ASK A SCIENTIST
Question: Why are yawns contagious?
Despite the mountains of research that have given us an understanding of nearly every part and process in our body, we still don't know why we yawn. But that lack of understanding hasn't stopped people from creating theories, and even superstitions, to help explain the basis-and contagious nature-of yawning.
Yawning is now understood to be an unusual reflex that occurs in three steps. During the first step, we open our jaws wide while taking a deep breathe in that lasts three to four seconds. To help blood flow during this process, it seems everything wants in on the act-we contract the muscles of our mouth, neck, and even the lower legs. Next, during the two-second extension phase, we stretch our limbs, close our eyes, and increase saliva production. Finally, we rapidly exhale.
While yawns can be relaxing and bring mild pleasure, the ancient Greeks and Arabs shared the belief that yawning was a danger sign of evil entering the body. Medieval Europeans had yet another belief-they felt that when a person yawns or sneezes, their soul leaves their body. Covering the mouth could therefore save one's soul. This may explain why we usually say, "bless you or God bless you" when someone sneezes.
Today there are three scientific theories commonly used to explain yawning. One suggests that yawning is a result of dullness, drowsiness, fatigue, or a transition between wakefulness and sleepy states. But there is one hitch to the "boredom theory." It fails to explain is why many musicians yawn before a performance, as do athletes before a marathon.
The second theory offers an evolutionary perspective. Naturalist Charles Darwin, nearly 20 years before the release of his work Origin of Species, wrote "...seeing a dog and horse and man yawn, makes me feel how much all animals are built on one structure." Later, Darwin suggested that yawning may be a method for non-verbal social communication, writing "baboons often show their passion and threaten their enemies in a very odd manner, namely, by opening their mouths widely as in the act of yawning." Yawning, Darwin thought, was a protective response used by many animals to show their teeth and scare off predators. Ultimately, then, Darwin felt yawning was merely a behavioral aspect and a "piece of useless physiology."
In contrast, the third theory to explain yawning is based on physiology. Hippocrates, the father of medicine, classified yawning as a "wind disease" and described it as "exhaustion of fumes preceding fever." By the 18th century yawning was proposed as a way to increase oxygen supply to help cool the brain. Many later studies have come to support this idea-after all, yawning is common in individuals who have low brain metabolism, take anti-depressants, are anemic, or are intoxicated.
But not so fast. If yawning is purely a way to meet oxygen demand, why don't we yawn when running or lifting weights? Our body needs lots of oxygen during those activities. Also left unexplained is why unborn babies yawn. They begin to do so as early as the 11th week-their lungs aren't even developed by that time.
So, no one really seems to know much about why we yawn. But what we do know is that yawns are contagious. Nearly half of us will yawn within five minutes of seeing someone else yawn, or even just reading about yawning. (Imagine my own battle against yawns while writing this piece!) Medical imaging has shown that during yawns, certain brain cells automatically begin to mimic other people's actions, something that neuroscientists refer to as a "mirror neuron system." Some biologists maintain that yawning is an unconscious group behavior that our ancestors might have developed, likely from birds, to communicate their alertness level, coordinate sleep schedules, or just show empathy.
Yawning is infectious in many animals as well. Chimpanzees are great contagious yawners. Researchers are now studying if yawning can spread in snakes, crocodiles, cats, and hippos as they all yawn for different reasons. Dogs, for example, yawn to maintain calm. As humans, we tend to pick up yawns from our pets too, be it dogs, hamsters, or cats. Now, as an experiment, try testing if you can induce a yawn in your pet.
Ask a Scientist appears Thursdays. Questions are answered by faculty at Binghamton University. Teachers in the greater Binghamton area who wish to participate in the program are asked to write to Ask A Scientist, c/o Binghamton University, Office of Communications and Marketing, PO Box 6000, Binghamton, NY 13902-6000 or e-mail firstname.lastname@example.org. Check out the Ask a Scientist Web site at askascientist.binghamton.edu. To submit a question, download the submission form(.pdf, 460kb).