“I THINK I’LL GO into wrestling. Blood, sweat, and fighting, that’s me.” I looked at my seven-year-old daughter as we left the playground, that hotbed of jungle activity after school. “Did something prompt this?” I pressed. My child has never expressed an inclination toward blood and sweat, and fighting is something she has historically interpreted as a verbal activity. We entered our destination, the grocery store, and she jumped on the rolling basket so she could ride it from the outside. Then: “It’s just — why are boys so … boyish? What makes them that way?”

As it happened, I had just finished Randi Hutter Epstein’s chatty and absorbing book, Aroused: A History of Hormones and How They Control Just About Everything, out in paperback from Norton. And that was how the old ladies examining the plums at Zabar’s ended up hearing a mother say, “Well, it starts with androgens. When an embryo is XY, it usually responds to hormones in the mother’s blood supply that tell it to grow testes and a penis. Do you want sweet potatoes for dinner? Later on, other hormones come into play.”

Hormones have been on my mind quite a bit of late. Earlier today I sat before a sea of papers, cross-hatched piles of notes and charts. Laundry remained unfolded on the couch. I wanted to eat a pound of cheese for lunch, and no sooner did something occur to me than I had forgotten it, lost in a fog of distraction. Put simply, I have PMS. I begin counting the days at this time, waiting for the near bacchanalian delight of my monthly blood flow. Oh, the release! Oh, the energy! The depression lifts, and I coast into clear-thinking waters! I will no longer study two pages of a newspaper article only to look up and say, “What the hell have I been reading?” I will follow things again — schedules, TV show story lines, a logical order of operations for housework. All this will be from a change in hormones. Two weeks later, my husband and child will marvel at how I’ve vacuumed the living room rug in a perfect pattern, scrubbed the bathtub to a gleaming shine, alphabetized the books by author and arranged them by subject. That’s ovulation. Then I will begin the downward slide once more to PMS. Hormones. They do seem to control everything.

Aroused begins at the turn of the century, when scientists tinkering in the lab began to discover hormones, and how they differ from neurotransmitters: if our nervous system is a highway of linked connections, our endocrine system is what Epstein calls “your wireless network.” Where the book really takes off is in its pointed examination of how social norms and sexual politics have interacted with new discoveries in science. Dr. Louis Berman declared a new field in the 1920s, which he called “Psycho-Endocrinology.” He went on to write a book called The Glands Regulating Personality, in which he suggested that women with irregular periods “will also be aggressive, dominating, even enterprising and pioneering — in short, [they have] masculinized ovaries.” To think: A woman has no chance of being enterprising if she bleeds like clockwork every month! There are plenty of other historical citations here that may make you laugh or cry. In 1924, two doctors, Harold Hulbert and Karl Bowman, from Boston Psychopathic Hospital — yes, that was a real place — were called in to examine Nathan Leopold and Richard Loeb, two teenagers being tried for what had been dubbed, “The Crime of the Century.” They had kidnapped, tortured, and murdered a 14-year-old boy. Other than depravity, there was no motive, but because the case converged with the burgeoning era of endocrinology, a shocked public and an eager defense attorney named Clarence Darrow sought answers in the new field. During the examination, which took eight days, reporters crouched outside the prison. Then Hulbert took the stand to declare Loeb suffered from “multigland syndrome” and Leopold had, among other pathologies, a calcified pineal gland, which accounted for his excessively high libido. Their chief instruments were a metabolimeter and an X-ray machine (the theory being that bones are pushed aside by oversized glands and would therefore show up on X-ray). The judge ultimately decided that it made no difference whether the endocrine experts were right or not. Murder was murder, and criminals could not seek absolution in their biochemistry.

It’s a problem that continues to fascinate us as a society: Does psychopathy excuse certain behaviors? Does it make a crime more or less understandable? Many experts debated the nature of notorious serial killer Ted Bundy’s character — was he a classic psychopath? — and felt they had zeroed in on a crucial question. But had they? Was it just about semantics, or were there important questions of science, treatment, and punishment on the line? Certainly it is interesting to see how the discovery of hormones kicked off a round of this debate, and it is gratifying to see a judge apply a moral code equally.

