Pheromones fascinate me, and not only the ones that mediate sexual attraction.

Several months ago I wrote a post about a study that suggests that airborne chemical signals also mediate stress and fear. Known as alarm pheromones, these chemicals are found in sweat and saliva. In that experiment, sweat from skydivers (collected from pads in their armpits) activated anxiety circuits in the brains of people who sniffed those pads later on. The fascinating theory: Emotions can be communicated by smell. It happens unconsciously.

That was just one study, and naysayers are quick to point out that a solitary experiment doth not prove human pheromones are real. But now there’s further proof.

Researchers at the Universities of Dusseldorf and Kiel in Germany recently published the results of study based on the sweat smells of 49 stressed-out students after a.) taking a final oral exam (stress sweat) and b.) exercising (sport sweat). Sniffing the pads that had been in students’ armpits, volunteers often couldn’t detect an actual odor. Nor could they tell whether they were smelling stress sweat and sport sweat. But it turns out that an area of their brains detected the difference. Only stress sweat — and not sport sweat — triggered brain activity in areas involved in the processing of social emotional stimuli (fusiform gyrus), and empathetic feelings (insula), attention (thalamus).

The implications are fascinating. Is stress contagious? In an emergency situation, it makes sense that we’re “wired” to perceive and respond to the stress of others. An odor that induces attention and anxiety may help a group to focus together or synchronize a fight-or-flight response.

It’s particularly interesting that neural circuits associated with empathy — not just attention — were activated. Are we naturally empathetic creatures? Then again, there’s no proof that the volunteers actually felt more understanding and compassionate when smelling stress sweat even if their brains go through some of the motions. I suspect empathy is context-dependent. Further experiments should look into whether volunteers really are more empathetic (more willing to help a person in distress, for instance) after smelling stress sweat compared to sport sweat. If so, this would be further proof that stress sweat is an alarm pheromone, which, by definition, changes the way we behave after we inhale it.

A thought: Could stress sweat induce compassion in autistic people?

And another thought: If feelings have smells, is happiness also inhalable, communicable?