True. Our sex is fixed and immutable—and not just at birth, but from the very moment of conception. That sex has implications for all the systems in our bodies, including our brains.

But in a sense, this is a trick question, because while we are undeniably and indelibly male or female from the very beginning, there are a variety of factors that contribute to the process by which we acquire our sex over the course of our lives. So, although you're always male or female, other factors are working on you at specific stages throughout your life to make you more or less that way.

What are those factors? Our genes are the unique cellular blueprint that makes us who we are, including our sex: The sex chromosome we get from our fathers at conception determines which sex organs we'll develop. An X chromosome from Dad means the baby will have two Xs and develop into a female. A Y chromosome means that there will be an XY complement, creating a boy. The sex organs we develop, in turn, release sex-specific hormones, which continue the process not only in the uterus but also during certain windows of time throughout life—puberty and menopause, for example—when hormone levels change precipitously. Those hormones also turn certain genes on or off, which further influences the sex-specific functions of our tissues, which is why more than one teenage girl has cursed her mother for the size (large or small) of her new breasts.

These genes are also why hormone levels vary from person to person. Those hormone levels affect our behavior. Individuals with high testosterone levels, for instance, are bolder, more aggressive, and more focused on a single goal. They smile less, have a higher libido, and are more likely to engage in extramarital sex.

There's one more factor influencing our sex—our experiences. A striking example of this is the conduct of some of the female soldiers at Abu Ghraib, the American-run prison in Iraq. Many of us were shocked—not just by the brutalities these women meted out, but at the discovery that women were just as capable of acts of humiliation and savagery as men. Clearly, experience is an important factor in modifying behavior.

True or False: Men's brains are bigger.

True. Whenever I lecture on this subject, nothing gets a more outraged response than this simple biological truth: Men's brains are bigger than those of women and weigh 10 percent more.

But size isn't everything. Women have more gray matter in certain parts of their brains and more intricate and extensive communications between brain cells than men, particularly in the frontal cortex. This is the area involved in judgment and decision making: the "executive center" of the brain. Some scientists think that this relatively more intricate system of neuronal interconnections explains why women's brains have a higher rate of blood flow. In fact, smaller brains may be more efficient. Ounce for ounce, women get more brain bang for the buck, possibly because of the greater degree of connectivity between cells.

And while it is true that male fetuses have more brain cells than female ones do, this may be the reason boys have more developmental defects than girls; it may require more energy to keep these larger brains in tip-top shape. It takes a lot of energy to drive a brain, especially a baby's brain, which has twice the number of working connections between cells as an adult's does. Boys, with their bigger brains, have significantly lower heart rates and lower body temperatures than girls; just when they need the energy to support their bigger brains, they fall behind! A higher number of boys have developmental disorders that become apparent in early childhood, such as mental retardation, expressive and receptive language disorders, stuttering, and autism; the energy deficit may explain why.

True or False: Women are better at multitasking, while men are better when concentrating on a single task from beginning to completion.

True. Ruben Gur, PhD, and Raquel Gur, MD, PhD, at the University of Pennsylvania in Philadelphia, measured blood flow and activity in men's and women's brains, and they found repeatedly that women use more parts of their brains when given a wide variety of verbal and spatial tasks. They believe that this may contribute to women's ability to focus on a number of different things at one time.

A new study has raised an important question: Women may be better at multitasking, but is multitasking really the most efficient way to work? Newer research shows that switching back and forth from one task to another takes precious seconds of reevaluation, and those seconds add up. As the researchers point out, in the best-case scenario, this makes you only slightly less efficient—but in the case of someone talking on a cell phone and driving, that fraction of a second may make the difference between life and death.

The conclusion I personally have come to is this: Multitasking is certainly helpful when you don't have any options, when your assistant is out sick or when you're trying to put dinner on the table while at the same time making sure your children are entertained and safe. But I find that when I need to concentrate on writing, it's helpful for me to turn off my phone and my e-mail program, with its constant "new mail" alerts, so that I can better and more purely concentrate on the task at hand.

True or False: The effects of our sex hormones (such as estrogen and testosterone) are restricted to the reproductive system.

False. There are two interesting things about hormones. The first is how many hormones play a role in sexing us—not just the sex hormones, as you might think, but others, like the ones we release when we're under stress.

