Testosterone – Part 1

The hormone that helps shape the male sex.

In this series of essays we’ll explore testosterone, the hormone that helps shape the male sex, through Carole Hooven’s book T: The Story of Testosterone, the Hormone that Dominates and Divides Us.

As Hooven writes:

Testosterone is present in our blood in minute quantities. Both sexes produce it, but men have ten to twenty times as much as women. Despite its insubstantial physical presence, T has managed to achieve a substantial reputation, dwarfing that of any other corporeal chemical. After all, T is an “androgen,” from the Greek “andro”—man—and “gen”—generating. If the Y chromosome is the essence of maleness, then T is the essence of masculinity, at least in the popular mind … There’s no doubt testosterone is responsible for the human male’s reproductive anatomy and physiology … Sex differences are everywhere. Some are big, some are small, some are uninteresting, and some are striking and in need of explanation. One very large sex difference is the level of testosterone over a lifetime. What role—if any—does that sex difference play in all the others? One uncontroversial role of T is to increase the height of men relative to women … Even if you hope that any disturbing conclusions about T’s effects are not true, the point I want to emphasize is that this has nothing to do with whether they are true. In general, 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. That might seem obvious, but it is something that it took me a long time to learn and put into practice.

Hooven explains the unique positioning of the testes:

Why aren’t the testes always stashed away inside the body, like the heart and brain? In all mammals, during embryonic development the testes start out in the abdomen, near the kidneys. And in most mammals, including humans, the fetal testes descend into the scrotum during the latter part of pregnancy, as a result of testosterone’s actions … One thing every guy knows about the scrotum is that it isn’t simply an inert ball bag. When a man wades into cold water, he can feel the muscles in the upper scrotum (the cremaster muscles) contract to pull the testes closer to the warmth of the body—sometimes pulled in so tight that it hurts. And when he smushes them under his hot laptop, the muscles relax and loosen their grip in an attempt to get them to hang low, farther away from the body. We know that the scrotum acts as a climate control system, keeping the testes at a temperature that optimizes sperm production—about four degrees lower than the temperature inside the body.

More generally, Hooven writes:

[A]dult men without testosterone are fatter and weaker and have smoother skin than other men because normal male levels of T reduce fat and increase muscle, bone strength, and body hair … We now know that the testes are part of the endocrine system, the network of glands that regulates basic processes of animal life, like growth, metabolism, hunger and thirst, reproduction, circadian rhythms, and body temperature, in addition to related behaviors like eating, sleeping, fighting, parenting, and mating. We also know that the testicular masculinizing agent is the hormone testosterone. Mammals have at least nine endocrine glands, but the testicles are the only visible, readily accessible ones. Female animals lack testes, so have much lower levels of testosterone than males. If we want to understand what it means to be a male human, and the ways in which boys and men are different from girls and women, we need to understand T … [C]hemicals, produced by special glands, traveled through the blood and affected distant tissues, regulating and coordinating bodily functions. But that was just the beginning, and discoveries in the field moved rapidly. In a short period beginning in 1929, the three estrogens (including estradiol, commonly called “estrogen,” along with the far less abundant estrogens, estriol and estrone) were discovered. Shortly after that came the discovery of testosterone … One physiologist at the University of Amsterdam, Ernst Laqueur, even cofounded a company called Organon (still in business today, as a subsidiary of the Merck pharmaceutical company) in close proximity to several slaughterhouses, providing him with easy access to the testicles of slaughtered bulls. In 1935, he collected 100 kg (about the weight of a baby elephant) of bull testes. From these, he harvested a minute 10 mg (less than the weight of a grain of rice) of a chemical that he injected into a castrated rooster to determine the extent to which it could regenerate his comb. (This had become a standard test to determine the extent to which a particular substance could induce masculine traits.) The chemical regenerated the cock’s comb, replicating the effects of testes transplantation. Laqueur dubbed it “testosterone.”

