Sex differences are not an arbitrary feature of Western culture, like the decision to drive on the left or
on the right. In all human cultures, men and women are seen as having different natures. All cultures
divide their labor by sex, with more responsibility for childrearing by women and more control of the
public and political realms by men. (The division of labor emerged even in a culture where everyone
had been committed to stamping it out, the Israeli kibbutz.) In all cultures men are more aggressive,
more prone to stealing, more prone to lethal violence (including war), and more likely to woo, seduce,
and trade favors for sex. And in all cultures one finds rape, as well as proscriptions against rape.37
• Many of the psychological differences between the sexes are exactly what an evolutionary biologist
who knew only their physical differences would predict.38 Throughout the animal kingdom, when the
female has to invest more calories and risk in each offspring (in the case of mammals, through
pregnancy and nursing), she also invests more in nurturing the offspring after birth, since it is more
costly for a female to replace a child than for a male to replace one. The difference in investment is
accompanied by a greater competition among males over opportunities to mate, since mating with
many partners is more likely to multiply the number of offspring of a male than the number of
offspring of a female. When the average male is larger than the average female (as is true of men and
women), it bespeaks an evolutionary history of greater violent competition by males over mating
opportunities. Other physical traits of men, such as later puberty, greater adult strength, and shorter
lives, also indicate a history of selection for high-stakes competition.
• Many of the sex differences are found widely in other primates, indeed, throughout the mammalian
class.39 The males tend to compete more aggressively and to be more polygamous; the females tend to
invest more in {347} parenting. In many mammals a greater territorial range is accompanied by an
enhanced ability to navigate using the geometry of the spatial layout (as opposed to remembering
individual landmarks). More often it is the male who has the greater range, and that is true of human
hunter-gatherers. Men's advantage in using mental maps and performing 3-D mental rotation may not
be a coincidence.40
• Geneticists have found that the diversity of the DNA in the mitochondria of different people (which
men and women inherit from their mothers) is far greater than the diversity of the DNA in Y
chromosomes (which men inherit from their fathers). This suggests that for tens of millennia men had
greater variation in their reproductive success than women. Some men had many descendants and
others had none (leaving us with a small number of distinct Y chromosomes), whereas a larger number
of women had a more evenly distributed number of descendants (leaving us with a larger number of
distinct mitochondrial genomes). These are precisely the conditions that cause sexual selection, in
which males compete for opportunities to mate and females choose the best-quality males.41
• The human body contains a mechanism that causes the brains of boys and the brains of girls to
diverge during development.42 The Y chromosome triggers the growth of testes in a male fetus, which
secrete androgens, the characteristically male hormones (including testosterone). Androgens have
lasting effects on the brain during fetal development, in the months after birth, and during puberty, and
they have transient effects at other times. Estrogens, the characteristically female sex hormones, also
affect the brain throughout life. Receptors for the sex hormones are found in the hypothalamus, the
hippocampus, and the amygdala in the limbic system of the brain, as well as in the cerebral cortex.
• The brains of men differ visibly from the brains of women in several ways.43 Men have larger brains
with more neurons (even correcting for body size), though women have a higher percentage of gray
matter. (Since men and women are equally intelligent overall, the significance of these differences is
unknown.) The interstitial nuclei in the anterior hypothalamus, and a nucleus of the stria terminalis,
also in the hypothalamus, are larger in men; they have been implicated in sexual behavior and
aggression. Portions of the cerebral commissures, which link the left and right hemispheres, appear to
be larger in women, and their brains may function in a less lopsided manner than men's. Learning and
socialization can affect the microstructure and functioning of the human brain, of course, but probably
not the size of its visible anatomical structures.
• Variation in the level of testosterone among different men, and in the same man in different seasons
or at different times of day, correlates with libido, self-confidence, and the drive for dominance.44
Violent criminals {348} have higher levels than nonviolent criminals; trial lawyers have higher levels
than those who push paper. The relations are complicated for a number of reasons. Over a broad range
of values, the concentration of testosterone in the bloodstream doesn't matter. Some traits, such as
spatial abilities, peak at moderate rather than high levels. The effects of testosterone depend on the
number and distribution of receptors for the molecule, not just on its concentration. And one's
psychological state can affect testosterone levels as well as the other way around. But there is a causal
relation, albeit a complicated one. When women preparing for a sex-change operation are given
androgens, they improve on tests of mental rotation and get worse on tests of verbal fluency. The
journalist Andrew Sullivan, whose medical condition had lowered his testosterone levels, describes the
effects of injecting it: “The rush of a T shot is not unlike the rush of going on a first date or speaking
before an audience. I feel braced. After one injection, I almost got in a public brawl for the first time in
my life. There is always a lust peak — every time it takes me unaware.”45 Though testosterone levels in
men and women do not overlap, variations in level have similar kinds of effects in the two sexes. Hightestosterone
women smile less often and have more extramarital affairs, a stronger social presence, and
even a stronger handshake.
