Blurb/Description: This book explores the science of testosterone (T), providing an extremely detailed and nuanced yet readable journey through its complex and often oversimplified role in shaping male and female biology, physiology, and behavior.
Summary: “A female scientist decides to specialize in researching testosterone” sounds like it could be the setup for a joke, but it’s actually the setup for one of the best books I’ve read in years. With a blend of scientific rigor, thoughtful intellectual exploration, and laugh-out-loud humor, Hooven provides a comprehensive exploration of testosterone (T), arguing that it is the single most important hormone driving the average biological and behavioral differences between the sexes.
The book moves from primate field observations (p. 7) to the deep science of endocrinology, explaining how T “organizes” the fetal brain (p. 86) and “activates” behaviors later in life. Using “natural experiments” like CAIS (p. 53) and CAH (p. 91), Hooven demonstrates that prenatal T exposure has a profound impact on everything from juvenile play preferences (p. 95) to adult sexual orientation (p. 203). The book tackles controversial topics head-on, detailing T’s role in the male-female sports performance gap (p. 106, 121) and its complex relationship with aggression (p. 177). Hooven’s central thesis is that while T is not destiny, we must honestly confront its influence. She argues against the idea that biology and culture are opposing forces, stating that “social progress depends on scientific progress” (p. 25) and that “the more we understand the forces that shape us, the more control we have over how we behave” (p. 182).
Highlights:Besides the humor (it’s really funny), the book’s greatest strength is its clear, accessible explanation of complex endocrinology, using vivid analogies like hormones as a “radio broadcast” (p. 46) or genes as “recipes” (p. 57). Hooven masterfully uses “natural experiments” like CAIS (p. 53) and CAH (p. 91) to illustrate how hormones shape development. She also does a great job of incorporating both science and our everyday experiences, ranging from men’s notoriously high sex drive to boys’ rough-and-tumble play style (including that of her own son, eleven years old at the time of writing.) The book also shines in its courage to address controversial topics, pushing back on the idea that “expressing that view carried perils” (p. 17) and arguing that we must not “discount the evidence that supports the hypothesis” (p. 23) just because it is “distasteful.”
Lowlights: None from my perspective; however, given the subject matter, there is extensive and detailed discussion of sexual behavior (in humans as well as other mammals), genitalia and their development, etc. This may not be for everyone, or everyone may not be in the mood for it at any given time (pun somewhat intended).
The cultural pushback against “dangerous ideas” (p. 19) regarding biological sex differences is a powerful illustration of ideological framing and desire bias, where people “discount the evidence that supports the hypothesis” (p. 23) if it’s “unpalatable” (p. 17).
The “organization/activation hypothesis” (p. 86) is a clear example of path dependency, where early-life hormonal exposure creates permanent, “bricklike effects” (p. 216) that are “difficult to eliminate” (p. 216) later.
The “Challenge Hypothesis” (p. 153, 174) and the “winner-loser effect” (p. 172) show a powerful feedback loop, where T levels “rise in response to threats to status” (p. 174) or competition, which in turn alters behavior.
The book’s emphasis on the “strikingly non-overlapping bimodal distribution” (p. 113) of T levels and the fact that men commit over 95% of homicides (p. 11) highlights the importance of understanding base rates.
The animal studies on “jacked-up lizards” (p. 150) and high-T sparrows (p. 152) illustrate biological trade-offs and dose-dependency, where “high T is not always better than low T” (p. 153).
You should buy a copy of Testosterone if: you want a scientifically grounded, courageous, and accessible explanation for how hormones influence human nature, behavior, and the differences between the sexes.
Reading Tips: Pay close attention to the “natural experiments” like CAIS and CAH, as the author uses them as recurring case studies to explain complex genetic and hormonal pathways. Don’t be intimidated by the endocrinology; the analogies are very effective.
IMPORTANT: the below commentary DOES NOT SUBSTITUTE for READING THE BOOK.
Page 5: The author recounts being the first researcher to observe weapon use by a nonhuman primate in the wild, an event later covered by Time magazine with the cringeworthy, anthropomorphic title “Wife Beaters of Kibale.” Despite the title, the author acknowledged the undeniable “similarities between Imoso’s disturbing behavior and domestic violence among humans.”
Page 6: Hooven describes a personal journey from a “comfortable” but unfulfilling job to pursuing a deeper question: “how has evolution shaped human nature?” Reading popular science books like The Selfish Gene and later Demonic Males by Richard Wrangham provided a new path, leading the author to quit their job and apply to graduate school to study evolutionary origins and chimpanzees.
Page 7: Hooven describes observing “contrasting patterns of chimp behavior”: relatively peaceful, “nurturing” females and “hierarchy-obsessed aggressive males.” Male aggression serves multiple purposes, including showing dominance and “mate guarding.” Aggression is also directed at females not in estrus, and the data suggests this “tends to increase a females sexual compliance in the future.” In fact, females were found to “preferentially mate with and bear offspring fathered by males who have been especially aggressive toward them.”
Page 8: This page contrasts the gentle, “patient” side of male chimpanzees – and men – with their capacity for “violence and cruelty.” The author juxtaposes this with the brutal civil war violence happening nearby and the concurrent news of President Clinton’s affair, noting he “risked everything for a few fleeting sexual encounters.” This behavior is a powerful example of the Planner-Doer problem, where present bias and a ‘hot state’ override long-term rational planning. She notes that “I spent long hours every day as the lone woman in a group of local men, and I trusted them with my life. But during that time, other men from the very same region of Africa were carrying out brutal acts against civilians.”
Page 10: The media and popular psychology often frame testosterone in simplistic, contradictory ways. Some on the left view Trump’s presidency as “testosterone-fueled,” while some on the right argue the opposite, lambasting certain conservatives as “low-testosterone” and “dilletantish.” Psychology Today even posited a “testosterone curse,” a “biological urge that sooner or later demands expression,” suggesting we should understand that “men are just animals” under its influence. These narratives are a clear example of how simplistic storytelling and ideological framing replace a complex scientific understanding, which I will discuss in more detail in the focus / attention mental model.
Page 11: The expert consensus is that testosterone’s “main job is to support the anatomy, physiology, and behavior that increases a male’s reproductive output.” The text then discusses sex differences, noting some are small while others are “large and meaningful.” A key example is that men are “far more physically aggressive than women in every pocket of the earth, at every age,” committing over 95% of homicides worldwide. This highlights the importance of understanding statistical base rates when discussing group differences.
Page 12: Using height as an example, this page illustrates two types of sex differences: a difference in average and a difference in variance. The graph shows that the distribution of male height “is wider than the distribution of female heights.” This greater variance means that while the average is different, “there is more variation in male heights,” leading to more men being at the “very tall and very short” extremes of the distribution.
Page 13: A sex difference is not a single concept; it can manifest in several ways. It can be “a difference in the average,” such as in some reading ability tests; “a difference in the variation,” as seen in IQ scores where there is greater male variation; or “a difference in both,” as is the case with height.
Page 17: We often make “poor decisions based on gut feelings rather than reasonable ones based on the proper evaluation of evidence” when a scientific hypothesis is “unpalatable.” This desire bias can drive us to irrationally reject conclusions we don’t like. The author notes that as they became convinced T “plays a central role in human sex differences,” they discovered that “expressing that view carried perils of its own.”
Page 19: Following comments by President Summers, the author observed a strong pushback against discussing biological hypotheses for sex differences. One professor claimed it was “crazy to think that it’s an innate difference,” insisting “It’s socialization.” The prevailing sentiment was that such “dangerous ideas” shouldn’t be discussed because they could harm “achieving gender equality.” This environment created powerful social disincentives for certain lines of research; Hooven, then untenured, felt “nervousness” and ultimately “gave up research and stuck to teaching.” This reminds me a lot of the discussion of luxury beliefs in Rob Henderson’s Troubled, where a curious recent development is many pockets of society denying basic, obvious factual reality as a status symbol.
Page 23: There is a common fear that biological explanations for behavior will be used to “reinforce the patriarchy,” but this concern is “irrelevant to the truth of scientific hypotheses.” The text warns:
“if you find a hypothesis distasteful, a red flag should immediately go up: there is a clear and present danger that you will discount the evidence that supports the hypothesis.”
This is a crucial defense against desire bias and an example of confirmation bias. Books like Testosterone Rex incorrectly argue that biological theories are “fruitless and dangerous” and that such thinking “squashes the hopes for sex equality.”
