Often a group of people will join in laughing when some members of the group are laughing at something, so that laughter feeds on itself and spreads contagiously to others. There may be no understandable cognitive reason for the joiners to begin laughing. This theory does not need to account for the contagiousness of laughter, since many human behaviors stimulate group imitation, laughter among them. A person may be stimulated to yawn, for example, by seeing another person yawn, or by reading this sentence, or by nearly any clue in the environment related to yawning. Similarly, it is natural to feel distressed when seeing another person, who is himself distressed. When you see another person smile, it may raise your own spirits. These are natural, social, following behaviors, of which laughter is one. It need not be a consequence of a theory of humor perception that people join in laughter with others; humor may not in fact be perceived or understood when this occurs. Instead, the giggle-fest must be understood as a particular case of the more general principles by which the class of social following behaviors are accounted for.