Laughter seems to involve a spasm of the diaphragm, but so does a hiccup, and laughter is not a hiccup. Both are involuntary, but that’s not much help.
Laughter has a stimulus, some visual or semantic event, even if only a memory, that triggers it. You don’t break out in laughter for “no reason.” Something makes you laugh.
Usually the triggering input is something unexpected, out of context, incongruous. Man slips on banana peel and falls. Ha-ha. That’s not supposed to happen. The context shift need only be very slight as with a pun, or a small reversal of semantic or attentional figure and ground, as in a riddle. A violation of expectations is funny. Sometimes.
At other times, violation of expectation is infuriating, as when a vending machine keeps your money and gives you no product. Not funny. Unless it happens to somebody else, perhaps. Violation of expectation can also provoke fear, even terror. So the incongruity hypothesis needs qualifications.
People laugh and giggle after smoking marijuana, possibly because the cognitive changes from smoking result in much relaxed expectations due to limited short term memory and short attention span. Under those conditions, it doesn’t take much to violate what few expectations are left.
Similarly perhaps, I am susceptible to fits of uncontrollable laughter when I am extremely tired. Again, there is a factor of diminution of cognitive faculties that accompanies extreme fatigue.
Tickling may produce laughter because of the incongruity of having your body stimulated in an unexpected way. The incongruity hypothesis is not an easy fit there, but it could be made to work.
Are clowns funny? They are for many children because they violate expectations of what is normal, both in the way they look and the way they act. But many children are afraid of clowns, so the violation of expectation hypothesis can only take us so far. Perhaps it must be a mild violation of context, or at least, one perceived as harmless. So safety seems to be a factor correlated with the incongruity hypothesis.
We laugh when something is funny, but for the most part, what's funny is culturally defined as whatever makes you laugh. Circular though that argument is, it suggests for the incongruity hypothesis that expectancies are culturally defined.
One group’s sacred ritual is another group’s comic farce. It all depends on what you expect and don’t expect in the normal course of things.
There are numerous physiological correlates to laughter in the brain, but that doesn’t tell us much, since we don’t know if they are causes or effects, or some mixture of both.
Likewise, laughter results in numerous changes in the body, but that doesn’t help us understand what laughter is or what causes it. Laughter can lead to tears, but that doesn’t make it the same as crying.
Another hypothesis is that laughter is a social phenomenon, possibly a form of communication. Why can’t you tickle yourself? Maybe because that is uncommunicative. It takes two to laugh.
Of course you can laugh when you are all alone, but according to the social hypothesis, it’s when you are remembering a social situation that makes you laugh.
Laughter is often contagious, additional evidence for its being a social, rather than strictly an individual phenomenon, and more reason to think that it serves specifically a communicative function. What is the communicative message? I don’t think it’s necessarily a conceptualized, linguistic proposition. It seems more like an implicit social understanding, like “We are together now.”
Despite the social hypothesis, a third component of laughter seems to be individual mood and temperament. I've met plenty of people I thought were humorless, yet everyone believes they have "a good sense of humor." I’ve never heard anyone say they have no sense of humor. Yet the plain fact is that some people are just not easily amused, while others can find almost anything funny. Humorless people perhaps feel under threat to the self, and if they are never safe, violations of expectations are fearful rather than funny.
Embarrassed laughter supports that hypothesis. Embarrassment arises from a violation of expectation, and can also produce laughter, if the personal threat is not too great. Phony laughter can pretend that the threat to self was insignificant, even when it wasn't.
Conversely, we might expect that people who are self-assured would have a more finely tuned sense of humor, meaning, they would laugh more readily at a wider range of incongruities. This could be tested experimentally, and probably has been.
I don’t think laughter is an emotion. And I don’t think emotions produce laughter, although laughter can produce happiness. We might try to make a grumpy person laugh just in order to elevate their mood. But the laughter is not the mood.
Any temperamental factor would be confounded with socialization, so there’s no way to get a clear picture of it.
Charles Darwin, in his fascinating book on the Expression of Emotions in Man and Animals (1898) Made the connection between laughter and aggression. We notice that laughter almost invariably involves showing teeth. Darwin surmised that it might be because laughter is similar to a self-defensive display of aggression.
I don't care for evolutionary "just-so" stories like that, but the conjecture is at least consistent with some of the other ideas I have put forward. If the perceived incongruity is felt to be threatening to the animal or person's self, then one natural response would be self-defense, and one way to demonstrate that is by showing teeth. It doesn't ring completely true to me, being based only on superficial observation of facial expression and not phenomenological analysis, but it is not unreasonable.
There is another idiosyncratic factor about what makes a person laugh, and it may be different from temperament. Perhaps it is socialization history. Often it is surprising what will make a person laugh.
For example, my wife, a well-educated, articulate, and thoughtful person, loves slapstick physical humor. When she sees somebody walk into a door and bang their head, in a comedic context, she might laugh until tears come to her eyes. Her laughter makes me laugh, but I look at her with bewilderment. Who is this person that thinks a bump on the head is so funny? It’s unfathomable.
For myself, I am partial to linguistic jokes. I love badly formed, ambiguous newspaper headlines, for example, clever captions to cartoons and sly puns. I also enjoy well-observed satire, which relies on good phenomenology. People bumping heads is just not funny for me.
I don’t think those kinds of differences are merely temperamental, but it is difficult to say what could account for them, other than, vaguely, “socialization.”
Do animals laugh? Many animals make noises, show emotional expression, and vocalize in situations that suggest to us that they are laughing. Chimpanzees especially seem to laugh and they have the cognitive capacity to understand when an expectation is violated. I have read that it is possible to tickle a rat and make it emit a special noise that can be heard with instruments. Is that laughter?
We can’t know if animals laugh because we lack sufficiently detailed intersubjectivity to understand their minds as well as we do with each other. I can be pretty sure when you are laughing because we are the same kind of animal and we know each other’s minds. Speculation about whether animals laugh is best put aside until we understand better what human laughter is.
What is Laugher?
One memorable account of the origin of laughter came from an eight year old boy. When I asked where laughter comes from, he said, "From God. Or maybe from my butt." The first answer says, “Laughter is a part of me that does not originate with me.” The second answer expresses the sudden, involuntary, and inexplicable quality of laughter.