What is yawning? And why do we do so much of it? Neuroscientist and yawn expert Robert Provine says it’s “ancient and autonomic.” It stems from early evolution and is common to many creatures—even fish do it. It’s autonomic in the sense that it stems from the brainstem, way down in the basement level of the brain where certain responses are so inbuilt they don’t even qualify as reflexes.

Yawning has many triggers, like boredom, sleepiness, and temperature, for example. A recent study concluded that there’s a “thermal window” (at around 20 degrees Celsius) for human yawning; as ambient temperature approaches body temperature or goes down near freezing, we yawn less. According to the paper, we may yawn to regulate the temperature of our brains. This isn’t the same as saying we yawn to take in extra oxygen, as evidence to date says we don’t. It means that yawning might act to draw brain-soothing ambient air in through the nose and mouth.

Copycat Yawning?

So far, scientists have observed “contagious yawning” in chimpanzees, humans, baboons, bonobos, wolves, and, to a certain extent, dogs and budgies. (Though my wife claims she once made a cat yawn by contagion.) Yawning feels good, so why not join in when someone else yawns? Well, you’re not really “joining in,” because you aren’t copying the yawn on any conscious level. It happens because you just can’t help it. If you become self-conscious about a yawn, it stops.

Studies have shown that seeing pictures or videos of yawning faces can provoke contagious yawning. What we might think of as the main component of a “yawn face”—a wide-open mouth—doesn’t even need to appear in the image for the trigger to work, “yawning eyes” can be enough to get us arching and gaping. If you yawn while reading about yawning, it’s not because you’re “picturing” a yawn. The response is more primal than that.

Some people with autism or schizophrenia don’t exhibit a yawn-contagion response. The same is true of children under the age of four years. This led to a bunch of theories about its relationship to empathy and the brain’s “mirror-neuron system” (MNS). The idea here is that MNS deficits might lead to missing hidden empathetic cues that trigger contagious yawning. The MNS seems to be involved in the process to some extent. fMRI scans on a range of people have shown that other parts of the brain also “light up” in response to images of yawning, perhaps more so than the areas normally associated with empathy.

Yawn and Run

Parts of the amygdala—a brain area associated with fear and heightened attention—light up in response to images of yawning. We yawn at times when we’re nervous, such as before a sporting performance.

So, perhaps we yawn at those times to prepare our brains for “fight or flight.” Maybe contagious yawning is a smart evolutionary shortcut for readying the brains of an entire group of hominins for swift action in response to a threat. (If that’s the case, then some older members would have been left behind, because older people are a little less susceptible to yawn contagion.) We hominins are social mammals; this kind of evolutionary refinement of an existing trait (general purpose yawning becoming contagious yawning) might have helped groups to survive.

Or maybe it’s a lot less deep than that. Laughing also feels good, and it too can be contagious. Like laughter, contagious yawning might help groups to bond—by signaling unselfconscious, relaxed sleepiness. Perhaps it has more to do with feeling safe than with feeling threatened.

Stragglers and Fakes

Contagious yawning is still a bit of a scientific mystery. We love to speculate about it and try to home in on the reason for it. But why should an evolutionary trait have one specific reason behind it? Often, traits survive (are “selected for” by natural selection) because they cover a number of bases. Sometimes, traits are just evolutionary “stragglers” whose original purpose has faded out, and because they don’t hurt anything, there’s no pressure to get rid of them.

One modern adaptation of yawning is not so contagious—fake yawning. You might do this as a less-than-subtle means of signaling that a conversation has dragged on too long. Why not engage in a scientific experiment next time you’re in a meeting with your boss? Lean back in your chair and yawn, then note down whether he yawns right back at you. Maybe there’s a scientific discovery in there, but probably no pay raise.