sizes of brain areas, and behavioral responses.
News about the differences between men's and women's brains popped up everywhere—from John Gray's Men Are from Mars, Women Are from Venus and Louann Brizendine's The Female Brain to Discovery Health and Newsweek. All of these sources said that brain differences, present from birth, are what make men and women distinct. Even a former president of Harvard said that, based on brain research, women didn't have the brains for math and physics.16
These new discoveries were accepted by the media and public, but they were publicized without rigorous analysis. Recent scrutiny questions the validity of the studies and the hidden assumptions beneath the results.
Brain Sex Rumors
New studies of brains use functional magnetic resonance imaging (fMRI) and positron emission tomography (PET), both of which measure blood flow and produce three-dimensional imaging. Researchers compared male and female brains and compared the brains of those doing puzzles to those meditating, those looking at happy pictures or at sad ones, or those just listening or responding actively in a conversation. They've measured the sizes of brain components and the blood flow to areas that were active; the results were pictures of brains filled with bright blobs of color indicating which sections were active.
These studies aimed to show that male and female brains are significantly different at birth in many areas. Supposedly:
Males have better right-hemisphere skills, such as those involved in art, music, and math, due to the testosterone male embryos receive; females are better at communicating, observing, and processing emotion.
Males are better able to systematize and are more aggressive than females, also because of this infusion of testosterone in male embryos; females have more collaborative and verbal brains.
Compared to men, women have the stronger ability to “mirror” others, feeling what others are feeling or sensing what others are thinking.
Men have a larger amygdala, dubbed the “instinctual core of the brain,” than women.
Women worry more than men because their anterior cingulate cortex is larger.
Women have more neurons for language processing and comprehension in the temporal lobe cortex than men have.
The corpus callosum, a pathway of 200 to 250 million nerve fibers between the right and left hemispheres, is larger in women than in men. This greater number of cross-brain connections means women are better at activities involving both sides of the brain, and men are better at activities requiring the focus of one side or the other.
But several authors have called these studies on the carpet. They scientifically challenged whether (1) our advanced instruments tell as much as we’d like, (2) the assumptions they tell are true, and (3) the studies are large enough for their statistics to have veracity.
For instance, when looking at the corpus callosum, some studies found that it is the same size in men and women, and other studies found that it's bigger in men. Studies also question whether having a larger right hemisphere means increased learning or more difficulty learning. And while the anterior cingulate cortex is the part of the brain that generates worry, this part is also involved with a wide variety of cognitive, motor, and emotional tasks, such as decision-making; so it makes as much sense to say that a larger anterior cingulate cortex means women think better than men, instead of that women worry more.
Cordelia Fine, author of Delusions of Gender, says that these studies are looking at the brain through traditional assumptions. It's just like 1915, she says, when studies “proved” that women couldn't judge political initiatives and couldn't vote, all because they had smaller upper spinal cords. The results of these studies are similarly biased because of what she calls neurosexism. 17
We are starting to get larger and more valid studies about gender and the brain, which we hope will clarify the issue. And we know some consistent male-female brain differences in animals and humans from previous decades of study.
What Do We really Know?
Boys and girls' brains are different at birth, but the differences are much smaller than we think, says Lise Eliot, PhD, in Pink Brain, Blue Brain: How Small Differences Grow Into Troublesome Gaps—And What We Can Do About It.18
In fact, before the eighth week of pregnancy, there is no male brain at all—everyone starts as a female. Around the eighth week, male embryos (meaning embryos with XY chromosomes) get a surge of testosterone. The testosterone changes the brain's original plan to create a uterus and ovaries in the female with XX chromosomes; the male embryo develops a penis and testicles instead.
Only a few physical pre-adolescent brain differences have been reliably proven to exist:
Boys’ brains are larger than girls’ brains. (This difference used to support the idea that men were smarter, but when you consider the brains of elephants, the logic fades. Larger brains are needed for larger muscles and to process more sensations.19)
Girls’ brains finish growing about one to two years earlier than boys’ brains. (Hormonal differences are key in this growth difference.)
Parts of the hypothalamus are different. The hypothalamus controls basic body cycles and is connected to the pituitary (or “master hormone”) gland. The areas of the hypothalamus that are different in males and females control circadian rhythms and reproduction.
There may be some subtle differences in boys and girls' sensory processing, language circuits, and frontal-lobe development, but overall, boys’ brains and girls’ brains are remarkably similar.
So what creates more gender-typical behavior? How babies are treated, say both Eliot and Fine. Our wonderfully plastic brains respond to gender-specific atmospheres. For example, if you believe you're better at math, the area of your brain that does math work will be larger. Even intentionally unbiased child-rearing practices have some sex stereotyping, according to Fine. Add Disney movies and sports stars to the mix, and it's a challenge to separate and study the natural brain from the nurtured one.
Hormones, such as testosterone, estradiol, oxytocin, and thyroid hormone, also affect both men and women's brains, causing differences in everything from pain response to aggression and emotional responses. In the next chapter, We'll talk about how the range of hormones affects functions of the brain.
Despite the popularity of studies that say sex differences are hardwired into brain structures, right now that assertion appears to be unproven. We have so much to learn about whether and how much gender behavior is influenced by differences in brains and hormones—even the best neuroscientists are still learning.
So when you're sorting through all the latest studies mentioned on Google News or articles in Newsweek, withhold your wholehearted approval until you've seen multiple large-sample studies. Some research might say you were born with limitations due to your gender, but others say that's not true. If you have to choose between thinking, “Oh well, that's just the kind of person I am,” or thinking “I can move beyond my limitations and perceptions,”
