It is common knowledge that yawns are “contagious.” But, why?

The simple answer is no one knows. However, there are several theories and research has been done to answer the question. Each person must decide which, if any, he believes.

Empathy

One theory is that contagious yawning is a sign of empathy. In a study in 2010, researchers from the University of Connecticut found most children did not begin so-called “contagious yawning” until they were around four years old—generally the age when empathy skills begin to develop. They also found that young people with autism, who may have trouble feeling empathy, were less likely to yawn contagiously than their peers without autism; children who showed more severe autistic symptoms were much less likely to yawn contagiously than those with milder symptoms.

In addition, a 2015 study found people with certain psychopathic traits may be less likely to catch a yawn from others. After taking a standard psychological personality test, 135 college students were shown 10-second video clips of different facial movements including yawning. The study found the less empathy a person had, the less likely he was to catch a yawn.

Age

However, a 2014 study from Duke University suggested that contagious yawning is not strongly related to variables like empathy, tiredness, or energy levels. Researchers at The Duke Center for Human Genome Variation found that contagious yawning may decrease as people age and may not be associated with empathy. “Age was the most important predictor of contagious yawning, and even age was not that important. The vast majority of variation in the contagious yawning response was just not explained,” said study author Elizabeth Cirulli, Ph.D., assistant professor of medicine at the Center for Human Genome Variation at Duke University School of Medicine.

A Warning Sign

Another theory claims that contagious yawning is a primitive form of unconscious communication and bonding that helped our ancestors stay safe and avoid danger long ago.

A 2010 study demonstrated that the number of times small parrots yawn contagiously increases when the temperature increases. In the study, 16 birds were exposed to four 10-minute periods of changing temperatures, and their yawning was found to positively correlate with ambient temperatures during the trial. The researchers said they think this behavior might serve as a warning sign to others of potential threats.

Social Mirroring

James Giordano, a neuroethicist and neuroscientist at Georgetown University said contagious yawning may be related to a phenomenon called social mirroring, where organisms imitate the actions of others. Other behaviors fall into this category, such as scratching, leg crossing and laughing.

Giordano said this behavior could be linked to mirror neurons in the brain. “What these neurons are involved in is matching what we sense and feel to the way we move,” Giordano said. “So if someone is seeing me scratch my face, they would know what it feels like. You may be compelled to do it too.”

Motor Function

The propensity to contagiously yawn appears to reside in the area of the brain that’s responsible for motor function, a 2017 study from England suggests. To study what’s going on in the brain when someone “catches” a yawn, the researchers observed 36 adults who were asked to watch video clips of other people yawning. Using transcranial magnetic stimulation (TMS), the researchers measured the participants’ brain activity during the experiments.

The researchers found that contagious yawning was linked to the levels of brain activity in a person’s motor cortex—the more activity in the area, the more inclined the person would be to yawn. When the researchers applied electrical currents to the area, the urge to yawn increased.