"We (men and women) are different, and vastly so, in every system of the body, from the skin that covers us, to the heart that beats within our chests, to the digestive system that processes the food we eat.
    Nowhere is this more true than in the brain, the 3-pound organ that houses all that makes us human: our passions, our insights, our appreciation of the created world, our entire intellectual and emotional lives.
    Men and women think differently, approach problems differently, emphasize the importance of things differently, and experience the world around us through entirely different filters."
    Whether the challenges are new or as old as time, the differences between us don't have to cause tension and conflict in our relationships. Ultimately, understanding them will help us to build closeness and commonality and to learn from each other. We are far more alike than we are different, and there is evidence to suggest that we can cultivate the aptitudes of the opposite sex, to the very great benefit of us all." From the back cover

I'd like to present you with an overview of the science as we currently understand it, via true-or-false statements that come from questions I've been asked at lectures and by my patients. As you read, I'd like to remind you that while there do seem to be gender-specific ways of thinking, remembering, and experiencing emotion, those differences do not necessarily connote superiority. Dr. Eric Kandel's groundbreaking research on learning assures us that our brains aren't set in stone, even if our sex is. If we learn from each other, then these differences become opportunities, not divisions. 

True or False: Sex is determined by our biology.

True and false. Although our sex is determined at the moment of our conception, and we stay that sex for the rest of our lives, we actually become more or less female or male over the course of our lives. Let's take a look at how this happens. 

The sex chromosome contributed by our fathers pushes us to form male or female sex organs. Those organs, in turn, release hormones that cause dramatic and sex-specific changes to every organ and tissue in our bodies -- including the brain -- and program them to respond in sex-specific ways down the line. Varying levels of hormones over the course of our lives continue the process of sexing us. 

In other words, our genes set us up for the sex we'll be, and our hormones salt the stew. The complex interaction between these two factors -- especially during specific windows when their levels drop or surge as they do during puberty and menopause -- make the two sexes different and each of us different from one another as well. 

Nature is only part of the explanation for the differences between us. In fact, one of the thorniest challenges faced by those of us who study gender differences is teasing out which differences are due to the genetic and hormonal components of our biology and which are the result of "nurture," or how we're conditioned and shaped by our environment. 

Society certainly believes men and women are different and expects sex-specific behavior from us. Even when children are young, parents encourage sons and daughters to do quite different kinds of activities, and in fact, boys and girls seem to enjoy quite different things. 

These very disparate paradigms of what it means to be male or female provoke important questions about the difference between the sexes. How many of the differences between us are the result of the gender roles that the society of the time imposes? Are our sex-specific talents, temperament, and world view inescapably hardwired into our central nervous system? Or is our sexually stereotyped behavior choreographed by our culture's expectations of us? 

Some of the differences between men and women are hardwired. But as soon as we're born, the environment works in powerful ways to interact with, and even change, our hardwiring to shape the way we act and interface with others. The idea that our experiences can change our brains means that the strands of conditioning and biology are more closely intertwined than we'd even thought. Treating your daughter like she's a girl may make her more so. The brain is never "done," but continues to grow and change as long as we provide it with inspiration. 

True or False: There are significant differences between the brains of men and women.

True. It seems self-evident that men and women would have different brains -- after all, what could be more fundamental about us than whether we're male or female? And yet, for most of medical history, doctors and scientists assumed that all the organs of men and women were the same, except for those directly involved in reproduction. Research suggesting otherwise is very new: Scientists first made the observation that there were differences in the physical structure of the brains of female and male rats a little more than three decades ago. It has now been confirmed that this is true not only in other species with two sexes, like songbirds and monkeys, but in our own as well: The anatomy of the brain and how it works are different in men and women. 

True or False: The brain has a sex at birth.

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.