Page 24: It’s a common belief that “biological explanations of sex differences lead to pessimism about progress, and a fatalistic acceptance of ‘ingrained’ gendered social norms.” Authors like Angela Saini, in Inferior, argue that “sexism and bias” have led science astray, and she concludes that “There are few psychological differences between women and men.” This viewpoint is obviously complete nonsense (to the point of being BS). That doesn’t mean that culture doesn’t exist (it’s one of our culture mental models), nor that biology is destiny.
Page 25: The author argues that testosterone does push the psychology of the sexes apart, but states “this isn’t bad news, it’s empowering information.” Acknowledging biological forces doesn’t require accepting negative outcomes like assault or discrimination. “On the contrary, social progress depends on scientific progress.” Understanding our nature equips us to “combat the expression of the darker parts of our nature,” a form of structural problem-solving for society.
Pages 26 – 27: The vulnerable position of external testes is explained as a biological tradeoff. The intense pain from an impact provides a powerful incentive to protect them (“you will try harder to avoid a similar situation”). The scrotum itself acts as a “climate control system,” contracting or relaxing to maintain a temperature about four degrees cooler than the body, which “optimizes sperm production.” This is a solution to a biological optimization problem.
Page 28: The study of hormones (endocrinology) has ancient roots. For millennia, it has been known “that the testes exert far-reaching influences on the appearance, behavior, and reproductive capacity of male animals.” As early as the 4th century BC, Aristotle reflected on the “changes wrought by castration” and noted the differences between life stages like “boyhood, manhood, and old age.” This early observational science “provided evidence that the testes were responsible for the maintenance of certain ‘masculine’ features.”
Page 30: Castrati were male singers who, beginning in the mid-sixteenth century, were castrated to preserve their high voices. This lack of testosterone had other significant physical effects. “You may know that T is what initiates the male pubertal height spurt; but you may be surprised to learn that it’s also needed to end it”. Because castrati lacked the testosterone to cause the cessation of bone growth, they would grow “quite tall;, with relatively long bones in the arms and legs” well past the normal age of puberty.
Page 31: Interestingly, “castrated men usually maintain an impressive head of hair into old age, since testosterone is responsible for most baldness”. Most castrati were treated as outcasts.
Page 33: The process of creating a eunuch was a “grisly procedure.” A “knifer” would “lop both the penis and the testicles”, which were preserved as the “bao” to be buried with the eunuch for masculinity in the afterlife. As if that wasn’t enough; “the knifer inserted a pewter needle into the urethra” to maintain the opening. Recovery was perilous; the patient was not allowed to drink for three days. At that point, if “the unfortunate wretch cannot make water he is doomed to a death of agony, for the passages have become swollen and nothing can save him.”
Page 36: Arnold Berthold conducted a landmark experiment in 1849 on cockerels. He castrated some, which failed to develop normal male traits. He then castrated another set but transplanted the testes back into their abdomens, noting “He put one testis from each bird into the abdomen of the other.” These birds developed normally. “He found that the castrated cockerels with transplanted testes ‘remained true cockerels'”, proving that the testes’ effect was communicated through the blood stream, not the nervous system. I can’t believe this worked… it’s a bit like grafting in plants.
Page 37: Berthold’s experiment was revolutionary because the transplanted testes were not connected to the nervous system. He concluded, “it follows that the results in question are determined by the productive functions of the testes, i.e , by their action on the blood stream, and then the corresponding reaction of the blood upon the entire organism”. This discovery directly challenged the dominant idea that the nervous system controlled these systemic changes and effectively “set in motion the development of the field of behavioral endocrinology”. Hooven describes herself getting “goose bumps every time I read those last few words and think about their significance.”
Page 41: After Bayliss and Starling isolated “secretin,” the great physiologist Pavlov attempted to replicate their experiment, hoping to disprove it. When he confirmed their findings, he famously exclaimed, “Of course they are right. It is clear that we didn’t take out an exclusive patent for the discovery of the truth”. In 1905, Starling coined a name for such substances: “These chemical messengers, however; or ‘hormones’ (from ormao, “I excite or arouse”) as we might call them… have to be carried… by means of the blood stream”.
Page 42: Scientists in the early 20th century worked to identify the “masculinizing agent” from testes. Ernst Laqueur, for example, collected 100 kg of bull testicles to harvest just “a minute 10 mg (less than the weight of a grain of rice) of a chemical”. This substance successfully “regenerated the cocks comb” in a castrated bird, which was the standard test. Laqueur “dubbed it ‘testosterone’.” By 1935, three different research groups had synthesized the hormone. Today, we know of about 75 hormones, secreted by both endocrine glands as well as intestinal and fat cells.
Page 43: The list of endocrine glands is extensive, going beyond the major ones to include the liver, heart, kidneys, skin, and brain, which all “secrete and respond to hormones.” Even gut bacteria are involved. “Every new discovery about the endocrine system surprises us with the inventiveness of natural selection in keeping us alive; healthy, and reproducing”. Understanding this complex system is critical, as “Finding the truth amid the hype around T will be impossible without a basic understanding of hormones in general”.
Page 44: While “Hormones are often imbued with all sorts of magical powers”, most people are unfamiliar with them. All multicellular organisms use hormones for chemical communication. In animals, they fall into two main categories: “protein hormones” (like insulin and melatonin, made from amino acids) and “steroid hormones”. This second category includes testosterone and estrogen, which “are all made from cholesterol”.
Page 45: Hormones act as “chemical messengers” circulating in the blood to keep the complex system of the body functioning. The body has two types of messengers. “While neurotransmitters convey information via electrical impulses in a point-to-point fashion via neurons, like trains traveling on a set of branching tracks, hormones send their chemical messages out far and wide, to any cell that is ‘listening'”. This is compared to a radio tower broadcasting a signal that can only be picked up by a tuner set to the correct frequency.s I double underlined this.
Page 46: The endocrine system works like a radio broadcast. Hormones are signals, but only “target cells” with the right receptors can “pick it up.” The “neuroendocrine system” describes the communication between the endocrine and nervous systems which coordinates behavior and physiology. “Ifr you’re hungry, hormones have sent signals about your energy status from your gut to your brain, where they alter how you feel, causing you to struggle to stay focused on testosterone while you crave a little snack.” LOL – and also, see Johann Hari’s Stolen Focus.
Page 47: When you consume glucose, like from an energy bar, it enters the bloodstream. This signals the pancreas to release insulin, which circulates and essentially “shouts to your cells ‘Heyl Open up! Glucose is at your door!'” Muscle cells and others respond by opening portals to let the glucose in, which is then used to produce ATP for fuel. This is a clear example of a biological feedback system. Later, after insulin has done its job, your “blood-glucose levels are low,” which your brain senses, triggering appetite.
Page 48: Hormones like insulin interface with the brain to coordinate the body’s needs, such as managing energy. Testosterone also “sends information to your brain,” but its focus is on “supporting reproduction.” For men, “successful reproduction has less to do with energy-sucking reproductive physiology, and more to do with finding, competing for, and attracting mates.”
Page 49: In the late nineteenth century, research on testicular secretions boomed. “Scientists could examine the effects of hormones on animal physiology anatomy and behavior without resorting to the laborious, minimally effective, and relatively cruel methods of transplanting entire organs”. This led to the synthesis of estrogen, progesterone, and testosterone. The “symbiotic relationship” between clinicians, researchers, and pharmaceutical companies helped the field of endocrinology flourish. Prior knowledge was based on castration, but this ignored the effects of testosterone exposure “before; or shortly after birth”.
Page 53: Complete Androgen Insensitivity Syndrome (CAIS) is introduced as a “variation” that challenges common assumptions. “With XY chromosomes and testosterone-producing testes, but without Ts actions, a person will have female primary and secondary sex characteristics.” (Primary characteristics are those present at birth, i.e. penis vs. vagina; secondary characteristics are those that develop in puberty, like breasts or a beard.) People with CAIS are externally indistinguishable from other females; their “appearance and behavior… are unexceptional; that is, they look and act like typical women or girls.” They often don’t know they have CAIS until they never experience menarche. The author admits that despite knowing the science, seeing an “ultrafeminine” person who had “testicles and testosterone” was something they “struggled to come to terms with.” As she puts it, “masculinity and femininity don’t always come in the packages we expect.”
