Testosterone – Part 2

How testosterone creates huge sex differences in sports performance.

Continuing this series of essays on testosterone, through Carole Hooven’s book T: The Story of Testosterone, the Hormone that Dominates and Divides Us, this concluding essay will explore how testosterone creates huge sex differences in sports performance.

As Hooven writes:

One thing that is not in dispute is the large sex difference in sports performance. We can rely on one athlete in particular not to sugarcoat the facts: seven-time Grand Slam winner John McEnroe. In a 2017 interview on National Public Radio, host Lulu Garcia-Navarro asked McEnroe why he had called Serena Williams the best “female player” in the world. Why hadn’t he called her the “best player in the world, period?” “Because,” replied McEnroe, “if she played in the men’s circuit she’d be like 700 in the world.” He went on to call her an “incredible player” but reiterated that she’d have no chance against the best men. Predictably, McEnroe was denounced as a sexist, and a clearly irritated Williams fired off a couple of stinging tweets in reply. Still, even Williams had expressed agreement with the underlying sentiment. When Williams appeared on The David Letterman Show in 2013, Letterman asked her what would happen if she played one of the top men’s tennis players. Williams explained how she would lose, badly, to the reigning world champ, Andy Murray: “Andy Murray, he’s been joking about myself and him playing a match. I’m like, ‘Andy, seriously, are you kidding me?’ For me, men’s tennis and women’s tennis are completely, almost, two separate sports. If I were to play Andy Murray, I would lose 6–0, 6–0 in five to six minutes, maybe 10 minutes. No, it’s true. It’s a completely different sport. The men are a lot faster and they serve harder, they hit harder, it’s just a different game. I love to play women’s tennis. I only want to play girls, because I don’t want to be embarrassed.” … [S]ex differences in testosterone contribute to men’s superior performance over women in most sports. In particular, what is the evidence that testosterone really confers an athletic advantage? Let’s start with those halcyon days when the genders are nearly equal on the track—childhood. Meet two fictional fraternal twins: Sofia and her brother Samuel. Sofia and Sam are quite ordinary. In elementary school, Sofia loved math, gossiping with other girls, playing Little League, and baking with her mom. Sam enjoyed drawing comics, playing the piano, and having laughter-infused wrestling matches with other boys. Despite being boringly average, Sofia and Sam are unusual in one respect. As they got older, they started to obsessively compete against each other in sports. At age six they ran against each other in the 30-meter dash. At ten they competed in the mile and the 25-yard freestyle swim and attempted to outthrow each other in the javelin. As they passed through the turmoil of adolescence, their passion for sports only intensified, and now in their twenties they are running marathons together, pole vaulting, and weight lifting—the works. No prizes for guessing who wins these contests of sibling rivalry. Yet up until around age ten, neither Sofia nor Sam could consistently outrun or outthrow the other, and neither had bragging rights in any sport. It wasn’t until Sam hit puberty at around age twelve that he started to really pull ahead. At fifteen, he was more than four seconds ahead of Sofia in the 30-meter dash. He could throw much farther, and with more accuracy. (The difference in throwing is vast. As one researcher put it: “Nearly every boy by age 15 throws better than the best girl.”) Long-distance swimming is one of the very few sports where Sofia has the advantage … [T]he most basic categorization, in almost every elite sport, is sex. And, until recently, the reason for that separation was obvious and uncontroversial. Women’s world records are consistently about 10 percent lower than those of men. For example, the marathon world record for women is two hours and fourteen minutes, around twelve minutes slower than the world record for men, two hours and two minutes, set by the Kenyan Eliud Kipchoge when he was thirty-three. This sex gap in performance means that in many events, thousands of male athletes are ahead of the very best female. In 2019, about twenty-five hundred men, almost one-third of the total number of men competing worldwide in the IAAF 100-meter event, beat the fastest women’s time. Without segregation, it’s not just that men would win—women would never even qualify for the competitions in the first place … [A]ll the evidence points to the same conclusion: a male level of T, in puberty and adulthood (with possible contributions from prenatal T), is the master key for superior performance in most sports.

