Now, a new study offers insight into why contagious yawning is such a powerful force.
Yawning when others yawn, the study suggests, is a sign of empathy and a form of social bonding. Kids don't develop this deeply rooted behavior until around age four, the study found. Kids with autism are half as likely to catch yawns. In the most severe cases, they never do.
Yawning might eventually help doctors diagnose developmental disorders. The work could also lead to a better understanding of the subtle ways that people communicate and connect.
"Emotional contagion seems to be a primal instinct that binds us together," said Molly Helt, a graduate student in clinical psychology at the University of Connecticut, Storrs. "Yawning may be part of that."
Contagious yawning is a different story. Only humans, chimpanzees and possibly dogs have been shown to do it.
Like contagious laughter and contagious crying, scientists have theorized that contagious yawning is a shared experience that promotes social bonding. Specifically, Helt said, it could diffuse stress after a period of being on high alert and spread a feeling of calm through a group.
"Yawning is a really big deal," Provine said. "We're dealing with something ancient, deep, and at the very root of our being. And psychologists have basically ignored it."
"It's a primal social bonding process," he added. "We're looking at the roots of empathy."