We've all "caught" yawning from other people, but why that happens is unclear, according to a psychologist who has researched the behavior.
"In short, we don't know why yawns are contagious," said Meredith Williamson, a clinical assistant professor at Texas A&M College of Medicine. "Researchers used to think that yawning was only signaling a need for sleep, but now they believe that it can communicate a shift in alertness or boredom."
One theory is that contagious yawning is related to empathy, and that people with higher levels of empathy yawn more often when someone else yawns, compared to people with lower levels of empathy or those with a mental disorder.
"Researchers have seen that yawning may not be as contagious to people with autism or schizophrenia," Williamson said in a university news release. "More research is being done to determine the cause of this."
She also noted that children under the age of 4 and older adults are less likely to yawn in response to somebody else yawning.
Yawning may be an unspoken form of communication, but it's not unique to people, Williamson added. Some species of primates and canines yawn in response to each others' yawns, and dogs will even yawn after a person yawns.