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Perhaps one such "why" involves biological premises. The steroid hormones secreted by the gonads (mainly androgens in males and estrogens and progestins in females), for instance, are not restricted to the lower half of the body and are known to have divergent effects on the brains of the separate sexes. Although the male and female brain may appear identical structurally (except for the male's being about 10% larger) (1), morphological differences abound.
The brain is organized by the presence or absence of androgens neonatally, long before it gets a chance to interact with its culture. In rodents, for instance, testosterone masculinizes the brain to prevent ovulatory capacity and establish male-typical behavior. This phenomenon is accomplished by aromatization, or the paradoxical conversion of testosterone to estrogen in the brain. Female rodents are protected from this estrogen-based masculinization by a protein in the blood called AFP. A small amount of estrogen does actually seep into the brain, however, and this might be responsible for manifesting feminine sexual behavior as an adult (2).
The primary mechanism by which these steroids appear to influence neuroanatomy, neurophysiology, and behavior is through binding to intraneuronal nuclear receptors in target brain areas and altering neuronal genomic expression. The effects of steroid binding are realized in alterations in regional cell growth, proliferation, or death, which may then influence cell number, size, or packing density. Early migrational patterns, dendritic growth, and neuronal myelination may also be modified (2).
One place where steroid binding appears to exert its effect is in the hypothalamus. The sexually dimorphic nucleus of the medial preoptic area (SDN-POA) is a sub-nucleus in the medial preoptic area that is approximately 2.5 times larger in males than in females. In addition, the presence of two sexually dimorphic cell groups has been confirmed in the preoptic-anterior hypothalamic area. There are four interstitial nuclei of the anterior hypothalamus and the two that are larger in the male brain are the INAH3 and INAH1. These hypothalamic findings are particularly noteworthy because the preoptic area has been shown to be sexually dimorphic in several other non-human species and more important, to be sensitive to prenatal or perinatal hormonal influences.
The SDN-POA can be enlarged in female rats through the administration of a synthetic estrogen (diethylstilbestrol) which does not bind to AFP, indicating that masculinization of this structure is dependent on the intracellular conversion of testosterone to estrogen. Estrogen, therefore, may mediate this sexual variation by preventing a developmental loss of neurons within the medial preoptic nucleus (2). Perhaps these hypothalamic discrepancies are also related to the contrasting natures of the male and female hypothalamus; the male's being constant and the female's being cyclic.
Neonatal testosterone also appears to be involved in the sexual differentiation of the cerebral cortex. Certain regions of the cortex are significantly thicker in the right hemisphere than in the left in male rats, whereas females showed a non-significant trend toward asymmetry in the opposite direction. This distinction appears to be mediated at least partly by androgen exposure, since neonatally gonadectomized male rats fail to show the right-left pattern of cortical asymmetry seen in intact males (2).
Perhaps yet another steroid-induced phenomenon lies in the differing proportions of white and gray matter seen in male vs. female brains. Researchers at the University of Pennsylvania Medical Center have recently reported that women have a higher proportion of gray matter to cranial volume while men have a higher proportion of white matter (gray matter is where computation takes place, while white matter is responsible for communication between groups of cells in different areas of the brain) (3). This finding complicates the earlier observations concerning the corpus callosum, a large body of nerve fibers that connects the right and left hemispheres of the brain. Those studies showed that women have a larger corpus callosum than men and therefore show a more bilateral representation of function which decreases specialization but integrates the two halves better. The corpus callosum, however, is composed of white matter, the tissue type now seen at lower overall proportions in women. The implication is that evolution has placed a priority on this structure in women and thus a superiority in their capacity to communicate between the different modes of perceiving and relating to the world (3).
But how does all of this relate to behavior and cognition? While there are no sex differences in general IQ on standardized tests of intelligence, men differ from women, on average, in a number of more specific abilities. Men typically excel in spacial abilities, quantitative abilities, and feats of strength while women tend to outperform men on tasks involving verbal abilities, perceptual speed and accuracy, and fine motor skills (4). The male advantages here reflect more specialized skill while the female advantages are inherently more integrative. Also, the sex difference in spacial ability does not appear in full force until puberty, raising the possibility that the hormonal changes characteristic of this stage may play some role in its ontogenesis.
In addition, the right-brain/left-brain distinction may shed some light on personal relations between the sexes. Although women have most commonly been deemed the more intuitive sex, it appears the opposite may be true. Since a man spends more time in his right brain, he may be more likely to sense a situation and take action intuitively. He responds immediately with sexual feeling to sex objects, bypassing the planning or long decision-making process that a woman may engage in. If a woman says something that a man does not want to hear, he may not respond verbally, but spatially as he moves to the TV to watch ESPN. A woman, on the other hand, may have long debates in her head before she comes to a decision and takes action. Such internal thinking and deliberation are left brain activities (5).
Because men do seem to have more difficulty with feelings, one might conclude that they must not be in the right half of their brain after all. On the contrary, however, they often compensate for their feeling impediments by replacing the uncomfortable feelings with other ones. Typical compensations are: they may feel sexual in situations that are not particularly sexual, they may resort to addictive behaviors as a distraction, or they may get angry in situations that might call for a different response (5).
There are, of course, no rules in delegating sex-specific behavior and deviations from the norm are more than common. In the meantime, the inconsistencies that plague this area of research should be regarded as evidence for the complexity of the underlying question involved and the variation intrinsic in the answer. It is my opinion, however, that in a society where gender blurs are increasingly more accepted and even applauded, the fact the stereotypes still permeate civilization and instill "fact-of-life" foundations across cultures is, in and of itself, testimony for the reality of sex differences in cognitive and emotional functioning. Indeed, most people who have had any sort of interaction with the opposite sex, whether it is in the sandbox or the wedding chapel, would agree that there are consistent differences in behavior that the nurture proponents simply cannot account for. The concept of a sexually dimorphic brain, therefore, should not be feared but should instead be regarded as an invaluable revelation that may prevent us from dismissing our observations of other-sex behavior as mere reflections of stupidity or "weirdness." It is useful to recognize that we may, in essence, be experiencing different realities! As we learn to stop underestimating the power of a hormone, we can also learn more about ourselves, the opposite sex, and our interactions. And, just as we must sometimes "agree to disagree," perhaps we must also be willing to "understand to not understand."
2)A Role for Ovarian Hormones in Sexual Differentiation of the Brain
3)Sex Differences Found in Proportions of Gray and White Matter in the Brain: Links to Differences in Cognitive Performance Seen
5)Left/Right Brain? Genetic Sex Differences
5)Left/Right Brain? Genetic Sex Differences
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