This amazing brain is formed in utero, and therein begins its journey into a "male" or "female" brain. The tissue and neurons are genetically wired to form in certain ways, and then hormones act on the fetal brain to make it a male or female brain.

Here's how it works in a nutshell:
In the mother's womb, hormones surge at different times to catalyze brain growth. Specifically, at between three and six months in the womb, the human brain of the fetal girl or boy is being bombarded with different hormones. When a forming brain gets bombarded with testosterone, certain cortical areas grow and become connected to other areas. Connections between cortical areas occur in what are called neural pathways. When a developing brain gets bombarded with estrogen and progesterone, certain other cortical areas grow and connect.

If the child in the womb is a chromosomal male child (XY), the mother's hormonal system reads this as "male" and makes sure the child's brain is bombarded with higher degrees of testosterone. If the child in the womb is a chromosomal female child (XX), the mother's hormonal system makes sure her brain is bombarded with higher degrees of female hormones. In the womb, the brain of the child is already being sexualized-feminized or masculinized-by hormones.

Boys and girls and men and women share the same hormones. The fetal boy does not only get testosterone and the fetal girl only estrogen. The brain development of all babies, no matter their sex, is stimulated by all the hormones, just as grown men have some estrogen and grown women have some testosterone. But the signals sent between the mother's ovaries and the fetus depend to a great extent on the XY or XX chromosome. A boy baby is going to get more testosterone, and thus a more "male" brain; a girl baby is going to get more estrogen, thus a more "female brain." By six months into the mother's pregnancy, the boy's and girl's brains have formed and will be mainly affected by his or her own sex hormones from then on.

Avoiding Stereotypes

As we move forward to explore exactly what we mean by "male" brain and "female" brain, it is important to recognize that human biological sciences don't aim to stereotype people. Biologists have proven a huge spectrum of brain difference among men and women. Within the parameters of the typical male brain is a wide spectrum of possible behaviors. For example, while many men are uncomfortable talking about their feelings, some men can discuss their feelings easily. The male brain (like the female brain) is multifaceted and is capable of doing and being many things at many times.

When we talk about men, we will avoid stereotyping. We will discuss biological trends lived by men and by women relating to men. In exploring these trends, we will often generalize about "men" and "women," but in each chapter of the book, we'll also consider exceptions to the rule. We'll explore how these exceptions happen in human evolution. I call them bridge brains - male brains and female brains that cross the genders. While biological trends help us to talk about men and women as biological groups, the bridge brains have much to teach us, for they often don't quite fit the group. Nature has always liked the exception as much as the rule.

Some Men You Know

On one end of the spectrum is the most male brain we can imagine - low serotonin, low oxytocin, small corpus callosum, small language center. This brain is called highly masculinized. It is defined by testosterone surges in the womb.

On the other end of the spectrum is the most female brain we can imagine - high serotonin, high oxytocin, large corpus callosum, many language centers. This brain is defined by lack of testosterone surges in the womb. In a woman, this brain might be called, in popular language, "very feminine." In males, we would call it a bridge brain.

To think about the very masculinized brain system, think of action heroes in movies. These men are singularly task focused. There isn't much cross talk between hemispheres. They like things that are loud and that blow up - they possess less aural neurons, so loud noises don't bother them. These men do not rely much on word production. In The Terminator, Arnold Schwarzenegger's character speaks perhaps thirty words in all.

These men emphasize aggression rather than direct empathy, possessing an enlarged amygdala with less neural signals to the frontal lobes; and they have high testosterone. Schwarzenegger's typical male character is relatively emotionless, though he is certainly compassionate with those to whom he has bonded. Possessing a smaller hippocampus, with low-range neural pathways to emotive centers, he does not rely heavily on personal memories or sensory detail.

He is very spatial and mechanical - he relies on objects moving in space with mechanical design - cars, trucks, guns, bullets, his own fists - enjoying more right-hemisphere cortical use, less left.

Action heroes give us an illustration of the very polar end of masculinization on the brain spectrum. Schwarzenegger is playing a stereotype, of course. Stereotypes generally tend to be the polar ends of spectrums because they are simplistic, and simplistic characters tend to be good ones on which to hinge simple, entertaining tales. Yet even as stereotypes, they are recognizable as the most "masculine" end of the brain spectrum.

. . . And Some Women

On the other end of the spectrum - the most neurally female end - we can picture the most talk-oriented, nonspatial woman. She would love soft things. She would have a hard time getting a basketball into a basket or trying to hit a softball with a bat. As a little child, she'll want to hold and care for dolls as much as possible, cooing to them and talking to them. She won't like loud noises, since she hears very well. She'll have a better-than-average sense of smell and taste. She'll probably love to read. Even as an adult, she won't be able to tell us how tall a building is by looking at it, but she'll have an acute color sense.

You might say, "He's stereotyping women now." I am, indeed, painting an extreme portrait of a polar end on the brain spectrum. I am describing women with almost no testosterone at all. These women are generally born without ovaries, thus their bodies produce very little testosterone. Since they had no ovaries, very little testosterone was secreted during gestation by their developing brains and bodies. Thus they have a very "female" brain, nearly untouched by the masculinization of testosterone.

Their spatial and mechanical design functions in the right hemisphere tend to be underdeveloped, so they don't rely on these as much as other women might. Many of these women even find driving a car more difficult. Their brains generally have more highly developed cortical areas for verbal, sensual, and tactile experience. With little testosterone, they have better than average hearing, making loud noises painful. They see a broader spectrum of color than even the average girl or woman. Galvanic skin-response tests show their under-the-skin nerves to be immensely sensitive. They have low pain tolerance.

With the highest testosterone male and the female who can't produce testosterone, we have illustrations at the extremes of the brain spectrum, useful, not to define us as human beings, but to help us see the outer boundaries of masculine and feminine brain development.

Ultimately, the work we do to understand the natural identity of boys and girls and women and men affects, not only individual people and relationships, but also our whole society. Through this work, we can move humanity into new successes at all levels. Fortunately, the new sciences are now showing us the unlimited potential of human nature. These sciences are not taking us back into old roles. In fact, quite the opposite. They liberate us to discover who and what a person really is. They free us to make love convincingly, to love compassionately, to honor the human soul, to notice human courage, to make marriages work again, and to care for others as human beings yearn to do.

From the book, "What Could He Be Thinking?" by Michael Gurian; Copyright (c) 2003. Reprinted by arrangement with St. Martin's Press. All rights reserved.