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Let's say it's getting late, you're studying with a group of friends around a table, and you see one of them stretch and yawn. You don't feel especially tired, but soon enough — without even realizing it — you yawn too.

Then, the yawn spreads around the table, with more and more of your friends picking it up unconsciously. This sort of contagion can happen with other things, like sniffing or laughter, and it has baffled psychologists for a while. Most scientists agree that these so-called echophenomena exist, but there are a couple different hypotheses about why they happen.

Several clues come from studies looking at what kinds of animals and people actually catch * yawns. We've only seen contagious yawning in certain species, like humans, chimpanzees, baboons, and, in response to us, dogs. These are all very social animals — creatures whose survival depends on working together and being sensitive to each other's needs.

And that might not be a coincidence. One idea is that contagious yawning is related to empathy, or the ability to put yourself into someone else's shoes. Various studies have repeatedly shown that people who have impaired social skills, including those with autism and schizophrenia, are less likely to yawn sympathetically than their peers. And kids below the age of four are also much less susceptible to contagious yawning than older children or adults.

Psychologists also point to experiments that find yawning is more contagious if the original yawner is someone close to you. Both chimpanzees and humans are more likely to yawn if they see someone familiar yawn, versus a stranger. Since we tend to be more empathetic to members of our own group, this bias suggests that contagious yawning could be rooted in empathy.

That being said, a more recent study in autistic children found that their inability to catch a yawn might be because they're not looking at people's faces, rather than a lack of empathy. In two sets of experiments with about two dozen children with autism, about 30% yawned contagiously — the same rate as developmentally normal children — if they were told to count the beards or glasses of the people yawning. In other words, if the kids were directed to look at the faces, they became just as likely to yawn.

But for scientists who think that empathy plays at least some part in contagious yawning, one hypothesis is that mirror neurons are involved in the brain. These neurons form networks and are thought to fire if a person is doing a task or watching someone else do the same task. This way, the thinking goes, the neurons “mirror” those actions and can help with mimicry, and maybe empathy.

In support of this, a handful of small fMRI studies have found that mirror neurons light up when volunteers watch videos or listen to sound recordings of other people yawning. But as appealing as this idea is, some scientists aren't so sure that mirror neurons are responsible. Other fMRI studies of contagious yawning haven't seen any mirror neuron activation.

And neuroscientists are still debating the function and location of mirror neurons in the human brain. A different way scientists explain contagious yawning is simply that motor areas of the brain, like the ones that control your facial movements, are easily excitable. A study published this past summer measured the excitability of motor neurons in three dozen people while they watched videos of others yawning. They used a technique called transcranial magnetic stimulation, or TMS, in which a coil that's generating a magnetic field is passed across the head.

And this magnetic field can be used to stimulate and measure electrical signals in neurons. The researchers found that people with more natural activity in those neurons were more likely to yawn. In fact, differences in neuronal activity explained about half of the variation in whether a person yawned or not.

And when some other scientists tested for correlations between empathy test scores and catching yawns, they also found that different people might just have inherently different susceptibility. In one study of more than 300 adults, researchers showed participants a 3-minute video of other people yawning. How likely a person was to yawn in response didn't have anything to do with how empathetic they were.

Instead, it was more related to age, with younger adults yawning more than middle-aged or older adults. Also, the scientists noticed that each person seemed to have a hardwired level of sensitivity to contagious yawning. Some people yawned a lot over multiple sessions, and others never opened their mouths at all.

These results jive well with the motor neuron excitability idea, although they still leave a lot of questions unanswered, like why we've evolved this reaction in the first place. In any case, for a lot of us, it doesn't take much to yawn if we see someone else do it. There's even a good chance a lot of you automatically yawned just while watching this video.

And if you haven't yet, I'll give you one more reason. Thanks for watching this episode of SciShow Psych! If you want to learn more about this kind of stuff, check out our video where Hank talks about a phenomenon called groupthink, which is basically how your friends can affect your opinions.

And don't forget to go to youtube.com/scishowpsych and subscribe! [♩OUTRO].