But How?

It's caused by a class of chemical signals, unoriginally called chemosignals, that are found in human sweat, tears and possibly other fluids. The extent to which chemosignals affect humans in day-to-day life is still under debate: It's hard to measure this kind of thing accurately, because a lot of the time the influence of chemosignals is subconscious.

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"Wait, why do I keep having flashes of gay pornography?"

That's what's so weird about it -- even though the MRI showed different parts of the women's brains working depending on which sweat they smelled, in every case the subjects claimed they couldn't tell the difference. Yet, when made to guess which was which (fear sweat vs. happy sweat, horny sweat vs. normal sweat), they were able to pick correctly (at least, at a rate better than chance). So it appears that a lot of what you just "sense" about people is nothing more than picking up chemicals in their fluids.

And make no mistake, these psychic nose-messages do affect us: Sweat collected from men about to go skydiving was shown to activate the "fear" sections in brains of people exposed to it. Women exposed to male fear-sweat also rated neutral faces as more "fearful" than when they were sniffing sweat unassociated with terror.

And psychotic faces to be more "murdery."

And get this -- the military is funding research into automated emotion detection systems that use chemosignals to detect suspiciously anxious people in public places. To picture the future, imagine a robot nose sticking into a human armpit, forever.

It's no secret that the neighborhood BBQ joint is relying on the smell of cooking meat to get you in the door, and that millions of new cars are sold every year purely because of the spell their scent casts over us. But your nose can make you buy things in endless, less obvious ways. Retailers call it "environmental fragrancing," and it really is nothing less than an attempt to brainwash you through your nostrils.

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"We sell a lot more HDTVs since management started piping in the smell of strippers and hot wings."

So, in the swimwear aisle, they'll give you a faint coconut smell. In the infant aisle, you'll smell baby powder. Big deal, right? Well, one experiment showed sales of men's and women's clothing nearly doubled when "masculine" and "feminine" scents were used nearby, an effect that disappeared when the smells were reversed.

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"I dunno, man, I smelled potpourri and just blacked out."

Another study in a Las Vegas casino found that areas piped with pleasant smells made 45 percent more money than those without them, although that might have just been all the gamblers moving over to a location that best masked the smell of desperation and old people.

But How?

As you'll find is a running theme with this article, your nose works on a subconscious level in a way that your eyes and ears don't. It has to do with the way your brain is wired -- think of your sense of smell as being able to avoid the filters that your other senses go through when you process them.

We can smell your soul, Devil Clown.

If you see a bear in the woods, that sight goes through all sorts of pathways in your brain that handle things like logic and language. You actually think, in your head, "That's a bear. It's dangerous. What's that they say about bears? That you should play dead? Or is it like a shark, where you're supposed to punch it in the nose?"

But if you are in the wild and you merely smell a bear, or the cologne of a crazy ex-boyfriend, it's a more physical reaction -- you'll feel it before you think it. That signal goes directly to the limbic system, the primitive part of the brain generally associated with emotions. This is why we can see a painting and immediately describe it in words: it's "colorful" or "religious" or "Wait, did you steal this from the Louvre?" But give us the smell of an orange, and the extent of our description will be: "It smells like an orange."

Via Wikipedia
Napalm can smell like victory, but only before noon.

So, if consumers can hear a few notes and instantly think of a particular brand of cat food, imagine what hitting someone with a smell can do. And putting scent in the store is just the beginning -- a lot of brands are now developing their own "brand scent," which is then infused into stores and products. In the past, your new car might have had a new car smell because it was made out of new plastic, glass and leather. Today, it's just as likely to have been infused with a new, improved new car smell, like Cadillac has been putting in its vehicles for almost a decade.

And they're doing it with everything -- the rubber in the handle of your razor may also have been infused with a scent, which is presumably what they decided to start doing when they ran out of room to cram more blades in there.

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"Honey, did you put my razor in your ass?"

You ever get ambushed by a memory? Like you're just sitting there, watching Magnum, P.I. reruns, and out of the blue you suddenly, vividly remember some completely unrelated event, like the day you spent at the zoo when you were 9, or something horrible that happened to you in kindergarten?

