from Susan Wenger
Just as watching someone yawn can induce the behavior in yourself, recent evidence suggests that laughter is a social cue for mimicry. Hearing a laugh actually stimulates the brain region associated with facial movements.
Mimicry plays an important role in social interaction. Cues like sneezing, laughing, crying and yawning may be ways of creating strong social bonds within a group.
Susan presents Happiness! at Mindcamp 2012.