Human Physiology   7/e   Vander/Sherman/Luciano
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Male and Female Brains

Nervous

Men Are from Mars, Women Are from Venus, the catchy title of the 1992 bestseller by John Gray, succinctly expresses an ancient dilemma. What--if anything--do men's and women's brains do differently? Ignoring their obvious differences in reproductive organs and body contours, there are certainly differences in the way men and women behave. Neurophysiologists have been seeking explanations for these behavior differences for decades. Some of the key questions involved include: Does the anatomy of a male brain differ from that of a female brain? How? Do structural differences mean that the brain operates differently in men and women? Are there differences in intellectual ability? Tantalizing preliminary evidence is suggesting answers to these questions, but as yet there are no final resolutions.

The general statement that men and women respond and behave differently under the same circumstances is true; demonstrable differences in behavior and in general brain operations have been carefully documented and described. For example, from the crib, male babies tend to be more aggressive and females more passive--tendencies that persist through life. As children, girls learn to read earlier than their male counterparts and are better at tasks involving the alphabet. As teens, boys tend to do better in mathematics and girls tend to excel at verbal tasks. As adults, women continue to exhibit superior verbal fluency and to better deduce the meaning of unfamiliar words from context cues than men. In spatial operations, men have the edge in such skills as negotiating a maze, reading a map, and quickly discriminating between right and left. Men also perform better than women when asked to visualize an object and imagine rotating it. On the other hand, women tend to perform better than men when asked to look at objects of different shapes, sizes, and colors, and then to group them in some order.

While certain generalities about behavioral differences between men and women appear to be well founded, it is important to remember that evidence for these findings is based on statistical averages. Not all boys are better at math, for example, than all girls; some girls outperform most boys. Likewise, some boys outperform most girls in the verbal domain. However, even if the findings cannot be applied to individual cases, most scientists agree that there are demonstrable differences in behavior between the sexes. It should not be surprising, then, that there are also distinct anatomical differences in the brains of men and women.

The fact that male and female brains do differ structurally in a number of ways was first demonstrated in the 1970s. For example, a specific region in the hypothalamus--called the sexually dimorphic nucleus--was found to have a distinctive synaptic pattern in each sex. In a series of experiments on monkeys, males castrated shortly after birth developed the female pattern, while females given injections of testosterone developed the male pattern. Based on these and later observations, scientists concluded that fetal testosterone is responsible not only for masculinizing the developing genitals, but the brain as well.

It has also been reported that women have more neurons in their temporal lobes than men. This may explain why the left temporal lobe in women is longer. In addition, the corpus callosum tends to be wider in women, particularly in the posterior regions, than in men. Brain scans reveal better connections between the hemispheres in women, which is consistent with the structural differences in the corpus callosum, and which supports the hypothesis that the female brain is less lateralized. Finally, clusters of neurons in the spinal cord that serve the external genitals are larger in men than in women. As with the sexually dimorphic nucleus of the hypothalamus, testosterone exposure before birth produces the male pattern of these neurons.

While the existence of structural differences in the male and female human brain is clear, it is less clear what they signify. To what extent do they account for differences in abilities and characteristics? And how big a role do environmental factors play in determining maleness or femaleness? It is possible that there will never be universally applicable answers to these questions. But for now it is safe to say that men and women are different in both their anatomy and their behavior.

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