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                                                                                SCIENCE, Page 42

COVER STORIES

Sizing Up The Sexes


Scientists are discovering that gender differences have as much
to do with the biology of the brain as with the way we are raised

By CHRISTINE GORMAN -- Reported by J. Madeleine Nash/Los Angeles


    What are little boys made of?
    What are little boys made of?
    Frogs and snails
    And puppy dogs' tails,
    That's what little boys are made of.

    What are little girls made of?
    What are little girls made of?
    Sugar and spice
    And all that's nice,
    That's what little girls are made of.

    -- Anonymous


    Many scientists rely on elaborately complex and costly
equipment to probe the mysteries confronting humankind. Not
Melissa Hines. The UCLA behavioral scientist is hoping to solve
one of life's oldest riddles with a toybox full of police cars,
Lincoln Logs and Barbie dolls. For the past two years, Hines and
her colleagues have tried to determine the origins of gender
differences by capturing on videotape the squeals of delight,
furrows of concentration and myriad decisions that children from
2 1/2 to 8 make while playing. Although both sexes play with all
the toys available in Hines' laboratory, her work confirms what
most parents (and more than a few aunts, uncles and
nursery-school teachers) already know. As a group, the boys
favor sports cars, fire trucks and Lincoln Logs, while the girls
are drawn more often to dolls and kitchen toys.

    But one batch of girls defies expectations and
consistently prefers the boy toys. These youngsters have a rare
genetic abnormality that caused them to produce elevated levels
of testosterone, among other hormones, during their embryonic
development. On average, they play with the same toys as the
boys in the same ways and just as often. Could it be that the
high levels of testosterone present in their bodies before birth
have left a permanent imprint on their brains, affecting their
later behavior? Or did their parents, knowing of their disorder,
somehow subtly influence their choices? If the first explanation
is true and biology determines the choice, Hines wonders, "Why
would you evolve to want to play with a truck?"

    Not so long ago, any career-minded researcher would have
hesitated to ask such questions. During the feminist revolution
of the 1970s, talk of inborn differences in the behavior of men
and women was distinctly unfashionable, even taboo. Men
dominated fields like architecture and engineering, it was
argued, because of social, not hormonal, pressures. Women did
the vast majority of society's child rearing because few other
options were available to them. Once sexism was abolished, so
the argument ran, the world would become a perfectly equitable,
androgynous place, aside from a few anatomical details.

    But biology has a funny way of confounding expectations.
Rather than disappear, the evidence for innate sexual
differences only began to mount. In medicine, researchers
documented that heart disease strikes men at a younger age than
it does women and that women have a more moderate physiological
response to stress. Researchers found subtle neurological
differences between the sexes both in the brain's structure and
in its functioning. In addition, another generation of parents
discovered that, despite their best efforts to give baseballs
to their daughters and sewing kits to their sons, girls still
flocked to dollhouses while boys clambered into tree forts.
Perhaps nature is more important than nurture after all.

    Even professional skeptics have been converted. "When I
was younger, I believed that 100% of sex differences were due
to the environment," says Jerre Levy, professor of psychology
at the University of Chicago. Her own toddler toppled that
utopian notion. "My daughter was 15 months old, and I had just
dressed her in her teeny little nightie. Some guests arrived,
and she came into the room, knowing full well that she looked
adorable. She came in with this saucy little walk, cocking her
head, blinking her eyes, especially at the men. You never saw
such flirtation in your life." After 20 years spent studying the
brain, Levy is convinced: "I'm sure there are biologically based
differences in our behavior."

    Now that it is O.K. to admit the possibility, the search
for sexual differences has expanded into nearly every branch of
the life sciences. Anthropologists have debunked Margaret
Mead's work on the extreme variability of gender roles in New
Guinea. Psychologists are untangling the complex interplay
between hormones and aggression. But the most provocative, if
as yet inconclusive, discoveries of all stem from the pioneering
exploration of a tiny 3-lb. universe: the human brain. In fact,
some researchers predict that the confirmation of innate
differences in behavior could lead to an unprecedented
understanding of the mind.

    Some of the findings seem merely curious. For example,
more men than women are lefthanded, reflecting the dominance of
the brain's right hemisphere. By contrast, more women listen
equally with both ears while men favor the right one.

