There is no shortage of theories about humor. Kant characterized it as a response to a sudden relief of tension. Freud claimed it was a socially acceptable outlet for aggression and the libido. More recently, incongruity theories have posited that humor is the result of resolving a subverted expectation (what Arthur Koestler called 'bisociation'.) Benign violation theory argues that humor is not about incongruity resolution so much as the recognition that a transgression is actually harmless. Evolutionary accounts have variously focused on humor's ability to foster physical fitness, cognitive development, social bonding, hierarchy regulation, and mate selection.
There are so many theories of humor, in fact, that they easily fill a textbook. Reviewing the literature leaves one both impressed with the sheer multitude of theories and overwhelmed by the disarray of the field. While most of the theories bear a family resemblance to one another (many note the element of surprise, humor's play-like qualities, its social nature), none of them seems to capture everything. It's not merely possible, but trivially easy, to come up with exceptions to each one.
Why is it that, after so many years and countless theories, we've failed to come up with a Unified Theory of humor? Is it because humor is so complicated that us mere mortals just haven't hit upon the right formula yet? Or perhaps we simply haven't collected enough data—with just the right set of studies, we could discover how all the pieces point to a single underlying rule? While humor is undoubtedly a complex phenomenon with a notable lack of solid empirical groundwork, there is a larger problem here: a single theory that can explain all of humor may not be possible.
Trying to come up with the one true explanation for humor may be akin to trying to figure out the one true function of oxytocin. This hormone plays a key role in everything from lactation and childbirth to orgasm and kidney function. Perhaps more notoriously, it moonlights a neurotransmitter, regulating maternal behavior, trust, empathy, pair bonding—even envy and schadenfreude. While research on this hormone is ongoing, one thing is clear: oxytocin has no one "true" function. Rather, it has many functions, which have become more elaborate and byzantine with time.
Demetri Martin's analytical lolcat, via Wired.com
So it is with humor and laughter. Think of all the things that make us laugh: A clever comment by an intelligent friend. A stupid comment by a less gifted friend. Being tickled or chased. Your boss tripping and falling (or better yet, attempting to dance). Polite laughter when chatting with a new acquaintance. Mirthless laughter at an ironic turn of events. Nervous laughter during an awkward pause, or when we have to complete an uncomfortable task (the most famous demonstration of this effect may be the Milgram experiment, where participants laughed while delivering an electric shock to someone in a neighboring room). Just because we're observing a single output doesn't mean the processes underlying them are the same, or even related to one another.
This may help explain why it is so easy to find fault with extant theories of humor: they mistakenly group partially overlapping behaviors under a single explanatory umbrella. Humor is not a unified phenomenon, but a cluster of related phenomena with multiple functions across many domains of human life. Mother nature is frugal (and a bit of a bag lady, frankly), and tends to jury-rig new functions out of existing materials. This habit is especially obvious with hormones, but it also seems to be at play with higher cognitive processes such as humor.
Psychology isn't like physics; it doesn't lend itself to unified theories. This is really too bad, because nice clean explanations can be quite satisfying. I hope what we'll see in the coming years is a series of interconnected theories, which reflect the complex cognitive and neural underpinnings of humor and humor-related phenomena. It may be messy, but abandoning the Unified Theory approach will allow us to give humor the rich, accurate, and nuanced treatment it deserves.