- Poor medical recommendations
- Increasing evidence of side effects and drug responses varying by cycle phase
- Feminism and Denying Biology: The push for equality should not mean denying meaningful sex-based differences, which has led to further neglect in understanding female experience.
7. The Pill: Flattening the Cycle—and the Self?
[59:32], [62:11], [64:21], [67:29]
- How the Pill Works: Mimics progesterone; suppresses ovulation and cyclic hormonal rhythm, putting women in a sort of hormonal “limbo” (progesterone dominant, but not true progesterone).
- Key Impacts:
- Suppressed libido, blunted emotional and sexual peaks and valleys.
- Progestins (synthetic) do not yield the same calming brain metabolites as natural progesterone.
- Strongly associated with increased risk of depression, especially in teens (up to 40%) and reduced relationship/sexual satisfaction.
- Dr. Hill argues: "On the pill, you’re not really yourself...it's masking an authentic part of yourself." [67:29]
- Macro Social Effects: Suggests birth control may contribute to the decline in men’s testosterone by muting cues of female fertility in society.
"Could oral contraceptives be the biggest unexamined mental health experiment that's ever happened?"
— Chris Williamson [68:15]
"Yeah, I think so."
— Dr. Hill [68:21]
8. Cycle Awareness: Cultural, Legal, and Social Implications
[83:43], [85:01]
- Medicine, law, and even “reasonable person” legal standards are based on a male reference point.
- Ignoring sex differences leads to negative ramifications for women, from ill-fitting seatbelts to misdiagnosed medication side effects.
- Push for a “new new feminism”: Embracing, not denying, sex differences as real and meaningful—without assigning value judgements, or fitting women into a broken, male-centric mold.
9. Practical Advice: Tracking and Harnessing the Cycle
[95:32]
- Dr. Hill encourages women to track their cycles for at least 2–3 months across relevant variables (energy, mood, cravings, libido, etc.):
- Identify your unique hormonal pattern—some women may feel best at unexpected times.
- Adjust lifestyle (nutritious eating, rest, self-care, exercise intensity) according to cycle phase.
- Better communication—with partners and with self—can enable smoother, more compassionate hormone transitions.
Notable Quotes & Memorable Moments
-
On self-understanding:
“Just start tracking your cycle and how you're feeling on things that are important to you...track that over the course of a couple of cycles to start to understand your relationship with your hormones.”
— Dr. Hill [95:32] -
On sex differences in research:
“Eight out of ten prescription drugs are pulled from the market during the first year of use because of unanticipated side effects in women, because we're not being tested enough.”
— Dr. Hill [81:48] -
On cultural narratives:
“We’ve all been led to believe that the path to our freedom...is to deny that biological sex matters...But the fact is, when we deny that, then...we have a bunch of medical research that has been tested on men and misapplied to women.”
— Dr. Hill [81:09] -
On biological design:
“If you tell me a woman's age and when she had the first day of her last period, I can tell you with a pretty decent degree of accuracy what's happening with her primary sex hormones. Whereas with men, I couldn't do the same thing at all.”
— Dr. Hill [54:19] -
On male expectations:
“There is this assumption...that whatever men do or can do or tend to do is the desirable thing or the set point...it is desirable to do what the men did. And the fact that we don't needs to be addressed so that women can do what the men did, not the other way around, that what the women do is in some way superior.”
— Chris Williamson [85:50]
Key Timestamps
- Introduction & Evolutionary Puzzle: [00:10–04:32]
- Functional Hormones & Phases: [05:39–12:27]
- Adaptive Reasons & Discriminative Senses: [12:27–17:16]
- Research Evidence & Replication Debates: [22:12–25:22]
- Behavior Shifts, Social Sensitivity: [30:53–36:21]
- Pair Bonding and Non-conceptive Sex: [36:21–43:49]
- Impacts of Modern Life on Cycling Women: [43:49–50:05]
- Comparing Male vs Female Hormonal Influences: [51:22–56:08]
- Birth Control, Authenticity, and Side Effects: [59:32–68:15]
- Societal and Macro Implications: [70:42–79:32]
- Sex Differences, Feminism, and Mismanagement: [81:09–90:51]
- Why Science Ignored Female Biology: [90:51–95:32]
- Practical Steps & Final Thoughts: [95:32–98:46]
Resources & Where to Learn More
- Dr. Sarah Hill’s website: sarahehill.com
- Instagram: @SarahEHillPhD
- Her books: This Is Your Brain on Birth Control & The Period Brain
Summary
This episode is a clarion call to re-examine not just how we think about women’s hormones, but how we construct science, medicine, and even workplaces. Dr. Hill’s message is clear: cycling hormones aren’t a design flaw, and honoring them unlocks more effective, healthier, and more authentic womanhood. This nuanced, evidence-driven discussion will challenge listeners—of all genders—to reassess the hidden, pervasive impact of the "period brain" on our world.