    Other revelations are bound to provoke more controversy.
Psychology tests, for instance, consistently support the notion
that men and women perceive the world in subtly different ways.
Males excel at rotating three-dimensional objects in their head.
Females prove better at reading emotions of people in
photographs. A growing number of scientists believe the
discrepancies reflect functional differences in the brains of
men and women. If true, then some misunderstandings between the
sexes may have more to do with crossed wiring than
cross-purposes.

    Most of the gender differences that have been uncovered so
far are, statistically speaking, quite small. "Even the largest
differences in cognitive function are not as large as the
difference in male and female height," Hines notes. "You still
see a lot of overlap." Otherwise, women could never read maps
and men would always be lefthanded. That kind of flexibility
within the sexes reveals just how complex a puzzle gender
actually is, requiring pieces from biology, sociology and
culture.

    Ironically, researchers are not entirely sure how or even
why humans produce two sexes in the first place. (Why not just
one -- or even three -- as in some species?) What is clear is
that the two sexes originate with two distinct chromosomes.
Women bear a double dose of the large X chromosome, while men
usually possess a single X and a short, stumpy Y chromosome. In
1990 British scientists reported they had identified a single
gene on the Y chromosome that determines maleness. Like some
kind of biomolecular Paul Revere, this master gene rouses a
host of its compatriots to the complex task of turning a fetus
into a boy. Without such a signal, all human embryos would
develop into girls. "I have all the genes for being male except
this one, and my husband has all the genes for being female,"
marvels evolutionary psychologist Leda Cosmides, of the
University of California at Santa Barbara. "The only difference
is which genes got turned on."

    Yet even this snippet of DNA is not enough to ensure a
masculine result. An elevated level of the hormone testosterone
is also required during the pregnancy. Where does it come from?
The fetus' own undescended testes. In those rare cases in which
the tiny body does not respond to the hormone, a genetically
male fetus develops sex organs that look like a clitoris and
vagina rather than a penis. Such people look and act female. The
majority marry and adopt children.

    The influence of the sex hormones extends into the nervous
system. Both males and females produce androgens, such as
testosterone, and estrogens -- although in different amounts.
(Men and women who make no testosterone generally lack a
libido.) Researchers suspect that an excess of testosterone
before birth enables the right hemisphere to dominate the brain,
resulting in lefthandedness. Since testosterone levels are
higher in boys than in girls, that would explain why more boys
are southpaws.

    Subtle sex-linked preferences have been detected as early
as 52 hours after birth. In studies of 72 newborns, University
of Chicago psychologist Martha McClintock and her students
found that a toe-fanning reflex was stronger in the left foot
for 60% of the males, while all the females favored their
right. However, apart from such reflexes in the hands, legs and
feet, the team could find no other differences in the babies'
responses.

    One obvious place to look for gender differences is in the
hypothalamus, a lusty little organ perched over the brain stem
that, when sufficiently provoked, consumes a person with rage,
thirst, hunger or desire. In animals, a region at the front of
the organ controls sexual function and is somewhat larger in
males than in females. But its size need not remain constant.
Studies of tropical fish by Stanford University neurobiologist
Russell Fernald reveal that certain cells in this tiny region
of the brain swell markedly in an individual male whenever he
comes to dominate a school. Unfortunately for the piscine pasha,
the cells will also shrink if he loses control of his harem to
another male.

    Many researchers suspect that, in humans too, sexual
preferences are controlled by the hypothalamus. Based on a study
of 41 autopsied brains, Simon LeVay of the Salk Institute for
Biological Studies announced last summer that he had found a
region in the hypothalamus that was on average twice as large
in heterosexual men as in either women or homosexual men.
LeVay's findings support the idea that varying hormone levels
before birth may immutably stamp the developing brain in one
erotic direction or another.