Hooven describes the role of hormones:

Finding the truth amid the hype around T will be impossible without a basic understanding of hormones in general, so let’s take a look at this vitally important class of chemical communicators, of which T is a member … All plants and animals—in fact, all multicellular organisms—have hormones. In animals, they fall primarily into two categories: protein hormones and steroid hormones. The protein hormones include insulin and melatonin and are made from amino acids (the building blocks of all protein). The steroid hormones include testosterone, other androgens such as dihydrotestosterone (DHT) and androstenedione, and estrogen; they are all made from cholesterol. (Cholesterol is important not least because it’s a major component of cell membranes.) Hormones are produced by various glands and tissues: melatonin is produced by the pineal gland; testosterone and estrogen are produced by the testes and ovaries (and other tissues); and insulin is produced by the pancreas. All of our hormones circulate in our blood and can be thought of as carrying information to various parts of the body. Since hormones go anywhere the blood goes, they circulate pretty much everywhere … In the complex system that is our body, this communication is carried out by “chemical messengers.” Animals have two main types of such messengers: neurotransmitters—facilitating communication in the nervous system (the brain and spinal cord)—and hormones. 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.” They are sent out by the hormone-producing glands (and other hormone-producing cells) into general circulation (the blood system), but their signals are only detected by cells that have specific receptors for them. The network of endocrine glands and cells makes up the endocrine system. Cells that can respond to a given hormone are the hormone’s “target cells.” The receptors for the protein hormones are embedded in the outer membranes of target cells, and receptors for the steroid hormones, including testosterone, are located on the inside of target cells … This is how hormones influence your brain and behavior. They facilitate communication between your body and brain (and vice versa), to coordinate your desires and actions with your physical needs. Hormones and neurotransmitters are there because the blind forces of evolution put them there, which is to say that they are ultimately in service of survival and reproduction … But how do a body and brain exchange information and make decisions about saving or spending energy? When to start and stop growing? Should we invest in staying healthy, or is there enough energy for playing, courting, fighting for mates, or making milk for our babies? … [Testosterone] acts in the body, and it also sends information to your brain. Like insulin, testosterone coordinates physical and behavioral processes, but rather than regulating blood sugar, its focus is on developing and supporting reproduction. For men, successful reproduction has less to do with energy-sucking reproductive physiology—growing and feeding a baby with one’s own body—and more to do with finding, competing for, and attracting mates. If T is relatively high (in the typical male range), it works in a man’s body to promote muscle growth and sperm production, and it also tells the brain what the body is up to. High insulin shouts, “Lots of energy down here, use it!” High T shouts, “Lots of sperm ready to go!” Testosterone helps males do what they need to reproduce—as castration throughout the ages has shown.

As Hooven explains, members of both sexes have the genes for lots of the same things, but they come to express different genetic traits because testosterone triggers some traits in males, but not in females, and also because males have a Y chromosome that females don’t have:

