Men and women differ not only in their physical attributes and
reproductive function but also in many other characteristics, including
the way they solve intellectual problems. For the past few decades, it has
been ideologically fashionable to insist that these behavioral differences
are minimal and are the consequence of variations in experience during
development before and after adolescence. Evidence accumulated more
recently, however, suggests that the effects of sex hormones on brain
organization occur so early in life that from the start the environment is
acting on differently wired brains in boys and girls. Such effects make
evaluating the role of experience, independent of physiological
predisposition, a difficult if not dubious task. The biological bases of
sex differences in brain and behavior have become much better known
through increasing numbers of behavioral, neurological and
endocrinological studies.
We know, for instance, from observations of both humans and nonhumans
that males are more aggressive than females, that young males engage in
more rough-and-tumble play than females and that females are more
nurturing. We also know that in general males are better at a variety of
spatial or navigational tasks. How do these and other sex differences come
about? Much of our information and many of our ideas about how sexual
differentiation takes place derive from research on animals. From such
investigations, it appears that perhaps the most important factor in the
differentiation of males and females and indeed in differentiating
individuals within a sex is the level of exposure to various sex hormones
early in life.
In most mammals, including humans, the developing organism has the
potential to be male or female. Producing a male, however, is a complex
process. When a Y chromosome is present, testes, or male gonads, form.
This development is the critical first step toward becoming a male. When
no Y chromosome is present, ovaries form.
Testes produce male hormones, or androgens (testosterone chief among
them), which are responsible not only for transformation of the genitals
into male organs but also for organization of corresponding male behaviors
early in life. As with genital formation, the intrinsic tendency that
occurs in the absence of masculinizing hormonal influence, according to
seminal studies by Robert W. Goy of the University of Wisconsin, is to
develop female genital structures and behavior. Female anatomy and
probably most behavior associated with females are thus the default modes
in the absence of androgens.
If a rodent with functional male genitals is deprived of androgens
immediately after birth (either by castration or by the administration of
a compound that blocks androgens), male sexual behavior, such as mounting,
will be reduced, and more female sexual behavior, such as lordosis
(arching of the back when receptive to coitus), will be expressed.
Likewise, if androgens are administered to a female directly after birth,
she will display more male sexual behavior and less female behavior in
adulthood. These lifelong effects of early exposure to sex hormones are
characterized as 'organizational' because they appear to alter brain
function permanently during a critical period in prenatal or early
postnatal development. Administering the same sex hormones at later stages
or in the adult has no similar effect.
Not all the behaviors that distinguish males are categorized at the
same time, however. Organization by androgens of the male-typical
behaviors of mounting and of rough-and-tumble play, for example, occur at
different times prenatally in rhesus monkeys.
The area in the brain that regulates female and male reproductive
behavior is the hypothalamus. This tiny structure at the base of the brain
connects to the pituitary, the master endocrine gland. It has been shown
that a region of the hypothalamus is visibly larger in male rats than in
females and that this size difference is under hormonal control.
Scientists have also found parallel sex differences in a clump of nerve
cells in the human brain--parts of the interstitial nucleus of the
anterior hypothalamus--that is larger in men than in women. Even sexual
orientation and gender identity have been related to anatomical variation
in the hypothalamus. Other researchers, Jiang-Ning Zhou of the Netherlands
Institute of Brain Research and his colleagues there and at Free
University in Amsterdam, observed another part of the hypothalamus to be
smaller in male-to-female transsexuals than in a male control group. These
findings are consistent with suggestions that sexual orientation and
gender identity have a significant biological component.
Hormones and Intellect
What of differences in intellectual function between men and women?
Major sex differences in function seem to lie in patterns of ability
rather than in overall level of intelligence (measured as IQ), although
some researchers, such as Richard Lynn of the University of Ulster in
Northern Ireland, have argued that there exists a small IQ difference
favoring human males. Differences in intellectual pattern refer to the
fact that people have different intellectual strengths. For example, some
people are especially good at using words, whereas others are better at
dealing with external stimuli, such as identifying an object in a
different orientation. Two individuals may have differing cognitive
abilities within the same level of general intelligence.