The most moving chapter of Epstein’s book follows the story of a baby named Brian Sullivan at birth in 1956. The baby was labeled a “hermaphrodite” in his earliest hospital records: he appeared to have both penile and vaginal tissue. At 18 months, the doctor — rather drastically, by today’s standards — performed an exploratory surgery and discovered the child had a vagina, uterus, and ovaries in his abdominal cavity. Without consulting Brian’s parents, he amputated the clitoris and then instructed them to begin raising the child as a girl. Brian became Bonnie. Wardrobe, activities, even room color were switched to the conventional “pink for girls.” And Bonnie promptly stopped talking. As Epstein reminds us, it was a dark time for people born with atypical genitalia. There was very little understanding of the complex processes underlying the hard-wiring of gender identity. Discovering a uterus inside a child did not, in fact, entail the discovery of the child’s gender identity. Later, after further research, it would become clear that an initial spurting of estrogen or testosterone into the bloodstream during pregnancy likely shaped at least some of this hard-wiring. So whatever disrupted Brian’s genital development may also have shaped whom he or she felt herself to be gender-wise. Bonnie’s life was again disrupted medically at eight when she was operated on to remove genital tissue from her abdomen. She was told that it was for her “stomach pains,” though she reports not having any pain at the time. She was admitted to Columbia Presbyterian Hospital in 1964, where photographers took photos of her naked, and where she endured pre-operative pelvic exams. “The fingers in her vagina and anus were mortifying. She felt like a freak,” reports Epstein, who met Bonnie, now called “Bo,” years later. It was many years before Bo unearthed her medical records, finding the crude designation “hermaphrodite” among the other abuses she endured. Eventually, she founded the “Intersex Society of North America,” in the hope of connecting with other people who had suffered similar sorts of mistreatment and aggressive interventions, and consequentially endured lives of loneliness and confusion. She also wanted to inform doctors on how better to manage intersex children. Soon, letters from lonely and discouraged intersex people flooded her post office box. A support network sprang up.

Aroused shows how hormone and surgical therapies hardened a binary gender system. “The data,” however, “were showing a much more complex picture of humanity,” writes Epstein. Christine Jorgensen, born George Jorgensen in 1926, transitioned at 26 from male to female with the help of surgery and hormones. The Daily News broke the story in 1956 and, before long, the Danish doctor who performed the surgery was inundated with requests for help. Americans turned to Dr. Harry Benjamin, an endocrinologist specializing in gender and sexuality. It was Benjamin who would clarify the notion of hard-wiring of the fetal brain toward gender identity as being dependent on hormonal influences in the womb, and would go on to write The Transsexual Phenomenon. (Transsexual is an outdated word, and has since been replaced with the more accurate “transgender.”) He, in turn, referred patients seeking surgery to the Johns Hopkins Gender Identity Clinic. Epstein navigates this topic well, focusing on the complexity of the issue. “[W]hat causes transgender identity in one person may not be the same as in another,” she writes.

Ultimately, the message of Aroused is that we know a lot about hormones, but definitely not everything, which might well have been its subtitle. Enter the quacks, frauds, careless researchers, and others looking for quick payoffs from new discoveries. In nearly every chapter, Epstein stresses the vulnerability of new scientific information about the maintenance of the human body to industry objectives. Hormones were especially vulnerable since they seem to regulate just about every major process, from growth to metabolism, sleep cycles to sex, parenting to the immune system, stress to lactation. From the “revitalizing vasectomies” and “epinephrine suppositories” of the 1920s, to the oxytocin supplements currently being sold as enhancers of both arousal and intimacy (despite no strong evidence that the hormone can be metabolized with a sublingual dose and despite the precise function of the hormone not being known), companies are eager to make a buck by turning new science into ill-gotten profit. Just as we are vulnerable to well-meaning doctors, we are vulnerable to our desires for treatments and cures.