The reason that boys develop a penis and girls do not, or that men have lots of coarse facial hair and women develop breasts, is not that the genes for these traits are possessed solely by one sex or the other. Females don’t have exclusive ownership of genes that result in the development of milk-producing breasts or wide hips, and males do not have sole rights to the genes for a deep voice or facial hair. 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. Humans have some genes that differ by sex, because males typically have a Y chromosome and females don’t. But the number of genes on the Y is minute—about seventy—relative to the twenty thousand to twenty-five thousand genes that populate the other twenty-two pairs of chromosomes. But don’t underestimate the power of the diminutive Y, which can pack quite a punch. One of its genes makes all the difference in the world … The forty-six chromosomes in each of our cells contain the whole human “genome”—all of our DNA (deoxyribonucleic acid). DNA is a molecule shaped like two long springs pushed sideways into each other. End to end, there’s about six feet of it in each cell, and all the DNA from all of your cells could stretch to the sun and back two hundred times. Your genes reside in your DNA and are strings of “chemical letters” (or “bases”) from a very small alphabet—A, C, T, and G—that provide the instructions for making proteins. So each gene is a kind of recipe for a protein. (In the jargon, the gene “codes for” the protein.) The gene lists the necessary ingredients and specifies the order in which they should be put together. But instead of combining butter, sugar, and flour, proteins are made by stringing together chemicals called amino acids. In humans, there are twenty-one types of amino acids to choose from. Take the gene for the hormone insulin, which calls for fifty-one amino acids to be strung together. Since there are only twenty-one types, some amino acids will be used more than once, just as the ingredients for one cookie might include ten chocolate chips and four walnuts. Cookie recipes need to be read, and then the ingredients need to be mixed and baked. In the case of genes, they are transcribed and then translated into proteins. This entire process is called gene expression … [Cells] specialize in making certain kinds of proteins, depending on the type of tissue in which they reside, and so rely only on certain sets of instructions. The instructions for the majority of the other proteins that could be made are crumpled up and ignored. (Literally: most of the DNA in every cell is squished up into something called chromatin, which is the DNA wound around proteins so that it is not open for translation.) By adulthood most of our cells have already differentiated (a tiny minority of stem cells remains). Each of our cells retains all our DNA—the whole genome—yet produces proteins from only a small selection of genes. Inside the cells in women’s facial skin, the recipe for making dark, thick hair is crumpled up in the back of the shelf—so most women have only a little bit of it on their face. But inside the cells in men’s skin, the same recipe is always out, prominently displayed, to be transcribed and translated over and over again. Typically, each sperm carries either an X or Y chromosome, and all eggs carry one X. Whether all the embryo’s cells inherit XY or XX sex chromosomes depends on whether the fertilizing sperm contains a Y or an X … Ultimately, the Y chromosome … cause[s] the cluster of cells that comprises the primordial gonad to form testes rather than ovaries … I’m throwing the terms “male” and “female” around here, but I haven’t yet explained what they mean. You might think you know the answer to that, too, as I thought I did before grad school. I believed that XX and XY chromosomes defined femaleness and maleness. But that’s not how it works, even though those chromosomes are distinctive of male and female mammals. XX and XY chromosomes are traits that are features of sex (in mammals), not ones that define sex. In humans, sex is usually determined at conception, based on whether the sperm contains a Y or X sex chromosome … What do all males (or females) have in common, if not sex chromosomes? Basically it’s the relative size of the sex cells or gametes. Males produce small, mobile gametes (sperm), and females produce larger, immobile gametes (eggs). Don’t take that too literally—my son doesn’t yet make sperm, but he’s still male. And although my ovaries are no longer regularly producing eggs, I’m no less female than when they were cranking them out on a monthly schedule. Rather, it’s the design plan for the gametes that counts … Making a female, in many ways, is easier than making a male—the external structures develop in the female direction in the absence of any hormonal signal.

Interestingly, male humans have nipples because all human embryos, regardless of sex, start developing along a similar pathway in the early stages of fetal development. In the early stages of embryonic development, both male and female embryos follow a similar developmental blueprint. At this point, the embryo has structures called mammary ridges or milk lines, which are the precursors to nipples and mammary glands. The differentiation of sexual characteristics begins later in fetal development. The presence of the Y chromosome and the SRY gene on it triggers the development of male-specific characteristics, such as testes and the production of male sex hormones like testosterone. However, by the time this differentiation occurs, nipples have already formed.  These male nipples hang around because there is no strong evolutionary pressure to eliminate nipples in males. Since nipples in males are generally neither beneficial nor harmful, they persist across generations as a neutral trait.

