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Laughter is a pleasant physical reaction and emotion consisting usually 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 rise from such activities as being tickled, or from humorous stories or thoughts. Most commonly, it is considered an auditory expression of a number of positive emotional states, such as joy, mirth, happiness, or relief. On some occasions, however, it may be caused by contrary emotional states such as embarrassment, surprise, or confusion such as nervous laughter or courtesy laugh. Age, gender, education, language, and culture are all indicators as to whether a person will experience laughter in a given situation. Some other species of primate (chimpanzees, gorillas and orangutans) show laughter-like vocalizations in response to physical contact such as wrestling, play chasing or tickling.

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.

The study of humor and laughter, and its psychological and physiological effects on the human body, is called gelotology.

Nature

Laughter might be thought of as an audible expression or appearance of excitement, an inward feeling of joy and happiness. It may ensue from jokes, tickling, and other stimuli completely unrelated to psychological state, such as nitrous oxide. One group of researchers speculated that noises from infants as early as 16 days old may be vocal laughing sounds or laughter. However, the weight of the evidence supports the appearance of such sounds at 15 weeks to four months of age.

Laughter researcher Robert Provine [es] said: "Laughter is a mechanism everyone has; laughter is part of universal human vocabulary. There are thousands of languages, hundreds of thousands of dialects, but everyone speaks laughter in pretty much the same way." Babies have the ability to laugh before they ever speak. Children who are born blind and deaf still retain the ability to laugh.

Provine argues that "Laughter is primitive, an unconscious vocalization." Provine argues that it probably is genetic. In a study of the "Giggle Twins", two happy twins who were separated at birth and only reunited 43 years later, Provine reports that "until they met each other, neither of these exceptionally happy ladies had known anyone who laughed as much as they did." They reported this even though they had been brought together by their adoptive parents, who they indicated were "undemonstrative and dour". He indicates that the twins "inherited some aspects of their laugh sound and pattern, readiness to laugh, and maybe even taste in humor".

Scientists have noted the similarity in forms of laughter induced by tickling among various primates, which suggests that laughter derives from a common origin among primate species.

The spotted hyena, another species of animal, was also known as the laughing hyena because of the way it sounds like when it communicates.

A very rare neurological condition has been observed whereby the sufferer is unable to laugh out loud, a condition known as aphonogelia.

Brain