Did you know? Why yawning is contagious, and why scientists still argue about it

Fair warning: this article might make you yawn. Not because it's boring, but because that's just how your brain works.

Yawning is one of the most "catching" things humans do. See someone yawn, and you follow. Hear someone yawn, and you follow. Even reading about yawning can do it. So why? The honest answer: scientists still don't agree.

Even reading about yawning is contagious

Psychologist Robert Provine put this to a proper test. In one experiment, people spent 5 minutes reading an article about yawning:

• 30% of those reading about yawning actually yawned, compared with 11% of those reading about hiccups.
• Count the people who felt the urge but held it in, and the figure jumps to 75%.

In a video test, 55% of viewers yawned while watching people yawn, compared with 21% while watching people smile.

Stranger still, Provine found that yawning faces with the mouth edited out were just as good at triggering yawns. So covering your mouth is good manners, but it won't necessarily stop the person next to you from joining in.

A typical yawn lasts about 6 seconds. If you've yawned since you started reading, yes, that was one.

Theory 1: A sign of empathy and social bonds

The most popular theory links contagious yawning to empathy, our ability to "feel" what others feel.

A study by Ivan Norscia and Elisabetta Palagi (PLoS ONE, 2011) observed more than 100 adults in everyday settings in Italy and Madagascar, such as on trains and at dinner tables. Yawns spread most easily between family members, then friends, then acquaintances, and least between strangers.

Not everyone is convinced. Researcher Andrew Gallup, who wasn't involved in the study, raised another possibility: maybe we simply notice loved ones yawning more, rather than feeling more empathy.

Then came a bigger study from Duke University (PLOS ONE, 2014). A total of 328 volunteers watched a 3-minute video of people yawning:

• 222 people (about 68%) yawned at least once.
• Some didn't yawn at all, while others yawned up to 15 times.
• Empathy scores showed no strong link with contagious yawning.
• The only clear factor was age: older people were less likely to catch yawns. Even so, age explained only 8% of the differences.

Researcher Elizabeth Cirulli said most of the variation between people was "just not explained." So the empathy theory is still alive, but it isn't the final word.

Theory 2: Mirror neurons (still debated)

Some scientists link contagious yawning to mirror neurons, brain cells that fire both when we do something and when we watch someone else do it.

An fMRI brain-scan study by Haker and colleagues (2013) found that an area at the front of the right side of the brain lit up when people watched videos of yawning. They saw this as support for a role of the mirror neuron system.

Provine is more skeptical. In his view, mirror neurons may fall short of explaining why we actually perform the yawn. Other work by Platek and colleagues found activity in brain areas tied to thinking about the self. In short, which brain areas are "responsible" is still up for debate.

Did you know? Babies start yawning as early as the 12th week in the womb. But contagious yawning only shows up several years after birth.

Theory 3: Yawning to cool the brain

This theory comes from Andrew Gallup and Gordon Gallup Jr. (2007). Their idea is that yawning helps cool the brain, and contagious yawning may help a group stay alert together.

Their evidence is intriguing:

• People breathing normally or through the mouth: 50% yawned while watching yawning videos.
• People told to breathe through the nose: 0%, no yawns at all.
• People holding a cold pack to the forehead also didn't yawn, unlike those with warm or room-temperature packs.

A follow-up study (Physiology & Behavior, 2014) compared pedestrians in Vienna, Austria and Arizona, USA. People in Vienna caught more yawns in summer, while people in Arizona caught more in winter. Contagious yawning peaked around 20°C and dropped off when it was very hot (about 37°C) or close to freezing.

In 2016, Gallup's team analysed yawns from 109 individuals across 19 species. Animals with bigger brains yawned longer. Humans averaged 6.5 seconds, mice just 0.8 seconds. Still, Gallup himself admits that whether yawning cools the brain "can still be debated", and some critics argue a yawn isn't strong enough to cool the brain at all.

Chimps and dogs catch yawns too

It's not just humans. Matthew Campbell and Frans de Waal of the Yerkes National Primate Research Center showed 9-second videos to 23 chimpanzees. The chimps yawned 50% more often when they saw members of their own group yawn than when they saw outsiders. That "in-group" pattern mirrors the human studies above.

Dogs are even more surprising, because they can catch yawns from humans. In a Biology Letters study (2008), 21 of 29 dogs (72%) yawned after watching a researcher yawn. When the researcher just opened and closed their mouth without yawning, not a single dog followed.

The study's authors cautioned that other explanations couldn't be ruled out, for example dogs yawning because they felt stressed.

So which answer is right?

So far, no single theory has won. Here's what scientists do know:

• Contagious yawning is real and easy to trigger, to the point that reading is enough.
• It's stronger between people who are close, but the link to empathy is still questioned.
• Temperature and breathing seem to play a part.
• Chimps and dogs show the same effect.

The answer may be a mix of several theories. One thing is certain: if you yawned a few times during this article, you're not alone.

Who's the quickest to make you yawn: a friend, family, or the cat at home?