Laughter

Laughter is a typically pleasant physical reaction and emotion consisting usually of rhythmical, usually 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, imagery, videos 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. Other than humans, some other species of primate 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 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 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

indicates that laughter is linked with the activation of the ventromedial prefrontal cortex, that produces endorphins. Scientists have shown that parts of the limbic system are involved in laughter. This system is involved in emotions and helps us with functions necessary for humans' survival. The structures in the limbic system that are involved in laughter are the hippocampus and the amygdala.
The December 7, 1984, Journal of the American Medical Association describes the neurological causes of laughter as follows:
Some drugs are well known for their laughter-facilitating properties, while the others, like salvinorin A, can even induce bursts of uncontrollable laughter.
A research article was published December 1, 2000, on the psycho-evolution of laughter.

Health

A link between laughter and healthy function of blood vessels was first reported in 2005 by researchers at the University of Maryland Medical Center with the fact that laughter causes the dilatation of the inner lining of blood vessels, the endothelium, and increases blood flow. Drs. Michael Miller and William Fry theorize that beta-endorphin-like compounds released by the hypothalamus activate receptors on the endothelial surface to release nitric oxide, thereby resulting in dilation of vessels. Other cardioprotective properties of nitric oxide include reduction of inflammation and decreased platelet aggregation.
Laughter has various proven beneficial biochemical effects. It has been shown to lead to reductions in stress hormones such as cortisol and epinephrine. When laughing, the brain releases endorphins that can relieve some physical pain. Laughter also boosts the number of antibody-producing cells and enhances the effectiveness of T-cells, leading to a stronger immune system. A 2000 study found that people with heart disease were 40 percent less likely to laugh and be able to recognize humor in a variety of situations, compared to people of the same age without heart disease.
Anecdotally, journalist and author Norman Cousins developed in 1964 a treatment program for his ankylosing spondylitis and collagen disease consisting of large doses of Vitamin C alongside laughter induced by comic films, including those of the Marx Brothers. "I made the joyous discovery that ten minutes of genuine belly laughter had an anesthetic effect and would give me at least two hours of pain-free sleep," he reported. "When the pain-killing effect of the laughter wore off, we would switch on the motion picture projector again and not infrequently, it would lead to another pain-free interval."

Communication

Laughter is often social and functional. A number of studies using methods of conversation analysis and discourse analysis have documented the systematic workings of laughter, for example in casual conversations, interviews, meetings, and therapy sessions. By closely examining recorded interactions, researchers have created detailed transcripts that indicate not only the presence of laughter but also features of its production and placement.
These studies challenge several widely held assumptions about the nature of laughter. Contrary to notions that it is spontaneous and involuntary, research documents that laughter is sequentially organized and precisely placed relative to surrounding talk, and, in the common case of overlapped laughter, relative to the laughter bouts of the conversation partner. Far more than merely a response to humor, laughter often works to manage delicate and serious moments. More than simply an external behavior "caused" by an inner state, laughter is highly communicative and helps accomplish actions and regulate relationships.

Causes

Common causes for laughter are sensations of joy and humor; however, other situations may cause laughter as well.
A general theory that explains laughter is called the relief theory. Sigmund Freud summarized it in his theory that laughter releases tension and "psychic energy". This theory is one of the justifications of the beliefs that laughter is beneficial for one's health. This theory explains why laughter can be used as a coping mechanism when one is upset, angry or sad.
Philosopher John Morreall theorizes that human laughter may have its biological origins as a kind of shared expression of relief at the passing of danger. Friedrich Nietzsche, by contrast, suggested laughter to be a reaction to the sense of existential loneliness and mortality that only humans feel.
For example: a joke creates an inconsistency and the audience automatically tries to understand what the inconsistency means; if they are successful in solving this 'cognitive riddle' and they realize that the surprise was not dangerous, they laugh with relief. Otherwise, if the inconsistency is not resolved, there is no laugh, as Mack Sennett pointed out: "when the audience is confused, it doesn't laugh." This is one of the basic laws of a comedian, referred to as "exactness". It is important to note that sometimes the inconsistency may be resolved and there may still be no laugh. Because laughter is a social mechanism, an audience may not feel as if they are in danger, and the laugh may not occur. In addition, the extent of the inconsistency has to do with the amount of danger the audience feels, and how hard or long they laugh.
Laughter can also be brought on by tickling. Although most people find it unpleasant, being tickled often causes heavy laughter, thought to be an reflex of the body.

Structure and anatomy

The forms of laughter are highly varied. Many laughs consist of sequences of bursts, such as ha-ha-ha or ho-ho-ho-ho. In extended laughter, several such bouts may occur, often interleaved with loud inhalations. Recent research has detailed aspects of the phonetic structure of laughter, especially as it occurs in interaction. While produced using the same vocal tract anatomy as speech, the phonetics and prosody of laughter differs from that of speech in many ways, such as usually having higher pitch. Laughed speech is also common, as in the example at right.
Not every imaginable laugh actually occurs. It is unnatural, and one is physically unable, to have a laugh structure of "ha-ho-ha-ho". The usual variations of a laugh most often occur in the first or final note in a sequence- therefore, "ho-ha-ha" or "ha-ha-ho" laughs are possible. Normal note durations with unusually long or short "inter-note intervals" do not happen due to the result of the limitations of our vocal cords. This basic structure allows one to recognize a laugh despite individual variants.
It has also been determined that eyes moisten during laughter as a reflex from the tear glands.