The scientific study of laughter as medicine was popularised by the 1970 publication of Norman Cousins's bestselling book, Anatomy of an Illness, which described the US journalist's use of laughter in recovery from ankylosing spondylitis, a painful disease causing inflammation of the spine.
Cousins watched comic films and read joke books, and claimed 10 minutes of laughter reliably gave him two hours of pain-free sleep, leading to the popular notion that laughter reduces pain - perhaps by stimulating the release of painkilling hormones called endorphins.
However, some scientists say Cousins's recovery could have been influenced by any number of factors, including the large doses of vitamin C he routinely took or certain personality traits. There has even been speculation the disease was misdiagnosed.
Since Cousins's book, several laboratory studies have examined the analgesic effects of laughter. In 1987, Rosemary Cogan of Texas Tech University and her colleagues found pain thresholds of college students were higher after listening to a comic tape than after listening to a dull narrative.
Other studies have shown similar results, but some have found pain thresholds also increase with negative emotional stimuli - a horror film or a Holocaust documentary - suggesting the analgesic effect might be due to general emotional arousal rather than laughter in particular.
In 1971, Dr William Fry of Stanford University showed laughter increases heart rate, blood pressure and oxygen consumption, and that these levels drop soon afterwards, providing a short-term relaxation response. However, a 1989 US study by Sabina White and Phame Camarena found no longer-term changes in the heart rate and blood pressure of volunteers after six weeks of laughter sessions.
By comparison, relaxation sessions over the same period lowered participants' heart rates and systolic blood pressure.
In the most famous study of laughter and immunity, published in 1989, a group led by Lee Berk of Loma Linda University tested blood samples taken from volunteers before, during and after they watched a comic video. After viewing, they found lowered levels of the stress hormone cortisol and enhanced levels of several immune system components.
Some scientists have pointed out methodological problems in this study - and in many studies of the health benefits of laughter - including small sample size and the absence of controls for other potentially influential factors, such as distraction, general emotional arousal and the expectations of volunteers.
Most importantly, laughter itself is rarely measured in these experiments - researchers simply assume it has occurred.