Sex differences in the brain begin in the womb. About midway through pregnancy, the testicles of a developing baby boy start churning out testosterone in huge quantities, comparable to what an adult man produces. These sex hormones bind to brain tissue and begin to transform it. Between 18 and 26 weeks gestation, the developing brain is permanently and irreversibly transformed. Israeli scientists Reuwen and Anat Achiron have found that if you do a regular ultrasound examination when a woman is 26 weeks pregnant, you can distinguish a female brain from a male brain. At 26 weeks!
Source: Reuwen Achiron, Shlomo Lipitz, & Anat Achiron. Sex-related differences in the development of the human fetal corpus callosum: in utero ultrasonographic study. Prenatal Diagnosis, 2001, 21:116-120.
This in utero study confirmed the findings of a previous anatomical study in which investigators examined the brains of babies which had died before birth. See: M. de Lacoste, R. Holloway, and D. Woodward, "Sex differences in the fetal human corpus callosum," Human Neurobiology, 1986, 5(2):93-6.
Once those changes have occurred, they are permanent. Research with both humans and with laboratory animals shows that you are born with a male brain or a female brain; postnatal experiences, even experiences as extreme as castration, will not change your brain from male to female, or vice versa.
Several lines of evidence support this assertion. One line of evidence is animal studies in which investigators have taken drastic measures to try to reverse the masculinization of the newborn brain. In one study, investigators delivered laboratory animals by Caesearean section and immediately castrated the males (within 20 minutes of birth). Despite being castrated, the castrated rat's brain remained masculinized throughout life. See: Christine Mack, Robert McGivern, Lynn Hyde, & Victor Denenberg: Absence of postnatal testosterone fails to demasculinize the male rat's corpus callosum. Developmental Brain Research, 1996, 95:252-254.
A second line of evidence comes from reports in humans. Of these, the most dramatic is surely the story of Brian/Brenda Reimer, "the boy who was raised as a girl." You can read Dr. Sax's essay on Brian/Brenda Reimer here.
Another line of evidence comes from human studies with transsexuals. Transsexuals -- individuals who have always felt themselves to "be" the opposite of their biological sex -- have been found to have brain anatomy which is indeed -- in some respects -- characteristic of the opposite sex. See Swaab et al., "Structural and functional sex differences in the human hypothalamus," Hormones and Behavior, 2001, 40:93-98.
What does it mean to have a "male brain" or a "female brain"? Let's consider sex differences in structure, sex differences in function, and finally sex differences in development.
Take a slice of the brain from somebody who's just died. Shave the slice very thin with a microtome (an instrument like a carrot peeler). Look at that translucent peel of brain tissue under the microscope. One group based primarily at the University of Lausanne in Switzerland has specialized in doing this work for more than ten years. They have employed computerized quantitative cytological techniques, using computer algorithms to measure the volume of individual nerve cells and the number of connections made by individual nerve cells, in specific areas of the brain. They have found that "fundamental gender differences exist in the structure of the human cerebral cortex."
Source: Theodore Rabinowicz, Jean MacDonald-Comber Petetot, Peter Gartside, David Sheyn, Tony Sheyn, & Gabrielle de Courten-Myers, "Structure of the Cerebral Cortex in Men and Women," Journal of Neuropathology and Experimental Neurology, January 2002, 61(1):46-57. The quotation comes from page 52 of this paper.
It is not the case that every area of a girl's brain differs from every area of a boy's brain. The Lausanne group studied cells in the cerebral cortex, the most advanced part of the brain. Specifically, they studied cells in Layer IV of the cerebral cortex, that part of the cortex which receives input from other cells.
One research team recently compared brain tissue from the brains of young girls and young boys. They found that sex differences in the structure of the brain were obvious, even in babies -- especially in babies. The differences in the photomicrographs of the brain tissue are so dramatic that they are readily visible to the naked eye.
Source: María Elena Cordero, Carlos Valenzuela, Rafael Torres, Angel Rodriguez, "Sexual dimorphism in number and proportion of neurons in the human median raphe nucleus," Developmental Brain Research, 124:43-52, 2000.
This finding alone should alert you that sex differences are real -- as opposed to, say, racial or religious differences. You can't tell by looking at a slice of someone's brain whether that person was Black or White or Asian; you can't tell whether that person was a Jew or a Christian or a Hindu or a Muslim. But you can tell whether that person was female or male.
A recent report from London, England documents consistent sex differences in brain anatomy which are visible to the naked eye (with the help of an MRI scanner). Male brains consistently show more hemispheric asymmetry: the left hemisphere looks different from the right hemisphere. In women, the two hemispheres are much more alike. In women, there is proportionately more grey matter, and less white matter; vice versa for men. Women have a higher concentration of grey matter in the neocortex (the phylogenetically 'newer' part of the cerebral cortex), whereas men had proportionately more grey matter in the entorhinal cortex, one of the 'older' areas of the brain.
