Why do we yawn, and why is yawning contagious?

Scientists do not agree on why we yawn. Several hypotheses exist, and the Cleveland Clinic describes the purpose of yawning as debated, with no clear consensus. The popular idea that yawning brings in more oxygen has been discarded. Contagious yawning, triggered by seeing or hearing someone else yawn, is thought to be linked to social bonds, though the exact reason is also unsettled. [1]

For years the main theory was that yawning draws in oxygen for the brain. Evolutionary biologist Andrew Gallup says research has explicitly tested this and found that breathing and yawning are controlled by different mechanisms. He points to marine mammals that yawn while submerged and not breathing. The Cleveland Clinic adds that a controlled lack of oxygen does not make people yawn more, and that a fetus yawns even though it receives oxygen through the umbilical cord. [2] [1]

The arousal hypothesis holds that yawning wakes the brain up. It fits the fact that tiredness and boredom are common triggers, and that yawning clusters around transitions between sleeping and waking. Gallup notes research linking yawns to increases in cortical arousal. The Cleveland Clinic adds that heart rate rises during a yawn, which may help a person stay alert. [2] [1]

The brain-cooling hypothesis proposes that yawning lowers brain temperature through the intake of air and changes in facial blood flow. Gallup says his studies in humans, other mammals and birds show that yawn frequency can be changed by altering ambient temperature or body temperature. In rats, rises in brain temperature triggered yawning, which was followed by drops in brain temperature. The Cleveland Clinic lists this as one of three main theories, alongside a communication hypothesis in which yawning signals boredom or stress to others. It says most researchers regard social interaction as a minor function that exists alongside a more significant one. [2] [1]

Contagious yawning is real and measurable. In an early experiment described in American Scientist, subjects yawned in 55 percent of cases while watching videos of yawns, compared with 21 percent while watching smiles. It has been documented only in highly social species, including humans, chimpanzees and lions, and people vary widely in how susceptible they are. [3] [2]

One explanation is empathy and mimicry. Researcher Matthew Campbell says people imitate others' expressions to feel what they feel, and that catching a yawn is a byproduct of how bodies and brains work, not a deliberate act. The Cleveland Clinic reports that susceptibility correlates with empathic skills in neurotypical people, is lower in people with autism spectrum disorder, and is higher for close loved ones than for strangers. Gallup, however, says other studies have failed to show a link with empathy and that the jury is still out. [4] [1] [2]

A second explanation is that contagious yawning synchronizes group behavior and raises vigilance. If a yawn signals low arousal in one individual, seeing it may make observers more alert, and the spread could heighten alertness across the group. In a study Gallup describes, people who had watched yawning videos became faster at spotting snakes, though not frogs. [2]