• Women's cognitive strengths and weaknesses vary with the phase of their menstrual cycle.46 When
estrogen levels are high, women get even better at tasks on which they typically do better than men,
such as verbal fluency. When the levels are low, women get better at tasks on which men typically do
better, such as mental rotation. A variety of sexual motives, including their taste in men, vary with the
menstrual cycle as well.47
• Androgens have permanent effects on the developing brain, not just transient effects on the adult
brain.48 Girls with congenital adrenal hyperplasia overproduce androstenedione, the androgen hormone
made famous by the baseball slugger Mark McGwire. Though their hormone levels are brought to
normal soon after birth, the girls grow into tomboys, with more rough-and-tumble play, a greater
interest in trucks than dolls, better spatial abilities, and, when they get older, more sexual fantasies and
attractions involving other girls. Those who are treated with hormones only later in childhood show
male patterns of sexuality when they become young adults, including quick arousal by pornographic
images, an autonomous sex drive centered on genital stimulation, and the equivalent of wet dreams.49
• The ultimate fantasy experiment to separate biology from socialization would be to take a baby boy,
give him a sex-change operation, and have his parents raise him as a girl and other people treat him as
one. If gender is socially constructed, the child should have the mind of a normal girl; if it {349}
depends on prenatal hormones, the child should feel like a boy trapped in a girl's body. Remarkably,
the experiment has been done in real life — not out of scientific curiosity, of course, but as a result of
disease and accidents. One study looked at twenty-five boys who were born without a penis (a birth
defect known as cloacal exstrophy) and who were then castrated and raised as girls. All of them
showed male patterns of rough-and-tumble play and had typically male attitudes and interests. More
than half of them spontaneously declared they were boys, one when he was just five years old.50
In a famous case study, an eight-month-old boy lost his penis in a botched circumcision (not by a
mohel, I was relieved to learn, but by a bungling doctor). His parents consulted the famous sex
researcher John Money, who had maintained that “Nature is a political strategy of those committed to
maintaining the status quo of sex differences.” He advised them to let the doctors castrate the baby and
build him an artificial vagina, and they raised him as a girl without telling him what had happened.51 I
learned about the case as an undergraduate in the 1970s, when it was offered as proof that babies are
born neuter and acquire a gender from the way they are raised. A New York Times article from the era
reported that Brenda (nee Bruce) “has been sailing contentedly through childhood as a genuine girl.”52
The facts were suppressed until 1997, when it was revealed that from a young age Brenda felt she was
a boy trapped in a girl's body and gender role.53 She ripped off frilly dresses, rejected dolls in favor of
guns, preferred to play with boys, and even insisted on urinating standing up. At fourteen she was so
miserable that she decided either to live her life as a male or to end it, and her father finally told her the
truth. She underwent a new set of operations, assumed a male identity, and today is happily married to
a woman.
Children with Turner's syndrome are genetically neuter. They have a single X chromosome, inherited
from either their mother or their father, instead of the usual two X chromosomes of a girl (one from her
mother, the other from her father) or the X and Y of a boy (the X from his mother, the Y from his
father). Since a female body plan is the default among mammals, they look and act like girls.
Geneticists have discovered that parents’ bodies can molecularly imprint genes on the X chromosome
so they become more or less active in the developing bodies and brains of their children. A Turner's
syndrome girl who gets her X chromosome from her father may have genes that are evolutionarily
optimized for girls (since a paternal X always ends up in a daughter). A Turner's girl who gets her X
from her mother may have genes that are evolutionarily optimized for boys (since a maternal X, though
it can end up in either sex, will act unopposed only in a son, who has no counterpart to the X genes on
his puny {350} Y chromosome). And in fact Turner's girls do differ psychologically depending on
which parent gave them their X. The ones with an X from their father (which is destined for a girl)
were better at interpreting body language, reading emotions, recognizing faces, handling words, and
getting along with other people compared to the ones with an X from their mother (which is fully
active only in a boy).54
• Contrary to popular belief, parents in contemporary America do not treat their sons and daughters
very differently.55 A recent assessment of 172 studies involving 28,000 children found that boys and
girls are given similar amounts of encouragement, warmth, nurturance, restrictiveness, discipline, and
clarity of communication. The only substantial difference was that about two-thirds of the boys were
discouraged from playing with dolls, especially by their fathers, out of a fear that they would become
gay. (Boys who prefer girls’ toys often do turn out gay, but forbidding them the toys does not change
the outcome.) Nor do differences between boys and girls depend on their observing masculine behavior
in their fathers and feminine behavior in their mothers. When Hunter has two mommies, he acts just as
much like a boy as if he had a mommy and a daddy