Sex differences in problem solving have been systematically studied in
adults in laboratory situations. On average, men perform better than women
at certain spatial tasks. In particular, men seem to have an advantage in
tests that require the subject to imagine rotating an object or
manipulating it in some other way. They also outperform women in
mathematical reasoning tests and in navigating their way through a route.
Further, men exhibit more accuracy in tests of target-directed motor
skills--that is, in guiding or intercepting projectiles.
Women, on average, excel on tests that measure recall of words and on
tests that challenge the person to find words that begin with a specific
letter or fulfill some other constraint. They also tend to be better than
men at rapidly identifying matching items and performing certain precision
manual tasks, such as placing pegs in designated holes on a board.
In examining the nature of sex differences in navigating routes, one
study found that men completed a computer simulation of a maze or
labyrinth task more quickly and with fewer errors than women did. Another
study by different researchers used a path on a tabletop map to measure
route learning. Their results showed that although men learned the route
in fewer trials and with fewer errors, women remembered more of the
landmarks, such as pictures of different types of buildings, than men did.
These results and others suggest that women tend to use landmarks as a
strategy to orient themselves in everyday life more than men do.
Other findings seemed also to point to female superiority in landmark
memory. Researchers tested the ability of individuals to recall objects
and their locations within a confined space--such as in a room or on a
tabletop. In these studies, women were better able to remember whether
items had changed places or not. Other investigators found that women were
superior at a memory task in which they had to remember the locations of
pictures on cards that were turned over in pairs. At this kind of object
location, in contrast to other spatial tasks, women appear to have the
advantage.
It is important to keep in mind that some of the average sex
differences in cognition vary from slight to quite large and that men and
women overlap enormously on many cognitive tests that show average
differences. For example, whereas women perform better than men in both
verbal memory (recalling words from lists or paragraphs) and verbal
fluency (finding words that begin with a specific letter), we find a large
difference in memory ability but only a small disparity for the fluency
tasks. On the whole, variation between men and women tends to be smaller
than deviations within each sex, but very large differences between the
groups do exist--in men's high level of visual-spatial targeting ability,
for one.
Although it used to be thought that sex differences in problem solving
did not appear until puberty, the accumulated evidence now suggests that
some cognitive and skill differences are present much earlier. For
example, researchers have found that three- and four-year-old boys were
better at targeting and at mentally rotating figures within a clock face
than girls of the same age were. Prepubescent girls, however, excelled at
recalling lists of words.
Male and female rodents have also been found to solve problems
differently. Christina L. Williams of Duke University has shown that
female rats have a greater tendency to use landmarks in spatial learning
tasks, as it appears women do. In Williams's experiment, female rats used
landmark cues, such as pictures on the wall, in preference to geometric
cues: angles and the shape of the room, for instance. If no landmarks were
available, however, females used the geometric cues. In contrast, males
did not use landmarks at all, preferring geometric cues almost
exclusively.
Hormones and Behavior
Williams also found that hormonal manipulation during the critical
period could alter these behaviors. Depriving newborn males of sex
hormones by castrating them or administering hormones to newborn females
resulted in a complete reversal of sex-typed behaviors in the adult
animals. Treated males behaved like females and treated females, like
males.
Structural differences may parallel behavioral ones. Lucia F. Jacobs,
while at the University of Pittsburgh, discovered that the hippocampus--a
region thought to be involved in spatial learning--is larger in several
male species of rodents than in females. At present, there are
insufficient data on possible sex differences in hippocampal size in human
subjects.
One of the most compelling areas of evidence for hormonally influenced
sex differences in humans comes from studies of girls exposed to excess
androgens in the prenatal or neonatal stage. The production of abnormally
large quantities of adrenal androgens can occur because of a genetic
defect in a condition called congenital adrenal hyperplasia (CAH). Before
the 1970s a similar condition also unexpectedly appeared in the offspring
of pregnant women who took various synthetic steroids. Although the
consequent masculinization of the genitals can be corrected by surgery and
drug therapy can stop the overproduction of androgens, the effects of
prenatal exposure on the brain are not reversed.
