Sophie Scott, a neuroscientist at the University College London, has found that laughter is contagious! Of course you and I already know that, but now it's been backed up by a real academic study.
Scott says that the sound of laughter triggers the brain to ready our facial muscles to laugh ourselves. Researchers measures brain response with volunteers in an MRI and found that the pre-motor cortical region of the brain responded to all sounds, but more to the positive cues than the negative ones. Scott thinks that's because people tend to avoid negative emotions. Makes sense to me!
Not only is laughter contagious, but it also makes people feel good. These two agents in combinations are what trigger dangerous levels of levity, spontaneous mirth infections, and epic epidemics of exuberance. I wonder what the critical mass of laughing people would be to have the entire world laughing at once? One giant Laugh In. Perhaps the better question is, what joke would make it happen? Probably not a Knock Knock joke, because not everyone has a door.