THE GIST

- Contagious yawning gets at the ancient, deep, and subconscious roots of empathy and social bonding.

- Yawns become contagious around age four.

- Kids with autism are less likely to catch yawns than other kids. The more severe their condition, the less common the behavior gets.

Watch someone yawn, and try not to yawn yourself. It can be impossible to resist. Even reading about yawning can make you do it.

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."

Inspiration for her study came when she tried to get her own autistic son to clear his ears on an airplane. She repeatedly yawned at him, hoping he would yawn back. He never did.

"The fact that autistic kids don't do it might mean they're really missing out on that unconscious emotional linkage to those around them," she said.

"The big thing people try to figure out in infant development is how we become humans who understand that humans have minds that are different from ours," she added. "Autistic people never sort of seem to understand that."

Fetuses begin yawning in the womb as early as 11 weeks after conception, said Robert Provine, a developmental neuroscientist at the University of Maryland, Baltimore County.

For reasons scientists still can't explain, spontaneous yawning continues throughout life. (Studies have shot down theories about yawning to bring in more oxygen.) In fact, all vertebrates yawn, including snakes and lizards.

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.

Although all vertebrates yawn, contagious yawning only exists only humans, chimpanzees and possibly dogs. Getty Images

To find out when in life the behavior develops, Helt read a story to 120 healthy kids, ages one through six. The kids were grouped by age, so that all of the one-year-olds heard one reading, all the two-year-olds heard another, and so on. There were 20 kids in each age group.

During each 10-minute story, Helt intentionally yawned every 90 seconds. A camera recorded whether the kids were watching her and if so, whether they yawned, too. She and colleagues repeated the experiment with 28 autistic kids, ages 6 to 15. Some of the children were further along the autistic spectrum than others.

Given four opportunities to catch a yawn, the researchers report today in the journal Child Development, none of the healthy one-year-olds did. Only one of the two-year-olds yawned back, and two of the three-year-olds caught a yawn.

There was a dramatic leap in the group of four-year-olds, where yawning spread to 9 out of 20 kids. That rate held steady for the older groups.

And it matches experiments in grown-ups, which find that between 40 and 60 percent of healthy adults yawn after seeing someone yawn, thinking about yawning or even reading the word "yawn."

In the second part of the study, Helt and colleagues found that contagious yawning happened half as often in kids with a mild versions of autism. Kids with the most severe diagnoses never caught yawns.

Besides offering potential for diagnosing and understanding autism, the new work gives overdue attention to a fundamental and unconscious behavior that the field of psychology has long ignored, Provine said. He studies a cluster of contagious behaviors, including emotional tears, and has written a book about laughter.

"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."