The numbers are unequivocal. Elite women are not catching elite men, nor maintaining their position. Men are ever so slowly pulling away. The biological gap is expanding.
But why does it exist in the first place?
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Next to a Yellow Pages–thick dictionary on the windowsill of David C. Geary’s corner office sits a woman’s skull. She is overlooking the campus of the University of Missouri. “You can see the cranium is small,” Geary says. He has a gaunt face and turquoise-tinted irises. A curve of gray hair that rises from the front of his forehead looks a bit like a question mark, lending his face an appropriately inquisitive air. “Her brain was only about a third the size of ours. That’s why she’s by the dictionary, she has to practice a lot,” he jokes. Geary is referring to his scale model of the skull of Lucy, the famous Australopithecus afarensis ancestor of modern man whose 3.2-million-year-old bones were found in Ethiopia.
Geary spends a lot of time thinking about brains. He is a cognitive developmental psychologist, and much of his career has been devoted to understanding how children learn math, a pursuit that landed him on the president’s National Mathematics Advisory Panel from 2006 to 2008. He is also a walking databank of sex differences.
Since he was a grad student at UC Riverside in the eighties, Geary has been interested in the evolution of human sex differences. But given the oft-fraught nature of research on biological sex differences—at least those that extend beyond genitalia—Geary waited until he had tenure to start publishing on human evolution. And then he exploded. He coauthored a thousand-page textbook that is nothing more than a compilation of the results of every serious scientific study of sex differences—from birth weight to social attitudes—that has been done in the last hundred years.
Though he may not have considered it before I showed up at his door, Geary’s most interesting contribution to the world of sports is his 550-page tome, Male, Female: The Evolution of Human Sex Differences. It is the first work to incorporate all of the studies—emphasis on all—done on human sex differences into the framework of sexual selection.
Charles Darwin first elucidated the principles of sexual selection, though it has received far less mainstream fanfare than his other brainchild, natural selection. Whereas natural selection refers to the changes in human DNA that are preserved or eradicated in response to the natural environment, sexual selection refers to those DNA changes that spread or die out as a result of the competition for and the choosing of mates. Sexual selection is the source of most human sex differences, and it is vital to the understanding of human athleticism.
Among the physical differences between the sexes, men are generally heavier and taller and have longer arms and legs relative to their height, as well as bigger hearts and lungs. Men are twice as likely to be left-handed as women—an athletic asset in a number of sports.* Men have less fat, denser bones, more oxygen-carrying red blood cells, heavier skeletons that can support more muscle, and narrower hips, which makes running more efficient and decreases the chance of injury—like ACL tears, which are epidemic in female athletes—while running and jumping. “Because they have a broader pelvis, women have a greater angle to their knee,” says Bruce Latimer, professor of anthropology and anatomy at Case Western Reserve University. “So they waste a lot of energy that goes into compression in the hip joint and it doesn’t help you move forward. . . . The broader the pelvis, the more wasted energy.”
One of the most pronounced physical differences between the sexes is in muscle mass. Men pack more muscle fibers into any given space in the body and have 80 percent more muscle mass in their upper body than women, and 50 percent more in their legs. As far as upper body strength, this translates to a three-standard-deviation difference in strength. That is, again, of a thousand men off the street, 997 would have a stronger upper body than the average woman.
“The differences in upper body strength are about what you see in gorillas,” Geary says. “That’s very big. Gorillas are the most sexually dimorphic of our close relatives. The males are about twice the size of the females. So the overall size difference is more than in humans, but the difference in upper body strength is similar.”
The reason for the similarity to gorillas reflects how sexual selection has shaped human (and gorilla) athleticism. If you want to know whether the male or female of a given species is bigger and stronger, one piece of information is particularly useful: which sex has the higher potential reproductive rate.
