That men and women are different, everyone knows that.
But, aside from external anatomical and primary and secondary sexual differences, scientists know also that there are many other subtle differences in the way the brains from men and women process language, information, emotion, cognition, etc.
One of the most interesting differences appear in the way men and women estimate time, judge speed of things, carry out mental mathematical calculations, orient in space and visualize objects in three dimensions, etc. In all these tasks, women and men are strikingly different, as they are too in the way their brains process language. This may account, scientists say, for the fact that there are many more male mathematicians, airplane pilots, bush guides, mechanical engineers, architects and race car drivers than female ones.
On the other hand, women are better than men in human relations, recognizing emotional overtones in others and in language, emotional and artistic expressiveness, esthetic appreciation, verbal language and carrying out detailed and pre-planned tasks. For example, women generally can recall lists of words or paragraphs of text better than men (13).
The "father" of sociobiology, Edward O. Wilson, of Harvard University (10), said that human females tend to be higher than males in empathy, verbal skills, social skills and security-seeking, among other things, while men tend to be higher in independence, dominance, spatial and mathematical skills, rank-related aggression, and other characteristics.
When all these investigations began, scientists were skeptical about the role of genes and of biological differences, because cultural learning is very powerful and influential among humans. Are girls more prone to play with dolls and cooperate among themselves than boys, because they are taught to be so by parents, teachers and social peers, or is it the reverse order?
However, gender differences are already apparent from just a few months after birth, when social influence is still small. For example, Anne Moir and David Jessel, in their remarkable and controversial book "Brain Sex" (11), offer explanations for these very early differences in children:
"These discernible, measurable differences in behaviour have been imprinted long before external influences have had a chance to get to work. They reflect a basic difference in the newborn brain which we already know about -- the superior male efficiency in spatial ability, the greater female skill in speech."
But now, after many careful controlled studies where environment and social learning were ruled out, scientists learned that there may exist a great deal of neurophysiological and anatomical differences between the brains of males and females.
Scientists working
at Johns Hopkins University, recently reporting in the "Cerebral Cortex"
scholarly journal (1),
have discovered that there is a brain region in the cortex, called inferior-parietal
lobule (IPL) which is significantly larger in men than in women. This area
is bilateral and is located just above the level of the ears (parietal
cortex).
Furthermore, the left side IPL is larger in men than the right side. In women, this asymmetry is reversed, although the difference between left and right sides is not so large as in men, noted the JHU researchers. This is the same area which was shown to be larger in the brain of Albert Einstein, as well as in other physicists and mathematicians. So, it seems that IPL's size correlates highly with mental mathematical abilities. Morphological brain differences in intellectual skills were suspected to exist by neurologists since the times of phrenology (although this was proved to be a wrong approach), in the 19th century. The end of the 20th century has witnessed the first scientific proofs for that.
The study, led by Dr. Godfrey Pearlson, was performed by analyzing the MRI scans of 15 men and women. Volumes were calculated by a software package developed by Dr. Patrick Barta, a JHU psychiatrist. After allowing for the natural differences in overall brain volume which exist between the brains of men and women, there was still a difference of 5% between the IPL volumes (human male brains are, on average, approximately 10 % larger than female, but this is because of men's larger body size: more muscle cells imply more neurons to control them).
In general, the IPL allows the brain to process information from senses and help in selective attention and perception (for example, women are more able to focus on specific stimuli, such as a baby crying in the night). Studies have linked the right IPL with the memory involved in understanding and manipulating spatial relationships and the ability to sense relationships between body parts. It is also related to the perception of our own affects or feelings. The left IPL is involved with perception of time and speed, and the ability of mentally rotate 3-D figures (as in the well-known Tetris game).
Another previous study by the same group led by Dr. Godfrey Pearlson (9) has shown that two areas in the frontal and temporal lobes related to language (the areas of Broca and Wernicke, named after their discoverers) were significantly larger in women, thus providing a biological reason for women's notorious superiority in language-associated thoughts. Using magnetic resonance imaging, the scientists measured gray matter volumes in several cortical regions in 17 women and 43 men. Women had 23% (in Broca's area, in the dorsolateral prefrontal cortex) and 13% (in Wernicke's area, in the superior temporal cortex) more volume than men.
These results were later corroborated by another research group from the School of Communication Disorders, University of Sydney, Australia, which was able to prove these anatomical differences in the areas of Wernicke and of Broca (3). The volume of the Wernicke's area was 18% larger in females compared with males, and the cortical volume the Broca's area in females was 20% larger than in males.
On the other hand, additional evidence comes from research showing that the corpus callosum, a large tract of neural fibers which connect both brain hemispheres, is enlarged in women, compared to men (5), although this discovery has been challenged recently.
In another research, a group from the University of Cincinnati, USA, Canada, presented morphological evidence that while men have more neurons in the cerebral cortex, women have a more developed neuropil, or the space between cell bodies, which contains synapses, dendrites and axons, and allows for communication among neurons (8). According to Dr. Gabrielle de Courten-Myers, this research may explain why women are more prone to dementia (such as Alzheimer's disease) than men, because although both may lose the same number of neurons due to the disease, "in males, the functional reserve may be greater as a larger number of nerve cells are present, which could prevent some of the functional losses."