Hooven then explains the effect testosterone has on the male brain:

Does testosterone affect boys’ brains to bias them to behave in typically masculine ways? Or are the T skeptics right, and our brains are closer to gender-neutral blank slates, to be written on by the pink and blue chalk of a gendered social world? You might wonder how we could ever find out, since both the hormonal and social explanations can seem equally compelling. Luckily, there is a large body of research that can give us important clues … I raised the question of whether testosterone affects the fetal brain, leading boys to behave in typically masculine ways. What makes that a challenging question to answer is that fetuses with a lot of T on the brain are typically born with normal male genitalia, and so are usually sexed as boys and likely subject to potentially masculinizing social influences. How could we know if T masculinizes behavior directly through its actions on the brain or indirectly through the body, or both? If we could somehow shoot a lot of T into the brains of fetuses and simultaneously ensure that the children were born looking just like girls, that would help to resolve the issue. The babies would then be subject to the usual feminizing social influences. If T had no direct effects on the brain, then the babies would grow up behaving like ordinary females. On the other hand, if the babies eventually played more like boys—like “tomboys” perhaps—then that would be evidence that high levels of T in utero had primed them for later masculine behavior. As you’ve probably gathered by now, we needn’t try to get this experiment through the ethical review board … [T]here are people who have experienced this naturally … [T]he object of intense study—which has made clear that boys’ and girls’ brains are not gender-neutral blank slates. In the early 1970s, Julianne Imperato-McGinley, an endocrinologist at Cornell Medical College in New York, heard of a group of “girls” in the Dominican Republic who became men at puberty. She and her research team trekked out to their remote village, accessible only by a dirt road, to meet them. Imperato-McGinley eventually studied thirty-three such people in two villages. Nineteen of them, according to her research, had been “unambiguously raised as girls.” … [G]ender roles were relatively strict in the Dominican Republic. By the time children were seven or eight years old, the expectations for boys and girls diverged sharply, and they played only in single-sex groups. Imperato-McGinley reported that boys were given wider latitude to “romp and play” and were expected to help out their fathers in the fields, planting and collecting crops and tending to livestock. The girls were supposed to help their mothers with the cooking and cleaning, getting water, and bringing food to the boys and men working in the fields. While the older boys and young men attended cockfights and visited the local bars, the older girls and young women tended to stay home with female relatives, looking after younger siblings. The villagers called the people whom Imperato-McGinley had come to see, who were raised as girls but who developed into men, guevedoces, sometimes translated as “penis at twelve” or “eggs [testicles] at twelve.” (Machihembras, “first women, then men,” was another name for them.) … Living as young girls between the ages of seven and twelve, they began to realize they were unusual. Other girls began to develop breasts, but the guevedoces did not. Testicles appeared instead, and the “clitoris” began to grow into a small penis. Seventeen of the guevedoces had transitioned from living as a girl to living as a boy at or after puberty, when they began to feel sexually attracted to girls … She believed her data demonstrated that “androgen (i.e., testosterone) exposure of the brain in utero, during the early postnatal period and at puberty has more effect in determining male gender identity than does sex of rearing.” She noted that “in addition to behavioral differences, androgen-induced sexual differences in brain morphology and function have been well-documented in animals.” Notably, rodents. Humans, it appeared, were just another animal in this regard … [I]n 1959 … a team at the University of Kansas Medical School, led by the legendary endocrinologist William C. Young [published a landmark paper] … In that paper, [Young] reported on an experiment that tested whether testosterone affects neural development during critical periods in utero or shortly after birth, in ways that promoted adult male sexual behavior. If testosterone had such an organizing action, then administering T to a female fetus should predispose it to adopt male sexual behavior when given T again as an adult. The idea was that the second, adult dose would activate the areas of the brain that had been organized by T during early development. Young and his team treated female guinea pig fetuses with T in utero by giving high levels of it to the pregnant moms, and they also removed the ovaries of the androgenized females after birth so that they could have complete control of their sex hormones. A female guinea pig who had been exposed to T in utero was born with something that looked like a penis—so her genitalia were clearly masculinized. But what about her brain? When she was given T in adulthood, would that androgenized female act like a male and try to mount a sexually attractive female? Or, when given the hormones that induce estrus—estrogen and progesterone—would she still bend into lordosis in the presence of a sexually attractive male? Young discovered that when a fetally androgenized female was given T in adulthood, she would behave like a male guinea pig: she would vigorously attempt to mount estrous females. But when that same female was given the estrus-inducing hormones estrogen and progesterone instead of T, she showed no interest in a normally sexy adult male and she failed to adopt the lordosis pose. Her fetally androgenized brain could not respond as that of a typical female to the usual hormones of estrus in adulthood. (And the fact that her ovaries had been removed had nothing to do with it. Non-androgenized female guinea pigs who had their ovaries removed did show lordosis when given female hormones.) Fetal exposure to high T had squashed the capacity for normal female sexual behavior. Since behavior is underpinned by the nervous system (the brain and spinal cord), Young concluded that high testosterone in utero had altered the female guinea pigs’ brains. 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 … In 1972 the basic results of Young’s paper were replicated in a study of rhesus macaque monkeys, and the evidence for the organization/activation hypothesis has been steadily accumulating in humans and other animals since then.

