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If it hasn't been said to you, you've probably heard it on TV or in a movie. Stay calm around strange dogs because they can smell fear and they might attack, but until recently, scientists hadn't looked to see if that was true.

Now we know that they can smell fear, but when they do, they don't get aggressive. They actually get scared, too. Dogs have amazing noses.

Their sense of smell is 10 to 100 thousand times better than ours, which is why they can help us detect traces of chemicals from bombs, drugs and even people buried under the snow. That's thanks to the sheer number of olfactory receptors they have. Up to three hundred million, compared to our measly six million, and to the fact that, relative to their size, the part of their brain that analyzes smells is about 40 times the size of ours.

But even so, dogs don't rely entirely on their sniffers. When they want to communicate with each other or with us, they usually can use body language and vocalizations as well. When we tell them they're such a good doggo, they process the tone of our voice and the words that we're speaking in different brain areas, just like we do when other humans talk.

Which means they do understand what we're saying to some extent, and we know that dogs are good at picking up on physical signals. In fact, studies have shown that they can differentiate between our emotions based off of our faces and body language. So for a while most scientists and dog trainers figured that dogs don't need to sniff out our feelings.

They can see it in our posture and hear it in our voice. It wasn't until recently that researchers looked directly into whether dogs can smell fear. When we're afraid, the sweat we produce contains different chemicals than when we're happy or sad, because the composition of our blood changes in response to hormonal signals.

So scientists can stick swabs in our armpits while inducing different emotions, and then see how dogs react to those odors. And they did this in a 2016 study published in Behavioural Brain Research, and then found that dogs' heart rates increased when they sniff sweat from people who watched a scary video, suggesting that they can smell fear, but the animals had the same reaction to sweat from people who worked out, so it wasn't clear if they were just sensing physical arousal. Then a study in Animal Cognition in 2017 did a similar experiment.

They had dogs sniff pads with sweat from people who watched scary or happy videos or unused pads, but this time the researchers watched the dogs' behavior afterwards and looked at heart rate. The dogs that smelled the happy sweat were friendlier to strangers. Their heart rates were generally higher than the control's, but lower than the dogs that had sniffed the fearful sweat.

Meanwhile, the dogs that smelled fear seemed stressed out, seeking more reassurance from their owners, and acting more wary of new people. And like the previous study, their heart rates went up. So the researchers concluded that the dogs were detecting emotions, and reacting by feeling a similar way.

Sniffing joy put them in a better mood, and the smell of fear didn't make them more aggressive—it scared them. So it seems like dogs can potentially smell fear and know how you're feeling, but rest assured the feeling will probably be mutual. Thank you for asking, and thank you especially to our Patreon President of Space SR Foxley.

Thank you, SR, for helping make SciShow possible. If you want to support this show head on over to patreon.com/scishow. ♪ OUTRO ♪.