Scientists Are Trying To Explain The Mystery Behind Why Yawning Seems Contagious

Few people seem immune to the contagious effect of yawning. Oftentimes, simply catching a glimpse of another person yawning is enough to trigger the urge in ourselves. Research has shown that a yawn can even spread through a phone call, as the sound of a yawn is enough to "infect" listeners. Yet, as well-documented as the "contagious yawn" phenomenon may be, there's no consensus among the scientific community to explain why it happens. Scientists aren't entirely sure why we yawn in the first place. The most popular explanation lies in our evolutionary drive to empathize with our friends and family.

Humans aren't the only animals to yawn. Almost all vertebrates yawn, including reptiles, birds, fish, amphibians, and mammals. Research in neuroscience has shown that the instinct to yawn originates in the brain stem, where it is triggered by the autonomic nervous system (as opposed to the controllable somatic nervous system). This means yawning is as involuntary as sneezing, breathing, and blinking. Whatever its evolutionary purpose, yawning has ancient origins in our deep primitive past.

Yet, while yawning is common among all vertebrates, contagious yawning has only been observed among highly social species, including humans and the other great apes with whom we've evolved. Contagious yawning also occurs more frequently between family members and loved ones. By unconsciously mimicking the behaviors of others, group members develop stronger interpersonal bonds and familiarity with one another. The same is true when we unconsciously mimic scratching our faces or crossing our arms. In other words, collective yawning signals to tired group members that they're safe among kin. This "social mimicry" theory is currently the most popular explanation, but it's not the only one.

For centuries, many people believed that yawning was a way for the body to expel built-up carbon dioxide and get fresh, new oxygen to their brains. This so-called "old oxygen" myth was disproven in 1987 when psychologist Dr. Robert Provine released the first study to demonstrate the lack of a correlation between yawning and oxygen deprivation. Since then, evolutionary biologists have developed a new theory of yawning: the thermoregulatory hypothesis.

Since the 2000s, researchers have used the thermoregulatory hypothesis to describe yawning as a way to cool off the brain. When we yawn, our jaw and face muscles flex involuntarily, drawing blood flow to the tissues around the skull. This isn't to deliver oxygen to the brain. Rather, the increased circulation helps flush warm blood away, replacing it with a cooler blood supply. Simultaneously, yawning draws air across the blood vessels in the nose, mouth, and sinuses. Similar to panting, this air exchange helps cool blood before it reaches the brain. The result is a cool, smooth-operating brain, and a feeling of increased alertness.

Thus, if yawning is a way to cool off an overheated brain, it's possible that our ancestors evolved to see yawning as something of a wake-up call. If one individual in the group yawned, this could be a sign that their mental acuity is slipping. Perhaps they're nodding off at guard duty, or maybe they experienced an inexplicable shiver down their spine telling them a predator is nearby. In any case, one yawn could be a sign that the group is in danger. As the yawn spreads through the group, so too does a collective, synchronized state of arousal. These days, yawning isn't a matter of life and death — but it certainly could have been for our ancestors.