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.

In their article, Wood and colleagues speculate about the evolutionary basis for this sex difference. Perhaps, since women are the primary child-rearers, their brains have become programmed to develop a larger SG, to prepare them to be sensitive nurturers. Prenatal sex hormones are known to alter behavior and certain brain structures in other mammals. Perhaps such hormones—or sex-specific genes—may enhance the development of females’ SG (or dampen the development of males’) leading to inborn differences in social cognition.

The best way to test this hypothesis is to look at children. If the sex difference in the SG is present early in life, this strengthens the idea that it is innately programmed. Wood and Nopoulos therefore conducted a second study with colleague Vesna Murko, in which they measured the same frontal lobe areas in children between 7 and 17 years of age.

But here the results were most unexpected: they found that the SG is actually larger in boys ! What’s more, the same test of interpersonal awareness showed that skill in this area correlated with smaller SG, not larger, as in adults. The authors acknowledge that their findings are “complex,” and argue that the reversal between childhood and adulthood reflects the later maturation of boys’ brains, compared to girls. (Adolescents’ brains undergo a substantial “pruning” or reduction in gray matter volume during adolescence, which happens about two years earlier in girls, compared to boys.)

However, in both studies, Wood and colleagues added another test that reminds us to be cautious when interpreting any finding about sex differences in the brain. Instead of simply dividing their subjects by biological sex, they also gave each subject a test of psychological “gender:” a questionnaire that assesses each person’s degree of masculinity vs. femininity—regardless of their biological sex—based on their interests, abilities and personality type. And in both adults and children, this measure of “gender” also correlated with SG size, albeit in just as complicated a way as the correlation between “sex” and SG size. (Larger SG correlated with more feminine personality in adults but less feminine personality in children.)

In other words, there does seem to be a relationship between SG size and social perception, but it is not a simple male-female difference. Rather, the SG appears to reflect a person’s “femininity” better than one’s biological sex: women who are relatively less feminine show a correspondingly smaller SG compared to women who are more feminine, and ditto for men.

This finding—that brain structure correlates as well or better with psychological “gender” than with simple biological “sex”—is crucial to keep in mind when considering any comparisons of male and female brains. Yes, men and women are psychologically different and yes, neuroscientists are uncovering many differences in brain anatomy and physiology which seem to explain our behavioral differences. But just because a difference is biological doesn’t mean it is “hard-wired.” Individuals’ gender traits—their preference for masculine or feminine clothes, careers, hobbies and interpersonal styles—are inevitably shaped more by rearing and experience than is their biological sex. Likewise, their brains, which are ultimately producing all this masculine or feminine behavior, must be molded—at least to some degree—by the sum of their experiences as a boy or girl.

And so, any time scientists report a difference between male and female brains, especially in adults, it begs the question, “Nature or nurture?” Is women’s larger SG the cause of their social sensitivity, or the consequence of living some 30 years in a group that practices greater empathetic responding? Wood and colleagues are among the few neuroscientists to analyze male-female brain differences for their relationship to gender type, as opposed to strict biological sex. Their findings do not prove that social learning is the cause of male-female differences in the brain, but they do challenge the idea that such brain differences are a simple product of the Y chromosome.

Are you a scientist? Have you recently read a peer-reviewed paper that you want to write about? Then contact Mind Matters co-editor Gareth Cook, a Pulitzer prize-winning journalist at the Boston Globe, where he edits the Sunday Ideas section.