Page 54: The case of CAIS demonstrates that sex chromosomes alone don’t determine physical traits. “The reason that boys develop a penis and girls do not is not that the genes for these traits are possessed solely by one or the other.” Instead, “Both sexes come genetically equipped to express almost all the traits typical of either sex. It’s just a matter of which genes are active, at what levels, in which bodies. based on hormonal signals. This same principle explains why some men develop breasts (gynecomastia) and some women develop dark facial hair.
Page 57: Gene transcription and translation are compared to having baking recipes. Cells specialize, keeping certain protein “recipes” available while others are “crumpled up and ignored.” This is literally true, as “most of the DNA in every cell is squished up into something called chromatin” so it cannot be translated. The specific recipes a cell uses are determined by its function; for example, “cells in the pancreas must sense blood glucose levels” while those “in bone must provide structural support.”
Page 58: Gene expression is explained further: in a woman’s facial skin, the gene “recipe” for “dark; thick hair is crumpled up in the back of the shelf.”“But inside men’s skin, the same recipe is… prominently displayed, transcribed and translated over and over again,” driven by higher testosterone levels. The page then introduces the “bipotential gonad,” the early embryonic structure that differentiates into either testes or ovaries based on genetic signals.
Page 59: Explains the genetic basis of sex determination. The primordial gonads differentiate based on “a loud request, in the form of high levels of a protein called SRY.” (SRY = sex-determining region of the Y chromosome.) This SRY protein is coded by the SRY gene, which is “located, unsurprisingly, on the Y chromosome.” The presence of the Y chromosome in the sperm is therefore “the decider” for whether the embryo’s cells inherit XY or XX chromosomes.
Page 62: Early fetal development is surprisingly flexible. “Not only do we all start out with bipotential gonads” that can become testes or ovaries, but “our early internal reproductive anatomy is also gender-convertible.” All fetuses begin with the the Wolffian (male) and Müllerian (female). After about eight weeks, hormonal signals determine the outcome: the Müllerian ducts degenerate if they receive a signal from the testes, while the Wolffian ducts develop in response to testosterone.
Page 63: The female developmental pathway is presented as the “default.” The Müllerian duct system “will develop without any specific hormonal stimulation,” unlike the male system. More strikingly, all external genitalia arise from the same embryonic structures: “The penis is a huge clitoris, and the scrotum… are basically fused labia.” Hormones act as “keys into locks” to direct this differentiation. In Jenny’s case (CAIS), the “lock” was broken; although her testes produced Müllerian inhibiting hormone (so no uterus formed), her Wolffian ducts also degenerated because they couldn’t respond to testosterone, resulting in a “blind vagina” (a vagina that just ends rather than connecting to a uterus).
Page 65: The androgen receptor is described as a “lock” and testosterone as a “key.” In Complete Androgen Insensitivity Syndrome (CAIS), the gene for the receptor has a mutation. Again compared to a recipe, it’s a significant typo like “two cups fluor’ instead of flour – a very different chemical. “With an androgen receptor that doesn’t work at all, the body has no way of hearing the usual high T signal–‘Hey! Develop male parts!'” The signal is there, but the radio has a broken tuner. This demonstrates testosterone’s primary “superpower”: its ability to “upregulate (or downregulate) gene transcription” and regulate protein production at different life stages. High levels of T tell “male relevant genes to upregulate.”
Page 66: Steroid hormones, like testosterone, work differently than protein hormones like insulin. Because steroids are “lipophilic (‘fat-loving’)”, they “float right through fatty cell membranes into the cell.” In contrast, protein hormones are “hydrophilic” and must bind to receptors on the cell surface. “Once inside the cell; T finds the androgen receptor,” creating a “hormone-receptor complex” that enters the cell’s nucleus and interacts with DNA to “upregulate the transcription of the genes” responsive to androgens. The text also notes that female external development is the “default” path, as “the external structures [ genitalia] develop in the female direction in the absence of any hormonal signal.”
Page 67: This page describes how steroid hormones are made, noting that “Cholesterol (a type of fat) is the grandparent of all steroid hormones.” Enzymes convert cholesterol down various pathways. It also explains a surprising fact about “Jenny,” who has androgen insensitivity: “Surprisingly, Jenny got all of the estrogen necessary for full feminization… from her high T levels.” This is possible because an enzyme called aromatase converts testosterone into estrogen. In a typical female, high T would “swamp the usual feminizing effects of estrogen,” but since Jenny’s body couldn’t respond to T, the estrogenic effects dominated.
Page 68: This page details the steroid production pathway, noting a key relationship: “In other words; testosterone is the precursor to estrogen.” An enzyme called aromatase is responsible for this conversion. Aromatase “converts testosterone and other; less active androgens into estrogen” and is found in many tissues, including fat, bone, and the brain. The levels of this enzyme vary, meaning “If you have more aromatase, more of the available testosterone will be converted into estrogen.” Again with trademark humor, Hooven notes “Bodybuilders are very aware of this and take specially altered androgens that can’t be aromatized; if they simply took high levels of T, much of it would be converted into estrogen, which would not give them the look they are going for.”
Page 70: Hooven discusses her then-eleven-year-old son, Griffin, to explore gendered play. While Griffin enjoys things often associated with girls, like dressing up and babies, he also has an “intense and complex” fantasy world focused on a villain “intent on destroying the universe.” The author was “perplexed by the juxtaposition of the sweetness… and the joy he took in playing out the mortal combat and violent destruction of planets.” This personal anecdote contrasts with studies like “Baby X,” which show that adults interpret behavior through a gendered lens – an example of the confirmation bias mental model.
Page 71: The text contrasts typical boy fantasy play (“heroes battling bad guys”) with girl fantasy play (“relationships, romance, and domestic concerns”). This is a real thing.
Page 72: Highlights that the “biggest difference in play between boys and girls is the amount of physical contact with other kids.” Boys engage in far more “pushing, hitting, tackling… and ‘rough-and-tumble’ play,” which is found across a diverse range of cultures, from industrial societies to hunter-gatherer populations. “Boys, it seems, are resistant to efforts to condition them away from battle and weaponry,” as shown by a preschooler who picked up a Barbie doll and began to “shoot bullets out of its head.”
Page 76: The case of Taman, who had high T but ambiguous genitalia, is used to correct a “simplification.” While testosterone is key, the development of male external genitalia “in utero” requires “extra stimulation” from a more potent androgen: “dihydrotestosterone (DHT).” This molecule, DHT, “is produced from testosterone with the help of the enzyme 5-alpha reductase.” DHT fits in the same androgen receptor “lock” as testosterone but turns the key “more easily and stays in the lock for longer,” driving the necessary gene transcription. Taman’s condition was caused by a mutation in the 5-alpha reductase enzyme.
Page 77: As with the previous CAIS example, Taman’s “penis later grew and his testicles descended” at puberty, as DHT isn’t necessary to masculinize genitals during puberty (only as a fetus).
Page 78: While it’s ethically impossible to run experiments injecting T into babies’ brains, “natural experiments” exist. The “Guevedoces” in the Dominican Republic, a group with 5-alpha reductase deficiency (like Taman, p. 76), became an “object of intense study.” These individuals are born with female-appearing genitalia but then masculinize at puberty. Their experiences “made clear that boys’ and girls’ brains are not gender-neutral blank slates.”
Page 83: Rodents are “ideal study subjects” because their sexual behavior “differs sharply between the sexes” and researchers can manipulate their bodies and environments. Experiments show hormones drive these behaviors; for example, a castrated male rat is indifferent to a female in estrus, but when his “normal T level is restored, he again responds to that female as an object of desire.”
Page 86: This page introduces the “organization/activation hypothesis,” a key concept for understanding testosterone’s lifelong effects. “If the brain is not masculinized prenatally, then the animal lacks the specialized neural anatomy that T can act on in adulthood to activate typical male behavior.” This idea, which demonstrates a biological path dependency, was initially controversial but has since been replicated in numerous mammals, including primates.
Hooven also notes “rats, and mammals in general, spend a surprising amount of time engaged in what looks like a frivolous activity – playing. All that tumbling and running around seems to waste valuable energy that could be directed towards other, more sensible activities like finding food… and small, inexperienced animals frolicking, oblivious to their surroundings, are the perfect targets for stalking predators.”
Page 87: Hooven notes that “fun” is a “proximate explanation” for play rather an “ultimate explanation” – my analogy (not hers, although I’m sure she’d agree) is that we have sex because it feels good, but that’s not why sex is selected for biologically; the “ultimate explanation” of sex is reproduction. (I like this proximate vs ultimate explanation concept and should use it more broadly.”
