It is long been known that seeing another yawn will bring others to yawning. Even seeing a picture of someone yawning or reading about someone yawning will elicit this behavior. It even works across species. Dogs yawn after a stranger yawns at a significantly higher frequency. One proposed reason for this "contagion" is the presence and function of mirror neurons.
Mirror neurons can be thought of as "empathy neurons". A concentration of mirror neurons are located in the inferior frontal gyrus and the inferior parietal lobe (although there may be other areas yet elicited) and collectively are referred to as the Mirror Neuron System (MNS). When we see another cry, it is easier for us to cry. When we see another laugh, we tend to laugh much more. It's mirror neurons that largely (perhaps entirely) allow us to mimic others' behavior. The NMS is also probably is a key component in language development. Incredibly interesting research (Norscia & Palagi) shows that the contagion of a yawn - is transmitted at a higher rate the closer they are psycho-socially related to us. We yawn more often when family members yawn, less so when friends yawn, acquaintances even at a lower percentage and strangers the least often. We "infect" others with our behavior the more empathy we have for them.
Young children with autism spectrum disorder will yawn less often (and even drops below their baseline yawning frequency) than other children without ASD. This is consistent with the empathy-requirement aspect of contagious yawning and with the diminished amount and/or function of the mirror neuron system with autism.
Some theorize that their are several different subtypes of mirror neurons. While the relative lack/health of one subtype may be related to ASD, disorders of other types may be correlated to narcissistic personalty disorder, sociopathy-psychopathic behaviors. One day in the relatively near future, I believe we will be able to quantify these subtypes and correlate them with different diseases/disorders. The next logical step will be to up-regulate gene expression or even selective transplanting of these mirror neuron subtypes into people who need them.
And all this from a yawn.
Secret # 1711: Charles Tayolor, Sierra Leone, Blood Diamonds and the Double Shhh! Sign
Secret # 21: Surprise + Fear + Empathy