Hormones are tiny drops in a very big ocean, interacting in complex ways with nerve cells and other chemicals in the human body, Aroused warns. Not only do we need to strive for more knowledge but also for accurate testing of that knowledge. We may have come far from the dark days when Blanche Grey, “The Fat Bride,” was put on display in the New York City Dime Museum — for suffering some type of thyroid or pituitary malfunction — or indeed from the days of nonconsensual clitoral amputation of the sort Bo Laurent endured, yet our society still struggles with accepting just how varied our gender spectrum is. My father, who is 80, says he has no trouble with understanding different sexual orientations, or transgender identity, but “nonbinary” is a bridge too far. Meanwhile, my daughter is growing up in a world where people are claiming the pronouns they feel best suit them, and she may be surprised to learn that there was ever a need for adjustment to the notion of a spectrum. We are currently reading A Wind in the Door, the sequel to A Wrinkle in Time, in which Madeleine L’Engle introduces the concept of more evolved creatures who communicate with one another by “kything,” which allows for a more complete understanding of another’s interior world. This strikes me as an apt metaphor for humanity’s expanding definitions of gender identity as science catches up to human experience. At their best, the biological sciences can bring us closer to understanding and helping one another, introducing our differences not as obstacles but bridges.

And then there’s menopause. “Mid-Life Psychosis,” screamed a New York Magazine cover one week in December last year. “45, female, and hearing voices.” The article examined a link between fluctuating hormones — specifically a drop in estrogen — during menopause, and late-onset schizophrenia. The author admits she was at first reluctant to take the topic on (she did not want to be responsible for furthering the trope of the crazy menopausal lady). But, she continues, it became clear that women were once again an underserved community: how hormones affect women’s minds and bodies is little understood because it is little studied. I am 44 myself and, while not yet in menopause, have begun to notice small changes. I still menstruate every month, but I find my moods a little less like clockwork, a bit more unpredictable. I notice increased restlessness, as well as a driving need for solid exercise every day to calm and center me. I notice a decrease in the symptoms of PMS, but find that those blue moods and sudden cramps might take hold of me at other times in the month. Naturally, I was both horrified and fascinated by the possible link between menopause and schizophrenia. Something is definitely happening, I thought. Please let them figure it out. A recent article linking cognitive decline and menopause made the rounds on Facebook, and I eagerly jotted down what I learned, hoping to glean tips on how to reduce my evidently gender-determined chances of developing dementia. I’ve already added Darcey Steinke’s new book on her own experience with menopause, Flash Count Diary, to my to-read-immediately pile.

Epstein’s dry wit is put to best use in her chapter on menopause: “Some women skip through the whole experience: their periods stop and that’s that. No erratic temperature changes, no mood swings, no brain fog, libido as good as ever. To those women, the rest of us must seem like cranky bitches.” I know how deeply affected my brain is by hormones, not only because of my dramatic monthly shifts and symptoms, but also because I experienced such a wrenchingly awful pregnancy that the memory never leaves my mind. At only six weeks, I began crying when the sun went down. At eight or nine weeks, my uterus began contracting, leaving me breathless, and I developed a painful rash. I spent much of my pregnancy deeply depressed, and I knew, even after I had seen my beautiful child, that I could never do it again. So, I expect to be one of the cranky bitches. I also want to be armed with knowledge. The trouble is, there’s conflicting data. Is it the decline in estrogen that makes women more likely to have heart attacks, strokes, dementia, osteoporosis, or is the reality more complex? Epstein reveals that studies don’t show any benefit to long-term hormone replacement therapy — that is, no reduction in these types of illnesses. However, she does offer some encouraging news: a few years of hormone therapy to ease the symptoms of menopause is unlikely to hurt you. The establishment has been back and forth on that point a few times, and, she acknowledges, who knows? It may swing the other way again. We have to muddle through with the research we’ve got at the moment. If that isn’t exactly comforting, Epstein’s writing so colorfully about the subject certainly is. I felt I had a friend guiding me through the terrifying waters of my own biology. In the meantime, I’ve got to go teach my daughter about estrogen. I sure hope she appreciates her generous stockpile of the mysterious but clearly crucial hormone.

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Leslie Kendall Dye is a freelance writer and actress based in New York City.