A Review of the Beneficial and Harmful Effects of Laughter
SOURCE: British Medical Journal 2013 (Dec 13); 347: f7274
R E Ferner, honorary professor of clinical pharmacology ,
J K Aronson, fellow 
Christmas 2013: Food For Thought
Laughter and MIRTH (Methodical Investigation of Risibility, Therapeutic and Harmful): Narrative Synthesis
Every December, the BMJ publishes hilarious Christmas research.
Their 2013 offerings are now online at:
Objective To review the beneficial and harmful effects of laughter.
Design Narrative synthesis.
Data sources and review methods We searched Medline (1946 to June 2013) and Embase (1974 to June 2013) for reports of benefits or harms from laughter in humans, and counted the number of papers in each category.
Results Benefits of laughter include reduced anger, anxiety, depression, and stress; reduced tension (psychological and cardiovascular); increased pain threshold; reduced risk of myocardial infarction (presumably requiring hearty laughter); improved lung function; increased energy expenditure; and reduced blood glucose concentration.
However, laughter is no joke — dangers include syncope, cardiac and oesophageal rupture, and protrusion of abdominal hernias (from side splitting laughter or laughing fit to burst), asthma attacks, interlobular emphysema, cataplexy, headaches, jaw dislocation, and stress incontinence (from laughing like a drain). Infectious laughter can disseminate real infection, which is potentially preventable by laughing up your sleeve. As a side effect of our search for side effects, we also list pathological causes of laughter, among them epilepsy (gelastic seizures), cerebral tumours, Angelman’s syndrome, strokes, multiple sclerosis, and amyotrophic lateral sclerosis or motor neuron disease.
Conclusions Laughter is not purely beneficial. The harms it can cause are immediate and dose related, the risks being highest for Homeric (uncontrollable) laughter. The benefit-harm balance is probably favourable. It remains to be seen whether sick jokes make you ill or jokes in bad taste cause dysgeusia, and whether our views on comedians stand up to further scrutiny.
— Robert Burton, The Anatomy of Melancholy (1621)
The BMJ has not dealt seriously with laughter since 1899, when an editorialist, following an Italian correspondent’s suggestion that telling jokes could treat bronchitis, proposed the term “gelototherapy” (in Greek gelōs means laughter; in Italian gelato means ice cream).  The journal had, a year before, described heart failure following prolonged laughter in a 13 year old girl. 
We searched Medline from 1946 to June 2013 and Embase from 1974 to June 2013, using the search term “laugh$.mp” (fig 1), removing animal studies and conference reports, and excluding papers on the Caribbean sponge Prosuberites laughlini and with authors called Laughing,  Laughter,  Laughton, or McLaughlin; none was particularly amusing. We discarded papers with opaque titles, such as “Gelotophobia and thinking styles in Sternberg’s theory”,  and publications that proved irrelevant, such as “Another exciting use for the cantaloupe”  (which described practising endoscopy on melons). We identified three classes of findings:
harms from laughter, and
conditions causing pathological laughter.
We discussed the uncertain cases.
Fig 1 Search strategy for papers
Dr Patch Adams advocated therapeutic clowning, declaring that “I have done vast numbers of clowning experiments . . . and have found friendliness and celebrating life to be the heavy artillery of the love strategy.”  However, the benefits of laughter have often been assumed rather than demonstrated.
We concentrated on mirthful or “unintentional” laughter, also called Duchenne laughter, since Duchenne first demonstrated that genuine laughter is characterised by contraction of both the zygomatic and the orbicularis oculi muscles (fig 2). 
Fig 2 The frontispiece of Duchenne’s study of the electrophysiology of expression, showing the facial muscles activated in mirthful laughter
Psychological and psychiatric benefits
Life satisfaction and laughter have been associated with one another,  but reciprocal causality has not been confirmed. Laughter can increase pain thresholds,  although hospital clowns had no discernible effect on distress in children undergoing minor surgery.  Perhaps surgical patients derive no advantage from being in stitches.