Because of a long gestation and breastfeeding period, a female gorilla can produce only one offspring about every four years. Male gorillas collect and defend harems of females and have a much higher potential reproductive rate. But for each male gorilla that has a harem, several other males are frozen out of breeding altogether. The result is that male gorillas compete fiercely for access to multiple females, and this “male-male competition” takes the form of fighting, or at least posturing to fight, and natural selection accentuates traits that make gorillas better fighters. “In species where females have a higher potential reproductive rate,” like seahorses, Geary explains, “the situation is reversed, and the females are bigger and more aggressive.” Not surprisingly, male seahorses, which care for eggs, prefer larger, stronger females.
In competition zones that are more difficult to patrol and defend physically—the sky, for example—the female’s choice of a mate becomes more significant and natural selection accentuates male traits such as the attractive coloration and melodic courtship songs that occur in birds. But in primates that are confined primarily to terra firma, like gorillas and ancestral humans, head-to-head fighting can be important and evolution accentuates brute strength.
All this implies some less-than-happy notions about humans, the earth-bound primates that we are, and men in particular: that certain traits were selected for in men so that they could hurt, kill, or at least intimidate one another, and that the men who were most successful at hurting, killing, or intimidating other men sometimes used that success to mate with multiple women and to have lots of children.
The weight of evidence supports both implications. Across hunter-gatherer societies, around 30 percent of men died at the hands of other men, in combat or in raids, which often were carried out in order to capture women. As Harvard psychologist Steven Pinker put it in a talk about his book The Better Angels of Our Nature, about the history and modern decline of human violence: “It turns out that [Thomas] Hobbes was right. Man’s life in the state of nature was nasty, brutish, and short.”
The second implication, that our ancestral man strove for multiple mates, is indisputable from the genetic evidence. Because fathers pass their Y-chromosomal DNA only to sons, and only mothers pass on a type of DNA called mitochondrial DNA, we can trace our maternal and paternal ancestors separately back through time. The findings in studies throughout the world are clear: no matter where scientists look, we have fewer male than female ancestors. It took far fewer Adams than Eves to spawn the world’s current population. (In some cases staggeringly so: 16 million Asian men—0.5 percent of the world’s male population—have a nearly identical portion of the Y chromosome that geneticists think probably came from Genghis Khan, who famously had hundreds of wives and concubines.)
Another pattern that holds across species and among primates that have intense male-male competition is that the physical abilities important to combat are bolstered, exclusively in males, via puberty. Puberty accentuates the qualities that a burgeoning adult is soon to need for reproduction. So if athletic traits, like throwing punches or rocks, are important to reproduction, they will be magnified during puberty. And here again, men follow the violent primate pattern to a tee. Whereas girls mature early and quickly, boys go through a puberty that is both late and long, giving more time for growth, and during which their athleticism explodes.
Up until the age of ten, girls and boys have similar bodies. Girls are taller and already have sl
ightly more body fat, but a number of athletic traits are nearly indistinguishable in boys and girls. Top running speed is almost identical in ten-year-old boys and girls, and close all the way until age fourteen, when boys very literally are on natural steroids.
At fourteen, the throwing gap, already wide, becomes a chasm. Boys develop stronger arms and wider shoulders, and by eighteen the average boy can throw three times as far as the average girl. Men also develop features that make them more difficult than boys and women to knock out: heavier brow ridges that protect the eyes and enlargement of the mandible that makes the face more resilient to blows. A glass jaw apparently did not cut it for ancestral men.
The testosterone surge of male puberty also stimulates the production of red blood cells, so men can use more oxygen than women, and it makes men less sensitive to pain than women*—just as it does to both animals and people who are given testosterone injections.
By around age fourteen, the average girl is closing in on her lifetime maximum sprint speed. World age-group records in sprinting are nearly identical for boys and girls at age nine, before puberty, when there is little biological reason for gender segregation in sports. By fourteen, however, the records are no longer in the same athletic universe.*
In some cases, women fare worse in certain athletic attributes after puberty. As estrogen causes fat to accumulate on widened hips, most girls experience a plateau or decline in vertical jump. And even the very leanest of adult female marathoners get down to around 6 to 8 percent body fat, double that of their male counterparts.
