MONTREAL—When we're under immediate stress—say, we are about to give a speech or about to be mugged—we either fight or flee, or so scientists have long preached. But some psychologists are now suggesting that this scenario may apply mainly to males. Men get antisocial under pressure, but women tend to react in the opposite way: they "tend and befriend," engaging in nurturing and social networking, perhaps as a way to protect their offspring, according to a theory proffered by neuroscientist Shelley Taylor of the University of California, Los Angeles. Here at the Cognitive Neuroscience Society 2010 annual meeting, psychologist Mara Mather of the University of Southern California presented powerful new support for Taylor's hypothesis in the divergent ways that stressed men and women respond to faces.

Mather and her colleagues asked male and female subjects to place their hand in ice water for three minutes, an activity that makes levels of the stress hormone cortisol shoot up over the next hour or so. Then these subjects—and a comparison group whose hands had been comfortably immersed in warm water—looked at angry or neutral faces while lying inside a brain scanner.

These conditions revealed a striking sex difference in the brain in the extent to which men and women process faces, and perform emotional assessments of others, under stress. The men under the influence of high cortisol levels showed less activity in a key face-processing region of the brain (the fusiform face area or FFA) than the unstressed men did, suggesting that stressful situations diminish the ability of men to evaluate facial expressions. By contrast, the brains of the women under strain worked harder on the faces: in these females, the FFA was more active than it was in women who did not experience the cortisol boost.

This sex difference was apparent not only in the face evaluation area, but also in a circuit of regions that enables people to internally simulate and understand the emotions of others. This circuit includes the insula, which governs feelings of empathy, and the temporal pole, which helps us understand others' states of mind. According to the researchers' analysis, stress appeared to increase the flow of information between these regions and the FFA in women, orchestrating a concerted response. But in males, cortisol worked to disconnect the brain's analysis of facial expression from its evaluation of others' emotions.

"Will stress increase females' accuracy in interpreting facial expressions—and fuel greater empathy?" Mather asks. Women do seek out affiliation during difficult or threatening times, she says, whereas men don't. Women's ability to read people's expressions more intensely and more accurately could partially underlie this propensity to bond under such circumstances—whether organizing a collective response to an earthquake or reaching out to a potential client during a tense negotiation. "When we start looking at stress, that is where the sex differences in behavior emerge," Mather says.