The XX Brain

Chapter 1
The Inner Workings of the Female Brain

John Gray's bestseller Men Are from Mars, Women Are from Venus, with which he coined that now famous metaphor, speaks to pop science's long-term fascination with the psychological differences between men and women. There's no shortage of comedy routines that enjoy poking fun at this infamous battle of the sexes. If we were so inclined, we could hop on the bandwagon, imagining a female brain that consisted of zones such as the must-have-chocolate-now node, the gossip gland, or the busy kids-and-marriage center. Of course, our male counterparts would possess their own set of similarly satirized areas, including a pair of power tool glands, a quick-firing lame excuse lobe, and the ever-defiant "getting lost and not admitting it" region.

The origins of behavioral differences between men and women have been a topic of conversation since ancient history. However, the idea that the brain could be the principal means by which males and females behave differently is strikingly modern, having only been accepted as a true determining factor in the 1960s. Before then, people were convinced that our genitals themselves were at the crux of the matter. Then in 1992, scientists made a very powerful discovery: Our so-called sex hormones, such as estrogen and testosterone, influence not only sexual behavior but brain function as well. In other words, the hormones inextricably involved with our sexuality turn out to be just as crucial in the overall functioning of our minds.

Although our gender and its hormones don't offer a universal explanation for our health or behavior, gender differences in the brain manifest themselves in many fascinating and often overlooked ways. This is in part due to the fact that hormones are made by our DNA, and as we know, our DNA is different depending on our sex. However, not many people know that the X chromosome is actually much larger than the relatively tiny Y chromosome, containing a lofty 1,098 genes as compared with the Y chromosome's 78. This means that a woman, wielding her double X, possesses over 1,000 more genes than a man, many of which are critical for both hormonal production and brain activity.

X Marks the Spot: The Estrogen-Driven Brain

All women are intuitively aware of the constant conversation between their brains and their hormones, and many of us find ourselves attributing our moods to them. Actually, our female hormones do have strong, deep effects on the brain that go far beyond the typical attributes of PMS and the variety of other ups and downs associated with our cycles.

Hormones are powerful chemicals involved in almost every process in the body and brain, including cellular metabolism, tissue growth, and recovery from injury. By doing so, hormones keep our brains acute, energized, and youthful. At the same time, they keep our bones strong, our gut active, and our sex life potent. They also affect our weight, immune function, and even how we turn food into fuel. Thanks to their all-encompassing roles, our hormones influence every aspect of our physiology, and subsequently, our health-physical and mental. When your hormones are out of whack, you feel it everywhere from your joints to your thoughts. Depending on whether they are in balance or going awry, we'll experience changes not only in a number of bodily functions but also in our cognitive power, mood, and mental alertness, right down to how we think, talk, feel, and remember.

While all hormones are important in this regard, most of the research points to 17§-estradiol, more commonly known as "estrogen", as a major, if not the major hormonal driver of women's brain health. Estrogen is a "master regulator" in the female brain, serving many roles that actually have nothing to do with reproduction, but rather everything to do with energy. Estrogen is key in the regulation of energy production and overall balance of various brain functions (homeostasis). This is particularly important in keeping brain cells healthy and active, as well as fostering brain activity in regions responsible for memory, attention, and planning .

Importantly, estrogen is also a neuro-protective hormone, playing a crucial defensive role in the brain by boosting the immune system, thereby shielding neurons from harm. And not only does it protect our nerve cells, but estrogen also encourages the formation of new connections between those cells. A well-connected brain is in turn more resilient and adaptable. Moreover, estrogen just so happens to be "nature's Prozac," too. Its levels influence the brain's production of gamma-aminobutyric acid (GABA), a chemical that soothes the nervous system, thanks to its calming properties, while also promoting the release of endorphins, the body's natural painkillers. Finally, our hormones all contribute to blood flow and circulation in the brain, which is crucial to ensuring the brain has an adequate supply of oxygen and nutrients.