Regarding the evidence:

David Handelsman [and other researchers] evaluated thirteen studies, published from 2005 to 2017, that met their high standards for eligibility. Only studies that reported on T levels sampled from blood (rather than saliva) were included, since this method is the most accurate, particularly for women. Handelsman concluded: “Circulating testosterone in adults has a strikingly non-overlapping bimodal distribution with wide and complete separation between men and women.” … We already saw a “bimodal” distribution in the first chapter—the distribution of adult height. A bimodal distribution has two peaks. And in the case of height, the male and female distributions look like two mountains with wide, overlapping bases. In other words, male and female heights significantly overlap, since some men are shorter than many women and some women are taller than many men. But with testosterone levels, that bimodal distribution has a “wide and complete separation,” like two mountains separated by a vast plain, as you can see in the figure below. In other words: a binary.

Hooven then returns to her hypothetical pair of fraternal twins, Sam and Sofia:

Sam’s advantages are largely due to T. And it all starts in puberty. Sam experienced heightened exposure to T at four different times. He had a big boost in T in the womb, which masculinized his reproductive system and brain. He also enjoyed a large elevation in T levels shortly after birth (the purpose of this “mini puberty,” as it’s called, is not well understood). His T soon dropped to baby Sofia’s rock-bottom levels, until the next big burst at puberty. During this critical period for physical development, Sam’s T level increased by twenty to thirty times, while Sofia’s T budged up slightly. And then there’s the rest of his life—Sam’s T levels peaked at around age twenty, plateaued for a few years, and then slowly declined. In Western populations, the average decline is about 1.2 percent per year after age forty … Puberty is a critical period for the development of traits that contribute to athletic success. In both of the twins, it was kicked into gear by the hypothalamus, an evolutionarily ancient, almond-sized structure deep in the vertebrate brain that acts as a kind of bridge between the nervous system and the endocrine system. The hypothalamus began to send pulsatile signals to the pea-sized pituitary directly below it, in the form of gonadotropin-releasing hormone (GnRH). In response, the pituitary began to send its own signals—called luteinizing hormone (LH) and follicle-stimulating hormone (FSH)—through the bloodstream down to Sofia’s ovaries and Sam’s testes. This system (depicted below) is called the hypothalamic-pituitary-gonadal (HPG) axis and is front and center in the control of the production of our sex hormones, along with eggs and sperm. HPG axis Sofia started puberty at a typical age for females, around age eleven. Soon after her ovaries received the signal from her pituitary gland, her pubic hair grew, breast buds appeared, pimples started to pop up, and her growth in height accelerated. She got her first period a year later. Sam started puberty at twelve and a half (boys start one to two years after girls), also developing pubic hair and oily skin. In addition, his penis and testicles enlarged, his voice began to crack and drop, and eighteen months later he experienced his first ejaculation. Sam was soon taller than his mom. Although Sofia’s growth spurt started before Sam’s, it also ended earlier.

