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Sex differences in the brain are sexy. As MRI scanning grows ever more sophisticated, neuroscientists keep refining their search for male-female brain differences that will answer the age-old question, “Why can’t a woman think like a man?” (and vice-versa).
Social cognition is one realm in which the search for brain sex differences should be especially fruitful. Females of all ages outperform males on tests requiring the recognition of emotion or relationships among other people. Sex differences in empathy emerge in infancy and persist throughout development, though the gap between adult women and men is larger than between girls and boys. The early appearance of any sex difference suggests it is innately programmed—selected for through evolution and fixed into our behavioral development through either prenatal hormone exposure or early gene expression differences. On the other hand, sex differences that grow larger through childhood are likely shaped by social learning, a consequence of the very different lifestyle, culture and training that boys and girls experience in every human society.
At first glance, studies of the brain seem to offer a way out of this age-old nature/nurture dilemma. Any difference in the structure or activation of male and female brains is indisputably biological. However, the assumption that such differences are also innate or “hardwired” is invalid, given all we’ve learned about the plasticity, or malleability of the brain. Simply put, experiences change our brains.
Recent research by Peg Nopoulos, Jessica Wood and colleagues at the University of Iowa illustrates just how difficult it is to untangle nature and nurture, even at the level of brain structure. A first study, published in March 2008 found that one subdivision of the ventral prefrontal cortex—an area involved in social cognition and interpersonal judgment—is proportionally larger in women, compared to men. (Men’s brains are about 10 percent larger than women’s, overall, so any comparison of specific brain regions must be scaled in proportion to this difference.) This subdivision, known as the straight gyrus (SG), is a narrow strip of cerebral cortex running along the midline on the undersurface of the frontal lobe. Wood and colleagues found the SG to be about 10 percent larger in the thirty women they studied, compared to thirty men (after correcting for males’ larger brain size). What’s more, they found that the size of the SG correlated with a widely-used test of social cognition, so that individuals (both male and female) who scored higher in interpersonal awareness also tended to have larger SGs.
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