There are obvious parallels here to the discussion of play in Johann Hari’s Stolen Focus. Hooven explains: “the ultimate explanation for why rats play is because it is a way for young animals to learn and practice adult behaviors that they need to survive and reproduce.” For males, this includes practicing for the “dominance arena,” as high status achieved via winning fights allows dominant males to “mate more.”
Page 88: Juvenile rat play (“rough-and-tumble”) is crucial for development. I wrote “LOL” in the margins here: “the description [of rat play], minus the ‘rat’ and ‘paws’ parts, seems very much like the way my own sun plays.”
Hooven expounds on the importance of play. “When male animals are experimentally prevented from playing in this way; they grow up to be evolutionary losers” who “readily submit to intruders and do not do well in the mating arena.” The page also shows a complex feedback loop: mom rats lick and groom their sons more if they have higher T. This differential grooming itself affects later adult behavior. “The vital lesson here is that hormones can sometimes affect behavior indirectly; by affecting social interactions, which can change behavior in turn.”
Page 89: The applicability of animal models to human behavior is questioned. While early testosterone exposure affects rat behavior, one can “argue that humans, immersed in a gendered social world, are an exception to the animal model.” For example, “Children can pretend a stick is a gun, a light saber; or a doll.” Furthermore, “Unlike rats, our genes are expressed within the context of a complex cultural environment, interwoven with diverse norms and practices.” Hooven also notes the diversity of human sexual behavior – we can “have sex any time of the day or year, with an endless variety of partners and physical configurations,” or we “can also choose not to have sex, even in the presence of an overwhelmingly attractive and receptive partner.”
Page 91: Introduces Congenital Adrenal Hyperplasia (CAH), a rare genetic disorder where female fetuses “are exposed to unusually high levels of testosterone.” This offers an opportunity to study early androgen exposure in humans. The disorder is caused by a mutation blocking the production of cortisol, which leads the pituitary to signal the adrenal gland to “work harder to produce cortisol,” resulting in an overproduction of precursor steroids, including T.
Page 92: When an enzyme needed to produce cortisol is missing, the adrenal gland can’t complete that pathway. “Instead, the adrenal gland sends all those precursors down the pathway that does work, the one that converts them into androgens.” This results in a flood of androgens (“More and more androgens are produced”). In girls, these high androgen levels cause masculinization, including an enlarged clitoris that may “look more like a penis.”
Page 93: This page explores the question: do girls with CAH “behave more like typical boys?” Before answering, it establishes a baseline for “average differences” in play. A classic experiment found that preschool boys and girls use different tactics in competitive situations. The boys “laughed and smiled even as they hit and shoved each other while fighting over rights to the viewer.” The girls, while “no less competitive,” “used different; less direct tactics.” The summary: “Overall; where boys used their bodies, girls used their words. Boys pushed, pulled, or hit their partners about six times more than girls did.”
Page 94: Describes the process of gender segregation in play. While kids under two play with “little attention to gender,” they soon “begin to gravitate toward their own kind.” This “gender segregation peaks… around ages eight to eleven.” This is driven by preferences for different play styles, such as “larger groups and rougher” play for boys versus “smaller groups and more domestic themes” for girls.
Page 95: Research on play preferences provides strong evidence for the effects of prenatal androgens. The text states there is “no bigger difference in the behavior of boys and girls than in the way they play.” A study comparing unaffected children to girls with CAH (who had high prenatal androgen exposure) found stark differences. Unaffected girls strongly preferred “girls’ toys” (60% of their time). In contrast, “The CAH girls played mostly with boys’ toys,” spending “44 percent with the boys’ toys” and only 21% with girls’ toys. Their preferences were “much more masculinized than their unaffected peers.” (Boys spent 70% of their time with boys’ toys and only 6 percent with girls’ toys; of course, it s difficult to separate out the culture mental model from biology here – “tomboy” girls who want to play baseball and go camping are much more accepted, by both parents and society at large, than effeminate boys who want to dress up and play with dolls.)
Page 96: While CAH girls show masculinized play, “boys who have CAH have not been found to differ from unaffected boys in their gender-typed behavior.” This suggests a nonlinear, dose-dependent relationship. It’s thought to be a “‘ceiling effect’ where exposure to T beyond some ‘male-typical’ level has no further masculinization” effect. This is consistent with evidence from adulthood, where women’s behavior and physiology are affected by small changes in T, but the same is not true for men.
Pages 99 – 100: Hooven argues against the socialization hypothesis I referenced on Page 95, citing CAH studies and “evidence from experiments in mammals… [showing] that when females are given high T in utero, their behavior is masculinized, and when males are deprived of it, their behavior is feminized.” She invokes the “scientific principle of parsimony,” suggesting the simpler theory is that humans are not an exception to this biological rule, rather than positing complex “epicycles” (like Ptolemy) to defend a socialization-only view. “Should we assume that humans are different from every other animal in which the relationship between T[estosterone] and masculinization of juvenile behavior has been observed, or is it more reasonable to assume that the human animal is subject to similar biological and evolutionary forces? […] it is a remarkable and unexplained coincidence that social forces have exactly reproduced the kinds of differences in play that would be predicted from endocrinology and evolution, in every human culture where they have been studied. The conclusion seems inescapable: as best we can tell, T makes boy brains.”
Page 105: Details a sibling rivalry between “Sofia” and “Sam.” Before puberty, “neither Sofia nor Sam could consistently outrun or outthrow the other.” However, after Sam hit puberty, he “started to really pull ahead,” becoming significantly faster and able to throw farther, illustrating a vast difference, especially in throwing: “Nearly every boy by age 15 throws better than the best girl.”
Page 106: The most basic categorization in elite sport is sex, which was “obvious and uncontroversial” because “Women’s world records are consistently about 10 percent lower than those of men.” In the marathon, this gap is around twelve minutes. This performance gap means “in many events, thousands of male athletes are ahead of the best female.”
Page 107: Discusses the performance gap in sports between men and women. While some argue “the gap in performance between elite men and women is closing in every sport,” the text counters that this is “mistaken.” After an initial narrowing following Title IX, “the gap hasn’t budged” in the last forty years.
Page 109: Makes a critical distinction: T is “not the master key to performance” WITHIN each sex. However, Hooven explains it’s a “subtle bait and switch” to claim T doesn’t explain differences between sexes. The conclusion is that “a male level of T… is the master key for superior performance in most sports.”
Page 110: Explains that T is hydrophobic (water-fearing) and must be carried in the blood by proteins. About 98 percent of T is “‘bound’ to a hydrophilic carrier protein” and cannot get inside cells, so it doesn’t have “biological action.” The other 2 percent is “‘Free T'” or “‘unbound'” T, which “can pass through cell membranes, interact with the androgen receptor,” and affect the body. Thus, “Free T” is the “biologically active” component.
Page 113: Describes the “strikingly non-overlapping bimodal distribution” of circulating testosterone between men and women. Unlike height, which also has a bimodal distribution but with “wide, overlapping bases,” the T distribution has a “wide and complete separation.” The two distributions are “like two mountains separated by a vast plain.” As Hooven puts it bluntly: “In other words, a binary.” This obvious reality has become quite controversial. See also Rob Henderson in Troubled on luxury beliefs.
Page 116: The massive divergence happens at puberty, with boys seeing 20-30x increase in T levels, while girls see only a little.”
Page 119: During puberty, T “promotes muscle protein synthesis.” It influences stem cells “to take the muscle route, and it actively discourages them from turning into fat.” In contrast, higher estrogen and lower T in females means they “convert more of her consumed energy into fat.” By their late teens, a male’s lean body mass will be about 1.5 times that of a female’s. Hooven further notes (back to the foreman analogy from elsewhere in the book) that “sex hormones act like the manager of a team of hormones, including growth hormone, insulin… and others.”
Page 120: It’s not just muscle, either. Bones have receptors for both T and estrogen, but estrogen “plays the dominant role, in boys as well as girls.” Recall that “all estrogen comes from testosterone.” Doctors learned about this by studying men with a rare mutation who “were unable to convert testosterone to estrogen” and consequently had weak, elongated bones, but testosterone injections had no effect.