Source: C. D. Good, I. Johnsrude, et al. Cerebral asymmetry and the effects of sex and handedness on brain structure: a voxel-based morphometric analysis of 465 normal adult human brains. Neuroimage, September 2001, 14(3):685-700.
Likewise, many studies demonstrate sex differences in the organization of the brain. For example, researchers at Johns Hopkins found substantial sex differences in the anatomy of the "higher association cortex" -- the part of the brain thought to be responsible for our most complex mental operations, "higher-order multimodal convergence integrating all aspects of mental function," including both cognitive and emotional experiences. Specifically, these researchers found that these areas of the brain were markedly asymmetric in men but not in women; and in men, these brain areas were larger on the left, whereas to the extent that there was asymmetry in the brain, women's association cortex was larger on the right.
Source: Melissa Frederikse, Angela Lu, Elizabeth Aylward, Patrick Barta, & Godfrey Pearlson. Sex differences in the inferior parietal lobule. Cerebral Cortex, 1999, 9:896-901.
Some brain structures that are prominent in females, such as the massa intermedia of the thalamus, are smaller or even entirely absent in males. In one study comparing the massa intermedia in women and men -- including only those men who had a massa intermedia -- the massa intermedia was, on average, 53% larger in the females, despite the fact that the male brains were on average 8% larger than the female brains.
Source: L. S. Allen & R. A. Gorski. Sexual dimorphism of the anterior commissure and massa intermedia of the human brain. Journal of Comparative Neurology, 1991, 312:97-104. These investigators also reported that the anterior commissure was 12% larger in female brains than in male brains.
Since the advent of functional MRI scans in the late 1980's, scientists have been able to study what areas of the brain are active when people engage in particular activities. These studies have shown that women and men process information, listen, read, and experience emotion in very different ways. We'll consider three examples: language, navigation, and emotion.
Language. Neuro-radiologist Joseph Lurito asked female and male volunteers to listen to a John Grisham novel. He mapped what areas of the brain "lit up" in each volunteer while they were listening. His findings, released in November 2001, were striking. Women use both the right and left hemispheres in processing language; men use only the left hemisphere. You can read a summary of Dr. Lurito's report, or examine images of brain activity in women vs. men.
Source: Michael Phillips, Mark Lowe, Joseph T. Lurito, Mario Dzemidzic, and Vincent Matthews. Temporal lobe activation demonstrates sex-based differences during passive listening. Radiology, 220:202-207, 2001.
Dr. Lurito's work is consistent with previous reports by Sally Shaywitz and her colleagues at Yale University. The Yale group has performed a series of experiments over the past ten years, studying what areas of the brain are activated when subjects read. In right-handed men, they found that just a small area in the left hemisphere -- the left inferior frontal gyrus -- "lit up" when men read. In right-handed women, the pattern of activation was remarkably different: both frontal lobes "lit up," and the activation was not confined to the inferior frontal gyrus but was more diffuse.
Source: Bennett A. Shaywitz, Sally E. Shaywitz, et al. (11 authors!) Sex differences in the functional organization of the brain for language. Nature, 16 February 1995, 373:607-609.
These studies illustrate a general principle in human sex differences. Men are more likely to use a small area of the brain, on just one side, for a particular task; women typically use more of the brain, on both hemispheres, for the same task.
Navigation. Psychologists have found that females and males use different strategies when they are navigating. Ask a woman how to get to a friend's house, and she may tell you something like, "Go down Elm Street till you see the McDonald's. Then make a left, go past the hardware store and the Exxon station, then you'll see the elementary school. Make a right just past the elementary school and go about another block till you see a split level house painted lime green, with these unbelievable fuchsia shutters and trim, can you believe it? It looks like a gingerbread house after the mold has gotten to it. That's their house." A man, giving directions to the same house, might say, "Go south on Elm Street about two miles, then turn left so you're heading east on Duke Street. After one mile on Duke Street, turn south again onto Scottsdale Boulevard. Their house is the fourth from the intersection, on the left." See the difference? Women typically navigate using landmarks that can be seen or heard. Men are more likely to use abstract concepts such as north and south, or absolute distance.
Source: N. Sandstrom, J. Kaufman, S. A. Huettel. Males and females use different distal cues in a virtual environment navigation task. Brain Research: Cognitive Brain Research, 1998, 6:351-360.
Those different strategies correlate with different brain regions. Neuroscientists have found that women and men use different areas in the brain when they are given problems which require navigational skills. Women use the cerebral cortex -- mostly the right parietal cortex -- while men do not use the parietal cortex but instead use primarily the left hippocampus, a nucleus deep inside the brain which is not activated in the women's brains during navigational tasks.
Source: Georg Grön, Arthur Wunderlich, Manfred Spitzer, Reinhard Tomczak, & Matthias Riepe. Brain activation during human navigation: gender-different neural networks as substrate of performance. Nature neuroscience, April 2000, 3(4):404-408.