Sheri A. Berenbaum, while at Southern Illinois University at
Carbondale, and Melissa Hines, then at the University of California at Los
Angeles, observed the play behavior of CAH girls and compared it with that
of their male and female siblings. Given a choice of transportation and
construction toys, dolls and kitchen supplies, or books and board games,
the CAH girls preferred the more typically masculine toys--for example,
they played with cars for the same amount of time that boys did. Both the
CAH girls and the boys differed from unaffected girls in their patterns of
choice. Berenbaum also found that CAH girls had greater interest in
male-typical activities and careers. Because there is every reason to
think parents would be at least as likely to encourage feminine
preferences in their CAH daughters as in their unaffected daughters, these
findings suggest that these preferences were altered by the early hormonal
environment.
Other researchers also found that spatial abilities that are typically
better in males are enhanced in CAH girls. But in CAH boys the reverse was
reported.
Such studies suggest that although levels of androgen relate to spatial
ability, it is not simply the case that the higher the levels, the better
the spatial scores. Rather studies point to some optimal level of androgen
(in the low male range) for maximal spatial ability. This finding may also
hold for men and math reasoning; in one study, low-androgen men tested
higher.
The Biology of Math
Such findings are relevant to the suggestion by Camilla P. Benbow, now
at Vanderbilt University, that high mathematical ability has a significant
biological determinant. Benbow and her colleagues have reported consistent
sex differences in mathematical reasoning ability that favor males. In
mathematically talented youth, the differences were especially sharp at
the upper end of the distribution, where males vastly outnumbered females.
The same has been found for the Putnam competition, a very demanding
mathematics examination. Benbow argues that these differences are not
readily explained by socialization.
It is important to keep in mind that the relation between natural
hormone levels and problem solving is based on correlational data.
Although some form of connection between the two measures exists, we do
not necessarily know how the association is determined, nor do we know
what its causal basis is. We also know little at present about the
relation between adult levels of hormones and those in early life, when
abilities appear to become organized in the nervous system.
One of the most intriguing findings in adults is that cognitive
patterns may remain sensitive to hormonal fluctuations throughout life.
Elizabeth Hampson of the University of Western Ontario showed that women's
performances at certain tasks changed throughout the menstrual cycle as
levels of estrogen varied. High levels of the hormone were associated not
only with relatively depressed spatial ability but also with enhanced
speech and manual skill tasks. In addition, I have observed seasonal
fluctuations in spatial ability in men: their performance is better in the
spring, when testosterone levels are lower. Whether these hormonally
linked fluctuations in intellectual ability represent useful evolutionary
adaptations or merely the highs and lows of an average test level remains
to be seen through further research.
A long history of studying people with damage to one half of their
brain indicates that in most people the left hemisphere of the brain is
critical for speech and the right for certain perceptual and spatial
functions. Researchers studying sex differences have widely assumed that
the right and left hemispheres of the brain are more asymmetrically
organized for speech and spatial functions in men than in women.
This belief rests on several lines of research. Parts of the corpus
callosum, a major neural system connecting the two hemispheres, as well as
another connector, the anterior commissure, appear to be larger in women,
which may permit better communication between hemispheres. Perceptual
techniques that measure brain asymmetry in normal-functioning people
sometimes show smaller asymmetries in women than in men, and damage to one
brain hemisphere sometimes has less of an effect in women than the
comparable injury in men does. My own data on patients with damage to one
hemisphere of the brain suggest that for functions such as basic speech
and spatial ability, there are no major sex differences in hemispheric
asymmetry, although there may be such disparities in certain more abstract
abilities, such as defining words.
If the known overall differences between men and women in spatial
ability were related to differing dependence on the right brain hemisphere
for such functions, then damage to that hemisphere might be expected to
have a more devastating effect on spatial performance in men. My
laboratory has studied the ability of patients with damage to one
hemisphere of the brain to visualize the rotation of certain objects. As
expected, for both sexes, those with damage to the right hemisphere got
lower scores on these tests than those with damage to the left hemisphere
did. Also, as anticipated, women did not do as well as men on this test.