    These prenatal fluctuations may also steer boys toward
more rambunctious behavior than girls. June Reinisch, director
of the Kinsey Institute for Research in Sex, Gender and
Reproduction at Indiana University, in a pioneering study of
eight pairs of brothers and 17 pairs of sisters ages 6 to 18
uncovered a complex interplay between hormones and aggression.
As a group, the young males gave more belligerent answers than
did the females on a multiple-choice test in which they had to
imagine their response to stressful situations. But siblings who
had been exposed in utero to synthetic antimiscarriage hormones
that mimic testosterone were the most combative of all. The
affected boys proved significantly more aggressive than their
unaffected brothers, and the drug-exposed girls were much more
contentious than their unexposed sisters. Reinisch could not
determine, however, whether this childhood aggression would
translate into greater ambition or competitiveness in the adult
world.

    While most of the gender differences uncovered so far seem
to fall under the purview of the hypothalamus, researchers have
begun noting discrepancies in other parts of the brain as well.
For the past nine years, neuroscientists have debated whether
the corpus callosum, a thick bundle of nerves that allows the
right half of the brain to communicate with the left, is larger
in women than in men. If it is, and if size corresponds to
function, then the greater crosstalk between the hemispheres
might explain enigmatic phenomena like female intuition, which
is supposed to accord women greater ability to read emotional
clues.

    These conjectures about the corpus callosum have been hard
to prove because the structure's girth varies dramatically with
both age and health. Studies of autopsied material are of
little use because brain tissue undergoes such dramatic changes
in the hours after death. Neuroanatomist Laura Allen and
neuroendocrinologist Roger Gorski of UCLA decided to try to
circumvent some of these problems by obtaining brain scans from
live, apparently healthy people. In their investigation of 146
subjects, published in April, they confirmed that parts of the
corpus callosum were up to 23% wider in women than in men. They
also measured thicker connections between the two hemispheres
in other parts of women's brains.

    Encouraged by the discovery of such structural
differences, many researchers have begun looking for dichotomies
of function as well. At the Bowman Gray Medical School in
Winston-Salem, N.C., Cecile Naylor has determined that men and
women enlist widely varying parts of their brain when asked to
spell words. By monitoring increases in blood flow, the
neuropsychologist found that women use both sides of their head
when spelling while men use primarily their left side. Because
the area activated on the right side is used in understanding
emotions, the women apparently tap a wider range of experience
for their task. Intriguingly, the effect occurred only with
spelling and not during a memory test.

    Researchers speculate that the greater communication
between the two sides of the brain could impair a woman's
performance of certain highly specialized visual-spatial tasks.
For example, the ability to tell directions on a map without
physically having to rotate it appears stronger in those
individuals whose brains restrict the process to the right
hemisphere. Any crosstalk between the two sides apparently
distracts the brain from its job. Sure enough, several studies
have shown that this mental-rotation skill is indeed more
tightly focused in men's brains than in women's.

    But how did it get to be that way? So far, none of the
gender scientists have figured out whether nature or nurture is
more important. "Nothing is ever equal, even in the beginning,"
observes Janice Juraska, a biopsychologist at the University of
Illinois at Urbana-Champaign. She points out, for instance, that
mother rats lick their male offspring more frequently than they
do their daughters. However, Juraska has demonstrated that it
is possible to reverse some inequities by manipulating
environmental factors. Female rats have fewer nerve connections
than males into the hippocampus, a brain region associated with
spatial relations and memory. But when Juraska "enriched'' the
cages of the females with stimulating toys, the females
developed more of these neuronal connections. "Hormones do
affect things -- it's crazy to deny that," says the researcher.
"But there's no telling which way sex differences might go if
we completely changed the environment." For humans, educational
enrichment could perhaps enhance a woman's ability to work in
three dimensions and a man's ability to interpret emotions. Says
Juraska: "There's nothing about human brains that is so stuck
that a different way of doing things couldn't change it
enormously."

    Nowhere is this complex interaction between nature and
nurture more apparent than in the unique human abilities of
speaking, reading and writing. No one is born knowing French,
for example; it must be learned, changing the brain forever.
Even so, language skills are linked to specific cerebral
centers. In a remarkable series of experiments, neurosurgeon
George Ojemann of the University of Washington has produced
scores of detailed maps of people's individual language centers.

    First, Ojemann tested his patients' verbal intelligence
using a written exam. Then, during neurosurgery -- which was
performed under a local anesthetic -- he asked them to name
aloud a series of objects found in a steady stream of
black-and-white photos. Periodically, he touched different parts
of the brain with an electrode that temporarily blocked the
activity of that region. (This does not hurt because the brain
has no sense of pain.) By noting when his patients made
mistakes, the surgeon was able to determine which sites were
essential to naming.