Page 121: Puberty demonstrates the permanent, path-dependent effects of testosterone. Boys’ higher T levels lead to larger muscles, and these muscles “apply greater load to the developing bones” during puberty, when “bone architecture is especially sensitive to mechanical load and develops in response to it.” The bones “respond by increasing mineral density and diameter.” These skeletal changes—“lengthening, enlarging, and strengthening—are, for the most part, permanent.” This contrasts with other T-induced changes, like higher hemoglobin levels (“Men’s levels are about 12 percent higher”), which are not permanent and can be changed in adulthood via hormone exposure.
Page 125: Athletes in the women’s division with DSDs (Disorders of Sex Development) and male-range T levels “tend to outperform women within the normal T range.” People with these conditions are “overrepresented in elite female sports by 140 times.” One study of Swedish Olympians found 37% had PCOS – 3x the general population rate.
Page 128: Discusses the issue of transgender women in sports. The dilemma arises because athletes who “experienced the physical benefits of male puberty” retain clear advantages. After a year of T-suppressing medication, some benefits decline (e.g., “hemoglobin plummets to female levels”). However, “bone size… doesn’t budge,” and evidence shows “muscle and strength are not completely lost.”
Page 131: The red deer mating season, or “rut,” is presented as a “spectacular illustration” of how “testosterone, sex, and aggression are related in wild animals.” The author describes a large stag, “Wisdom 11,” with huge antlers that are clearly weapons: “sprouting straight up from his head, above his eyes, were two shorter branches; perfectly positioned to poke out the eyes of a rival.” Violence is driven by the incentive to acquire a harem.
Page 133: Describes the mating behavior of stags. Harem-holders like “Wisdom 11” must defend their hinds. Other stags, called “‘SFs’ (for ‘sneaky fuckers’),” (LOL) will “make a daring dash into the edge of Wisdom 11’s territory” hoping to mate while the dominant stag is distracted. This “lower-risk strategy” benefits younger stags, allowing them to “practice being bold, but avoid the high risk of injury” from a direct challenge. Hooven also notes the time element – unlike humans (who can mate anytime), “stags can smell when hinds are in their short window of estrus, which can last for as little as two hours, and it draws those without harems of their own from near and far.”
Page 134: Red deer stags prefer to avoid physical conflict, due to the risks and energy consumption of fighting – it’s “best reserved for when the rewards… are worth the risk.” They first try intimidation by “displaying their size, weaponry, and intentions.” Roars are “‘honest signals'” of fighting ability because they “can’t be faked” and better roars are produced by stronger males. “Physical battles happen only when each party believes that victory is within his reach, or when desperation.. drives one of them to do something a little crazy.” Hooven does note that “strength isn’t everything. Skill and grit matter, too.”
Page 137: The behavior of red deer highlights the power of incentives. Outside the mating season, stags’ “competitive stakes are low” and they don’t fight viciously. For stags, the rut is “something akin to a reproductive jackpot.” One stag fathered “fifteen calves, which was a whopping 25 percent of the total… born that season.” Because the incentives are different, the behaviors are different; hinds “don’t need to be so aggressive.” Interestingly, “a busy, dominant harem holder doesn’t have time for luxuries like eating during the rut, when he spends only about 5 percent of his time feeding.”
Page 138: Therefore, stags must store fat in the offseason and stay injury-free. Intriguingly, after the rut, “the testes slowly shut down, and blood T drops to very low levels.” This is described as “like a temporary castration.” In the absence of mating prospects, “sperm and T aren’t needed,” and the stags “chill.”
Page 139: Describes the seasonal growth of antlers, which are initially covered in a “soft, velvet coating” that provides a blood supply. As T rises in August and September, it “increases the calcification of bone.” High T also “cuts off the blood supply” to the velvet, causing it to be shed, revealing the sharp tips in preparation for the rut.
Page 141: Uses a child’s excitement on Halloween to illustrate the correlation vs. causation mental model. We might assume “the sugar caused the silliness,” but what if the “high energy… was just the excitement of Halloween?” This illustrates a correlation (sugar and excitement) where the sugar is not the cause. The text notes that “experiments along these lines have actually been done, and it turns out that it is a myth” that sugar makes kids hyperactive.
Pages 142-143: Describes “remove and replace” experiments on stags. When castrated, stags’ “aggression, and status, plummeted.” They “couldn’t compete” and lost sexual interest. To determine if this was due to T’s effect on the brain or horns, researchers later implanted the castrated stags with testosterone, which increased aggressive behavior even outside the normal rutting period, confirming T’s role. They found testosterone is “necessary but not sufficient” for sexual behavior; the “proper environmental stimulus” (like fertile females) must also be present. However, T’s effect on aggression is separate. Stags given T implants “began to pick fights with nearby males” even during the off-season. The evidence “strongly suggests that T promotes aggression in these animals by acting on the brain.” This makes evolutionary sense, as establishing dominance “pays off even when mates are not directly at stake” by securing future resources.
Page 144: Red deer are used as a “compelling example” of testosterone’s (T) effects in male animals, as they are “seasonal breeders.” Fluctuations in T cause dramatic changes, turning stags from “spermless, weaponless, relatively peaceful” creatures into “highly sexed, aggressive, and dangerous” ones. This illustrates sexual asymmetries common in mammals: females must “use their bodies to host and feed their developing offspring,” while males are free to “invest the ‘extra’ time and energy into the pursuit of additional mates.” This asymmetry results in a “predictable; sex-based behavioral pattern” where males tend to compete for mates, while females prioritize acquiring resources and selecting fit mates.
Page 150: An experiment with lizards illustrates the downside of excessive testosterone. Control (“Goldilocks”) lizards spent time “basking in the sun” and eating. In contrast, “jacked-up lizards” with high T “more than doubled the amount time out and about, patrolling their territories, and attacking other” lizards. Though they secured territories, they “failed miserably in their preparations for the fall mating season,” having “spent their precious energy early and unwisely.” Half of the high-T lizards died, while the “Goldilocks guys were rested, fattened up, and prepared” for the arrival of females.
Page 151: Discusses sparrow mating. While pairs form, there is “a fair amount of cheating,” with about a quarter of chicks fathered by neighboring males. The male mate must “‘mate-guard'” his partner. T levels are highest during the initial “male-male competition” but then drop to a “‘Goldilocks level'” just sufficient for mating and raising young.
Page 152: Details a study by John Wingfield on song sparrows, illustrating the trade-offs of high T. When male birds who were “busy dads” received extra testosterone, “other activities took on a greater appeal.” Instead of “devoting time and resources to their children,” the high-T dads “went out singing at all hours,” defended territory, and tried to “score new females.” Consequently, these fathers “neglected their families, and their chicks ups were more likely to die of starvation.” I do wonder if there is a human parallel here. Anyway, “territorial intrusions… caused residents’ T to shoot up to maximum levels.” At least in sparrows, there’s a feedback loop here – not only can testosterone cause aggression, but aggression can elevate testosterone.
Page 153: The “Challenge Hypothesis” proposes that testosterone levels are adaptive and fluctuate based on social context. “T levels fluctuate depending on whether a male needs to breed, care for his family, or fight off rivals.” (Again, to be clear, we’re talking about sparrows.) Cues from the social environment, such as “threats from rival males, sexy females, or the squawking of hungry young,” signal to males when the cost of high T is worth it. This framework shows that “high T is not always better than low T, and depending on the circumstances, it can be fatal.”
Pages 156 – 157: Turning to humans, while women are “capable of anger and inflicting pain,” there is a significant difference in how male and female aggression is expressed. Hooven notes once again: “T’s central role in male violence is well established for many non-human animals. Could men really be exceptions?” Behavior is “always a product of interactions between biology… and its environment”; it’s never just one or the other, reflecting a multicausal reality. For many male animals, testosterone’s (T’s) primary role is to “coordinate male sexual anatomy, physiology, and behavior.” This coordination often includes aggression as a key component for successful reproduction.
Page 159: The outcomes of intimate-partner violence differ significantly by sex, even if the rates of aggression are similar. Men cause more damage, due not only to their strength but also the empathy mental model. Men show less empathy on average (“men are less able to [understand how others are feeling] than women, across cultures”), a finding mirrored in other male animals – chimpanzees, bonobos, gorillas, elephants, dogs, and wolves.
