His, Hers, and Yours: Diversity in Brain Structure

Joyce Norman, Psychology (photo ??)
It's a sexual truism that bigger doesn't mean better, and though the organ in question isn't usually a brain, it could be. In the third Conversation on Diversity, psychology professor Joyce Norman delivered a talk on some of the latest research regarding brain structure and gender. Describing the material as "very complex and very controversial, " she surprised many in the audience with the apparently irrefutable fact that male brains are "significantly" larger than female brains, even after adjusting for differences in relative body size. Males also have more brain cells than do women, about 16 percent more, or approximately 4.5 billion neocortical neurons. But are men smarter? No.

Brandishing half of an actual, though plasticized, human brain of indeterminate gender, Norman was careful to emphasize that the organ varies tremendously from person to person, even among members of the same sex. And size is only moderately correlated with intelligence, Norman said.

Neanderthals had larger brains than we do, she pointed out, but obviously they weren't smarter. Research with magnetic resonance imaging on the brains of twins indicates a 94 percent heritability factor. It turns out that brain size is mostly a function of genes. Interestingly, though, the difference isn't evident at birth. Instead, said Norman, it starts to emerge between the ages of two and five years. So what happens? Unfortunately, no one really knows for sure.

Norman cited research that suggests brains are "sculpted" by experience and stimulation. In the 1960s, studies on rats showed that their cerebral cortexes expanded when they were placed in environments with lots of toys. When they were removed from those environments, however, their brains returned to normal size. Other research, on the brains of young violinists, has shown greater than normal development of the part of their brains stimulated by the use of their left hands for fingering. "We know that human brains don't add cells [in response to stimulation] the way the brains of songbirds do," Norman said, but cell death, selective pruning of synapses, and the growth of "white

matter," or myelin, all play a part in the shaping process. Other studies with twins suggest that random environmental events account for most of the variability in surface structure. In general, there is a 2 percent loss of brain cells over a human lifetime and the regression correlates fairly well with age. Men and women both lose cells with age, but from different parts of the brain. In men, the loss occurs primarily from the frontal and temporal lobes; in women, from the thalamus and hippocampus. Overall, however, men lose slightly more brain cells than do women.

Questioned whether metabolic differences between men and women might have an effect on brain size, Norman responded that some researchers have speculated that the brain is an energy guzzler, others that growth is related to perinatal exposure to testosterone, and still others that size differences are essentially meaningless. The prevailing view now is that size differences are adaptations to the environment that helped our species survive.

As for gender differences in cognitive skills, Norman cautioned her audience not to go overboard in their interpretation of data showing that among 17-year-olds, boys tend to score lower than girls in tests of reading and writing and that girls demonstrate less mechanical and mathematical ability than boys. Referring to the commander of the next space shuttle, she said, "Eileen Collins is an exception worth noting."


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