Why do we laugh?
Jacob Schor ND, FABNO
December 26, 2106
Laughter is a social phenomenon. Though people can laugh while alone, they are about 30 times more likely to laugh when they are with other people.  Laughter serves, at least in part, a deeply social function.
Current theory suggests that laughter evolved from a pattern of breathing that mammals, particularly primates, use during ‘play behavior’ to communicate playfulness and safety. Laughter says something to the effect, “This wrestling is just a game. We’re having fun. There is no need to be afraid.” During play, laughter encourages both cooperation and competition. Play and the shared communication through laughter, strengthens positive bonds between individuals. Laughter prolongs how long either children or chimpanzees will play. Laughing directly triggers conscious and unconscious positive emotional responses in both those who are laughing and also in those who just hear laughter. 
Humans begin laughing spontaneously in response to triggers during early infancy. It’s not just humans that laugh, it’s a trait shared by other apes. Evolutionary biologists suspect that the evolution of laughter goes back millions of years; our ancestors were laughing long before we were even human.  Current thought is that the evolutionary origins of human laughter can be traced back at least 10 to 16 million years to the last common ancestor of humans and modern great apes. Laughter is not a learned behavior but rather a fundamental communication skill deeply ingrained in our DNA.
Laughter increases pain tolerance and signals social status but its most important function is probably to create social bonds between individuals. Laughter is both the glue and the lubricant that allows us to live in complex social structures and create cooperative alliances.
Laughter has evolved to communicate and signal complex messages between individuals and to the larger group.
A recent paper by Bryant el al, published in April 2016 suggests that laughter indicates friendship status.  Listeners were asked to judge friendship status of pairs of individuals who were either friends or strangers to each other simply by hearing short snippets of their laughter together. The listeners were reliably able to distinguish friends from strangers based on the sound of their laughter even when they were from vastly different cultures.
In the distant evolutionary past, laughter, along with other simple sounds were triggered by primal emotions. We cried when distressed, we roared when angry. As the human brain learned to vocalize voluntarily, our lives got complicated. Our brains grew in complexity and early humans learned to mimic laughter (and other vocalizations, leading the way to speech and language). Being able to create social bonds through laughter, even if faked, no doubt provided a survival advantage.
Chimps will produce imitation laughter when they hear spontaneous laughter in other chimps.  Yet this fake laughter is recognizable as not being the real thing. Spontaneous laughter is higher pitch, shorter duration and comes in shorter laugh bursts than volitional laughter. Most of us can distinguish between the two quite reliably.
Laughter between friends tends to be spontaneous while strangers use volitional laughter. Our ears can tell the difference. Well obviously it’s not our ears, it’s our brains that can perceive the difference, but you know what I mean. In an evolutionary sense one would want to detect deception; an authentic laugh makes one part of the group, a faked laugh means someone is still partly a stranger. Yet even the canned laughter played on TV sound tracks works to win us over and it is difficult not to laugh along with it. 
In our modern world, laughter rarely fits into a single category; it is typically a blend of spontaneous and volitional types. Choosing to laugh along with others in a group is giving your assent to belonging to that group. Given how estranged people often are in their modern lives, it starts to make better and better sense why laughter can elicit so many health benefits.
There is a term, aphonogelia, in the medical literature that describes a rare condition in which people are unable to laugh audibly. Max Levin MD described the case of a patient he had seen in 1931.  This is apparently a very rare condition. Although aphonogelia is listed in the medical dictionaries, it does not show up on PubMed and there seem to be few if any descriptions published since Levin’s article. One recent article in the popular press describes a public figure who apparently does not laugh, but the article totally lacks scientific objectivity and it is difficult to tell whether this claim is valid or only meant to call the individual’s humanity into question.
Our capacity to laugh is both a deeply defining human trait yet at the same time a capacity that we share with our broader evolutionary family.
“Laughter connects you with people. It’s almost impossible to maintain any kind of distance or any sense of social hierarchy when you’re just howling with laughter.” John Cleese 
Enough of the ideas in this article were borrowed directly from an article written by Jordon Raine to the degree that a specific acknowledgement is in order:
Jordon Raine The evolutionary origins of laughter. The Conversation http://theconversation.com/the-evolutionary-origins-of-laughter-are-rooted-more-in-survival-than-enjoyment-57750 Accessed 12-17-2016
Provine RR and Fischer KR. Laughing, Smiling, and Talking: Relation to Sleeping and Social Context in Humans. Ethology, Vol 83; 4
Jan-Dec 1989 295–305
3. edited by Eckart Altenmüller, Sabine Schmidt, Elke Zimmermann.
“The Evolution of Emotional Communication: From Sounds in Nonhuman Mammals to ...”. Oxford University Press, Oxford, Jan 24, 2013 –
Davila Ross M, Owren MJ, Zimmermann E. Reconstructing the evolution of laughter in great apes and humans. Curr Biol. 2009 Jul 14;19(13):1106-11.
5. Bryant GA, Fessler DM, Fusaroli R, et al. Detecting affiliation in colaughter across 24 societies. Proc Natl Acad Sci U S A. 2016 Apr 26;113(17):4682-7.
6. Aping Expressions? Chimpanzees Produce Distinct Laugh Types When Responding to Laughter of Others. Emotion 11(5):1013-20 · February 2011
DOI: 10.1037/a0022594 ·
PMID: 21355640 DOI: 10.1037/a0022594
7. Gregory A. Bryanta, C. Athena Aktipisc, The animal nature of spontaneous human laughter. Evolution and Human Behavior. Volume 35, Issue 4, July 2014, Pages 327–335.
8. Platowa MJ, Haslamb SA, Botha A, et al. It’s not funny if they’re laughing”: Self-categorization, social influence, and responses to canned laughter. Journal of Experimental Social Psychology. Volume 41, Issue 5, September 2005, Pages 542–550
9. Max Levin, M.D. INABILITY TO LAUGH AUDIBLY: APHONOGELIA
Arch NeurPsych. 1931;25(1):157-159. doi:10.1001/archneurpsyc.1931.02230010169012
10. Leibovich, Mark. Al Franken faces Donald Trump and the next four years. New York Times, Dec 13, 2016.
11. John Cleese Explores the Health Benefits of Laughter
in Comedy, Health, Psychology| April 29th, 2015