Gender and the Brain

Brain Size and Structure

One of the first topics to be considered was the comparative size of male and female brains. Male brains were noted to be larger than female brains were. The average brain of boys at birth was found to be 12 percent to 20 percent larger than that of girls. As adults, men were found to have brains that weighed 11 percent to 12 percent more than the average weight for women. This early research was used to label women as intellectually inferior. It was later revealed, however, that when the size of the brain was compared with body weight, there was relatively no difference between the brain sizes of the sexes.

Brain structure was the next topic to receive considerable research attention. Looking for differences in brain structures to explain cognitive differences between males and females became a focal point. The human brain is complex. The cerebral cortex, the outermost part of the brain, contains 10 to 15 thousand million neurons and billions of synaptic connections. The cortex is divided into two halves, the right and left hemispheres. Connecting the two hemispheres is a mass of nerve fibers made up of 200 to 250 million axons called the corpus callosum. These fibers connect hundreds of millions of neurons between the two brain hemispheres.

The hypothalamus, considered a primitive part of the brain, is located at the base of the brain and connects to the master endocrine gland called the pituitary. The hypothalamus is the controller of many basic life functions such as temperature regulation, appetite, and sex drive. Scientists have found sex differences in several nuclei including the interstitial nucleus of the anterior hypothalamus. This clump of nerve cells was found to be larger in men than in women. Further, gender identity and sexual orientation have also been linked to variations in the hypothalamus. A particular cell group of the interstitial nucleus was shown to be twice as large in heterosexual men than in homosexual men and women. Another part of the hypothalamus was found to be smaller in male-to-female transsexuals than in a male control group. These research findings prompted great debate about whether there is a biological basis for homosexuality.

In the past 10 years, neurobiologists have reported structural differences between the sexes that have been linked to women's advantage in language-related tasks and male's advantage in visuospatial skills. Some studies indicated that when processing language, activation in the men's brains was confined to the left hemisphere, whereas both the left and right hemispheres of women's brains were strongly activated. Further, it has been said that men's right hemisphere was more specialized for spatial tasks compared with that of females. The research suggests that women have greater integration between the brain's two hemispheres (bilateral processing), whereas male brains are said to be more lateralized or [Page 344]specialized. Further, this concept of bilateral processing has been supported by findings regarding the size of the corpus callosum.

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