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Biology 202, Spring 2005
Third Web Papers
The brain can be divided into three sections: the forebrain, the midbrain, and the hindbrain. The forebrain is the most interesting of all three due to the fact that throughout one's life, the forebrain grows until it forms two cerebral hemispheres, which make up about 85% of the entire brain. (1) The thick coating over the cerebral hemispheres is called the cerebral cortex, which is the location of many studies involving gender differences. According to one study done on the brains of male and female rats, some areas of the female cortex were shown to be more developed than those of the male cortex. (1)The same study showed that the thickness of the cerebral cortex was different in males than in females. In 95% of the cases, the female rat demonstrated symmetry between the thickness of the right and left cortex. However, only in 60% of the cases did the male rat show symmetrical cortexes. (1) Marian Diamond suggests that the common asymmetry of the male cortexes could be useful for the behavior male rats exhibit of finding their female partners and defending their territory. Though many early studies in gender behavior have been (and continue to be) done on rats, technology has rapidly advanced in the last decade, allowing a great deal of research to be carried out on live human brains using functional Magnetic Resonance Imaging (fMRI), volumetric measurements of certain sections of the brain, and positron-emission tomography (PET) (2).
In general, men have about 4% more brain cells than women. However, women are shown to have more connections between brain cells than men. (3) It has also been noted that although men use the dominant hemisphere of their brain (usually the left) to master language, women actually use both sides, giving them a general advantage over men. Further research has shown that women have larger and deeper limbic systems than men, therefore enabling them to be more emotional, better able to express their emotions, and connect more easily to others. (3) Instead, men seem to be better at perception of space, and general cognition. One explanation of this discrepancy can be found in a recent study done by University of California, Irvine with the University of New Mexico. The researches found that men have 6.5 times more "gray matter", centers in the brain that process information, than women, and that women have more 10 times more "white matter", connections between these centers. (4) The study also found that 84% of the regions that had gray matter in female brains were in the frontal lobe, compared to only 45% percent in male brains. (4) Therefore, the gray matter is more spread out in the male brain than in the female brain. The study also shows that women have a larger corpus collosum than men, which is why women are better at combining information from both sides of the brain while men are better at general overall cognition.
In addition to the amount and location of gray or white matter in the brain, there are other aspects that influence the variation in behavior between men and women. In a recent study done at Johns Hopkins University, researchers discovered a region of the parietal cortex, called the inferior-parietal lobule (IPL), is much larger in men than in women. (2)The right IPL is found to be associated with the perception of emotions, as well as the perception of bodily and spatial interaction, while the left IPL is involved with the perception of time, speed, and mental rotation. (2) In the study,the researchers found that in men, the left IPL is larger than the right one, and that the exact opposite is true in women. This could potentially explain why women are generally more emotionally sensitive than men, and why men are generally more mathematical than women.
Another area of the brain that has also been shown to be significantly different in male and female brains is the hippocampus, which is involved with memory storage and spatial navigation. (5) Numerous studies performed on rats have shown that male rats tend to find their way through mazes using directional information while female rats tend to navigate mazes by using the memory of landmarks. One study executed by Tracey J. Shors from Rutgers University found that when they exposed male rats to a series of electric shocks, there was an increase of connection between neurons in their hippocampus and they learned tasks more quickly. (5) The same experiment done on female rats showed the opposite effect, with a decrease in the females' ability to learn tasks. This study demonstrates the way in which different genders respond under stress, and it has interesting implications for the ways male and female children should be taught in school.
It may seem that gender determination is pretty clear-cut, but in reality that is not necessarily the case. There are many cases of Gender Identity Disorder (GID) in which a person feels deeply associates themselves with the opposite sex and consistently feel as though they were born the "wrong sex".(6) For example, a male with GID tends to dress in female clothing and prefers playing with dolls than participating in sports games. On the contrary, females with GID tend to dress in male clothing and enjoy playing rough sports games. Both sexes who have this disorder have also shown to be extremely uncomfortable with their biological genitalia and often express the wish that they were more like the opposite sex in physical structure.(6) One interesting aspect about gender disorders is that the subject of the disorder usually has a strong sense very early on in childhood that they are the wrong sex.
This raises the question of how much of our gender is determined at birth as opposed to the environment in which we grow. It is an interesting question that has caused much discussion about the nature versus nurture debate. Through experiments involving male and female infants, it has been noted that boys tend to choose toys such as trucks and cars, while girls tend to choose dolls. A study done by Melissa Hines of City University London and Gerianne M. Alexander of Texas A&M University on vervet monkeys showed that, when presented with a selection of toys, the male monkeys spent more time playing with toys such as trucks and cars, while the female monkeys spent more time playing with dolls and other feminine toys. (5) The study also showed that both the male and the female monkeys spent an equal amount of time on the gender-neutral toys such as coloring books. (5) Since monkeys are not influenced by societal pressures like humans, this study suggests that the differences between male and female behavior could be innate instead of environmentally constructed.
Many scientists say that the differences between male and female brains can be explained by evolution. (2) During the primitive stages of humans, men and women each had distinct roles in society that corresponded to which function their brains performed better. For example, men hunted for food and protected their families while women gathered food close to their homes, cared for the children, and looked after the house. According to Professor David Greary, a researcher of gender differences at the University of Missouri, USA, "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 the home." (2) In addition, the tendency of women to be more developed verbally can be explained by the fact that women socially interacted more than men due to their responsibilities of raising children.
Obviously there are other factors that control behavior of the sexes other than the composition of the brain, such hormones and environment. However, one wonders how much genes play a role in the differences between male and female brains. Dr. Eric Vilain, an assistant professor of genetics at the David Geffen School of Medicine at UCLA performed a study in 2003, in which he extracted RNA from 10-day old male and female mice in order to compare those genes that had different stages of action. (7) Dr. Vilain found variation in 54 genes, half of which were more operational in females than in males. (7) He said this information can help explain some of the behavioral differences between the sexes, and hopes to prove with further research that sexual identity and sexual attraction are not in fact voluntary, but rather predetermined at birth.
So, in fact we have found that there are biological differences between male and female brains. Does this mean that one sex is superior to another? Not at all. As Dr. Godfrey Pearlson says, "To say... 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 at language skills. Only when we look at very large populations and look for slight but significant trends do we see the generalizations." (2) In addition to the problem of generalizing, it is important to remember that the biological makeup of the brain is not the only aspect that determines the variation in gender behavior. Hormones and environmental influences, such as society and culture, also play a part in the conceptualization of gender.
Examining differences between male and female brains allow us to explore the diversity in human behavior. However, it does not determine it. The notion that our brains are composed in the way they are because of gender roles in primitive societies does not exclude the possibility of alteration in the brain's composition to accommodate future roles played by the modification in our perception of gender. Furthermore, ignoring the biological differences in male and female brains to avoid stereotyping of gender is wrong. Instead, we should use the information available to us to learn something about gender determination and gender behavior. Even though there is a great deal of evidence pointing towards the conclusion that gender is primarily determined at birth, my guess is that since general human behavior is determined by a combination of genetics and environmental influences, gender behavior can be explained very much in the same way.
1)Male and Female Brains, a speech given by Marian Diamond
2)Are There Differences between the Brains of Males and Females?, essay by Renato M.E. Sabbatini
3)Male-Female brain differences, a general overview
4)Today @ UCI, "Intelligence in men and women is a gray and white matter"
5)Scientific American, "His Brain, Her Brain"
6)At Health, Gender Identity Disorder
7)Daily Bruin, "Research shows gender linked to gene activity"
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