Why Do We Yawn?


No one knows why we yawn. There are lots of theories, some talk about it signalling tiredness or getting oxygen to the brain, others mention clearing out stale air from the lungs and reducing blood carbon dioxide levels. Most are baloney. But, one thing that is certain, yawns can be infectious. Catch sight of someone yawning, and nine times out of ten, you will yawn yourself within a few seconds. But, that still doesn’t really answer the question, why do we yawn? Is such an infectious yawn a message to others in the group that it’s time for bed? Probably not, otherwise why yawn first thing on getting out of bed? Either way researchers have found that people with autism spectrum disorder don’t tend to succumb to an infectious yawn.

Atsushi Senju of the Centre for Brain and Cognitive Development at Birkbeck, University of London has shown for the first time that children with some degree of autism are not susceptible to contagious yawning. Autism Spectrum Disorder is a developmental disability that severely affects social interaction and communication including empathy. Report published in the August issue of the Royal Society journal Biology Letters.

This would seem an obvious result given that contagious yawning is thought to share similar cognitive and neural mechanisms as empathy.

Senju and colleagues from the University of Tokyo showed videos of people yawning or making mouth movements to 24 children with autism spectrum disorder and to 25 non-ASD children. Both groups of children yawned the same number of times while watching the video of general mouth movements, but the non-ASD children yawned more when watching the video of people yawning.

“This is the first report that a neuropsychological or psychiatric condition can selectively impair contagious yawning, sparing spontaneous yawning,” explains Senju, “Our study confirms the prediction of ‘empathy theory’, by demonstrating that individuals with autism, who show atypical developments in empathy, also show selective impairment in contagious yawning.”

None of this answers the question of why do we yawn in the first place? Apparently, yawning becomes contagious at around one to two years of age, although unborn fetuses also yawn (presumably not contagiously though!) and can be triggered in animals by stimulating the hypothalamus in the brain with injected dopamine, excitatory amino acids, nitric oxide, and neuropeptides. None of this really explains why we yawn. The empathy angle perhaps points to an ancient benefit in group behaviour, but what that benefit is, science does not yet know.

For more on simple experiments and the power of yawn, check out the neuroscience for kids page at Washington U.

By the way, did you notice while reading this whether you yawned? Hopefully, it was not merely boredom that did it…


30 thoughts on “Why Do We Yawn?

  1. Unfortunately Dennis, yawning is much more complicated than that. People yawn after yawning, people yawn even when their lungs are fully inflated, people yawn when they see other people yawn. There are confounding factors in making an attempt at a definitive statement such as yours. You might think you have the answer, but don’t you think others have already suggested and tested your hypothesis many times?

    Indeed, people yawn while exercising even though they may be pounding a treadmill, breathing heavily and get lots of oxygen into fully expanded lungs.

  2. Comment sent by email from Dennis Regan:

    “A Yawn is a Stretch of the muscles around the lungs. The same way that we stretch when we wake up and get out of bed, our lung muscles do the same thing. From the shallow breathing that we do while sleeping or getting tired our lung muscles feel the need to stretch and a yawn is an uncontrollable stretch. Have you ever tried to stop a yawn from happening? It is impossible. The lungs feel the need to stretch and they do it whether we like it or not!

    And there is your answer. Hope you and your readers find it fun and informational.”

  3. @Absta “We” vomit when “we” see someone else vomit? Speak for yourself. Not everyone reflexively vomits at the site or smell of another person’s vomit. Certainly vomiting is not anywhere near as contagious as yawning. But, what’s the “danger” that reflexive yawning is protecting us from, I’d be interested to know…

  4. We yawn when we see someone else yawn just like we vomit when we smell/see someone else’s vomit. It’s most likely a ‘just in case’ response that our body does to protect itself from possible dangers.

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