Studies of Olympians show that an important trait of female athletes in certain sports is that they don’t develop the wide hips that many other women do. If elite female gymnasts go through a significant growth spurt in height or hips, their career at the top level is essentially over. As they increase in size faster than strength, the power-to-weight ratio that is so critical to aerial maneuvers goes in the wrong direction, as does their ability to rotate in the air. Female gymnasts are pronounced over the hill by twenty, whereas male gymnasts are still early in their careers. China was stripped of an Olympic gymnastics medal from the 2000 Games in Sydney when the International Olympic Committee determined that female gymnast Dong Fangxiao was two years younger than the minimum competition age of sixteen. It is safe to say that we will never see a similar scandal in men’s gymnastics.
The advantage, then, that some female athletes have comes from certain traits that are more typical of men, like low body fat and narrow hips.
It now appears that a primary reason why women in track and field gained on men in the 1970s and ’80s—and what the Nature papers did not account for—was because they were making up for the lack of an SRY gene by simply injecting testosterone. Beginning in the 1960s, the competition of the Cold War spilled into sports, and the systematic doping of girls, often without their knowledge, was widespread in countries like East Germany. Since that era, top women in the most explosive events have gotten worse. Seventy-five of the top eighty women’s shot put throws of all time, for instance, came between the mid-1970s and 1990, predominantly from Eastern Bloc countries. That eightieth performance was a throw from East Germany’s Heidi Krieger, who decades later testified in court about systematic doping in East Germany. By that time she was Andreas Krieger, having chosen to live as a man after enormous doses of steroids, which are simply testosterone analogues, pushed her body in that direction. To this day, nearly all women’s world records in sprint and power events are from the 1980s, a testament to the powerful effect of male hormones on female athletes. Once the era of extreme doping ended, the performance gap between humans with and without an SRY gene stretched anew. It is now clear that the genetic advantage of men over women in most sports is so profound that the best solution is to separate them.
As Alice Dreger, professor of clinical medical humanities and bioethics in the Feinberg School of Medicine at Northwestern University and an authority on the history of sex testing in sports, told me: “The reason we have females separated in sports is because in many sports the best female athletes can’t compete with the best male athletes. And everybody knows that but nobody wants to say it. Females are structured like a disabled class for all sorts of, I think, good reasons.”
The difficulty in determining who is granted access to that class was evident at the 2009 world track and field championships when Caster Semenya, a young and unheralded South African 800-meter runner, looked over her muscled shoulder and tore away from the field en route to the world title. Semenya’s competitors derided her in the world media. “Just look at her,” sneered Russian Mariya Savinova, the fifth-place finisher, in reference to Semenya’s narrow hips and armored torso. Just looking at her, though, does not give an answer.
After the world championships, it was reported that Semenya has internal testes and no ovaries or uterus, and high levels of testosterone. (Semenya never confirmed or addressed that report.) So where, if true, should that leave her? To start breaking down sport classifications by specific biological traits, “you’d have to run international competitions like the Westminster Dog Show, with competitions for every breed,” says Myron Genel, the Yale professor of pediatrics. María José Martínez-Patiño, the Spanish hurdler, had both a Y chromosome and an SRY gene, but because she was insensitive to testosterone she was ultimately allowed to compete against women.
Before the 2012 London Olympics, faced with continuing controversy over the Semenya case, the IAAF and the International Olympic Committee announced that sex would be determined based on testosterone levels. Not just the amount that is produced, but the amount the body can use.