Hooven then describes the connection between testosterone and aggression:

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 toward other, more sensible activities like finding food; or it could just be conserved, by resting. And small, inexperienced animals frolicking, oblivious to their surroundings, are the perfect targets for stalking predators. So why do they do it? You might be thinking, “Well, obviously, because it’s fun!” And that’s right: “Because it’s fun” is what biologists call a “proximate” explanation, which specifies the mechanisms (psychological, biochemical, or even social) that underpin a particular trait or behavior. But there’s also an “ultimate” explanation that addresses the evolutionary history of the trait. 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. Such play behavior increases reproductive success, and so over evolutionary history it has become a prominent feature in the young of many different mammalian species. For many vertebrate males, success in the mating arena depends on success in the dominance arena. As with other crucial adult skills, like finding food and avoiding predators, dominance skills don’t magically materialize once the hormones of adolescence kick in. Juvenile play allows animals to develop adult skills. Among adult male rats, dominance pays off. High status is achieved via winning sometimes vicious fights, which cause the loser to behave submissively. Dominant males who can win fights mate more. Male rats are more aggressive than female ones (who can also be quite aggressive, such as when defending their kids). This sex difference is the product of an evolutionary history in which aggression paid greater reproductive benefits for males than for females … [D]espite the vast gulf between Rat and Man, it’s unsurprising that sex differences in rat play mirror those of boys and girls in some important ways. When male animals are experimentally prevented from playing this way, they grow up to be evolutionary losers. They are poor fighters who readily submit to intruders, have a lowly dominance status, and do not do well in the mating arena … One of the world’s experts in the neurochemical and hormonal underpinnings of sexual behavior, in humans and other animals, is James Pfaus, a professor of neuroscience and psychology at Concordia University in Montreal. Pfaus, who has spent much of his career investigating the relationships between human and animal sexuality, points out that the basic systems that mediate sexual responses in other mammals are largely retained in humans due to common evolutionary descent: Identification of common neurochemical and neuroanatomical substrates of sexual responding between animals and humans suggests that the evolution of sexual behavior has been highly conserved [i.e., preserved over evolutionary time] and indicates that animal models of human sexual response can be used successfully as preclinical tools. A “preclinical tool” is an initial study, usually on animals, that evaluates the effectiveness of a particular treatment, like drugs or surgical procedures, before it is tried out on humans … Without research on rats and other animals, modern medicine would not exist … [T]he overall picture supports the organization/ activation hypothesis for humans. The evidence suggests that sex differences in play behavior in particular are to a significant extent due to differences in prenatal T exposure.

