Laughter is a physical reaction in humans 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 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 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.