“How am I supposed to work?” I asked, staring dolefully at my husband. Our cat had been hit by a car the night before and was at the animal hospital, clinging to life. Even though I was on tight deadlines, I could think of nothing else.

“Just put it out of your mind,” he responded. “There’s nothing we can do.”

I almost strangled him.

My gal pals commiserate when I tell them this story later on because it seems as though we’d be incapable of that kind of detachment. Yet male friends look at me blankly, waiting for the punch line.

The fact is, men process their strong emotions differently from women and tend to act out and take action, which may explain why my husband—once he’d made sure our vet had the situation under control—was able to move on, while all I could do was obsess.

The contrasting ways men and women behave have provided plenty of fodder for Letterman and Leno. But beyond the Mars-Venus stereotypes, it’s not just the social conditioning we receive from the time we parachute down the birth canal that accounts for sex differences. Our brains are hardwired differently, and these anatomical variations in architecture and function illuminate some of the reasons why men and women seem to come from different planets.

Men and women, to be sure, “are more alike than they’re different, and even when there are variations, there is a significant overlap between the sexes,” says Nancy Andreasen, a psychiatrist and neuroimaging expert at the University of Iowa. But advances in such techniques as PET scans and functional MRIs (fMRIs) have enabled scientists to peer deep inside the brain and actually film it while a subject is thinking and processing information, giving them new insights into sex differences in storing memories, making decisions, and solving problems.

These findings offer tantalizing hints that even gender behavior differences once attributed solely to nurture—women are more emotionally attuned, while men are more physically aggressive—stem in part from variations in our neural circuitry.

“Neuroscience helps us to understand the underpinnings of behavior,” says Sandra Witelson, a neuroscientist at McMaster University in Hamilton, Ontario. “Sex differences in the brain in all likelihood translate into cognitive differences. And even when the differ­ences are small, we don’t know how that plays out in daily life.”

The brain is divided into two hemispheres that play different roles in perception and behavior. The right side is relatively more involved with visual and spatial control, while the left is the seat of language. There is evidence that the male brain uses either one hemisphere or the other and relies on specialized brain regions when performing a task. Women, meanwhile, call on both hemispheres regardless of the task, resulting in greater communication between the two; they also enlist more brain regions to process information. When at rest, male minds appear to be more attuned to the “external world,” while there may be a “differential tilt toward the internal world” in female brains, says Larry Cahill, a neurobiologist at the University of California at Irvine.

Like many-handed Hindu goddesses, women are better jugglers (in my experience), sweeping through their lives performing several tasks at once, while men seemingly do things sequentially—a division of labor that certainly prevails in our household. Mornings find me feeding our animals, watching The Today Show, glancing at the newspaper, fixing coffee, sticking English muffins in the toaster, and emptying the dishwasher while my husband fries sausages. When they’re done, he whips up a fruit smoothie in the blender.

This distinct gender variation in the way we harness our brainpower may also illuminate why men are much more vulnerable to such developmental disorders as cerebral palsy, dyslexia, autism, and attention-deficit/hyperactivity disorder; in most cases, the increased incidence may be several times greater. Because women are less specialized, their brains seem to be more resilient—they recover faster and more easily from the effects of strokes—and better at compensating if one region is damaged.

Even when men and women do the same chores equally well, they may use different brain circuits to get the same result. In a 2005 Harvard University functional imaging study of working memory—that short-term memory we use to carry on conversations or remember telephone numbers—a group of volunteers were given verbal attention tasks while inside the scanning machine. This required that volunteers listen to prompts through a headset and respond accordingly.

While male and female participants performed about the same in terms of accuracy and reaction time, the neural pathways that were activated were different: Women showed greater activity in both hemispheres in the prefrontal cortex—in regions implicated in language functions and higher-level cognitive functions—than men. “We didn’t see sex differences in how well volunteers performed, but men and women used their brains differently to get the same results,” says Jill Goldstein, a professor of psychiatry and medicine at Brigham and Women’s Hospital and Harvard Medical School.

Men and women may take different routes, but they arrive at the same destination at about the same time. “It’s like two different automobiles,” says Witelson. “Each has a motor, a steering wheel, and brakes. But one is a Volvo and the other is a Lexus.”

DOES SIZE MATTER?

Recent research may also provide some clues to a long-standing paradox. Men’s brains are bigger than women’s—about 100 grams heavier on average—even when their larger proportional size is factored into the equation. Brain volume is correlated with intelligence, according to a 2005 study (pdf), yet both sexes score about the same on IQ tests.