The researchers made measurements on slices of brains of 17 deceased persons (10 males and seven females), such as the cortex thickness and number of neurons in several places of the cortex.
Other researchers, led by Dr. Bennett A. Shaywitz, a professor of Pediatrics at the Yale University School of Medicine, discovered that the brain of women processes verbal language simultaneously in the two sides (hemispheres) of the frontal brain, while men tend to process it in the left side only. They performed a functional planar magnetic resonance tomographic imaging of the brains of 38 right-handed subjects (19 males and 19 females). The difference was demonstrated in a test that asked subjects to read a list of nonsense words and determine if they rhyme (7). Curiously, oriental people which use pictographic (or ideographic) written languages tend also to use both sides of the brain, regardless of gender.
Although most of the anatomical and functional studies done so far have focused on the cerebral cortex, which is responsible for the higher intellectual and cognitive functions of the brain, other researchers, such as Dr. Simon LeVay, have shown that there are gender differences in more primitive parts of the brain, such as the hypothalamus, where most of the basic functions of life are controlled, including hormonal control via the pituitary gland. LeVay discovered that the volume of a specific nucleus in the hypothalamus (third cell group of the interstitial nuclei of the anterior hypothalamus) is twice as large in heterosexual men than in women and homosexual men, thus prompting a heated debate whether there is a biological basis for homosexuality (6). Dr. LeVay wrote an interesting book about the sex differences in the brain, titled "The Sexual Brain" (6).
According to the Society for Neuroscience, the largest professional organization in this area, evolution is what gives sense to it. "In ancient times, each sex had a very defined role that helped ensure the survival of the species. Cave men hunted. Cave women gathered food near the home and cared for the children. Brain areas may have been sharpened to enable each sex to carry out their jobs". Prof. David Geary, at the University of Missouri, USA, a researcher in the area of gender differences, thinks that "in evolutionary terms, developing superior navigation skills may have enabled men to become better suited to the role of hunter, while the development by females of a preference for landmarks may have enabled them to fulfill the task of gathering food closer to home." (2) The advantage of women regarding verbal skills also make evolutionary sense. While men have the bodily strength to compete with other men, women use language to gain social advantage, such as by argumentation and persuasion, says Geary.
Author Deborah Blum, who wrote "Sex on the Brain: The Biological Differences Between Men and Women" (12), has reported the current trend towards assigning evolutionary reasons for many of our behaviors. She says: "Morning sickness, for example, which steers some women away from strong tastes and smells, may once have protected babes in utero from toxic items. Infidelity is a way for men to ensure genetic immortality. Interestingly, when we deliberately change sex-role behavior -- say, men become more nurturing or women more aggressive -- our hormones and even our brains respond by changing, too."
During the development of the embryo in the womb, circulating hormones have a very important role in the sexual differentiation of the brain. The presence of androgens in early life produces a "male" brain. In contrast, the female brain is thought to develop via a hormonal default mechanism, in the absence of androgen. However, recent findings have shows that ovarian hormones also play a significant role in sexual differentiation.
One of the most convincing evidences for the role of hormones, has been shown by studying girls who were exposed to high levels of testosterone because their pregnant mothers had congenital adrenal hyperplasia (4). These girls seem to have better spatial awareness than other girls and are more likely to show turbulent and aggressive behaviour as kids, very similar to boys'.
"No", says Dr. Pearlson. "To say this means that men are automatically better at some things than women is a simplification. It's easy to find women who are fantastic at math and physics and men who excel in language skills. Only when we look at very large populations and look for slight but significant trends do we see the generalizations. There are plenty of exceptions, but there's also a grain of truth, revealed through the brain structure, that we think underlies some of the ways people characterize the sexes."
Dr. Courten-Myers concurs: "The recognition of gender-specific ways of thinking and feeling -- rendered more credible given these established differences -- could prove beneficial in enhancing interpersonal relationships. However, the interpretation of the data also has the potential for abuse and harm if either gender would seek to construct evidence for superiority of the male or female brain from these findings."
The conclusion is that neuroscience has made great strides in the 90s, regarding the discovery of concrete, scientifically proved anatomical and functional differences between the brains of males and females. While this knowledge could in theory be used to justify misogyny and prejudice against women, fortunately this has not happened. In fact, this new knowledge may help physicians and scientists to discover new ways to explore the brain differences in the benefit of the treatment of diseases, the personalized action of drugs, different procedures in surgeries, etc. After all, males and females differ only by one Y chromosome, but this makes a real impact upon the way we react to so many things, including pain, hormones, etc.
Renato
M.E. Sabbatini holds a doctorate in neurophysiology by the Faculty
of Medicine of the University of São Paulo at Ribeirão Preto,
Brazil, and was a guest scientist and post-doctoral fellow at the Max Planck
Institute for Neurobiology in Munich, Germany. He is currently chairman
of medical informatics and adjunct professor at the Faculty of Medical
Sciences of the State University of Campinas, in Campinas, Brazil; associate
editor and chairman of the editorial board of "Brain
& Mind" Magazine.
Email: sabbatini@nib.unicamp.br