Sam might be able to get away with just producing sperm, but his extra muscle and larger body size help him compete for the status or resources that will help him to attract mates and provide for his offspring. So in addition to developing his reproductive system, high T helps Sam to grow a bigger body than Sofia’s, including stronger and longer bones and increased muscle mass … T influences stem cells—those that haven’t yet decided whether to become fat or muscle—to take the muscle route, and it actively discourages them from turning into fat. Higher T in Sam also enlarges those muscle fibers, resulting in stronger, larger muscles. Sofia’s higher estrogen and lower T means that, compared to her brother, she converts more of her consumed energy into fat rather than muscle. Across puberty, while both Sam and Sofia put on fat, Sofia’s rate of fat accumulation is twice as fast as Sam’s! When their bodies cease their pubertal development and have stabilized in their adult forms in their late teens, Sam’s lean body mass (everything except fat) is about one and a half times that of Sofia’s. While that fat will fuel her ability to produce children, when it comes to sports, for the most part, the extra fat just means she has more dead weight to carry than her brother. It’s as if Mother Nature has come along and strapped a few bags of flour on Sofia, giving Sam nothing to carry. This means Sofia is not going to win a pull-up contest against her brother. It’s worth taking a moment here to emphasize that Sofia is not “biologically destined” to lose that pull-up contest. Differences in all kinds of habits, even in words of encouragement from the parents, might allow Sofia to beat her brother in any number of sports. If Sam blobbed out on the sofa all day, playing video games and eating cupcakes, and Sofia spent her time training and eating healthily, she’d improve her prospects immensely. There’s no question that environment, upbringing, culture, and personal habits affect what people are capable of athletically. But all else being equal, Sam is the one likely to enjoy the sporting advantage. And not just because he has more muscle; bone differences matter, too … Sam’s larger and stronger bones are a result of his increased muscle and higher T levels. In puberty, bone architecture is especially sensitive to mechanical load and develops in response to it. And the muscles of pubertal boys apply greater load to the developing bones than do those of girls. Stronger and bigger muscles constantly pulled on Sam’s bones, causing them to respond by increasing mineral density and diameter. All of these effects—lengthening, enlarging, and strengthening—are, for the most part, permanent. Sam would have a bone strength advantage (not to mention height) over Sofia even if he reduced his T to match hers in adulthood. Plus, the elevation in T at puberty increases Sam’s hemoglobin levels, as it does in nearly all adult mammals. (Men’s levels are about 12 percent higher than women’s.) Hemoglobin is a protein inside red blood cells that carries oxygen from the lungs to working muscles, fueling their action and increasing endurance, among other benefits … [H]igh T will also maintain Sam’s secondary sex characteristics, most relevantly his bone strength and increased muscle mass. In addition, his high T will keep his hemoglobin levels pumped up, giving him greater aerobic power. And with T continuing to keep his fat low, along with his intense training, Sam’s athletic dominance over Sofia is assured.

As Hooven explains:

Sports are sex-segregated because of the advantages that male puberty brings, together with their continued maintenance by T during adulthood. Without sex segregation, people who didn’t go through male puberty would effectively be shut out of elite competition … The dilemma arises because some athletes who … want to compete in the women’s category have also experienced the physical benefits of male puberty, and those benefits don’t all disappear with the testosterone reduction that comes with a male-to-female gender transition. When evaluated after a year of T-suppressing medication, along with increasing estrogen, many of the sports-related benefits of high T decline significantly. For example, hemoglobin plummets to female levels. But bone size (including height, of course) doesn’t budge, and much of T-induced bone strength is also retained. Experts and activists debate the question of just how much strength and muscle volume drop, but evidence shows that male-typical levels of muscle mass and strength are not completely lost, although the changes vary widely among individuals. In some trans women, no muscle at all is lost, and in others, levels drop considerably. One finding is clear, and that is that the muscle gain in female-to-male transgender people, who move from female to male levels of T, is significantly larger than the muscle lost in the other direction.

This explains why a biological male’s reducing their testosterone levels if they want to complete with biological females is not a solution to the problem of sports mis-match following puberty: testosterone at puberty has already supercharged his body in ways subsequent lower testosterone levels won’t take away. Similarly, as Hooven writes: “Testosterone’s bricklike effects are the reason that physically transitioning in the male-to-female (MtF) direction is so much harder than the reverse (FtM). Many of the secondary sex characteristics that T produces in puberty, like 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.”

The result is that, as a 2024 report by the United Nations states, “Policies implemented by international federations and national governing bodies, along with national legislation in some countries, allow males who identify as women to compete in female sports categories. In other cases, this practice is not explicitly prohibited and is thus tolerated in practice. The replacement of the female sports category with a mixed-sex category has resulted in an increasing number of female athletes losing opportunities, including medals, when competing against males. According to information received, by 30 March 2024, over 600 female athletes in more than 400 competitions have lost more than 890 medals in 29 different sports.”

