Laughter is an expression of amusement. It is a sound that can be heard. Strong laughter can sometimes bring an onset of tears or even moderate muscular pain as a physical response to the act. Laughter is a part of human behavior regulated by the brain. It is used as a signal for being part of a group - it signals acceptance. The study of humor and laughter and its psychological and physiological effects on the human body is called geletology.
Most psychologists concentrate on question what does laughter mean? It is undoubtedly true that in most cases it means amusement. However, quite often it shows negative intentions of a person. This issue is mostly analysed by psychoanalysts. For Freud, laughter is an "economical phenomenon" whose function is to release "physic energy" that had been wrongly mobilised by incorrect expectations. Other psychologists like Karen Horney claim that it is release of tension or even self-defence. For example, when someone is asked for a difficult question and doesn't know how to answer it, he or she often may begin laughing and joking.This proves Lange-Bard theory that physical response comes after emotion (we are sad because we cry, we are happy because we laugh...). After discovering this fact many researches were made and some laughter therapies invented. It was noticed that laughter increases blood pressure, heart rate, changes breathing, reduces level of certain neurochemicals, strengthens immune system. Even humor therapy for cancer patients were developed. So now laughter is very important to clinical psychologists too.