Both methods work. But the two methods are very different, and the brain areas involved are completely separate. The same principle has been demonstrated in laboratory animals, such as rats and monkeys. Again, sex differences in performance are small, but the brain areas involved are completely different: in female laboratory animals, spatial tasks involve the phylogenetically newer neocortex, whereas males use the phylogenetically ancient subcortical nuclei.
Source: B. Kolb & J. Cioe. Sex-related differences in cortical function after medial frontal lesions in rats. Behavioral Neuroscience, 1996), 110:1271-1281. See also: R. L. Roof, Q. Zhang, et al. Gender-specific impairment on Morris water maze task after entorhinal cortex lesion. Behavioral & Brain Research, 1993, 57:47-51.
This example illustrates a general truth about sex differences in cognitive function. The differences in what women and men can do is small; the difference in how they do it is large.
Emotion. Neuroscientists at Harvard University have used sophisticated MRI imaging to examine how emotion is processed in the brain of children between the ages of 7 and 17. In young children, they found that emotional activity was localized in primitive subcortical areas of the brain, specifically in the amygdala. That's one reason why it doesn't make much sense to ask a 6-year-old to tell you why she is feeling sad. The part of the brain that does the talking, up in the cerebral cortex, doesn't connect to the part of the brain where the emotion is occurring, namely the amygdala. In adolescence, brain activity associated with emotion moves up to the cerebral cortex. So, the 17-year-old is able to explain what she is feeling, and why, in great detail and without much difficulty. But that change occurs only in girls. In boys, the locus of emotional control remains stuck in the amygdala. Asking a 17-year-old boy to talk about his feelings is about as productive as asking a 6-year-old boy to talk about his feelings.
Source: William Killgore, Mika Oki, and Deborah Yurgelun-Todd. Sex-specific developmental changes in amygdala responses to affective faces. NeuroReport, 2001, 12:427-433.
This finding is consistent with a recent report from Germany, showing that in adult women, brain activity associated with emotion occurs mainly in the cerebral cortex, whereas in adult men, emotional activity is still ‘stuck' in the amygdala.
Source: Frank Schneider, Ute Habel, et al. Gender differences in regional cerebral activity during sadness. Human Brain Mapping, 2000, 9:226-238.
The differences in brain structure and function just discussed are relatively new findings. By contrast, scientists have known for decades that girls develop faster than boys. The advent of more advanced technology has confirmed this understanding. The major advance in the past two decades has been in the recognition that sex differences in the pace of brain development are of larger magnitude and longer duration than previously thought.
There are three basic methods employed to measure brain development. The first, and most intuitive, is anatomic. We know what the major structures of the adult brain look like (the amygdala, thalamus, hippocampus, cerebellum, cerebral ventricles, etc.). Those structures have a very different appearance in the young child's brain. So, one simple measure to determine the degree of brain maturation is to examine how closely the size of different brain structures in the child's brain approximates the size and shape of those structures in the adult brain. Caviness and associates used MRI scans to compare the development of boys and girls. They found that the brain of the 17-year-old boy looks like the brain of the 11-year-old girl.
Source: V. S. Caviness, D. N. Kennedy, et al. The human brain age 7-11 years: a volumetric analysis based on magnetic resonance differences. Cerebral Cortex, 1996, 6:726-736.
The second way to measure brain development is by degree of myelination. Myelin is the waxy material that insulates the axons (nerve fibers) whereby neurons communicate with one another. The newborn baby's brain has almost no myelin; the adult's brain is filled with myelin. Degree of myelination is very easy to measure with MRI scans. Such a study was published in 1994. According to this study, girls' brains were three to four years ahead of the boys' brains from ages 7 - 22; the men did not catch up with the women until age 29.
Source: F. M. Benes, M. Turtle, et al. Myelination of a key relay zone in the hippocampal formation occurs in the human brain during childhood, adolescence, and adulthood. Archives of General Psychiatry, 1994, 51:477-484.
A third way to measure brain development is by electrophysiological activity, using electro-encephalograms (EEGs). The EEG activity of the mature adult brain is quantitatively and qualita-tively different from the EEG activity of the young child's brain. The adult EEG is more complex and multimodal. Anokhin and associates (2000) did a sophisticated analysis of the EEG's of girls and boys, to determine how those EEGs changed over time and at what point they began to look like adult EEGs. Anokhin found that the EEG patterns of the 17-year-old boys resemble those of the 11-year-old girls; in other words, using this method, boys are six years behind girls, and the differences between the sexes increased as children got older.
Source: A. P. Anokhin, W. Lutzenberger, et al. Complexity of electrocortical dynamics in children: developmental aspects. Developmental Psychobiology, 2000, 36:9-22.
There is no one "best" way to measure brain development. But, whichever method you use, the message is the same: girls develop substantially faster than boys, and boys don't catch up until well after the age of high school graduation.
No one would seriously propose putting 2nd-graders and 5th-graders in the same English class with the same curriculum and the same assignments. But, when we put a typical 11-year-old boy and a typical 11-year-old girl in the same 5th grade English class, we are essentially doing just that: putting together two children who are at very different maturational levels.
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