Of course men and women are different. Boy, are they different. In every sphere of life, it seems, the sexes act, react or perform differently. Toys? A little girl daintily sets up her dolls, plastic cups and saucers, while her brother assembles his Legos into a gun -- and ambushes the tea party. Navigating? The female tourist turns her map every which way but right, trying to find the way back to that charming bistro, while her boyfriend charges ahead, remembering every tricky turn without fail. Relationships? With spooky intuition, women's acute senses pick up subtle tones of voice and facial expressions; men are insensitive clods who can't tell a sad face until it drenches them in tears. Cognition? Females excel at language, like finding just the right words to make their husbands feel like worms; males can't verbalize even one good excuse for stumbling home at 2 a.m.
Stereotypes? Maybe -- but as generalizations they have a large enough kernel of truth that scientists, like everyone else, suspect there's something going on here. As Simon LeVay, the Salk Institute neuroscientist who in 1991 discovered structural differences between the brains of gay and straight men, put it recently, ""There are differences in the mental lives of men and women.''
The mind, of course, is just what the brain does for a living. So if LeVay is right, those mental differences must arise from differences in that gelatinous three-pound blob. For a decade neuroscientists have been discovering evidence of differences. Although the findings are tentative and ambiguous, at the end of the day, relaxing over beers at a neuroscience conclave, most specialists agree that women's and men's brains differ slightly in structure. But the studies have been frustratingly silent on whether the anatomical differences in their brains make men and women think differently. But now -- drumroll, please -- thanks to an array of new imaging machines that are revolutionizing neuroscience, researchers are beginning to glimpse differences in how men's and women's brains actually function.
With new technologies like functional magnetic resonance imaging (FMRI) and positron emission tomography (PET), researchers catch brains in the very act of cogitating, feeling or remembering. Already this year researchers have reported that men and women use different clumps of neurons when they take a first step toward reading and when their brains are ""idling.'' And, coming soon to a research journal near you, provocative studies will report that women engage more of their brains than men when they think sad thoughts -- but, possibly, less of their brains when they solve SAT math problems. ""Now that we actually have functional brain data, we're getting lots of new insights,'' says Richard Haier, professor of pediatrics and neurology at the University of California, Irvine, and leader of the SAT study. ""Even at this early point we have data to support the idea that men and women in general have brains that work differently.'' The latest studies:
In men's idling brains, the action was in the temporal-limbic system (diagram). This primitive region controls highly unsubtle expressions of emotion, such as fighting.It is often dubbed the ""reptilian'' brain. In most of the women's supposedly idling brains, the neurons were buzzing in the posterior cingulate gyrus, an evolutionarily newer addition to mammals' brains. Not even the researchers are sure what all of this shows. For one thing, 13 men and four women showed activity more like the other sex's. But the real problem is that ""thinking of nothing'' is nearly impossible. Volunteer (and coresearcher) Lyn Mozley admits that ""some of the time I was probably thinking, "When is this going to be over?' '' What the PET scans may actually be showing is that, when told to think of nothing, men fixate on sex and football, while women weave together strings of words. But if,in men, the pilot light is always on in neurons that control aggression and action, it may explain why they're more violence-prone than women.
In all 19 men, one region in the left inferior frontal gyrus (that's behind the left eyebrow) lit up like Las Vegas. So far, so good: for more than a century scientists have known that the left brain controls language. But in 11 of the 19 women, that area plus one behind the right eyebrow lit up. The right side of the brain is the seat of emotion. Perhaps women are more felicitous with language because they draw on feelings (right brain) as well as reason (left brain) when they use words. The Yale team made one more intriguing find. In eight of the women -- 42 percent -- the brain worked like the men's. ""That some of the women's brains looked like the men's is true of all these sex studies,'' says neuropsychologist Melissa Hines of UCLA. ""Girls play with boys' toys more than boys play with girls', for instance. Males for whatever reason are more exclusively channeled into one way of behaving'' -- and, possibly, thinking.
Well, of course. Evolutionarily speaking, it makes sense that a man would have to be hypervigilant about men's faces; otherwise he would miss the first hint that another guy is going to punch him. Being oblivious to a woman's emotions won't get him much worse than a night on the sofa. The Gurs may even have stumbled on why women can't understand why men find it so hard to be sensitive to emotions. According to the PET scans, women's brains didn't have to work as hard to excel at judging emotion. Women's limbic system, the part of the brain that controls emotion, was less active than the limbic system of men doing worse. That is, the men's brains were working overtime to figure out the faces. But the extra effort didn't do them much good.
