Hank: In the late 19th century, American entomologist Joseph Lintner discovered a small army of male silk moths crowding a female outside his window. This was … not something Lintner was used to seeing, and he guessed that the frenzy of male attention had to do with some kind of chemical perfume the female was emitting.

He was right, and although there wasn’t yet a name for it yet, Lintner was watching the actions of pheromones -- chemical signals that help some species communicate. Since then, scientists have found that lots of living things -- especially social insects -- use pheromones to some degree. But there’s at least one species that continues to puzzle pheromone researchers... Humans.

I don’t want to dash anyone’s hopes of chemically attracting a Valentine this year, but I’m just going to put this out there: so far no one has definitively identified a human pheromone. Though that hasn’t stopped loads of companies from marketing so-called pheromone perfumes -- what they say is like, sexual mojo in a bottle. Of course, just because we haven’t found any doesn’t mean they can’t exist. But even if they do, they might not hold that much sway over our behavior.

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Pheromones are made of all different kinds of molecules, and they’re usually detected by smell. Depending on the species, they might travel through air or water, or just be deposited straight onto the recipient. The goal is to prompt some kind of behavioral or psychological reaction. From observing dogs in heat to angry swarms of bees, humans have long suspected that animals were capable of quietly communicating certain messages like “hey, let’s mate”, or “help, I’m dying,” or “danger, danger!”

But it wasn’t until 1959 that technology finally allowed Nobel-winning German chemist Adolf Butenandt and his associates to separate and chemically identify the first official pheromone -- called bombykol -- which is released by female silkworm moths to attract mates. Butenandt and his crew showed that exposure to a mature lady moth induced a particular behavior in a receiving male moth -- in this case, a whole lot of excited wing fluttering. They then isolated and synthesized bombykol molecules and showed that they had exactly the same effect on males.

That’s more or less how new pheromones are identified today -- by pinpointing specific behaviors and then nailing down the exact molecules causing them. But before we get into whether humans might have them, and whether it even matters, let’s talk about how they work in other animals -- and even plants. We tend to think of pheromones as aphrodisiacs, but the truth is, different species actually use them to silently communicate all sorts of different things. Though, yeah, a lot of them do boil down to sex, like the androstenone in boar saliva that makes fertile sows ah, assume the mating position when they catch a whiff.

But other pheromones induce aggression or alarm, advertise territorial boundaries, promote parent-offspring bonding, or just generally keep social behaviors in check. Pheromones fall into two broad categories -- releaser pheromones are fast-acting, and produce short-term behavioral changes, like repelling or attracting other individuals. Primer pheromones, on the other hand, work at a slower pace, and cause longer-lasting hormonal changes in behavior or development.

But while pheromones are supposed to aid in communication within a species, sometimes they can backfire and draw the attention of a different species. If this attention damages the signaling animal in some way -- say by attracting predators or warning prey, that chemical is known as a kairomone. This might happen, for instance, when army ants -- a highly social species -- lay down a pheromone chemical trail so that wandering ants can find their way home. But sometimes, that chemical trail also attracts a snake and leads it right to their door.

Other kairomones seem to alert prey to the presence of predators, causing them to scatter, or prepare to defend themselves. And other times, one species might actually mimic another species’ pheromones in a kind of deadly romantic trickery -- the way female American bolas spiders imitate the love scent of a certain kind of moth to lure randy males into their sticky ball-like webs.

Some plants use this kind of deception, too. Not only do some orchid flowers look like bees, but they’re also able to mimic female bee sex pheromones to attract males. Then, as the males try to get it on with the flowers, they inadvertently pick up and transport pollen. Which is something we humans can use to our advantage!

For years, entomologists have used pheromones to control agricultural pest insects -- for example, releasing synthetic female hormones of certain moth species can confuse males and disrupt the mating process, which helps control the population. Pheromones may even help save human lives -- some researchers are looking at using these signaling chemicals to disrupt the life cycle of a nasty little parasitic nematode that spreads disease in humans and other species.

So pheromones may help us humans protect crops or fight parasites, and yeah, that’s all pretty cool, but by now you’re probably wondering: if they can affect other animals so much, can pheromones score you a date? Well... no, probably not, but to be honest, we’re not 100 percent sure. Turns out the idea of human pheromones is a very tricky business. It’s likely that we do have pheromones, just because we’re related to so many other mammals that do. But, remember, no one has specifically identified a human pheromone molecule.

