Think your anger is as plain as the scowl on your face? If you're talking to a stressed-out man, that may not be the case.

A new study finds that when men under stress looked at angry faces, they seemed to disengage, at least according to brain scans showing lower activity in brain areas responsible for processing other people's emotions and facial expressions. In contrast, stressed-out women showed more activity in those brain regions.

The findings, appearing in the Oct. 6 issue of the journal NeuroReport, could represent a neurological basis for findings that women tend to be more emphatic than men. Researchers don't know exactly why these differences occur, but one reason may be the interaction of sex hormones with stress hormones.

"Very rarely do we see sex differences in the studies that we have in our lab, but now that we've started to do things where we're looking at how stress affects cognitive processes, the stress differences are really jumping out at us," said Mara Mather, a cognitive psychologist at the University of Southern California and a co-author of the paper. "We see differences that don't show up unless people are stressed."

Two genders, under stress

Psychologists have long noted that stress affects men and women differently. Women tend to seek out social support, while men are more likely to withdraw. To find out if brain differences are influencing these behaviors, Mather and her colleagues focused on the amygdala, an almond-size structure buried deep in the brain that helps process fearful stimuli like angry faces. The researchers also looked at the insula, another deep brain structure that assists with empathy, and the temporal pole, a brain area tucked behind the ears involved with understanding another person's state of mind.

The researchers measured levels of the stress hormone cortisol from saliva samples collected from 47 volunteers. Next, half of the men and half of the women were asked to immerse their hands in nearly freezing water for up to three minutes, while the others dipped their hands in comfortably warm water.

Both groups then had their brains scanned using functional magnetic resonance imaging (fMRI), which measures blood flow to different brain areas. The participants completed some tasks unrelated to the study, before giving another saliva sample for stress-hormone levels. After that, the researchers continued the brain scan while participants watched images of 160 faces, 80 angry and 80 neutral.

Stress and the brain

The researchers found that the men and women who had been given the ice water were equally stressed by the experience, judging by their cortisol levels. But how that stress affected their brains was very different. When the stressed-out men viewed either neutral or angry faces, their brains showed a decrease in activity in the fusiform facial area, which helps with facial recognition. The signal measured in their FFA was about 0.75 percent, compared to almost 1.5 percent for the men in the warm-water group. Frazzled women, by contrast, seemed more attuned to recognizing facial expressions. Their FFA signal measured almost 1.5 percent, compared with 0.75 percent among the relaxed women.

The researchers also noticed a hormonal correlation: The higher the testosterone levels to begin with, the lower the FFA activity when stressed. They found no comparable fluctuations based on estrogen levels. The finding supports the theory that hormones may be at the root of the differences in men and women's responses to stress.

The interactions between emotion-processing areas like the right temporal pole, insula and inferior fontal gyrus also differed by gender. The researchers looked at a measurement called functional connectivity, which reveals the extent to which brain areas simultaneously become active. Men showed less functional connectivity between these areas when stressed, while women showed more. It seems that when women are stressed, social and emotional areas of the brain go on alert, perhaps reflecting a tendency to reach out. The same areas in men's brains seem to disengage.

The researchers don't know whether these brain differences are innate or a product of socialization, and they can't yet say if the decreased activity in stressed men causes them to actually become less engaged and empathic, or if they compensate in some other way. However, Mather said, other research does find gender differences in the way men and women act when stressed. The current study meshes with those findings, she said.