The brain-imaging studies are the latest, and the highest-tech, periscope into sex differences in the brain. Yet no matter how ""scientific'' it gets, this research serves as ammunition in society's endless gender wars. When Raquel Gur gave a talk to M.D.-Ph.D. students in Illinois about sex differences in brains, a group of women asked her to stop publicizing the work: they were afraid women would lose 20 years of gains if word got out that the sexes aren't the same. They had good reason to worry. Among the choicer passages from recent pop-science books: Male brains ""are not so easily distracted by superfluous information.'' ""A woman may be less able to separate emotion from reason.'' And ""the male brain is a tidier affair,'' as Anne Moir and David Jessel write in their 1991 book ""Brain Sex.'' The subject of sex differences in the brain attracts almost as much inflammatory rhetoric as the ""science'' of racial differences in IQ.
Even before scientists caught images of the brain thinking or emoting, there were hints that men's brains and women's differed. As long ago as 1880, English surgeon James Crichton-Browne reported slight differences in the brain anatomy of men and women -- a slightly larger gaggle of neurons here in one sex, and there in the other. But by far the most frequent finding through the years has been that the bundle of nerve cells through which the left side of the brain talks and listens to the right -- it's called the corpus callosum -- is larger in women than in men. In perhaps the best study of this kind, in 1991 UCLA neuroendocrinologists Roger Gorski and Laura Allen examined 146 brains from cadavers and found that the back part of women's callosum is up to 23 percent bigger than men's.
This brought neuroscientists as close as they ever get to jumping up and down in public. It fit their cherished idea that, in male brains, the right and the left side barely know what the other is doing, while in women there's practically nonstop left-right neural chitchat. If women's brains are paragons of holism, while men's are a house divided, it could explain findings both serious and curious. Women's language ability better survives a left-brain stroke -- perhaps because they tap the language capacity of the right brain. Women tend to have better language skills -- perhaps because the emotional right brain enriches their left-brain vocabulary. And women have better intuition -- perhaps because they are in touch with the left brain's rationality and the right's emotions simultaneously.
There is just one problem with these tidy explanations. A bigger corpus callosum matters only if it has more neurons, the cells that carry communications. After all, fat phone cables carry more conversations only if they contain more wires. But despite years of searching, scientists cannot say for sure that women's corpus callosum has more neurons.
The quest for other anatomical differences has been only a little more successful. In rats, biologists have found 15 regions that differ in size between males and females. Finding such differences in humans has been much tougher. But in November, at the annual meeting of the Society for Neuroscience, Sandra Witelson of McMaster University in Ontario reported results from a study of nine autopsied brains. (She gets them from people with terminal cancer who bequeath their brains to science.) Women, despite having smaller brains (on average) than men because their whole bodies are smaller, have more neurons. The extra 11 percent are all crammed into two layers of the cerebral cortex whose job is to understand language and recognize melodies and tone of voice.
Neuroscientists know of only one force that can prune and stimulate, kill and nourish the brain's gaggles of neurons: sex hormones. Before birth, a fetus's brain is bathed in sex hormones -- different ones, in different amounts, depending on whether the fetus is male or female. Ethics prevents scientists from experimenting to see how a fetus's brain would change if its hormone exposure changed. But nature has no such compunctions. Girls with a rare birth defect called CAH, which made them churn out high levels of the male hormone testosterone as fetuses, score better than the average female on spatial tests. (The extra testosterone exposure also masculinizes their genitals.) As girls, they prefer cars and trucks and other toys that boys usually grab. Other girls were exposed to male-like levels of testosterone before birth when their mothers took the hormone DES to prevent miscarriages. As children, the DES girls did better than their normal sisters on rotating a figure in space and other tasks at which boys outdo girls. Finally, boys with a syndrome that makes them insensitive to testosterone are better at language than their unaffected brothers. But they are less adept at spatial tasks -- the typical female intellectual pattern. Hormonal effects, wrote psychologist Doreen Kimura of the University of Western Ontario in 1992, ""appear to extend to all known behaviors in which males and females differ . . . [such as] problem solving, aggression and the tendency to engage in rough-and-tumble play.''
Are these hormonal effects present at birth, or do they result from how a child is raised? The feminized boys (whose cognitive abilities resemble women's) and the CAH girls (minds like boys) are not physically normal. Their parents know they are different, and likely treat them differently from their sisters and brothers. Perhaps even more crucial, the hormonally abnormal girls might identify, psychologically, not with girls but with boys, aping their behavior and preferences. Similarly, the feminized boys might identify with girls. So if the girls play like tomboys and do better in math than normal girls, and the boys have superior language tasks compared with their normal brothers, it is impossible to tell whether the reason is hormones alone or life's experiences, too. Only Hines's DES girls are pure products of prebirth hormones: they looked like ordinary little girls, so people treated them as girls and they saw themselves as female. Their male-like cognitive function is the only one ever found that cannot be easily explained away as the result of nurture.