Even though women’s brains are smaller, they have some distinct advantages that may level the playing field. Women have more gray matter—areas of neuron cell bodies. Studies consistently show that women surpass men on verbal and memory tasks, and the superior temporal cortex, one of the brain areas responsible for language, is 29 percent larger in females. Blood flow is about 15 percent higher in the female brain, which offsets the cognitive losses of aging, and women’s nerve cells are also more tightly packed together, which suggests that the neurons may function more efficiently.

A postmortem analysis of human brain tissue, for example, conducted by Witelson and her colleagues at the Michael G. DeGroote School of Medicine at McMaster, revealed that women’s neurons were 11 percent denser than men’s in the prefrontal cortex and in a region of the temporal cortex that is involved with language processing, comprehension, and memory. “Little girls develop language skills earlier than boys, and they’re better able to make fine phonetic discriminations between speech sounds,” says Witelson. “These microscopic anatomical differences may have something to do with it.”

This may be part of the reason why women do better in school—it’s not merely because they’re conditioned to be people pleasers or to sit in a chair without squirming. According to behavioral studies, even in kindergarten and first grade, girls are more articulate than boys, their handwriting is more legible, and they’re quicker at answering questions, says Louann Brizendine, a neuropsychiatrist at the University of California at San Francisco (UCSF) and author of The Female Brain. Over time, this pattern appears to become more pronounced: Females now outnumber males in college, and more women go on to graduate.

The male brain, by contrast, is filled with more white matter, which consists of longer neuron fibers coated with a fatty myelin sheath that communicate with more distant regions of the brain. White matter also contains fibers from inhibitory neurons that block the dispersal of information in the cortex, which enhances local processing. The bulkier white matter may be what gives men their ability to focus intently on work and tune out distractions, as well as their clear-cut superiority when it comes to spatial reasoning, says Ruben Gur, a psychologist and director of the Brain Behavior Laboratory at the University of Pennsylvania in Philadelphia. Little wonder, then, that men still predominate in fields like architecture, organic chemistry, physics, and neurosurgery. There are only a handful of female chess grandmasters, and girls are less likely than boys to sit transfixed for hours playing video games.

Men are faster at mentally rotating an object—imagining what something looks like from a different perspective—and have an internal compass that enables them to squint up at the sun to figure out cardinal directions. Women, on the other hand, rely on landmarks—bear right at the church, turn left after the fire station—to find their way around.

Because it takes hundreds of thousands of years for the genetic evolution of natural selection to occur, these sex differ­ences in navigation strategies probably have their roots in the Stone Age. Women most likely watched the kids and foraged close to home, using familiar sights to find their way back. Men, meanwhile, were hunters and ventured into unknown territory scavenging for food. In all likelihood, they figured out where they were by estimating the distance they had already traveled and their orientation in space—what neurobiologist Larry Cahill calls “dead reckoning.”

Over the millennia, men and women apparently evolved different neural pathways to get around, and men mastered the use of geometric cues to navigate unfamiliar terrain. In a 2000 German study, for example, men sped through a three-dimensional virtual-reality maze much faster than women, averaging two minutes and 22 seconds compared with three minutes and 16 seconds for women.

Brain imaging techniques revealed that men found their way out of the maze using the left hippocampus, a memory storage region that also governs spatial mapping in the physical environment. Women employed their right parietal and prefrontal cortices, which are linked to visual identification and reasoning. The women’s use of the prefrontal cortex, say researchers, suggests that they relied on landmarks and pictured the objects in their minds, while the men used both landmarks and geometric cues, like shapes and angles, to escape the maze.

These spatial skills may have enabled men to navigate distant lands across unmarked desert sands and vast uncharted oceans. In one famous 1916 incident, New Zealand native Frank Worsley, a member of the ill-fated Antarctic expedition led by explorer Ernest Shackleton, relied almost solely on his internal compass to save the lives of 28 men marooned on an island in the Antarctic.

Worsley set off from the remote island in a 22-foot lifeboat across the South Atlantic Ocean, eventually traveling 800 miles to one of the South Sandwich Islands near the tip of South America, where there was a whaling station where they could get help. Worsley had only been able to take sightings of the sun four times during the stormy 17-day trek, and the rest had been based on dead reckoning.

Neuroscientist Sandra Witelson. Image courtesy of Melissa Hines.

ANGER MANAGEMENT

The way males and females handle their anger or emotionally upsetting situations—women may feel sick to their stomach, while men tend to act out—may also stem from fundamental differences in how their brains have evolved.