The presumed positive effects of laughter on wellbeing have been harnessed in serious mental disorders, without much evidence of benefit. [12-14]
Some psychoanalysts believe that a joke can substitute for interpretation — provided that the patient appreciates the joke. Others, however, view jokes as undesirable, because they circumvent resistance to psychic exposure and may be regarded as seductive.  Ken Dodd pertinently observed that Freud, who thought that laughter conserved psychic energy, never played second house Friday night at the Glasgow Empire.
Laughter reduces arterial wall stiffness  and improves endothelial function.  So perhaps it relieves more than one kind of tension. Laughing lowers your risk of myocardial infarction,  and reduces recurrence after myocardial infarction in diabetes.  So, reading the Christmas BMJ could add years to your life.
Laughter induced by a clown improved lung function in patients with chronic obstructive pulmonary disease.  One of the study’s authors was a clown, something only alleged of other studies.
In healthy people, “genuine laughter” for 15 minutes increased energy expenditure by up to 167.2 kJ (40 kcal).  Laughter induced by a comedy show attenuated the postprandial increase in glucose in diabetes by 2.5 mmol/L compared with a “monotonous lecture.”  A day of merriment could therefore consume over 8360 kJ (2000 kcal), improve glycaemic control, and cure obesity.
Lord Chesterfield said of the act of procreation that “the pleasure is momentary, the position ridiculous, and the expense damnable.” His first proposition has been refuted,  but a pioneering study has confirmed the second.  A clown, dressed as a chef de cuisine, entertained would-be mothers for 12-15 minutes after in vitro fertilisation and embryo transfer. His saucy jokes were a recipe for success — the pregnancy rate was 36% in those whom he entertained compared with 20% in the controls (adjusted odds ratio 2.67; 95% confidence interval 1.36 to 5.24).25 Clowning may cost effectively vitiate Lord Chesterfield’s last proposition.
Sometimes life imitates art: “A surgeon proceeded to read [to me] the diverting history of ‘The Lady Rohesia’ [from The Ingoldsby Legends], and how she was cured of her quinsy . . . The story caused me to laugh, and this led to the bursting of the [tonsillar] abscess, and to my cure without the use of cold steel.” 
Laughter has no consistent effect on immune functions, such as the activity of natural killer cells. [27, 28] Work on how laughter affects IgE production by seminal cells in atopic eczema  has sown the seeds for further studies.
Humour weakens resolve and promotes brand preference,  so the prudent response to the drug rep’s spiel would be “Don’t make me laugh.”
Hearty laughter can cause syncope, [31-35] perhaps by a neural reflex response to the increase in intrathoracic pressure that accompanies intense laughter. [36, 37] Syncope after laughing has accompanied bilateral carotid stenosis in Takayasu arteritis.  Laughing can cause conduction anomalies  and arrhythmias.  A woman with long QT syndrome and a history of torsade de pointes took ziprasidone, collapsed, and died after intense sustained laughter.  Laughter in Angelman’s (“happy puppet”) syndrome can cause asystolic arrest, apparently of vagal origin.  Laughing fit to burst can cause cardiac rupture. 
The quick intake of breath that accompanies laugher can provoke inhalation of foreign bodies. 
In patients with asthma, laughter sometimes triggers an attack,  but cough after laughing is commoner than a good wheeze. [46, 47] Asthma was once perceived as a psychological disorder,  but Gillespie noted that laughter probably had a physical rather than a psychological effect, and that even hollow (non-Duchenne) laughter could trigger an asthma attack. 
Laughter can cause pneumothorax. [50, 51] Pilgaard-Dahl syndrome, named after two Danish revue actors, is pneumothorax in middle aged smokers induced by laughter.  If the YouTube video we have watched  is representative, non-Danish speakers are not at risk.
Interlobular emphysema can reportedly result from “efforts of parturition and of defaecation, by the lifting of heavy weights, during coitus, by paroxysms of rage, excessive laughter, and hysterical convulsions.” 
Exhaled airflow — from sneezing, whistling, and laughing, for instance — potentially disseminates infection. Paper tissues may reduce spread.  So, we suspect, might laughing up your sleeve.
