For some people, it’s nearly impossible to resist a yawn when one’s colleague or dinner date does the same. Why is this? It’s thought to have to do with social bonding (only humans and chimpanzees do it). Humans also have other contagious responses: for instance babies cry when they hear others crying, and we all tend to laugh when others do (this is why laugh tracks on TV shows work so well, according to the study authors). Researchers at the University of Connecticut studied children as young as one year old to look at how child development — both normal and abnormal — affects contagious yawning.

They found that contagious yawning jumped in frequency when children were at least four years old.

In the study, the researchers read stories to kids from one to six years old. Every 90 seconds, the reader intentionally yawned; the team tracked how often the kids returned the yawn. They found that contagious yawning jumped in frequency when children were at least four years old. None of the one- year-olds yawned in response to the reader, but the older a child was, the more likely they were to yawn. Five- and six-year-olds contagiously yawned about 40% of the time, which is close to how often adults do.

In the second part of the experiment, Molly S. Helt and her team studied children who had Autism Spectrum Disorder (ASD), of varying degrees of severity, and compared them to kids of normal development. They wanted to see if having ASD, which is known to affect social bonding and communication, might affect contagious yawning. The results showed that indeed it did: Kids with ASD were about one quarter as likely to yawn in response to the experimenter as normal children. And children who suffered the most severe form of autism never yawned contagiously. (In case you were wondering, when the experimenter was not present, the groups of children all yawned — spontaneously — about the same amount.)

So what does this study actually tell us about contagious yawning? First of all, it seems to come with age, and probably, social experience. This is borne out by the fact that children with ASD yawned so much less than normal children. Helt and her team write that contagious yawning may, in the future, be used as a simple and inexpensive biomarker for certain neurological issues (like autism, and perhaps other disorders). The study shows nicely how little phenomena that we often don’t think about can have interesting and significant clinical implications.

The study was published in the September/October issue of Child Development.