Page 160: Women’s motivation for violence are more often self-defense; men’s are often to keep a mate from straying. Meanwhile, “Indirect aggression” is described as “unique to our species” and distinct from direct confrontation. It “involves using language to get others to do your dirty work,” such as spreading rumors or orchestrating social exclusion. This behavior is noted as familiar if you “know anything about high school girls,” and while men also engage in it, “girls and women seem to have an affinity for it.”
Page 161: A critical distinction is made between “reactive” and “proactive” aggression, which maps closely to the intuitive vs. cognitive systems. Reactive aggression is an immediate, hot response to a perceived threat, like if “you walk into your bedroom, discover your partner all twisted up in the sheets… hair tousled, naked, and with company. … You scream expletive-filled threats.” Pro-active aggression is a “cold calculation,” such as planning to “exact your revenge by posting” revealing photos online.
Page 165: Examining the base rates for violence reveals a clear sex difference. Men dominate arrest statistics for violent crime, comprising 80-85% of offenders in the U.S. for violent crimes and 90% for assaults. The key insight is that “the more risky, extreme, and cruel the violence, the larger the sex difference,” with psychologist John Archer noting that men are more likely “to escalate aggressive exchanges to dangerous levels.” See also Rob Henderson’s discussion in Troubled of the structural problem solving mental model as it relates to the military – taking a bunch of young men at the most dangerous times in their lives and preventing them from doing anything stupid.
Page 167: Sexual selection, which drives male-male competition, has not been “mysteriously switched off” in the human lineage. Hooven notes that in “traditional” societies, the sex difference in reproductive variance can be large. Our modern culture – which forbids having multiple wives and discourages fathering children out of wedlock – serves as a constraint. Rob Henderson has something to say about these cultural mores in Troubled (and in fact, I believe I might have first heard about T from one of his posts.)
Meanwhile, unlike seasonal breeders such as deer and lizards, men and chimpanzees (our closest relatives) are always “on the go.” They must be “ready to mate and to compete” at any time, but must also “avoid the costs of maintaining T levels that are cranked up to the max.”
Page 169: In humans, male T levels tend to “drop when they are in a romantic pair bond and involved in caring for their offspring.” Money quote: the evidence suggests this “decline in T heps direct attention away from rivals and new sexual prospects and toward one’s mate and children.”
Page 170: Social hierarchies, common in primates and humans, exist to “reduce the intensity and frequency” of violence because “Everybody knows who is most likely to win a fight.” Maintaining this status quo provides a powerful incentive to respond to challenges, even seemingly minor ones. The example of a drunk man is analyzed as an evolved response from our past in small hunter-gatherer societies, which “averaged about fifty to a hundred people” and where “strangers rarely” appeared – a trait adaptivity issue (in group vs out group behavior).
Page 171: Discusses the evolutionary context of male violence, noting the “lack of anonymity” in ancestral groups where behavior had “long-lasting reputational consequences.” In that environment, “backing down from an evenly matched competitor could damage hard-won status.” This programming persists.
Page 172: Short-term changes in testosterone (T) levels are an “adaptive response” to competition, creating a feedback loop known as the “winner-loser effect.” This was famously demonstrated in a study of fans watching the 1994 World Cup final between Brazil and Italy. After Brazil won, the T levels of the Brazilian fans “stayed the same or increased,” while the T levels of the losing Italian fans “dropped.” This response is found even in non-physical competitions like chess and video games.
Page 173: The “winner-loser effect” is clearly mediated by testosterone (T) in many nonhuman animals. In male Syrian hamsters (which her son Griffin had as a pet), a loss causes a drop in T and makes the hamster “cower in fear,” even against a non-aggressive opponent. This effect, which typically lasts about a month, can be chemically altered; if the loser “is given extra T shortly after a defeat, it will block the typical loser response.” This hormonal feedback system is adaptive: for a loser, “overconfidence is generally a bad idea,” and the drop in T helps them stay alive by making them more cautious. This is an example of the trait adaptivity mental model.
Page 174: A very interesting page that I’ll discuss in the focus/attention mental model.
Whether the context is “playing hockey, playing a video game, arguing, or even throwing punches,” men may feel their “status or reputation is on the line.” This sense of challenge or threat can trigger a rise in testosterone levels, at least temporarily. Scientists refer to this as the Challenge Hypothesis, which predicts that “male T levels rise in response to threats to status or resources.” In essence, testosterone responds dynamically to competition, serving as a kind of biological fuel for dominance-related behavior.
However, the passage also emphasizes that researchers still have many unanswered questions. It remains unclear “the purpose of such T changes, the biochemical mechanism involved, the characteristics of the men in whom the effects occur, and the kinds of social interactions that can influence T levels.” Scientists are actively studying how personality traits might mediate the link between testosterone and aggression. A team led by Canadian psychologists Shawn Geniole and Justin Carré has conducted several studies examining what happens when men’s testosterone levels are experimentally increased. Their findings suggest that not all men respond to testosterone in the same way.
Geniole and Carré’s research found that “dominance-oriented” men (those who are “assertive, forceful and self-assured”) tend to pursue status and power more aggressively. If that dominance orientation is combined with “a lack of self-control,” then a rise in testosterone is much more likely to produce aggressive behavior. In other words, it is the combination of personality and biology that matters most. The researchers observed that dominance-oriented men who are also impulsive—“lacking self-control”—are the ones who typically respond to testosterone spikes with aggression, such as when “a man anticipates competition.”
The passage concludes that testosterone may indeed increase aggression, but only in a subset of men whose personalities make them more prone to it. This helps explain why previous studies have produced inconsistent results. The connection between hormones and behavior is not universal but conditional, depending on both temperament and context. As Hooven notes, since these studies often involve “only a small number of men,” and the biological and social factors are highly complex, the findings remain provisional. But they illuminate an important link between competition, status, and human nature.
Page 175: Details an experiment using the Point Subtraction Aggression Paradigm (PSAP), a “well-validated tool to measure aggression.” After being shown a video that led them to believe they were competing against another man, participants could either “earn points for themselves” (which they could later exchange for cash) or “press a different key that reduces the other player’s points,” an action that “doesn’t benefit the participant and only harms the other player.” Intriguingly, “the men who received the T gel behaved more aggressively – that is, they store more points from the other guy. And like the previous experiment, the effect occurred primarily in the dominant, impulsive men.”
Page 176: I find this page absolutely fascinating as well. The effect of T depends on the efficiency of its receptors, not just the level of the hormone. “It’s not all about T level; the effectiveness of the receptor matters, too.” The androgen receptor gene contains a “CAG repeat”; fewer repeats mean a more efficient receptor. “All else being equal, the people who should have the biggest response to a given amount of T are those who have the lowest number of CAG repeats.” This efficiency has been linked to aggression, as men with short repeats “responded with increased impulsive, dominance-oriented aggression” to a T gel. These men also “got more pleasure from being aggressive.”
Page 177: Hooven argues that testosterone (T) doesn’t cause aggression outright but shifts several underlying systems in ways that make aggression more probable. It works by subtly tuning emotional and motivational circuits rather than flipping an “anger” switch.
When T levels rise, empathy tends to drop, so an individual becomes less sensitive to others’ pain or distress cues. At the same time, motivation and reward sensitivity increase, which means that the act of confrontation or dominance can start to feel more compelling or rewarding. Meanwhile, fear and pain perception decline, lowering the natural brakes that usually discourage risky or violent behavior. The combined effect is a brain and body state more inclined to push through fear or hesitation and to persist in conflict.
When T levels fall, these effects reverse: fear and pain regain influence, empathy returns, and retreat or avoidance becomes more adaptive. So testosterone doesn’t directly make someone violent; it shifts the balance of internal signals so that dominance or aggression feels easier and less costly to pursue.
Page 181: A very important page for the mental model. Contrasts different cultural approaches to violence, specifically the “culture of honor” found in places like the “American South.” This culture, which historically has “higher rates of violent crime,” taught that “Male children were trained to defend their honor without a moment’s hesitation –lashing out against their challengers with a savage violence.” This is starkly different from Singapore, which has one of the lowest murder rates in the world. Singapore’s culture is one of “law-abidingness.” There’s also not much poverty.