Central nervous system harms
Cataplexy, often allied to narcolepsy (Gélineau’s syndrome),  is characterised by sudden loss of muscle tone provoked by laughter and other stimuli.  It is apparently difficult to elicit during medical consultations,  perhaps because “laughing by itself” is a much less powerful stimulus than “laughing excitedly.”  The combination of muscle weakness induced by laughter and the ability to hear during an episode distinguishes cataplexy from sleep apnoea.  In one case, cataplexy induced by laughter affected only the right side of the body;  this patient presumably could still laugh on the other side of her face.
Laughter, like many pleasurable things, including ice cream, chocolate, and sex (separately, and perhaps together), may precipitate headaches.  The Chiari malformation and colloid cysts of the third ventricle are occasionally associated with laughter induced headache. [63, 64]
A woman with a patent foramen ovale laughed uproariously for three minutes, became aphasic, and had a cerebral infarct. 
A good belly laugh can make a hernia protrude, aiding diagnosis in children  — rapture unmasking rupture. By contrast, failure to laugh is an important sign of intra-abdominal infection in children.  Laughter is an unusual precipitant of Boerhaave’s syndrome, spontaneous oesophageal perforation. 
Laughing can dislocate the jaw.  Rectus sheath haematoma is described as an adverse reaction to side splitting “laughter therapy.” 
Urinary tract harms
Laughing like a drain can cause stress incontinence.  It can also cause “enuresis risoria” (“giggle micturition” or “giggle incontinence”), [72, 73] a consequence of uncontrolled detrusor contraction induced by laughing, for which methylphenidate has been advocated. 
Pathological causes of laughter
Laughter has its serious side. We have identified many disorders associated with unprovoked laughter, for example, gelastic seizures (seizures manifest by laughing — true nervous laughter; web table).
Limitations of the study
We limited our search to “laugh$,” and did not explicitly seek cacchinations, cackles, chortles, chuckles, giggles, grins, guffaws, smiles, smirks, sneers, sniggers, teehees, or titters; we also ignored sources of laughter (comedy, drollery, humour, jest, jocularity, whimsy, wit, and wisecracks).
Embase and Medline do not yet index some potential sources of information, including HUMOR: International Journal of Humor Research, Therapeutic Humor, Cahiers de recherche de CORHUM-CRIH, the European Journal of Humour Research, and the Israeli Journal of Humor Research (yet). 
We categorised effects as beneficial or harmful, a usually clear-cut distinction; some effects, however, such as lowering the threshold for seduction, could not be unequivocally categorised.
Some readers may ignore the benefits of laughter — that would be serious; others may dismiss its harms — we call them the laughing cavalier.
Our review refutes the proposition that laughter can only be beneficial. However, invoking a pharmacological classification,  the harms occur during prolonged overdose (toxic effects), occur immediately after exposure, and are most dangerous in those with susceptibility factors. We infer that laughter in any form carries a low risk of harm and may be beneficial.
These conclusions are necessarily tentative. It remains to be seen whether, for example, sick jokes make you ill, if dry wit causes dehydration, or jokes in bad taste cause dysgeusia, and whether our views on comedians stand up to further scrutiny.
Contributors: REF proposed a systematic review of the benefits and harms of laughter and conducted the initial search and classification.
JKA checked that work and collated the data in the web table. Both wrote and revised the manuscript. REF acts as guarantor.
Funding: None was required.
Competing interests: The authors’ interests are unrelated to mirthful laughter and are generally not conducive to it; their senses of humour sometimes conflict.
Both authors have completed the ICMJE uniform disclosure form at www.icmje.org/coi_disclosure.pdf and declare: no support from any organisation for the submitted work; no financial relationships with any organisations that might have an interest in the submitted work in the previous three years; no other relationships or activities that could appear to have influenced the submitted work.
Ethical approval: None was required.
Data sharing: Dataset of references available from the corresponding author at firstname.lastname@example.org.
The lead author (the manuscript’s guarantor) affirms that the manuscript is an honest, accurate, and transparent account of the study being reported; that no important aspects of the study have been omitted; and that any discrepancies from the study as planned (and, if relevant, registered) have been explained.
This is an Open Access article distributed in accordance with the Creative Commons Attribution Non Commercial (CC BY-NC 3.0) license, which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, provided the original work is properly cited and the use is non-commercial. See: http://creativecommons.org/licenses/by-nc/3.0/.
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