It seems hard to watch someone yawn and not to yawn ourselves. Even reading about yawning can make you do it. Now, a new study has found why yawning has such a powerful force. Yawning when others yawn, the study suggests, is a sign of pity and a form of social connection. Kids don’t develop this deeply rooted behavior until around age four, the study found. Kids with autism (自闭症) are less likely to catch yawns. In the most serious cases, they never do. Yawning might help doctors to see whether the children are developing rightly. The work could also lead to a better understanding of the ways that people communicate and connect. "Emotional infection seems to be a born thing that connects us together," said Molly Helt, a graduate student in psychology at the University of Connecticut. "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 emotional connection with people around them," she said. "The biggest thing people try to figure out after birth is how we become humans and understand that humans have minds that are different from others’," she added. "Autistic people never seem to understand that." Like infectious laughter and crying, scientists have found that yawning is a shared experience that promotes social connection. Helt said it could fight stress after a period of being nervous and spread a feeling of calm through a group.Which of the following is NOT true about yawning?