You didn't know it, but you smelled something that was connected to the memory. You can even use this to your advantage, strangely enough. If you smell a certain scent when you're studying for an exam, like say you are baking a pie while you read your notes, and then bring the smell back on exam day, like if you bring the pie with you, science says your score will improve.

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Mmmmm ... fractal geometry.

And even if your instructor forbids pie in the classroom, your nose can still help you out: Being exposed to a particular smell while learning during the day, and then again while in a deep sleep that night, can improve your memory the next day.

But How?

The part of your brain that processes smell, the olfactory bulb, has strong physical links to the hippocampus and amygdala, brain areas that are heavily involved in memory formation and encoding. This close relationship means that while visual memories tend to decay quickly -- you'll lose 50 percent of them within months -- the majority of new smell memories will still be there a year later. So you can improve visual memories (like the ones you gained by reading) by associating them with a scent, like less attractive people latching onto good-looking friends in order to skip the line at a nightclub.

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"Do NOT let the nose in. Last time he snorted all of our whiskey."

The trick works best if the smell you're latching onto is appropriate -- studies showed that people remembered book passages better when they read them in the presence of scents that "matched up" to the reading topic. So, the fragrance of blueberry pancakes might not be the best choice when you're studying for that History of Genocide test in the morning. And as for smell helping you when you sleep, we've said before that the brain uses deep sleep to encode memories that formed during the day, and it seems like this works even better when the brain is helped along by a nose-based reminder.

So, before dozing off on the night before the History of Genocide exam, instruct your roommate to fill the apartment with "genocide smells." Don't be any more specific than that -- the fun part is seeing what he comes up with.

But How?

It's down to something called olfactory conditioning: You pair up a smell with something that causes a reaction, and later on that smell will cause a repeat of that reaction, even if the original cause is gone. And we're not talking about obvious things, like the way Rambo freaks out at the smell of napalm. You can actually change the blood glucose levels of diabetics by introducing a smell that you've paired up with giving them insulin in the past.

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Like the smell of sugar if it's pressed into their nose, and they sniff really hard.

As for the feelings of anxiety, for whatever evolutionary reason, we are a lot better at detecting and remembering smells when we're in situations that feel dangerous, stressful or emotionally charged. Maybe it helped us remember the smell of bears.

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Fun fact: An angry grizzly smells a lot like you pooping yourself.

But the connection between smell and emotion gets stranger still. Your sense of smell can actually ...

There are many things generally accepted as risk factors for depression: genetic predisposition, bad diet, lack of exercise, living in Oregon. But there's another depression predictor that's less well-known: how well you can smell things.

You would think that people who develop anosmia, or lack of smell, get off relatively easy in the big scheme of things. At least visiting public toilets must be easier, and that pretty much cancels out the bad stuff, right? But anosmic people have a high risk of severe depression, and it's not just because they're sad about losing one of their five senses. It's much weirder than that.

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It's because they have horse souls.

Studies of depressed people have shown that they tend to have a reduced sense of smell and a smaller olfactory bulb. In fact, one of the easiest ways to bring about depression in lab rats (in order to say, test antidepressant drugs on them, or just because you really hate rats) is to remove their olfactory bulb. De-smelled rats immediately become lethargic, listless and uninterested in rewards or grooming, and they start listening to Morrissey on their teeny tiny little rat headphones.

And if that doesn't have you nervously sniffing the air to find out whether you'll want to get out of bed tomorrow, a bad sense of smell and small olfactory bulb are also correlated with schizophrenia.

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Don't tell him, but that painting is made out of ketchup.

But How?

That is how strongly your moods are tied to your sense of smell. Scientists don't perfectly understand the link between sense of smell and mental illness, but we know that depression and schizophrenia both involve persistently flat moods. Apparently if one link in that chain that connects smell and emotions breaks, the whole thing falls apart. Or it could work in the other direction -- if your sense of smell disappears after a lifetime of smell/emotion connection, and all of these subconscious emotional triggers disappear along with it, the flat numbness that results could bring about the depression.

Whatever the connection, it's so strong that scientists are now making "scratch and sniff" tests to predict people's future risks for schizophrenia, obsessive-compulsive disorder and other illnesses. No, really.

"No, ham! I meant ham!"