Why do we yawn? And why are yawns so contagious?
It isn't about oxygen. Here is what experiments say about why we yawn, why yawns spread from face to face, and why even dogs catch ours.
Did you just yawn a little while reading the title? That’s perfectly fine. In fact, it is one of the clues scientists have followed to work out what yawning is for.
About six seconds, and catching
In 1986 psychologist Robert Provine decided to take the humble yawn seriously. He asked people to simply think about yawning and timed what happened. Yawns turned out to be surprisingly regular, lasting about six seconds on average (5.9 seconds). Seeing someone else yawn was a powerful trigger, but thinking about yawning, or just reading about it, set people off too. So if you’re yawning right now, you’re in good company.
Not a gulp of oxygen
A popular explanation says we yawn because the body needs more oxygen or wants to get rid of carbon dioxide. In 1987 Provine’s team tested this directly with college-age volunteers. Breathing pure oxygen did not make people yawn less, and breathing air with extra carbon dioxide did not make them yawn more, even though both changed how fast they breathed. In a second study, exercise that doubled people’s breathing rate did not change their yawning either. The researchers concluded that yawning and breathing seem to be run by separate controls.
A little cool breeze for the brain?
One idea that keeps gathering support is that a yawn helps cool the brain. It is still a hypothesis, but several experiments point the same way.
- Breathing and cold packs (2007). Andrew Gallup and Gordon Gallup showed people videos of others yawning. About 48% yawned along when breathing normally or through the mouth, but nobody did while breathing only through the nose. With a cold pack held to the forehead, contagious yawning dropped to 9%, compared with 41% with a warm or room-temperature pack.
- Rats (2010). Temperature probes in the front of the rat brain showed that yawns and stretches happened while brain temperature was rising, and that it settled back to its starting level afterward.
- Seasons (2011). Pedestrians outdoors in a desert climate in the United States looked at pictures of people yawning. In mild winter weather (22°C), 45% yawned; in hot early summer (37°C), only 24% did.
In 2016 Gallup’s team compared yawns across mammal species. Species with heavier brains and more neurons in the cortex tended to have longer yawns, and primates generally yawned longer than other mammals. The researchers had predicted this link, on the view that yawning boosts blood flow and cooling in the brain.
Why yawns spread between friends
Over a year, Ivan Norscia and Elisabetta Palagi watched adults go about their lives and noted who yawned after whom. Yawns spread most often between family members, then friends, then acquaintances, and least often between strangers. Closeness mattered more than gender or nationality.
It is tempting to read that as empathy, a sign that we tune in to the people we care about. But a 2017 review by Jorg Massen and Andrew Gallup found the evidence for an empathy link inconsistent and inconclusive, and pointed to other factors that can muddy the results, such as where people’s attention is and whether they hold back a yawn in company. “You yawned because you care” is a sweet idea, not a settled fact.
Even dogs catch our yawns
In a 2008 study, 29 dogs watched a person yawn or make other mouth movements. Twenty-one of them yawned after seeing the yawn, and none yawned at the other mouth movements. The researchers say more work is needed to understand why.
In short
Yawning isn’t about oxygen. It may help cool the brain, it spreads most easily between people who are close, and it can even jump from us to our dogs. If you yawned a few times while reading this, you have just joined a long line of experiment participants.
Sources
- Yawning as a stereotyped action pattern and releasing stimulus. Provine RR. Ethology 72, 109–122 (1986)
- Yawning: no effect of 3–5% CO2, 100% O2, and exercise. Provine RR, Tate BC, Geldmacher LL. Behavioral and Neural Biology 48, 382–393 (1987)
- Yawning as a brain cooling mechanism: nasal breathing and forehead cooling diminish the incidence of contagious yawning. Gallup AC, Gallup GG. Evolutionary Psychology 5(1) (2007)
- Yawning and stretching predict brain temperature changes in rats: support for the thermoregulatory hypothesis. Shoup-Knox ML, Gallup AC, Gallup GG, McNay EC. Frontiers in Evolutionary Neuroscience 2, 108 (2010)
- Contagious yawning and seasonal climate variation. Gallup AC, Eldakar OT. Frontiers in Evolutionary Neuroscience 3, 3 (2011)
- Yawn duration predicts brain weight and cortical neuron number in mammals. Gallup AC, Church AM, Pelegrino AJ. Biology Letters 12, 20160545 (2016)
- Yawn contagion and empathy in Homo sapiens. Norscia I, Palagi E. PLoS ONE 6, e28472 (2011)
- Why contagious yawning does not (yet) equate to empathy. Massen JJM, Gallup AC. Neuroscience & Biobehavioral Reviews 80, 573–585 (2017)
- Dogs catch human yawns. Joly-Mascheroni RM, Senju A, Shepherd AJ. Biology Letters 4, 446–448 (2008)