All these effects start taking place inside our brains from the very moment of conception, during the development of the embryo in the womb. Over time, circulating hormones play an important role in the sexual differentiation of the brain. Androgens (male hormones like testosterone) produce a "male" brain, while a lack of those androgens, with a subsequent increase in estrogens (female hormones) in their place, produce a "female" brain.

Though these differences are subtle, if you were to closely inspect the brains of men and women, as I so often do for my work, you might notice them. For example, depending on which type of hormone is more abundant in your brain (estrogen in women, testosterone in men), you may be making more or less of certain neurotransmitters, the chemical messengers that the brain uses for signaling, communication, and information processing. Generally, men's brains produce more serotonin, the "feel good" neurotransmitter involved in mood, sleep, and even appetite. Women produce more dopamine instead- a brain chemical responsible for one's drive and reward-motivated behavior.

Even more intriguing is the fact that some parts of our brains are "sexually dimorphic," which means they are built a bit differently from each other depending on gender. For instance, the fact that men and women don't see things in the same way turns out to be as literal an observation as it is a figurative one. Deep within the visual cortex, the part of the brain that is responsible for processing visual information, we find one good example of why men and women don't always see eye to eye. While men possess more M cells, those cells responsible for movement detection, women have more P cells, the cells in charge of detecting objects and shapes. (Could this explain women's superior ability to find things in the fridge?)

Moving on to our ears, women also generally hear better than men, in part because we have 11 percent more neurons in the primary auditory cortex, the part of the brain that decodes sound. Additionally, although men have generally bigger brains by virtue of having generally bigger bodies, women have thicker cerebral cortices that seem better connected inter. In particular, in women's brains, the hippocampus (the memory center of the brain) and the amygdala (the emotional center of the brain) are more tightly connected to the frontal cortex, which is in charge of abstract thinking, planning, and reasoning.

As a result gender differences in brain connectivity are particularly pronounced in the limbic system, the part of the brain that includes the hippercampus and amygdala mentioned above, and that resonates with the experiences of love and affection, thereby responding to the innumerable factors involved in having a family. This part of the brain is responsible for generating the motivations and emotions that govern parental instincts, everything from nursing children to protecting them, not to mention the impulse to engage and play with them. If you have children, perhaps you have tiptoed into your children's rooms at night to check their breathing, or to deposit a kiss on their foreheads before being able to sleep yourself. Or maybe you have found yourself smiling at the thought of reading your children their favorite bedtime story, in spite of the fact that you've probably read that very same tale over a hundred times already. Those are all signs of the limbic lobe in action. Men have it too, but women possess its qualities in spades. Suddenly some familiar cultural stereotypes don't seem quite so strange, do they?

It is important to point out that although male and female brains are to some extent wired differently and exhibit some biochemical differences, this doesn't have a big effect on behavior. to be perfectly clear, there is nothing in our biology to justify the gender gap in equality, wages, or opportunities. There is also no scientific basis for a "gendered brain." Blue versus pink, Barbie versus Lego, businessman versus secretary-these are all social constructs that have nothing to do with how our brains are built or operate. Unfortunately, though, results from scientific studies have all too often been manipulated to imply that one gender, the male gender, is better or more intellectually advanced than the other. You may have caught wind of the idea that "brilliance in math is a male phenomenon," a bias that disregards the fact that men have had access to higher education for significantly longer than women-not to mention that there are plenty of brilliant female mathematicians in spite of these obstacles: Ada Lovelace, Emmy Noether, and Katherine Johnson, to name but a few. The truth of the matter is that men and women are equally intellectually capable, though we may arrive at our results by way of somewhat different neural routes.

All that said, from a raw biological perspective, men and women are different to some degree. Such diversity produces gender-specific health risks and vulnerabilities. In particular, closer to my research and more urgent to consider, a growing body of literature shows that male and female brains age differently, in part because of changes in hormonal quantity and quality.

Our brains go through a series of hormonal transitions throughout our lives as we progress from childhood to puberty, and eventually on to loss of fertility and the onset of menopause. While puberty is accompanied by an explosion of hormonal power, the loss of fertility in women can be more of a blow than anticipated. If you consider estrogen as fuel for the brain, rather than for babies, the magnitude of the change becomes a lot clearer.