Hooven then explains that:

Congenital adrenal hyperplasia (CAH) is a rare genetic disorder, present in about one out of every fifteen thousand births, that affects the health of both males and females, but significantly affects the behavior of girls only. CAH fetuses are exposed to unusually high levels of testosterone, but in areas where families have access to modern medical care, the hormonal imbalance is typically corrected shortly after birth. (CAH is called a “disorder” here because it has effects on health that require medical treatment.) CAH girls, then, differ from unaffected girls in that they were exposed to high T levels (typically not as high as male levels) throughout their fetal development. The ways in which the behavior of these girls differs from that of other girls offers an opportunity to investigate the effects of early androgen exposure on the developing brain in humans. The disorder is caused by a mutation in one of the genes …  If the organization part of the organization/activation hypothesis applies to humans, then girls who have had high levels of androgens during an early critical period should also show higher levels of male-typical behavior in childhood, similar to the effects seen in rats and monkeys. So do CAH girls behave more like typical boys? Before getting into that question we should have average differences in the behavior of boys and girls firmly in mind. Rather than relying on anecdote or personal experience, let’s start by looking at one classic experiment that elegantly illustrates differences in how boys and girls tend to interact in social groups. Researchers divided eighty preschoolers, ages four and five, into groups of four kids each, all single sex. They told the children that they would be able to watch a cartoon through a viewer. But there was a catch—in each group, only one kid could watch the cartoon through the viewer at a time, and two others had to cooperate to run the viewer. One had to turn a crank, and another needed to keep pressing down on a light switch. The fourth just had to hang out. The researchers gave instructions to each single-sex group, and then left the small children on their own. Apparently, the boys enjoyed the whole project more than the girls. They laughed and smiled even as they hit and shoved each other while fighting over rights to the viewer. Girls were no less competitive about the movie viewing time—they just used different, less direct tactics to get what they wanted. Girls used more “unfriendly” commands than the boys, but they were also more likely to offer their position with the viewer or the crank to other girls. Boys were more likely to use physical contact to assume a desired position. Overall, where boys used their bodies, girls used their words. Boys pushed, pulled, or hit their partners about six times more than girls did. It’s not that boys and girls don’t ever use the same strategies—of course they do. Some girls are just as physical as many boys, and some boys are gentler and more apt to use verbal persuasion. But overall, as is found in study after study, boys are far more likely to compete physically to get what they want … Before the age of about two or three years old, kids toddle around with little attention to gender. But for most kids, as soon as they come to understand that they are a boy or girl, in diverse cultures all over the world, they begin to gravitate toward their own kind. The overwhelming majority of children’s playmates are members of their own sex, and this gender segregation peaks around the ages of eight to eleven. Younger kids appear to be drawn to the sex that is playing in a way that they find appealing, leading to fairly loose patterns of segregation. But as kids develop, playing with one’s own sex, no matter what they are doing, becomes more important. Preferences for larger groups and rougher, more active play, versus for smaller groups with more talking and domestic themes, drive boys and girls into same-sex groups. Sex differences in children’s play presage differences that have been amply confirmed in adults. We can see the seeds of sex differences in aggression, parenting, social hierarchies, and preferences for people versus things in our younger selves. In studies of how the exposure to high levels of prenatal T in CAH affects children’s behavior, play gets the most attention. And that’s not surprising. Play is what kids like to do with their spare time, and there’s no bigger difference in the behavior of boys and girls than in the way they play. For example, in a 2005 study, researchers studied two groups of three- to ten-year-old children—with and without CAH—and gave them the opportunity to play with a variety of toys. The children could choose from toys that, according to previous research, were strongly preferred by one sex or the other, or equally by boys and girls. The “girls’ toys” included a cosmetics kit, dishes, and dolls with different outfits to put on; the “boys’ toys” included building logs and blocks, a gun, a tool set, and various vehicles; and the “neutral” toys included puzzles and crayons and paper for coloring. (To be clear, these toys were not selected because they were thought to be appropriate for, or naturally appealing to, one sex or the other; they were selected because they were consistently preferred by either boys or girls in previous studies.) The boys and girls who did not have CAH—the “unaffected” children—made unsurprising toy choices. Boys spent most of their time playing with the boys’ toys, and girls spent most of their time playing with the girls’ toys (remaining toy-time was with “gender neutral” toys). The more interesting results were the comparisons between the toy choices made by unaffected girls and those with CAH. The CAH girls played mostly with boys’ toys. They spent only 21 percent of their time with girls’ toys but 44 percent with the boys’ toys. In contrast, the unaffected girls showed the reverse pattern: they spent 60 percent of their time with girls’ toys and only 13 percent with the boys’ toys (unaffected boys spent 70 percent of their time with boys’ toys and only 6 percent with girls’ toys). The girls with CAH showed play preferences that were much more masculinized than their unaffected peers. (See the graphic depicting the results below.) These results are typical of findings from the research of CAH and behavior in two main respects. First, CAH girls’ play is masculinized. Second, CAH girls don’t play just like boys, but they play more like boys than do unaffected girls. CAH girls’ play is midway between that of typical girls and typical boys.