A 2002 study using MRI scans showed that brain areas keeping aggression and impulsive behavior in check were relatively larger in women than in men. Female brains had a significantly greater volume than males of orbital frontal cortex, the seat of cool-headed decision making behind the forehead, in proportion to the amygdala, a more primitive, almond-shaped structure deep inside the brain that pumps us up by stimulating the fight-or-flight reaction and getting the adrenaline flowing.

The implication here is that in an emotionally charged situation, women are better equipped to keep a tighter rein on their tempers, says Gur, who conducted this research. Because they have a bigger braking mechanism than males, he adds, women will try to defuse a volatile situation rather than fly off the handle.

We need look no further than the latest in celebrity bad behavior for an example. When Paris Hilton was arrested on suspicion of drunk driving in the early morning hours last September, the heiress was “cooperative,” according to police sources. But alcohol suppresses the frontal cortex, magnifying an inability to control anger or aggression. Like when Mel Gibson got pulled over...well, we all know the rest.

When Larry Cahill and his colleagues at UC Irvine conducted a series of experiments to track sex differences in the brain’s ability to store memories, they came to similar conclusions. Researchers found that the amygdala, which also processes emotional memories, acts differently in men and women. In one study, volunteers were shown a series of graphically violent films while their brain activity was measured using a PET scan.

To process the most disturbing material, men fired up the amygdala’s right hemisphere, which is more in tune with the outside world and communicates with regions that control sight, such as the visual cortex, and motor coordination, like the striatum. Women, on the other hand, activated the left hemisphere, which concentrates more on the body’s inner environment and is connected to the insular cortex, where sensory information is translated into emotional experiences, and to the hypothalamus, the master regulator of such basic functions as metabolism.

“When men are presented with an emotionally provocative stimulus, part of the motor system is activated, which may be why men try to resolve the situation by acting on the environment,” says Witelson. “But in women, the hypothalamus is activated, which controls digestion, so it may not be surprising that when a woman is really upset, she feels weak and nauseated and can’t sleep.”

We also know that the brain’s right hemisphere distills the essence of a situation, the central idea, while the left side mulls the finer points and tracks the details. Consequently, this right-left amygdala division may also illuminate why women remember every excruciating detail of a blowup they had on their honeymoon—where they were, what they were wearing, the time of day—while their husbands barely recall the tiff.

In a subsequent UC Irvine experiment, Cahill and his colleagues gave study participants a drug, the beta blocker propranolol, which blunts emotionally charged memories by blocking signals to the amygdala. Then they were shown an emotionally disturbing movie about a boy run over by a car. A week later, when volunteers were quizzed about the film’s content, the males with the beta blocker had trouble remembering the gist of what they saw—the boy being run over by a car. In females, the drug did the converse, interfering with their ability to recall peripheral details, like the fact that the boy had been carrying a soccer ball.

The results were “striking,” says Cahill, “because I assumed sex wouldn’t make a difference. But we have to stop assuming men and women are basically the same because they’re not, which represents a fundamental change in how neuroscience has been doing business—a major zeitgeist change is afoot.”

And as the lines between gender roles continue to blur, our brain architecture may eventually be sculpted to reflect these societal changes, though that structural transformation will take hundreds of generations. “We’re living with brains that evolved in response to conditions over thousands of years, but the brain is a fast learning machine,” says Louann Brizendine of UCSF. “It changes every nanosecond to adjust to what is happening in the environment.“

New behaviors, like learning a new language or how to play the piano, can alter our neural circuitry. Someday soon, Brizendine says, it’s conceivable that the pathways for caring for newborns and doing housework may be activated in men, while women warriors will become hardwired to coolly face danger.

Still, these studies are “in their infancy,” says Melissa Hines, a professor of psychology at the University of Cambridge in England and author of Brain Gender. “We don’t really have a clear overall picture yet.” But the hope is that this research could bring into sharper focus the reasons for the deep gender divide in the incidence of disabling ills that plague millions, says Harvard’s Jill Goldstein, and that understanding could lead to more effective treatments and better methods of prevention.

Vast pieces of the puzzle are missing, in part because imaging techniques are still rather primitive. “To see the brain in action, test subjects need to lie down on a plank inside a scanner or be wired up to a PET machine, which hardly mimics the real world,” says Brizendine. “When we can run around all day long with a miniature MRI scanning apparatus strapped to our head, which could happen in the not too distant future, then we’ll be able to see real sex differences in the brain.”