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Can People Really Smell Fear?

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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
How Does Autopilot Work on an Airplane?
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How does autopilot work on an airplane?

Joe Shelton:

David Micklewhyte’s answer is a good one. There are essentially a few types of features that different autopilots have. Some autopilots only have some of these features, while the more powerful autopilots do it all.

  • Heading Hold: There’s a small indicator that the pilot can set on the desired heading and the airplane will fly that heading. This feature doesn’t take the need for wind correction to desired routing into account; that’s left to the pilot.
  • Heading and Navigation: In addition to holding a heading, this version will take an electronic navigation input (e.g. GPS or VOR) and will follow (fly) that navigation reference. It’s sort of like an automated car in that it follows the navigator’s input and the pilot monitors.
  • Altitude Hold: Again, in addition to the above, a desired altitude can be set and the aircraft will fly at that altitude. Some autopilots have the capability for the pilot to select a desired altitude and a climb or descent rate and the aircraft will automatically climb or descend to that altitude and then hold the altitude.
  • Instrument Approaches: Autopilots with this capability will fly preprogrammed instrument approaches to the point where the pilot either takes control and lands or has the autopilot execute a missed approach.

The autopilot is a powerful computer that takes input from either the pilot or a navigation device and essentially does what it is told to do. GPS navigators, for example, can have a full flight plan entered from departure to destination, and the autopilot will follow the navigator’s guidance.

These are the majority of the controls on the autopilot installed in my airplane:

HDG Knob = Heading knob (Used to set the desired heading)

AP = Autopilot (Pressing this turns the autopilot on)

FD = Flight Director (A form of navigational display that the pilot uses)

HDG = Heading (Tells the autopilot to fly the heading set by the Heading Knob)

NAV = Tells the autopilot to follow the input from the selected navigator

APR = Tells the autopilot to fly the chosen approach

ALT = Tells the autopilot to manage the altitude, controlled by the following:

VS = Vertical Speed (Tells the autopilot to climb or descend at the chosen rate)

Nose UP / Nose DN = Sets the climb/descent rate in feet per minute

FLC = Flight Level Change (An easy manual way to set the autopilot)

ALT Knob = Used to enter the desired altitude

This post originally appeared on Quora. Click here to view.

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Big Questions
What's the Difference Between Vanilla and French Vanilla Ice Cream?
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While you’re browsing the ice cream aisle, you may find yourself wondering, “What’s so French about French vanilla?” The name may sound a little fancier than just plain ol’ “vanilla,” but it has nothing to do with the origin of the vanilla itself. (Vanilla is a tropical plant that grows near the equator.)

The difference comes down to eggs, as The Kitchn explains. You may have already noticed that French vanilla ice cream tends to have a slightly yellow coloring, while plain vanilla ice cream is more white. That’s because the base of French vanilla ice cream has egg yolks added to it.

The eggs give French vanilla ice cream both a smoother consistency and that subtle yellow color. The taste is a little richer and a little more complex than a regular vanilla, which is made with just milk and cream and is sometimes called “Philadelphia-style vanilla” ice cream.

In an interview with NPR’s All Things Considered in 2010—when Baskin-Robbins decided to eliminate French Vanilla from its ice cream lineup—ice cream industry consultant Bruce Tharp noted that French vanilla ice cream may date back to at least colonial times, when Thomas Jefferson and George Washington both used ice cream recipes that included egg yolks.

Jefferson likely acquired his taste for ice cream during the time he spent in France, and served it to his White House guests several times. His family’s ice cream recipe—which calls for six egg yolks per quart of cream—seems to have originated with his French butler.

But everyone already knew to trust the French with their dairy products, right?

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