Hines's work has been seized as proof that biology is destiny, but other research undermines that dogma. For one thing, the overlap between men's and women's scores on just about every psychological test is huge. Any randomly chosen woman might do better at a ""male'' skill than a man, and vice versa. ""This [overlap] is also true of brain structures,'' says UCLA's Gorski. More important, the nature-nurture dichotomy is simplistic. Nurture affects nature; experience, that is, affects biology (page52). The brain is so malleable that rats raised in a cage filled with toys and mazes grow more connections between their neurons than rats raised in a bare cage. Also in rats, mothers sense their sons' testosterone and lick them more than daughters; that causes more nerve cells at the base of the tail to grow. The human brain is malleable, too: in people whose hands were amputated, scientists reported last year, the part of the brain that once registered feelings from the missing hand vanishes.
Is it farfetched to wonder whether parts of girls' brains grow or shrink, while different parts of boys' expand or shrivel, because they were told not to worry their pretty heads about math, or because they started amassing Legos from birth, or because . . . well, because of the vastly different experiences boys and girls have? ""Surely the more complex social interactions among humans also sculpt the developing nervous system,'' argues psychologist Marc Breedlove of UC, Berkeley. ""The studies provide no evidence favoring either nature or nurture.'' But, he adds, ""there's one thing I know that testosterone does to masculinize [men's] brains. It causes them to be born with a penis. And everybody treats the baby differently [than they do a girl].I'm sure that affects the development of the brain. Is that a biological effect or a social effect? It's both.''
The recent PET scans and FMRIs are silent ""on how the brains of men and women get to be different,'' says Irvine's Richard Haier. The scans probe adults, whose brains are the products of years of living, feeling, thinking and experiencing. Children have not yet been scanned in the service of science. But in studies of fetal brains (from miscarriages) and newborns' (from stillbirths), ""none of the sex differences in [the brain] have been reliably detected,'' says Breedlove.
The powerful new techniques of brain imaging are just beginning to be trained on the age-old question of what makes the sexes different. As the answers trickle in, they will surely challenge our cherished notions of what makes us think, act and feel as we do -- and as members of the opposite sex do not. But if the first tantalizing findings are any clue, the research will show that our identities as men and women are creations of both nature and nurture. And that no matter what nature deals us, it is we -- our choices, our sense of identity, our experiences in life -- who make ourselves what we are.
Twenty-two male and 22 female students were PET-scanned while they solved SAT math problems. By detecting areas of the brain using the most blood, PET pinpoints active regions.
Male students with high SAT scores showed intense activity in the temporal lobes (red spots at top and sides), compared with men with average scores. In men, it seems, ability is related to how hard the brain works.
Brain activity in the mathematically gifted women was less intense than in the high-SAT men, even though their scores were comparable. These women expended no more neural effort than the average-SAT women.
In one study, 61 men and women were told to "think of nothing." After they were injected with a radioactive tracer, the volunteers let their brains idle while researchers took PET scans--images that show which regions of the brain are active. Bright spots indicate high levels of activity.
Women had more activity in the cingulate gyrus, an evolutionarily recent region that controls complex expressions of emotions, such as showing anger by looks, not punches.
Men had more activity in their temporal limbic system. This evolutionarily ancient region controls emotions linked to action, especially aggression.
With all the differences in how men and women feel, act and perform on cognitive tests, you'd expect lots of differences in their brain structures. But although neuro-scientists have found 15 such differences in rats, they've identified only a few in humans.
This region of the cerebral cortex helps control hearing, memory and a person's sense of self and time. ..CN.-MEN
In cognitively normal men, a tiny region of the temporal lobe behind the eye has about 10 percent fewer neurons than it does in women. ..CN.-WOMEN
Women have more neurons in this region, which understands language as well as melodies and speech tones.
This bundle of neurons is the main bridge between the left brain and the right, carrying messages between them. ..CN.-MEN
A man's corpus callosum takes up less volume in his brain than a woman's does, suggesting the two hemispheres communicate less. ..CN.-WOMEN
In women, the back part of the callosum is bigger than in men. That may explain why women use both sides of their brain for language.
This collection of nerve cells also connects the brain's two hemispheres. It is smaller and appeared earlier in evolution than the corpus callosum. ..CN.-MEN
In men, the commissure is smaller than it is in women, even though men's brains are, on average, larger than women's. ..CN.-WOMEN
The larger commissure in women may be another reason their two cerebral hemispheres seem to work in partnership on tasks from language to emotional responses.
Men and women were asked whether the expressions on actors' faces were sad or happy and were monitored by PET scans as they decided.
Men did as well as women -- 90 percent right -- in identifying happy male and female faces. But they were worse at sensing sad women.
Overall, women were better able to judge facial expressions of both sexes. The PET scan showed their brains required less energy than the men's to decide.