Page 182: Although homicide rates have fallen dramatically over the centuries, the consistent pattern is that men commit most acts of violence. This suggests that both biological and environmental factors shape aggression. Cultural norms, social expectations, and legal systems can amplify or suppress violent behavior, yet the biological distinctions between the sexes (partly influenced by testosterone) remain. What sets humans apart is our capacity to think ahead, weigh consequences, and restrain primal impulses. Money quote:
“Although humans are not held captive by evolutionary forces, genes, or hormones, we still experience their profound influences. But we are unique… in some important respects, including our ability to carefully consider the consequences of our actions and inhibit our baser instincts. The more we understand the forces that shape us, the more control we have over how we behave. As I have stressed… behaviors that show sex differences are often heavily influenced by culture… but one can’t solve a problem if one newmisunderstands its causes. Frank talk about T will help us appreciate how changes in the environment can rein in problematic male behavior. It is within our power to close or widen sex differences in aggression – but the underlying tendencies producing those differences precede culture, and they exist because of testosterone. No good can come from denying that.”
Page 183: The “Coolidge effect” is introduced through a (likely apocryphal) story about President Calvin Coolidge. The effect is the “renewed sexual interest and capacity to respond to a new potential sexual partner.” In the story, Mrs. Coolidge notes a rooster mating “dozens of times” a day and asks for this to be told to the president. When the president learns it’s “a different hen each time,” he replies, “Tell that to Mrs. Coolidge.”
Page 184: An experiment on rats illustrates the role of dopamine in sexual behavior. When a male rat is separated from an “estrous female rat” by a barrier, his dopamine levels jump about 50% in the “appetitive” phase. When the barrier is removed, allowing the rats to mate (the “‘consummatory’ phase”), the male’s dopamine “now doubles from baseline and stays very high until he ejaculates.”
Page 185: Dopamine is “crucial for the expression of all kinds of motivated behavior,” including sex. This is demonstrated by the “Coolidge effect” (not explicitly named) in male rats. After mating to exhaustion, a male rat’s dopamine is depleted and he shows no sexual interest. However, when a new female is introduced, “he gets a whiff of her fresh, estrous scent, and his dopamine starts to climb back up,” providing the motivation needed to “pursue her, mate, and ejaculate again.”
Page 186: The “Coolidge effect” is an example of the trait adaptivity mental model. The logic is that having more sex with the same female is less likely to drive reproductive success than sex with a new female. This effect relates to the “stereotype of men as being more eager than women for new sexual partners.” The text asserts that “not only is that stereotype accurate; but T has lots to do with it.” Hooven: “when it comes to having sex, men want to do it more often, and with a greater numer of people.”
Page 188: During puberty, juvenile growth allows young males to increase in size before energy shifts toward developing secondary sex characteristics like enhanced muscle mass and body composition, supported by rising sex hormones that also spark sexual and romantic interests. Early adolescents often experience crushes that evolve into more sexual tones, with most kids starting to take an interest in others but not yet engaging sexually; by age 14 in the United States, around 90% of boys and 20% of girls have masturbated, while the first real mouth-to-mouth kiss typically occurs a year or so later, and by age 18, the majority of teens have had sex. Rising sex hormones prepare the body for reproduction by influencing mate seeking, courtship, sex, and child care, affecting gene transcription to coordinate gradual changes across multiple body systems, including the brain, with long-lasting effects. In girls, testosterone levels remain low while progesterone and estrogen increase, clearly impacting sexual behavior in female mammals including humans, whereas in boys, testosterone rises sharply starting around ages nine to ten, heightening excitement about sexual possibilities. Overall, these hormonal shifts stimulate sexual desires and capacities, with notable increases in puberty and declines accompanying old age.
Page 190: Cultural norms around sex vary dramatically, from polygamy to the legality of homosexuality (“In ancient Rome, it was acceptable… in much of the Middle East, homosexuality is illegal”) to attitudes on promiscuity (women are “slut-shamed,” men are “praised as studs”). Despite this variation, an evolutionary perspective predicts a clear “biological signal”: “We would expect men to be more open to casual sex” and “prefer a higher number of sexual partners,” as male reproductive success is limited only by “the number of one-night stands they can get – and have the stamina for.” Hooven notes the nonlinearity of potential reproductive success: Genghis Khan “probably had hundreds of children.” Many of his children were proficient procreators as well, and “around one in two hundred of all men alive today have [Genghis Khan] as an ancestor.”
Page 191: The text compares reproductive strategies across species, highlighting how humans fall between extremes. Male red deer can leave offspring unattended because their young are self-sufficient soon after birth, whereas songbirds must care for their helpless chicks together. Human infants, by contrast, need prolonged parental care, though they can technically survive without fathers, their chances improve when men stay involved. This explains why pair bonding and cooperative parenting are common in human societies. For women in hunter-gatherer groups, limited fertility rates and the heavy energy cost of childrearing made reliable male investment a key factor in reproductive success.
Page 192: For most men, staying with one mate and helping raise their children is a safer and more rewarding reproductive strategy than constantly seeking new partners. Across human societies, monogamy is the dominant mating pattern, though men often gain more reproductively from having multiple mates than women do. Evolutionarily, humans maximize offspring survival when both parents cooperate as a team, supported (when possible) by extended family or community networks. Even without such help, humans have psychological adaptations like romantic and parental love that motivate commitment, cooperation, and sacrifice in raising children. Finally, women are biologically compelled to invest heavily in offspring, whereas men face more flexibility and can reproduce successfully through either long-term pair bonds or short-term mating opportunities.
Page 193: The stereotype that men are “hornier than women” is supported by large-scale data. A massive internet survey of over 200,000 people from 53 nations (including the US, Pakistan, Brazil, and China) established a clear base rate. The results showed that “on average, men have a significantly higher libido than women.” The study also used the “Sociosexual Orientation Inventory” to measure “willingness to have casual sex,” with the character Samantha from Sex and the City cited as an example of someone who would be at the “top of her sociosexuality class.”
Page 194: Men consistently score higher on “sociosexuality” than women, though the size of this difference varies by culture. “Unsurprisingly, men scored higher than women” in all countries. Money quote:
“This greater desire for sex without commitment among men is one of the largest of all psychological sex differences in humans.”
In wealthier, more gender-equitable countries (like Iceland), the gap is smaller because women express more sexual freedom. In less equitable countries (like Pakistan), the gap is larger. This suggests the “results from ‘cultural forces superimposed on biological predispositions.'” In contrast, the sex difference in libido (sex drive) “didn’t vary much by country” – as Hooven notes, “that libido is less culturally malleable shouldn’t be surprising. Our environment strongly affects what we believe and how we act, but its impact on our natural drives, like those for food, sleep, and sex, is far weaker.”
Page 195: To overcome unreliable self-reports, researchers use more objective methods, like the penile plethysmograph. Hooven humorously notes that “the penis never lies. At least, that’s what sex researchers seem to think.” (Maybe that’s where Bryan Johnson got the idea.) Sex researchers “[strap] a ring onto a man’s member that registers changes in its circumference as he watches nature documentaries or pornography, or listens to erotic recordings or classical music.” (In the case of Bauji from Mirzapur, though, that might be one and the same. Literally nobody reading this will get that reference, but you should email me if you do.)
Page 196: The page explains the Coolidge effect in human males, where sexual arousal diminishes with repeated exposure to the same stimulus, such as in pornography, but quickly rebounds when a new partner or actor is introduced, as measured by penile tumescence, ejaculate volume, and other physiological indicators. This phenomenon is strongly supported by research and appears in men regardless of sexual orientation, contrasting with weaker or absent evidence in women. Men demonstrate this effect through behaviors like out-masturbating women, being the primary consumers of pornography, and more frequently fantasizing about sex with strangers, multiple partners, or infidelity. They are also more likely to use prostitutes and take risks driven by sexual urges, leading to poor decisions (such as the famous Zoom call). Hooven, with trademark humor: “men are the main consumers of pornography worldwide, which allows them to view naked, apparently sexually eager women (or men) without working for it. (And men prefer to dispense with the ’emotional foreplay’ that women like in their pornography.”
Page 197: Men’s greater average sex drive and preference for sexual novelty is argued to be an adaptation, and “there is little doubt that testosterone is part of the mechanism.” This is supported by human data: “large increases in men’s T levels… will increase sex drive, sexual arousal, and sexual function.” T levels also respond to social cues; for example, T tends to drop in new fathers and men in committed relationships (“lower T in this situation is associated with being a more devoted partner”). And: “Conversely, higher T is more characteristic of men who are keeping an eye out for opportunities to cheat.”