The second is how many systems these hormones affect. Yes, estrogen is responsible for menstrual periods, but did you know that it also has a profound effect on the way women learn, think, and remember? For instance, estrogen may be one of the keys to the earlier questions about the differences between schizophrenia in men and women. Here's a more pedestrian example: I tell patients with young girls to keep an eye on their daughters' sneakers. The hormonal changes that announce puberty and bring on a girl's first menstrual period will cause a sudden surge in her growth and a leap in her shoe size as well.

All of the hormones in the body have far-reaching effects, which is why it's so important to take note when differing levels of them are found in men and women.

True or False: Boys and girls develop on different schedules.

True. One of the most important ways in which our brains are shaped is not through growth, but the programmed death of a large number— about half—of the neurons originally produced as the brain forms. This pruning process goes on from the final month of pregnancy and continues long after birth. Synapses, or connections between cells, that don't get reinforced by stimulation from the outside world atrophy and eventually disappear. The connections that are stimulated grow stronger and become permanent. You have to use it, or you lose it, and practice makes perfect.

It's a mysteriously wasteful process. Why don't we simply make what we need to begin with? I like to think that we're choosing the neurons that function optimally, like choosing the prettiest and healthiest flowers out of a bunch for a bouquet.

This brain tailoring process is part of what makes us unique: Our experiences—the stimulation we're exposed to, or protected from— have a very real impact on who we become. If we don't have appropriate input during these times, the systems can be impaired forever, and there are all too many examples of abused and neglected children who are cut off from interaction during crucial developmental windows and will never develop normal language skills as a result. Less tragically, it's what makes the differences between siblings and even identical twins who carry the same genetic information.

New information also tells us that how and when this brain tailoring occurs between the ages of 6 and 17 is different for boys and girls. There are major differences in when boys and girls prune and expand the connections in their brains, and in which areas they tend, as well as in the numbers of connections between the two halves of the brain in boys and girls. The hormones that surge during puberty (testosterone in boys, estrogen in girls) play a major role in these processes, as they have very different effects on brain function. These hormonal differences may be the reason for the different pace of development in pubescent boys and girls.

True or False: We treat boys and girls differently.

True. Of course, the society and culture in which we raise boys and girls has a tremendous impact on their outcomes. A landmark study done in the seventies showed that women tended to coo at babies dressed in pink jumpsuits, while men tossed those in blue up into the air. People tend to talk to girls, while they encourage boys to play with mechanical toys and objects, often from a very young age. In fact, this research leaves us unable to tell what comes first. Do the sex-specific innate areas of the brain make one sex function differently from the other? Or is it the impact of gender-specific behavior, induced by the societal roles we are asked to play? Gender bias may be even more important than we once thought, if the structure of our brains is in play.

True or False: There's no difference in the ways
that men and women solve spatial problems.

False. There is evidence that men and women solve spatial problems differently. For example, precisely how men and women find their way through new and even familiar environments was tested in an interesting experiment done in Canada using a virtual reality maze. Not only did men and women activate completely different areas of their brains to negotiate the space, but they used different strategies to find the exit. Women used landmarks to guide them, while men used what was called "Euclidian information" to place themselves within the structure and navigate through it.

The researchers wondered whether the two sexes could use either strategy equally well and simply chose one over the other for reasons that weren't understood. They found that each performed less well when using the methods of the opposite sex, and they concluded that there is a real sex-specific difference in the way spatial problems are solved.

Apparently, in both animals and humans, the right brain houses the material we need to navigate our way through both familiar and new environments. It's risky business to relate the number or size of areas that are activated in response to a specific challenge to the success of how well the individual meets the challenge. But it is interesting to know that while both sexes activated part of the right brain, men used part of the left brain as well to do spatial tasks, which women didn't.

Men excel in their ability to imagine, for example, how a figure would appear if rotated in a space of two or three dimensions. Success with problems in advanced mathematics correlates with our ability to understand and manipulate three-dimensional relationships. Kerrin Christiansen and Rainer Knussmann from the University of Hamburg in Germany showed that higher testosterone levels in men correlated with men's enhanced spatial ability. However, they also found that those levels corresponded with their diminished ability for verbal expression—an area in which women excel.

