Before we can figure out why yawns are so contagious, we have to deal with a deeper question…

Why the heck do we yawn in the first place?

Scientists don’t know for sure. For a long time, the leading theory was that yawns pull in extra oxygen, although that’s a bit unsatisfactory. Normal breathing seems to do a pretty good job of keeping the blood oxygenated, after all.

A 2014 study seemed to consolidate another leading theory, pushing it to the front of the pack. The study was published in the journal Physiology and Behavior, and it makes a pretty convincing case that yawning is the body’s method of cooling down the brain.

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Or, to be way less clear, as the study says, “The thermoregulatory theory of yawning posits that yawns function to cool the brain in part due to counter-current heat exchange with the deep inhalation of ambient air.”

Researchers figured that, if yawns really are just a way to cool down an overheated brain, we wouldn’t yawn as much in colder environments. The results of the study seemed to bear this out.

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“The proportion of [subjects] reported yawning was significantly lower during winter than in summer,” the study reports. It concludes that “the underlying mechanism for yawning in humans, both spontaneous and contagious, appears to be involved in brain thermoregulation.”

Okay, weird, but say that’s true and we do yawn to keep our brains cool.

Why would that make yawns so contagious?

Most scientists lean on evolutionary theory to explain this phenomenon. First of all, humans aren’t the only animals that “catch” the yawns from one another. Scientists have also observed this behavior in baboons, bonobos, chimps, and even wolves and dogs. These are all social animals, just like humans.

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Plus, according to a report in the journal NeuroImage, fMRI studies show the amygdala lighting up when subjects see images of other people yawning. The amygdala handles that whole fight-or-flight response. So maybe, the thinking goes, yawn contagion is an evolutionary phenomenon that prepares entire communities for action, literally allowing cooler heads to prevail.

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Huffington Post

(Actually, there’s an interesting paradox about that idiom. The amygdala overrides logic and reason with emergency responses; if yawning is indeed part of the brain’s preparation for battle or retreat, and if yawns really exist to keep our brains cool, it might be the “hotter heads” that we want to prevail.)

Of course, there’s a simpler evolutionary explanation for contagious yawning. We tend to yawn when we’re sleepy; we tend to only get sleepy when we feel safe.

Maybe group yawning is a tool of social bonding.

The group that yawns together isn’t worried about eating one another.

There’s also a third theory making the rounds these days. A 2017 study from the journal Adaptive Human Behavior and Physiology questions whether yawns are actually contagious in the first place. Researcher Rohan Kapitány explained his reasoning in an interview with PsyPost:

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Reaching Utopia

“The belief that yawns are contagious seems self-evident, but there are some very basic reasons for why we might be mistaken in this. If we fail to dissect that which we think we know, we might end up with conclusions that do not reflect reality. In this instance, the literature hasn’t questioned the basic features of contagious yawning, and ended up with a wide range of unstandardised methodologies and conclusions.”

This particular study failed to uncover a causal relationship between one yawner and the next. That is, it left the door open for the fact that yawns aren’t even contagious in the first place. It’ll take a lot more research to prove that surprising theory, but whatever the scientists come up with, you won’t catch us yawning over the research.