Over one hundred published studies since the late 1960s report on the effects of CAH on gendered behavior and have employed paradigms similar to those in the 2005 study on toy preferences discussed above. These studies confirm that the play behavior of girls with CAH is masculinized. That is, relative to girls with normal levels of androgens in utero, girls who were exposed to higher levels of androgens play more like boys. Compared to age-matched unaffected girls, CAH girls gravitate toward rough-and-tumble play; choose toys such as trucks, planes, and blocks; and have a greater preference for playing with boys. And this tendency toward masculine behavior and preferences also extends into adulthood—CAH girls grow up to be women who are more likely to prefer male-typical professions like carpentry, that involve working primarily with things, rather than female-typical professions like teaching, that involve more interactions with people … [Also,] [c]ompared to heterosexual women, lesbians are more likely to be attracted to male-dominated occupations—such as truck driving, building contracting, and appliance repair—that have more to do with things than people. And gay men are overrepresented in female-dominated occupations—such as hairdressing, nursing, and interior design—that have more to do with people than things. In other words, relative to heterosexual people, homosexual people are more interested in careers that are typical of the opposite sex … Evidence from experiments in mammals, from guinea pigs and rats to rhesus macaque monkeys, shows that when females are given high T in utero, their behavior is masculinized, and when males are deprived of it, their behavior is feminized. Prior to puberty, the behavior that is most affected is play style—T-exposed females play more like males. This makes perfect sense in the light of evolutionary theory. Males and females have different reproductive interests, which are served by different play styles in infancy. And research on the behavior of people who have natural differences in testosterone’s actions or levels … strongly suggests that humans are no exception … The conclusion seems inescapable: as best we can tell, T makes boy brains … [S]exual asymmetries that are common across mammals—most relevantly, the males are able to produce offspring at a faster rate than females. This asymmetry begins with the size and quantity of eggs (large and limited) versus sperm (small, plentiful, and continuously produced) and continues with the nature of the female versus male mammalian bodies. Females must use their bodies to host and feed their developing offspring, during which time they are unable to produce another offspring. But most male mammals contribute only DNA to each offspring and are free to invest the “extra” time and energy into the pursuit of additional mates. These differences lead to a predictable, sex-based behavioral pattern: males tend to prioritize competing for mates, and females prioritize acquiring the resources they need for health and survival and selecting fit males with whom to mate … Behavior is always a product of interactions between an animal’s external environment and its biology, including its genes. And—to repeat one of the main points of this book—testosterone’s primary job is to coordinate male sexual anatomy, physiology, and behavior in the service of reproduction. For many male animals that must compete for mates, like the red deer of Rum, one of the behaviors that most directly supports reproduction is aggression. T’s central role in male violence is well established for many nonhuman animals. Could men really be exceptions?

On the flipside of aggression, perhaps, is crying.  And as Hooven writes:

Women do cry far more than men on average, even though rates of crying among male and female infants don’t differ. For the most part, as girls progress from childhood through adolescence, their crying doesn’t change much. But the same cannot be said for boys, whose tears seem to dry up on the way to manhood. For me, the grass is greener on the other side. I wish I had a little more insulation. Women not only cry more but they have higher rates of depression, and having lower T may have something to do with it.

In the next essay in this series, we’ll examine how testosterone creates huge sex differences in sports performance.