The Scientist: Gordon G. Gallup, Jr., Ph.D., an evolutionary pyschologist at the State University of New York at AlbanyThe Answer: Most people have experienced the sensation of yawning when they see someone else yawn, whether they are tired or not. It could be in reaction to a friend or family member, or to a stranger they see on the subway. In fact, people often yawn when they simply see a picture of someone yawning or read about the phenomenon. But despite decades of research, scientists still aren’t 100 percent sure why and how contagious yawns happen.The leading theory is that contagious yawning evolved as a way to keep packs of animals, including humans, more vigilant. If one member got tired and yawned, others would yawn in response to signal that they all needed to be more alert as they scanned for lurking predators.Several studies have linked contagious yawning with people’s capacity for empathy, that is, their ability to interpret what others are feeling. People are more likely to yawn in response to their close friends and family members, rather than a stranger. Humans can catch yawns from chimpanzees, or from animals they have a strong connection to, like their dogs. On the other hand, people with autism or schizophrenia—who can’t read others’ emotions well—don’t catch the yawns.Contagious yawning doesn’t develop until ages three to five years. So, while babies spend a lot of time yawning, especially in their first few months, they don’t do it when others do. It’s something we grow into. Yawning yet?MORE: Why People in Arizona Yawn More