Damage to the right hemisphere, however, had no greater effect on men than
on women.
The results of this study and others suggest that the normal
differences between men and women on rotational and line orientation tasks
need not be the result of different degrees of dependence on the right
hemisphere. Some other brain systems may be mediating the higher
performance by men.
Patterns of Function
Another brain difference between the sexes has been shown for speech
and certain manual functions. Women incur aphasia (impairment of the power
to produce and understand speech) more often after anterior damage than
after posterior damage to the brain. In men, posterior damage more often
affects speech. A similar pattern is seen in apraxia, difficulty in
selecting appropriate hand movements, such as showing how to manipulate a
particular object or copying the movements of the experimenter. Women
seldom experience apraxia after left posterior damage, whereas men often
do.
Men also incur aphasia from left hemisphere damage more often than
women do. One explanation suggests that restricted damage within a
hemisphere after a stroke more often affects the posterior region of the
left hemisphere. Because men rely more on this region for speech than
women do, they are more likely to be affected. We do not yet understand
the effects on cognitive patterns of such divergent representation of
speech and manual functions.
Although my laboratory has not found evidence of sex differences in
functional brain asymmetry with regard to basic speech, movement or
spatial-rotation abilities, we have found slight differences in some
verbal skills. Scores on a vocabulary test and on a verbal fluency test,
for instance, were slightly affected by damage to either hemisphere in
women, but such scores were affected only by left hemisphere damage in
men. These findings suggest that when using some more abstract verbal
skills, women do use their hemispheres more equally than men do. But we
have not found this to be true for all word-related tasks; for example,
verbal memory appears to depend just as much on the left hemisphere in
women as in men.
In recent years, new techniques for assessing the brain's
activity--including functional magnetic resonance imaging (fMRI) and
positron emission tomography (PET), when used during various
problem-solving activities--have shown promise for providing more
information about how brain function may vary among normal, healthy
individuals. The research using these two techniques has so far yielded
interesting, yet at times seemingly conflicting, results.
Some research has shown greater differences in activity between the
hemispheres of men than of women during certain language tasks, such as
judging if two words rhyme and creating past tenses of verbs. Other
research has failed to find sex differences in functional asymmetry. The
different results may be attributed in part to different language tasks
being used in the various studies, perhaps showing that the sexes may
differ in brain organization for some language tasks but not for others.
The varying results may also reflect the complexity of these
techniques. The brain is always active to some degree. So for any
activity, such as reading aloud, the comparison activity--say, reading
silently--is intended to be very similar. We then 'subtract' the brain
pattern that occurs during silent reading to find the brain pattern
present while reading aloud. Yet such methods require dubious assumptions
about what the subject is doing during either activity. In addition, the
more complex the activity, the more difficult it is to know what is
actually being measured after subtracting the comparison activity.
Looking Back
To understand human behavior--how men and women differ from one
another, for instance--we must look beyond the demands of modern life. Our
brains are essentially like those of our ancestors of 50,000 and more
years ago, and we can gain some insight into sex differences by studying
the differing roles men and women have played in evolutionary history. Men
were responsible for hunting and scavenging, defending the group against
predators and enemies, and shaping and using weapons. Women gathered food
near the home base, tended the home, prepared food and clothing, and cared
for small children. Such specialization would put different selection
pressures on men and women.
Any behavioral differences between individuals or groups must somehow
be mediated by the brain. Sex differences have been reported in brain
structure and organization, and studies have been done on the role of sex
hormones in influencing human behavior. But questions remain regarding how
hormones act on human brain systems to produce the sex differences we
described, such as in play behavior or in cognitive patterns.
The information we have from laboratory animals helps to guide our
explanations, but ultimately these hypotheses must be tested on people.
Refinements in brain-imaging techniques, when used in conjunction with our
knowledge of hormonal influences and with continuing studies on the
behavioral deficits after damage to various brain regions, should provide
insight into some of these questions.