<Table> <Tr> <Td> </Td> <Td> This article includes a list of references, but its sources remain unclear because it has insufficient inline citations . Please help to improve this article by introducing more precise citations . (January 2009) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article includes a list of references, but its sources remain unclear because it has insufficient inline citations . Please help to improve this article by introducing more precise citations . (January 2009) (Learn how and when to remove this template message) </Td> </Tr> <P> Laughter is a physical reaction in humans and some other species of primate, consisting typically of rhythmical, often audible contractions of the diaphragm and other parts of the respiratory system . It is a response to certain external or internal stimuli . Laughter can arise from such activities as being tickled, or from humorous stories or thoughts . Most commonly, it is considered a visual expression of a number of positive emotional states, such as joy, mirth, happiness, relief, etc . On some occasions, however, it may be caused by contrary emotional states such as embarrassment, apology, or confusion such as nervous laughter or courtesy laugh . Age, gender, education, language, and culture are all factors as to whether a person will experience laughter in a given situation . </P> <P> Laughter is a part of human behavior regulated by the brain, helping humans clarify their intentions in social interaction and providing an emotional context to conversations . Laughter is used as a signal for being part of a group--it signals acceptance and positive interactions with others . Laughter is sometimes seen as contagious, and the laughter of one person can itself provoke laughter from others as a positive feedback . This may account in part for the popularity of laugh tracks in situation comedy television shows . </P>

Where does laughter come from in the body