Hooven also explains the interesting reasons testosterone levels in males drop when they take part in child care:

[I]n humans, children are more likely to survive and thrive when their fathers provide care, which helps to explain why many men invest time and energy in their children. And, just like song sparrows, a human dad’s T levels tend to drop when they are in a romantic pair bond and involved in caring for their offspring. The links between T changes and behavior in men aren’t as tight as in nonhuman animals, but the evidence is consistent with the idea that this decline in T helps direct attention away from rivals and new sexual prospects and toward one’s mate and children … Prior to the surge of sex hormones in puberty, most kids think anything to do with sex is gross—possibly because they feel its pull and they’re not emotionally ready to handle it. Beginning at ages six to seven in girls and seven to eight in boys, androgens are already on the rise. This happens well before the testes and ovaries get up and running, churning out their main products: sperm and eggs, respectively, and sex hormones. These early androgens are produced not by the gonads but by the adrenal glands (which, to remind you, sit atop the kidneys and produce cortisol, among other hormones). These “adrenal androgens” rise until our early twenties, when they begin their slow decline. The main adrenal androgen is a bit of a mouthful—dehydroepiandrosterone, more compactly known as DHEA. In both boys and girls, DHEA is released into the blood and is converted into low levels of testosterone (this conversion happens in “peripheral tissues” like the liver, kidneys, and brain). It is this adrenally derived T that causes the first appearance of slight pubic hair, acne, and body odor in both sexes. It may also be responsible for the earliest of crushes and hints at sexual feelings. But these adrenal androgens aren’t enough to stimulate the growth spurt, and true puberty doesn’t start until a few years later … Considering all human societies for which we have reliable records, a monogamous partnership is the typical form of mating arrangement. But having said that, men generally benefit more reproductively than women do from increasing their number of mates, and polygyny (many wives) is much more common than polyandry (many husbands). From an evolutionary point of view, the best way for humans to maximize the survival of their offspring is for both parents to stay home; that is, to work as a team. Parents in the distant past relied on lots of help from extended family or other community members. But today, many parents lack this kind of support … This greater desire for sex without commitment among men is one of the largest of all psychological sex differences in humans. To give you a better idea of the size of the difference in sociosexuality averaged across all of the countries, if you were to select a woman at random, there would be a 70 percent chance that she would be less interested in casual sex than a male picked at random … T may drop in newly paired men who are in committed relationships, and lower T in this situation is associated with being a more devoted partner. Conversely, higher T is more characteristic of men who are keeping an eye out for opportunities to cheat. When the baby comes, T may drop even more. This drop in T, along with other hormonal changes, may serve to make the hard work of parenting feel less like a chore and more like a rewarding and enjoyable activity (the balance there is, of course, often tipped in the other direction). In addition … it may dampen one’s drive to compete with other males and seek out other mates … A man’s hormonal response to becoming a father depends heavily on his culture and the amount of time he interacts with his children. For example, dads in the Hadza foragers of Tanzania (who are generally socially monogamous) frequently hold, feed, and play with their babies, while dads in the neighboring Datoga pastoralists (who practice polygyny) are more likely to leave this to the moms and other caregivers. You can guess which dads have the lowest T levels—the Hadza. The T levels of the Hadza dads were found to be almost 50 percent lower than their childless fellows, while the T levels of the Datoga dads were no different from Datoga men without kids … If the idea of sex—either with a partner or solo—holds little appeal, or if sexy thoughts are few and far between, you might have “low sexual desire.” This is only a problem if it bothers you. Lots of factors affect your interest in sex, like your age, mental and physical health, and whether you’re in a relationship.

This concludes our essay series on estrogen and testosterone.