Every person yawns. So do many other vertebrate （有脊椎的） animals. including snakes. Dogs, cats, sharks, and chimpanzees. While yawning is contagious （会传染的）. not everyone catches a yawn. Around 60-70% of people yawn if they see another person yawn in real life or in a photo or even read about yawning. Contagious yawning also occurs in animals, but it doesn't necessarily work the same way as in people.
Scientists have proposed many theories for why we catch yawns. Probably the most popular theory is that yawning serves as a form of nonverbal communication. Catching a yawn shows you're accustomed to a person's emotions. Scientific evidence comes from a 2010 study at the University of Connecticut. which concluded yawning does not become contagious until a child is about four years old. when empathy （共情） skills develop. In the study, children with autism, who may have impaired empathy development, caught yawns less often than their peers.
Studying contagious yawning in other animals may provide clues to how people catch yawns. Contagious yawning in animals may serve as a means of communication. Siamese fighting fish yawn when they see their mirror image or another fighting fish, generally just prior to an attack.
Contagious yawning is linked to temperature, in both animals and people. Most scientists speculate it is a thermoregulatory behavior, while some researchers believe it is used to communicate a potential threat or stressful situation. A 2010 study of budgerigars （虎皮鹦鹉） found that yawning increased as temperature was raised near body temperature.
People commonly yawn when tired or bored. Similar behavior is seen in animals. One study found the brain temperature in sleep deprived rats was higher than their core temperature. Yawning reduced brain temperature, possibly improving brain function. Researchers will do further study on this.