Testosterone levels are not on a continuous spectrum. A typical woman will make less than 75 nanograms of testosterone per deciliter of blood. For men, the range is typically 240 to 1,200. So the low end of the male range is still more than 200 percent higher than the high end of the female range. In 2011, the NCAA—informed by a think tank held with the National Center for Lesbian Rights—determined that any man who undergoes sex reassignment surgery to become a woman must sit out a year while lowering her testosterone levels before she can compete on a women’s team. Thus, testosterone has been deemed the source of the male athletic advantage. Though it may not be the only one.
When I spoke with endocrinologists who work with androgen-insensitive women, they all felt that XY women with androgen insensitivity—that is, like Martínez-Patiño, they can use no testosterone at all—are overrepresented, not underrepresented, in sports.
At the 1996 Atlanta Summer Olympics, the last that had cheek swabs, 7 women out of the 3,387 competitors—or about 1 in 480—were found to have the SRY gene and androgen insensitivity. The typical rate of androgen insensitivity is estimated to be between 1 in 20,000 and 1 in 64,000. Over five Olympic Games, an average of 1 in every 421 female competitors was determined to have a Y chromosome. So women with androgen insensitivity are vastly overrepresented on the world’s largest sporting stage. Perhaps, then, something about the Y chromosome other than testosterone may be conferring an advantage.
Women with androgen insensitivity tend to have limb proportions more typical of men. Their arms and legs are longer relative to their bodies, and their average height is several inches taller than that of typical women. Like Erika Coimbra, a 5'11" Brazilian volleyball player and 2000 Olympic bronze medalist who is one of the few athletes with androgen insensitivity whose names have ever been made public. (Two of the endocrinologists I spoke with said that XY women are also overrepresented in modeling, because they are often very feminine in appearance in addition to being tall with long legs. Before her personal medical information unfortunately landed in the press, the tall, blond Coimbra had been dubbed the “Brazilian Barbie Doll.”)
The increased height of XY women who are insensitive to testosterone may result from an extended growth period because they don’t heed hormonal stop messages or from genes on the Y chromosome that influen
ce height. Men who have an extra Y chromosome tend to be very tall. Dave Rasmussen, the tallest member of Tall Clubs International, is a 7'3" XYY male whose parents are 6'4" and 5'9".
The overrepresentation of XY women only “scratches the surface of intersex conditions in sport,” as a paper in the British Journal of Sports Medicine put it. Jeff Brown, a Houston endocrinologist who works with some of the best athletes in America—his patients have fifteen Olympic gold medals, collectively—has treated numerous female Olympians with a condition called partial 21-hydroxylase deficiency, which can run in families and causes overproduction of testosterone.* In Brown’s estimation, the condition is highly overrepresented among female athletes. “The question would be, does that put them at an advantage over someone who doesn’t have it,” Brown says. “Of course, the answer is yes. But that’s God given. . . . I’ve seen it in jumpers, sprinters, and distance runners.”
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No scientist can claim to know the precise impact of testosterone on any individual athlete. But a 2012 study that spent three months following female athletes from a range of sports—including track and field and swimming—showed that the elite-level competitors had testosterone levels that consistently remained more than twice as high as those of the nonelites. And there are powerful anecdotes as well.*
Joanna Harper, fifty-five, is a medical physicist who was born male and later transitioned to living as a woman. Harper also happens to be a nationally accomplished age-group runner, and when she started hormone therapy in August 2004 to suppress her body’s testosterone and physically transition to female, like any good scientist, she took data. Harper figured she would slow down gradually, but was surprised to find herself getting slower and weaker by the end of the first month. “I felt the same when I ran,” she says. “I just couldn’t go as fast.” In 2012, Harper won the U.S. national cross-country title for the fifty-five-to-fifty-nine age group, but age and gender-graded performance standards indicate that Harper is precisely as competitive now as a female as she was as a male. That is, as a female Harper is just as good relative to women as she was relative to men before her transition, but she’s far slower than her own former, higher-testosterone self.
The Sports Gene: Inside the Science of Extraordinary Athletic Performance Page 8