Page 198: When a baby is born, a father’s testosterone (T) levels often drop further, making parenting feel more rewarding and reducing the urge to seek new mates. This hormonal shift mirrors patterns seen in some bird species, where lower T promotes caregiving over competition. How much a father’s T changes depends on his culture and level of hands-on involvement with his children. Among the Hadza of Tanzania, highly involved fathers have about 50 percent lower T levels than childless men, while neighboring Datoga men (who rarely engage in childcare) show no such drop. Physical interaction with children, such as feeding or playing, consistently lowers T, though it can still spike temporarily in response to a baby’s cries to encourage protective behavior.
Page 199: Low libido is a common issue, particularly for women; “about twice as many women as men report that they aren’t particularly interested in sex.” This varies significantly by culture: one large study found low libido was most common in women in the Middle East and Southeast Asia (43%) and least common in Northern Europe (26%). Though doctors “frequently prescribe testosterone ‘off-label'” to women for this, “there’s little reason to think it works.”
Page 201: Hooven notes that there aren’t any exclusively gay rats known, but “nonexclusive homosexual behavior is common among animals, including rats.”
Page 202: Lesbians are more likely than heterosexual women to choose traditionally male-dominated jobs involving things rather than people, such as trucking or construction. Conversely, gay men are overrepresented in female-dominated fields like nursing, hairdressing, and interior design, which focus more on people than objects. Overall, homosexual individuals tend to be more interested in careers typical of the opposite sex. They also tend to see themselves as less traditionally masculine or feminine than heterosexual counterparts. These gender-atypical preferences usually appear early in life, rather than emerging in adulthood.
Page 203: Childhood gender-atypical behavior is strongly associated with adult homosexuality, and this “has been found in diverse cultures.” Boys who shun “rough-and-tumble play” and prefer dress-up “are more likely to grow up to be gay.” A classic study that followed “feminine” boys into adulthood found that “75 percent of the men in the gender-atypical group were gay or bisexual,” whereas “All of the men in the gender-typical group were heterosexual.”
Page 204: The link between childhood play and adult orientation is hypothesized to be influenced by prenatal T exposure. For “tomboys,” the idea is “they had high T exposure in the womb, which inclined them toward masculine play.” There’s some evidence for women, but in men, the evidence is weak to nonexistent.
Page 206: One popular method for “estimating T levels in utero” is the “digit ratio,” which is the “ratio of the index finger to the ring finger.” Women tend to have longer index fingers; men have longer ring fingers. This difference is present in utero and can be manipulated by altering T levels; girls with CAH (who experience high prenatal T) tend to have “lower (meaning shorter index relative to the ring finger)” ratios, more typical of males.
Page 207: Examines the 2D:4D digit ratio as a potential, though noisy, signal of prenatal testosterone exposure. The text notes conflicting findings, such as some studies (but not all) finding lesbians have a lower ratio.
Page 209: The author argues that gay male culture acts as a natural experiment, revealing innate male sexuality without the influence of patriarchy or female preferences. This subculture is described as “full of handsiness and groping and objectification… and the ruthless pursuit of yet another conquest.” While many gay men have loving, committed relationships, the prevalence of anonymous hookups is seen as revealing. Hooven notes that “it’s not likely that culture explains why gay men are relatively promiscuous… the wider straight culture doesn’t encourage gay male promiscuity – generally quite the opposite.” The author contrasts this with lesbian culture, noting there is “nothing to stop lesbians from having their own bathhouses and anonymous hookups, but they don’t.” The conclusion drawn is that “the obvious explanation is that men are more motivated to have sex and have a stronger preference for a variety of partners. Gay men have more sex simply because they can: it’s not a ‘gay’ thing, it’s a ‘man’ thing.”
Page 215: Testosterone is described as a “foreman” for the “construction/remodeling” project of turning a boy into a man. This metamorphosis “requires tight coordination between the nervous, endocrine, and reproductive systems.” T “recruits a team of hormones” (like growth hormone, insulin, and estrogen) to manage the process. Stopping T “won’t undo all its earlier work,” as some changes are permanent (“Once they’re up, the brick walls… are also difficult to renovate”) while others, like facial hair, require “ongoing maintenance.”
Page 216: Differentiates between T’s “maintenance” effects and its “bricklike effects.” These “bricklike” (permanent) changes are “the reason that physically transitioning in the male-to-female (MtF) direction is so much harder than the reverse (FtM).” Characteristics produced by T in puberty, such as “broad shoulders, square jaw, and greater height, are obvious cues to the male sex and are difficult to eliminate or even significantly remodel or reduce.”
Page 217: Various hormones affect voice, but “none are as impactful as male levels of T in puberty.”
Pages 218-219: the larynx contains the vocal folds, two bands of tissue that vibrate to produce sound. Their tension, shape, and spacing determine pitch. Testosterone during male puberty causes these tissues to thicken and lengthen, the larynx to grow larger, and the vocal tract to widen, resulting in a deeper voice. Hooven compares this to a rubber band: thicker and looser bands vibrate more slowly, producing lower tones, while thinner and tighter ones create higher pitches. T also enlarges facial bones and nasal cavities, and lowers the larynx, enhancing vocal resonance and projection. Once these changes occur, they cannot be reversed by blocking T or adding estrogen, though female-to-male transgender individuals typically develop deeper voices within a few months of starting testosterone therapy, even if not as deep as typical male voices.
Page 221: Why do we lack hair? The idea is “survival of the sweatiest” on the sunny savanna.
Page 222: T is responsible for the development of male-pattern body hair, the “most obvious biological marker of sexual maturity and biological maleness” as one researcher puts it. The growth of terminal hair follicles (the coarser, adult hair vs the “vellus” hair that kids have) requires DHT (dihydrotestosterone).ss DHT binds to androgen receptors and increases transcription of genes that produce terminal hair.
Page 228: Puberty blockers for gender-dysphoric children are a relatively new treatment, so long-term research is limited. Known short-term risks include slower physical development, reduced bone density, and potential emotional challenges from falling out of sync with peers. While growth typically resumes after stopping blockers, delayed puberty may cause lasting effects on bone strength and emotional adjustment. Puberty blockers are generally temporary and reversible, but starting cross-sex hormones afterwards (especially testosterone) creates permanent bodily changes and lifelong medical dependence. The text concludes that about 95 percent of children who take puberty blockers eventually continue with hormonal transition.
Page 230: A personal account of gender dysphoria and starting testosterone (T). The individual felt an immediate positive effect (“Immediately after my first shot, I felt like I had been injected with an instant antidepressant”) and experienced expected physical changes like a dropped voice and hair growth, allowing them to “easily pass as a guy.” However, despite loving weightlifting and looking masculine, they remained unhappy, concluding: “No matter how well I was passing, eventually I knew that I could never get enough surgeries to feel completely comfortable as a trans man.” This realization led to detransition. The individual also noted: “Lots of the problems I had before T were still there after T.”
Page 231: “I slowly realized that my body was not the problem… now I understand that I can never actually leave my body, despite changing how other people see me, so I wish I could have just learned to be happy with myself.” Some changes from T were permanent (“I still have a masculine voice”), while others needed active management, such as laser hair removal.
Page 251: Hooven argues against “T fatalism.” Money quote: “Among healthy humans, with rare exceptions, there is not one gene or even one hormone that unilaterally causes anyone to act in any particular way or that sets anyone up for a specific kind of future.” Our behavior and agency are powerful: Hooven, discussing her familial tendency to anxiety and depression, “my behavior has a big impact on whether I feel the symptoms.”
Page 257: Discusses a 2018 Washington Post opinion piece titled “Why Can’t We Hate Men?” Hooven critiques this “vilification of half the human population,” contrasting the trend of “inspiring young girls to shoot for the stars” with men being “blamed for simply existing, for their inherent toxicity.” It argues against forgetting “the virtues that nature seems to have preferentially handed out to men…. they might sometimes feel the need to confidently explain the obvious, but they also put their lives on the line for others and are massively overrepresented in the most dangerous occupations.” She cites the Ugandan men who protected her in the jungle, noting this book wouldn’t exist without them.
Page 259: Humans are unique among animals, even our close relatives, chimps. “Sex differences in humans are small compared to many other species,” partly because “human children tend to do better with two parents” and “male-male competition isn’t as intense in humans.” Most importantly, humans have “evolved big brains and the capacity to reflect on our choices” and built sophisticated societies. The agency and culture mental models. This knowledge and self-reflection give us a “power over ourselves that no chimp comes close to having.”
Overall Rating: ★★★★★★★ (7/7)
