You might not notice a smell when you walk into your home.

But if you visit a stranger’s house, you’ll probably detect a lot more funk in the air. That’s because their microbes stink!

At least, they probably stink in a different way from your own. People have different kinds of bacteria crawling all over them, and those determine how you smell. But the weird thing is, research shows that those scents can influence who you’re friends with.

Chances are, your best friend probably smells a lot like you. And here at SciShow, we’ve sniffed out the reasons why. [intro music] We all have BO. It’s just that some of us have more than others.

A quality that makes some researchers describe you as “intensely odiferous.” That natural variation in BO comes from the number and type of bacteria living around your apocrine sweat glands. See, your sweat doesn’t actually smell like anything. It’s the bacteria near it that make a big stink.

When your sweat comes out of the glands in your armpits, the microscopic residents of Pitt City, like staph, eat it. Yeah, your sweat is like a delicious fruit smoothie for armpit bacteria. And by the time they’re through with it … let’s just say it’s not odorless anymore.

Staph microbes turn sweat into sulfurous thioalcohol. And we know from rotten eggs and volcanoes that sulfurous stuff has a stench to it. So your BO comes from the bacteria in your armpits.

But not just your armpits. A similar process happens on other parts of your body, too. And because you might have different microbes living in other places, they smell a little different.

The ones on your feet turn lactate from your sweat into diacetyl. This adds a certain acidy smell, and might be why your feet smell like a stinky cheese. Your genes determine which microbes can live alongside you. genes create an immune system that allows some bacteria to thrive on your particular fleshy meat sack and not others.

And to prove that this creates a unique smell, researchers in the UK and Austria gathered a bunch of identical and fraternal twins and sniffed them … for science! Specifically, both kinds of twins were people who didn’t live together. So the identical twins had genes in common, but didn’t co-foster a dog or anything like that that would make them smell like each other.

The researchers had the participants put cotton pads under their armpits for a night. Then they recruited people to sniff five of the cotton pad samples and asked them to identify the identical twins. And the sniffers were pretty good at it!

I mean, only 42 of the 113 successfully correctly identified the twins with the same DNA, but that’s still more than would have randomly put them together. To those participants, the identical twins literally smelled like identical samples. Like, sometimes the sniffers were given two odor samples from the same person among the five options, and they paired those together at roughly the same rate that they paired the identical twins together.

So the identical twins smelled similar. But the fraternal twins? Not so much.

And that means other factors like being raised together probably aren’t enough to make identical twins smell as similar as they do. At the end of the day, your microbial community is a major contributor to your personal aroma, deodorants and perfumes aside. And you probably like that smell.

In 1980, an experimenter in what was then West Germany asked participants to judge their own odor against other people’s musk. And the result was often “predominantly pleasant.” Many of the participants liked their own smells and were indifferent to other people’s smells. Meaning that, yeah, we do prefer our own brand.

But apparently, that goes for more than just our own aroma. Your friends probably have a similar scent. A study headed by Cornell University recruited high school students to participate in some research on human recognition of odor.

But it wasn’t just them. The participants were also asked to bring a friend to the study. And I know that sounds like the beginning of a questionable tupperware party, but there really was a scientific reason for it.

The purpose was to figure out how smells help us recognize both ourselves and our friends. It even showed how they contribute to which friends we choose in the first place. The study found that we seem to have a predisposition for friends that we think smell like us.

After smelling t-shirts belonging to themselves, their friend, and some random people, some of the participants even thought their shirt was their friend’s. And a more recent study published in 2022 confirmed that people who smell like each other are better friends. Here’s how they figured it out: They recruited pairs of “fast friends” who just clicked right off the bat.

They literally went on social media to find people who mutually described their origin story that way. Then the researchers sussed out their scents using both the human sense of smell and electronic noses. An eNose is a device that can detect certain chemicals and show how they combine to make specific scents.

It’s a high tech way to identify smells that the plain old human noses were sniffing in the same study. The sniffers had to smell 40 pairs of odors. 20 of them were fast friends, and the other 20 were randomly paired together. And both the human and electronic noses found that the fast friends smelled more like each other than the random pairs.

But, just like in the twin study, they had to account for the influence of spending a lot of time together. So to get rid of that factor, they also studied strangers who had chemistry. They put two people who had never met before in a room together.

After their interaction, the researchers asked them to rate how much they had in common, how pleasant the interaction was, and if they felt like they clicked. The pairs who clicked with each other also smelled like each other. Now, this isn’t hard and fast evidence that you have to smell like your buddy or else you’re not really friends.

Some people just don’t have a smell preference. But it does provide a great excuse when you don’t click with someone and don’t want to see them again. You can just tell them, “it’s not you, it’s your bacteria.” Thanks for watching this episode of SciShow!

If you think it’s weird that you probably smell like your best friend, wait until you learn about why many people like the smell of gasoline. And grass. And old books.

We made a big compilation of SciShow videos on scents. Follow your nose to some fascinating science on our favorite odors. Or, you know, just click on the link down in the description [ OUTRO MUSIC ]