The yawn reflex is often described as contagious: if one person yawns, this will cause another person to "sympathetically" yawn. Observing another person's yawning face (especially his/her eyes), or even reading about or thinking about yawning, can cause a person to yawn. However, only about 55% of people in a given audience will respond to such a stimulus; fewer if only the mouth is shown in a visual stimulus.The proximate cause for contagious yawning may lie with mirror neurons, i.e. neurons in the frontal cortex of certain vertebrates, which upon being exposed to a stimulus from conspecific (same species) and occasionally interspecific organisms, activates the same regions in the brain. Mirror neurons have been proposed as a driving force for imitation which lies at the root of much human learning, e.g. language acquisition. Yawning may be an offshoot of the same imitative impulse. A 2007 study found that children with autism spectrum disorders, unlike typical children, did not yawn after seeing videos of other people yawning; this supports the claim that contagious yawning is based on the capacity for empathy.
To look at the issue in terms of evolutionary advantage, if there is one at all, yawning might be a herd instinct. Other theories suggest that the yawn serves to synchronize mood behavior among gregarious animals, similar to the howling of the wolf pack. It signals tiredness to other members of the group in order to synchronize sleeping patterns and periods of activity. This phenomenon has been observed among various primates. The threat gesture is a way of maintaining order in the primates' social structure. Specific studies were conducted on chimpanzees and stumptail macaques. A group of these animals was shown a video of other conspecifics yawning, and both chimpanzees and stumptail macaques yawned also. This helps to partly confirm a yawn's "contagiousness".
Gordon Gallup, who hypothesizes that yawning may be a means of keeping the brain cool, also hypothesizes that "contagious" yawning may be a survival instinct inherited from our evolutionary past. "During human evolutionary history when we were subject to predation and attacks by other groups, if everybody yawns in response to seeing someone yawn, the whole group becomes much more vigilant, and much better at being able to detect danger."