There are a few well-known studies that seem to suggest we have them, like a famous 1994 t-shirt sniffing experiment that suggested women preferred the scent of men with different immune system genetics than their own. There have also been lab trials involving androstenone, that male pig pheromone that we just talked about that makes the lady pigs go wild and also happens to be found in human sweat. But these kinds of studies have had lots of problems -- they often involve small sample sizes, poor experimental design, or just results that can’t be replicated.

In fact, some researchers think the field of human pheromone work has become so controversial, even sensational, that we should scrap all past studies and basically start over from scratch. And there’s another problem with all this: we might not even have the right parts to detect pheromones in the first place. All vertebrates have a main olfactory system in some form of a nose, but many animals, including snakes, lizards, and rodents, also possess a special extra sniffing organ called the vomeronasal organ, or VNO, that they use to detect pheromones. But humans probably don’t have one -- at least, not one that works. We might start out with a VNO as fetuses, but most biologists agree that it disappears during fetal development. And even though some adults seem to have a kind of leftover vomeronasal cavity, it doesn’t have sensory neurons and probably doesn’t work.

Still, in other animals, both kinds of sniffers ultimately send their signals to the same place -- the amygdala, a major memory and motivation hub in the brain. And in animals that do have a VNO, we see a lot of overlap in pheromone and smell inputs. And some species actually do use their main noses to detect pheromones. So just because we humans and other primates don’t have a functional VNO doesn’t mean we don’t have pheromones. We certainly do send some messages via smell -- like, human newborns know the scent of their mother’s breasts from across the room. Humans and other mammals give off a Pigpen-style cloud of personal molecules that give us unique scent profiles. Which is why you might be able to tell who someone is just by their smell. But smells aren’t the same thing as pheromones.

Still -- even if they aren’t technically pheromones, these smells might act in a similar way by influencing our mating choices. A particular blend of things like secretions, bacteria, immune system genetics, food particles, might make you more or less attractive to me. But the attractiveness of body odor also involves a lot of nuance, and how we feel about a smell often depends on our previous experiences with it.

That’s because the receptor cells in our nasal cavities shoot information to our brain’s main olfactory bulb, which relays it to the parts of the limbic system, right next to the memory-making hippocampus and emotion-inducing amygdala. Which means: scent is also rooted in memory. You might suddenly feel happy when you smell pancakes because that’s what your family made on lazy Sunday mornings growing up, or feel attracted to the scent of sweat and sawdust because your sweetheart is a sexy lumberjack. In other words, we often respond to scents because of their context, not necessarily because of something about the molecules they’re made of. Which is what makes them different from pheromones, when it is all about the molecules.

Plus, humans are all about conscious choice. Like, I don’t care if you smell like the best thing on earth, if I watch you kick a kitten I’m not going to feel any attraction to you. And losing my sense of smell it would suck, but unlike for some animals that use pheromones, it wouldn’t exactly destroy my life cycle, or limit my social behavior too much. I would still have control over my choices. So if pheromones do influence our behavior, the effects are likely much more subtle. This fact, combined with the knowledge that humans are also greatly influenced by sight, sound, memory, learning, context, and social norms, is yet another reason why the potential existence and influence of human pheromones remains a bit of a mystery.

So, what’s the next step in figuring out if we have them? Well, so far most human pheromone work has focused on the sex end of things, but as you might’ve noticed, humans are really complicated -- especially adult humans. That’s why French researchers are looking instead at babies, and how chemicals secreted by nursing mothers affect sucking and mouth motion behavior in newborns. If a molecule from that secretion can be identified, synthesized, and shown to induce sucking and rooting behavior in any baby, then we may have our first official human pheromone. Either way, in the end, if you’re looking to attract a date, I’d save money on anything marked as a pheromone boost and just work on polishing up your conversation skills.

But in the meantime, we here at SciShow may be able to help you on the dating front with these awesome Valentine’s Day cards! Though they probably won’t arrive at your house by Valentine’s Day if you order them now. You are kind of too late. But, Valentine’s Day in our hearts is all year-round, and then you can have them for next year, too. We made a thing, and I hope that you like them. Your fellow nerdy friends might appreciate them. If you want to keep getting smarter with us, you can to go to youtube.com/scishow and subscribe!

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