Laughter is a physical reaction in humans 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. Some other species of primate (apes, chimpanzees, gorillas, bonobos 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.
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 its appearance at 15 weeks to four months of age.