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Can People Really Smell Fear?

Fictional characters, and even real-life folks, often talk about animals and people—particularly snarling dogs and knife-wielding lunatics—being able to “smell fear” on people. No one ever seems to be able to describe just what fear smells like, though.

The lack of detail—Is it musty? Does it have a hint of vanilla?—leads one to think that the smell of fear is more metaphorical than literal. But scientific evidence suggests that fear might really have a chemical component that we sense through our noses without even realizing it. The matter is far from settled, though.

Right Under Your Nose

In 2009, a team of German researchers collected sweat from two groups of students, one where the sweat came from exercising on bikes and one where it came from the stress of waiting to give a graded oral presentation.

A third group of students, lying in fMRI scanners and wearing modified oxygen masks, then smelled air odorized by the two groups’ sweat. Asked about what they were smelling, the students didn’t even notice an odor in half of the trials. When they did notice they were smelling something, they were unable to tell the difference between the two sources and rated both as low in intensity, weakly pleasant, unfamiliar, and having no effect on their own emotions.

The brain scans told a very different story, though. After smelling the sweat of the students nervously waiting for their exams, the smellers’ brains showed increased activity in areas that are involved with empathy and processing social signals and the emotional states of other people. The sweat from exercise didn’t cause these same activations, suggesting that the nervous students’ sweat contained some sort of chemical signal of their anxiety that triggered a response in the smellers’ brains without registering as the sensory experience of a smell.

That same year, a pair of psychologists at Rice University collected sweat from different volunteers while they watched horror movies or slapstick comedies, and then asked other volunteers to smell the sweat while they looked at images of faces that switched expressions from happy to ambiguous to fearful. As the faces morphed, the volunteers were asked to indicate whether they thought the expressions were happy or fearful.

The smellers were more likely to judge the ambiguous faces as fearful after being exposed to the the horror watchers’ sweat than when they smelled the comedy watchers’ sweat or a control sweat. That behavioral change suggests that not only did the sweat contain some chemical signal that communicated emotion, but also affected people’s visual perception of emotions and biased them towards the one being communicated by the sweat (that second part is consistent with other findings that emotional cues from faces and voices can regulate each other).

Just last month, Dutch psychologists found evidence that fear-induced sweat not only biases someone who smells it toward seeing fear, but might also push them to feel it themselves. Volunteers either watched scenes from a scary movie or from Jackass, and their sweat was collected. People in another group were then exposed to the smell of one of the sweats while they took a visual test that asked them to find a target object on screens full of different items. While this was going on, the researchers recorded their facial expressions and tracked their eye movements.

The people who got the horror sweat made facial expressions suggesting fear or anxiety shortly after they were exposed to the sweat. The Jackass-sweat smellers, meanwhile, made disgusted faces. (This was determined by comparing their faces to “distinctive facial-muscle signatures” associated with emotions. For more on face reading, check out Paul Ekman and his facial action coding system.) The two sweats also appeared to have affected the smellers’ behavior, with the horror sweat smellers attempting to acquire more sensory information while exposed to the sweat by taking bigger sniffs and scanning more and fixing their gaze less during the visual exercise.

What the Nose Knows

What all this, and a bunch of other studies looking at the same sort of thing, suggests is that humans might not communicate by just sight, sound, and touch. Like other animals, we might also use chemical signals embedded in our sweat, and maybe elsewhere, to let each other know about our emotional states.

That has been a contentious idea for a very long time, with some people claiming “definitive evidence of human pheromones,” and others saying “no, not so fast.” While there’s a good amount of evidence for behavioral and physiological changes in people in response to “chemosignals,” no one has been able to nail down just what the chemicals are that trigger these responses, and how people detect them. The vomeronasal organ, which many animals use to detect pheromones, is present in some humans, but doesn’t appear functional. When scientists can find them in people, the genes that code for their receptors don’t seem to have done their job, and their sensory neurons have little or no connection with the central nervous system.

Those are two very important dots that need to be connected before the line between someone’s sweaty armpit and someone else’s schnoz can be drawn clearly.

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Big Questions
Why Is the American Flag Displayed Backwards on Military Uniforms?
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In 1968, famed activist Abbie Hoffman decided to crash a meeting of the House Un-American Activities Committee in Washington by showing up in a shirt depicting the American flag. Hoffman was quickly surrounded by police, who ripped his shirt off and arrested him for desecration of the Red, White, and Blue.

Hoffman’s arrest is notable today because, while it might be unpatriotic to some, wearing the American flag, burning it, or otherwise disrespecting it is not a violation of any federal law. In 1989, the Supreme Court ruled that it would be unconstitutional to prosecute any such action. Still, Americans have very fervent and strict attitudes toward displaying the flag, a longstanding symbol of our country’s freedom. According to the U.S. Flag Code, which was first published in 1923, you shouldn’t let the flag touch the ground or hang it upside-down. While there’s no express prohibition about reversing the image, it’s probably a safe bet you shouldn’t do that, either.

Yet branches of the U.S. military are often spotted with a seeming mirror reflection of the flag on their right shoulder. If you look at a member in profile, the canton—the rectangle with the stars—is on the right. Isn’t that backwards? Shouldn’t it look like the flag on the left shoulder?

The American flag appears on a military uniform
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Not really. The flag is actually facing forward, and it’s not an optical illusion.

When a service member marches or walks forward, they assume the position of a flagpole, with the flag sewn on their uniform meant to resemble a flag flapping in the breeze. With the canton on the right, the flag would be fluttering behind them. If it were depicted with the canton on the left, the flag would be flying backward—as though it had been hung by the stripes instead of the stars nearest to the pole. The position of the flag is noted in Army Regulation 670-1, mandating the star field should face forward. The official term for this depiction is “reverse side flag.”

As for Hoffman: His conviction was overturned on appeal. In 1970, while at a flag-themed art show in New York, he was invited to get up and speak. He wore a flag shirt for the occasion.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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Big Questions
What Causes Sinkholes?
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Mark Ralston/AFP/Getty Images

This week, a sinkhole opened up on the White House lawn—likely the result of excess rainfall on the "legitimate swamp" surrounding the storied building, a geologist told The New York Times. While the event had some suggesting we call for Buffy's help, sinkholes are pretty common. In the past few days alone, cavernous maws in the earth have appeared in Maryland, North Carolina, Tennessee, and of course Florida, home to more sinkholes than any other state.

Sinkholes have gulped down suburban homes, cars, and entire fields in the past. How does the ground just open up like that?

Sinkholes are a simple matter of cause and effect. Urban sinkholes may be directly traced to underground water main breaks or collapsed sewer pipelines, into which city sidewalks crumple in the absence of any structural support. In more rural areas, such catastrophes might be attributed to abandoned mine shafts or salt caverns that can't take the weight anymore. These types of sinkholes are heavily influenced by human action, but most sinkholes are unpredictable, inevitable natural occurrences.

Florida is so prone to sinkholes because it has the misfortune of being built upon a foundation of limestone—solid rock, but the kind that is easily dissolved by acidic rain or groundwater. The karst process, in which the mildly acidic water wears away at fractures in the limestone, leaves empty space where there used to be stone, and even the residue is washed away. Any loose soil, grass, or—for example—luxury condominiums perched atop the hole in the ground aren't left with much support. Just as a house built on a weak foundation is more likely to collapse, the same is true of the ground itself. Gravity eventually takes its toll, aided by natural erosion, and so the hole begins to sink.

About 10 percent of the world's landscape is composed of karst regions. Despite being common, sinkholes' unforeseeable nature serves as proof that the ground beneath our feet may not be as solid as we think.

A version of this story originally ran in 2014.

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