    Several complex sexual differences emerged. Men with lower
verbal IQs were more likely to have their language skills
located toward the back of the brain. In a number of women,
regardless of IQ, the naming ability was restricted to the
frontal lobe. This disparity could help explain why strokes that
affect the rear of the brain seem to be more devastating to men
than to women.

    Intriguingly, the sexual differences are far less
significant in people with higher verbal IQs. Their language
skills developed in a more intermediate part of the brain. And
yet, no two patterns were ever identical. "That to me is the
most important finding," Ojemann says. "Instead of these sites
being laid down more or less the same in everyone, they're laid
down in subtly different places." Language is scattered randomly
across these cerebral centers, he hypothesizes, because the
skills evolved so recently.

    What no one knows for sure is just how hardwired the brain
is. How far and at what stage can the brain's extraordinary
flexibility be pushed? Several studies suggest that the junior
high years are key. Girls show the same aptitudes for math as
boys until about the seventh grade, when more and more girls
develop math phobia. Coincidentally, that is the age at which
boys start to shine and catch up to girls in reading.

    By one account, the gap between men and women for at least
some mental skills has actually started to shrink. By looking
at 25 years' worth of data from academic tests, Janet Hyde,
professor of psychology and women's studies at the University
of Wisconsin at Madison, discovered that overall gender
differences for verbal and mathematical skills dramatically
decreased after 1974. One possible explanation, Hyde notes, is
that "Americans have changed their socialization and educational
patterns over the past few decades. They are treating males and
females with greater similarity."

    Even so, women still have not caught up with men on the
mental-rotation test. Fascinated by the persistence of that gap,
psychologists Irwin Silverman and Marion Eals of York University
in Ontario wondered if there were any spatial tasks at which
women outperformed men. Looking at it from the point of view of
human evolution, Silverman and Eals reasoned that while men may
have developed strong spatial skills in response to evolutionary
pressures to be successful hunters, women would have needed
other types of visual skills to excel as gatherers and foragers
of food.

    The psychologists therefore designed a test focused on the
ability to discern and later recall the location of objects in
a complex, random pattern. In series of tests, student
volunteers were given a minute to study a drawing that contained
such unrelated objects as an elephant, a guitar and a cat. Then
Silverman and Eals presented their subjects with a second
drawing containing additional objects and told them to cross out
those items that had been added and circle any that had moved.
Sure enough, the women consistently surpassed the men in giving
correct answers.

    What made the psychologists really sit up and take notice,
however, was the fact that the women scored much better on the
mental-rotation test while they were menstruating. Specifically,
they improved their scores by 50% to 100% whenever their
estrogen levels were at their lowest. It is not clear why this
should be. However, Silverman and Eals are trying to find out
if women exhibit a similar hormonal effect for any other visual
tasks.

    Oddly enough, men may possess a similar hormonal response,
according to new research reported in November by Doreen Kimura,
a psychologist at the University of Western Ontario. In her
study of 138 adults, Kimura found that males perform better on
mental-rotation tests in the spring, when their testosterone
levels are low, rather than in the fall, when they are higher.
Men are also subject to a daily cycle, with testosterone levels
lowest around 8 p.m. and peaking around 4 a.m. Thus, says June
Reinisch of the Kinsey Institute: "When people say women can't
be trusted because they cycle every month, my response is that
men cycle every day, so they should only be allowed to negotiate
peace treaties in the evening."

    Far from strengthening stereotypes about who women and men
truly are or how they should behave, research into innate sexual
differences only underscores humanity's awesome adaptability.
"Gender is really a complex business," says Reinisch. "There's
no question that hormones have an effect. But what does that
have to do with the fact that I like to wear pink ribbons and
you like to wear baseball gloves? Probably something, but we
don't know what."

    Even the concept of what an innate difference represents
is changing. The physical and chemical differences between the
brains of the two sexes may be malleable and subject to change
by experience: certainly an event or act of learning can
directly affect the brain's biochemistry and physiology. And so,
in the final analysis, it may be impossible to say where nature
ends and nurture begins because the two are so intimately
linked.