The smell of fear (sweat) - and anxiety

Women who've smelled sweat from frightened men are more likely to interpret facial expressions as fearful (as in, I think, "threatening" rather than "frightened") - but only when the expression is ambiguous / neutral.

"Our findings provide direct behavioral evidence that human sweat contains emotional meanings," Chen said. "They also demonstrate that social smells modulate vision in an emotion-specific way." (From the press release 6 March 2009).

From their paper Fear-related chemosignal modulates fear recognition in ambiguous facial expressions - by Wen Zhou, Denise Chen, Department of Psychology MS-25, Rice University:

"We show in two experiments that the chemosignal of fearful sweat biases women toward interpreting ambiguous expressions as more fearful, but has no effect when the facial emotions are more discernable. Our findings provide direct behavioral evidence that social chemosignals can communicate emotions and demonstrate that fear-related chemosignals modulate visual emotional perception in an emotion-specific way -- an effect of olfaction in humans that has been hitherto unsuspected."

New Scientist also recently reported different experiments by Lilianne Mujica-Parodi, a cognitive neuroscientist at Stony Brook University in New York and colleagues, showing that when "stress sweat" was inhaled by two separate sets of volunteers, their amygdala was activated. That's the "primitive" part of the brain which deals with fear, anxiety and other emotions. Perspiration created in unstressed conditions didn't however produce the same result. (Second-Hand Stress: Neurobiological Evidence for a Human Alarm Pheromone)

Another New Scientist article, on yet more sweaty studies by Bettina Pause and colleague  at the University of Dusseldorf, Germany, said that sweat from students 1 hour before their final exams "had a different effect on brain activity [compared with sweat from exercising], lighting up areas that process social and emotional signals, as well as several areas thought to be involved in empathy", in volunteers who sniffed it. "This may allow fear to spread quickly and speed our ability to flee danger."