There's excellent evidence that this spatial ability is hardwired into the human brain at birth and is the result of the sex-specific hormones we secrete during development. Baby girls who have an adrenal condition called congenital adrenal hyperplasia, or CAH, suffer from a buildup of masculinizing hormones as a result. These girls can appear very masculine (the enlarged clitoris of such a girl may be mistaken for a penis at birth), prefer roughhouse play, and are more likely to be sexually attracted to women than females without the disorder. They also have a better ability to solve spatial problems than unaffected females.

More evidence comes from studies of twins. Baby girls placed near a male fetus in utero seem to benefit from the additional boost of testosterone to which their brothers expose them: They have enhanced ability, for example, to solve three-dimensional problems compared with their singleton sisters.

Does this mean, as has so often been claimed, that boys and men are better at math? Maybe not. A 1990 study by psychologist Janet Hyde, PhD, looked at math ability in adolescent girls and boys and found that while boys did score better, the difference was in a mere few percentage points, not a significant majority. She did find, however, that there were more males at both extremes of the spectrum—the very high end of competence and at the lowest. Again, it's hard to extrapolate much of relevance there without more research. Most of all, it's impossible with what we know at the present time to decide how much ability people can cultivate by virtue of good teaching.

If girls are "worse" at math, it may have something to do with their socialization. Girls who took a math test believing that the test contained a gender bias (that they'd do worse, simply because they were girls) did worse than those who were told no such thing. In addition, girls in single-sex educational environments tend to do better in math than those in coed classes, perhaps because the implicit comparison with boys is removed.

Cultural stereotyping and discrimination are too subtle, insidious, and entrenched for us to dismiss them, even in supposedly gender-neutral, bias-free surroundings. For instance, did you have a female math or science mentor when you were growing up? I was lucky enough to get tremendous encouragement (and a wealth of medical knowledge) from a world-class researcher and doctor who happened to be a woman, but I know that I was privileged. An older doctor I know once joked that she first met a female doctor on the eve of her med school graduation—herself! This is changing; the women in science I know take a great deal of pride in extending a hand to their younger counterparts, but the overall question of whether young girls and women get the same opportunities in mathematics and the sciences is still a troublesome one.

Question: Why Has It Taken Us so Long to Study
the Differences between Men and Women?

One of the very first questions I get when I'm lecturing about the implications of the sex differences between men and women is, "Why are we only just learning about this now?"

Because, for much of the time that we've been doing real medical research, we haven't studied women, we've studied only men.

But why?

It's a good question, and one that deserves a real answer. At all levels of scientific investigation, researchers believed that males were easier and better study subjects than women. They weren't wrong. Studying women is more complicated than studying men, and, in medical research, a "complication" translates to "expense." Men aren't subject to the same hormonal fluctuations over the course of the month that women are. Those fluctuations, which affect many of the body's systems, introduce variables that need to be taken into account when tabulating results. For instance, body temperature, a baseline measurement, rises in women at ovulation. Normal variations like these, which don't appear in men, can impact results.

It's also harder to do any kind of medical testing on women of childbearing age. Certain medications and techniques simply aren't appropriate because they may compromise fetal development— particularly in the very early stages (before a woman even knows she's pregnant)—or interfere with a woman's ability to conceive at all. And a pregnancy can throw a monkey wrench into collecting data for a long study. Too many changes take place over the course of a pregnancy for that person to remain a stable medical subject, even if what we're testing is safe.

It's still difficult to do the work we need to do to know what the differences are between women and men. It's difficult because women are more complicated test subjects and because it's difficult to get funding for research into this fascinating area. It's my hope that continued inquiry—driven by men and women—will illuminate some of these differences and that this new awareness will provide us with the strategies we need to move forward.

In the chapters to come, we'll look at some of the ways that men and women are different, and how those differences complicate and enervate the relationships we're in with one another. As the song says, the very beginning is a very good place to start, so in the next chapter, we'll explore what happens when two people meet and fall in love.

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© 2005 by Marianne J. Legato. All rights reserved. No Part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any other information storage and retrieval system, without the written permission of the publisher.

Tags: Gender Studies, Brain

About the Author

Marianne J. Legato, MD, FACP, is a professor of clinical medicine at Columbia University, where she founded and heads the Partnership for Gender-Specific Medicine. One of the world's foremost experts on gender medicine and winner of many awards for her work, she is the author of The Female Heart, What Women Need to Know, and Eve's Rib. More