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The two women on stage stretch their calf muscles and lean into the starting blocks. They look into the middle distance, their faces flat with concentration. A man steps onto the stage behind them and shouts, "Three, two, one: yoga off!" As the women glare at each other and begin stretching, the audience erupts with laughter.

This was a recent scene performed by Washington Improv Theater (WIT)--a set of D.C.-based comedy troupes that invents plays on the fly. According to longtime WIT member Topher Bellavia, the secret to creating a successful scene is to mirror the world in a slightly askew way. The "yoga off," for example, may remind people of times when they felt judged by others in an exercise class. The improv troupe exaggerated that--exposing the absurdity of competitive relaxation to get a rise out of their audience.

"Our basic mantra is 'truth in comedy.' People want to see themselves on stage, and they want to see people they know on stage," says Bellavia, who also teaches a class on improv comedy. "When they see that, it resonates with them and it causes them, for some reason, to laugh."

Exactly what people will find funny, however, often surprises Bellavia and his fellow actors.

"As a performer, [audience laughter] can be very disorienting," says WIT member Anna Trester. "Sometimes I'll do something that I don't even intend to be funny, and it will be hysterical to someone."

Like these comedians, some psychologists are attempting to pinpoint exactly what people laugh at, and why. And while scientists have been investigating humor since at least the 1960s, only recently have they brought the data together into encompassing theories of humor, says University of Western Ontario psychology professor Rod Martin, PhD, author of "The Psychology of Humor: An Integrative Approach" (Academic Press, 2006).

"There is a lot of research out there--I found over 4,000 articles, peer-reviewed journal articles, on the psychology of humor--but it hardly ever gets mentioned in textbooks or scholarly books," Martin says.

What these studies are adding up to is the idea that incongruity--when an idea or an object is out of place--is the heart of humor, Martin says. Truth plays an important role as well: The juxtaposition of the two things often gives people a new insight into a familiar situation, he notes. In fact, much of the enjoyment of humor may come from seeing familiar situations with new eyes.

Weighty humor

A simple case of incongruity recently inspired chuckles from the actors in a WIT improv class. Bellavia lined up his students and asked them to step forward and say something that isn't true. Noah Kanter--who at 5 feet and 6 inches was the shortest man in the room--stood in front of his classmates and complained about the trials of being extremely tall. "I get so tired of people asking me, 'How's the weather up there? Do you play basketball?'" said Kanter.

Such a statement is bound to incite mirth, says Martin. In fact, psychologists have long known that a mismatch between expectation and reality can lead to humor, he notes.

Perhaps the first person to illustrate this was Ball State University psychology professor Lambert Deckers, PhD. In the 1970s, Deckers asked people to judge the weight of different objects that turned out to be either much lighter or much heavier than they initially looked. The participants then attempted to lift the items, and, upon discovering their true weight, they smiled and even laughed. Interestingly, one iteration of the experiment, published in a 1974 issue of the Journal of Psychology (Vol. 86, No. 2, pages 309-312) found that heavier-than-expected objects are funnier than lighter-than-expected ones. A follow-up experiment showed that the bigger the difference, the funnier the moment.

In all of the experiments, however, a simple discrepancy between expectation and reality resulted in feelings of amusement, Martin says. However, this brand of humor only works in a playful atmosphere where the incongruity represents no real threat, he notes.

"There could be things that are incongruous, but they aren't funny," says Martin. " Someone walking down the sidewalk and being hit by a car is incongruous, but not funny."

Mental gymnastics

But we don't always need Styrofoam weights to get a laugh. In fact, a more common way to set up expectations and then foil them is through jokes, says humor researcher Jyotsna Vaid, PhD, a Texas A&M University psychology professor.

Consider this joke: A woman walks into the bar with a duck on a leash. The bartender says, " Where'd you get the pig?" The woman says, " This is not a pig. It's a duck." The bartender says, "I was talking to the duck."

This joke first sets us up to believe the duck is the pet, and then reverses that meaning, so the woman is the duck's pet, Vaid explains.

"In 'garden path' jokes, you think you are being led in one way but the real humor comes in being tricked," she says.

The experience of feeling two incongruous frames of reference activated simultaneously may be the key to enjoying these jokes, according to a recent study by Vaid and her colleagues, published in the Journal of Pragmatics (Vol. 35, No. 1, pages 1,431-1,449).

In the study, Vaid used a computer to present jokes one sentence at a time to 30 college students. After each sentence, the computer flashed three letters on the screen, and the participants determined whether they formed a word. The words tapped into schemas activated by either the initial or true meaning of the joke. For instance, the word " pet" would be easily accessible to participants who were thinking of the duck as the woman's pet, while they'd be quicker to recognize "sow" once they realized the bartender was insulting the woman.

Vaid found that garden path jokes do, in fact, succeed in tricking listeners. After reading the joke's first line, the participants took twice as long to recognize words consistent with the joke's final meaning than they did with words consistent with the joke's first meaning.

However, the researchers also found a surprising result: The two schemas remained equally active through the end of the joke. Immediately after the punch line, for instance, participants took about the same amount of time--100 milliseconds--to recognize words related to either of the joke's meanings.

This suggests that punch lines can actually serve to reinforce incongruity, rather than resolving it, says Martin.

"Punch lines are a mechanism in jokes that allow another schema to be activated," says Martin. "You have these two incompatible schemas activated at the same time, and you have to look at the same situation from two different points of view."

A follow-up study by Vaid, however, found that a few seconds after the joke, the participants showed slowed access to the joke's first meaning.

"To get a garden path joke you do have to suppress the initial meaning, but this isn't true for all jokes," said Vaid. "Puns, in particular, require a concurrent activation of the two meanings. Puns aren't enjoyable if you don't keep the two meanings in mind."

Truth in comedy

The fun of going through mental gymnastics to get to the true meaning of a joke probably accounts for much of our enjoyment of humor, says David Ritchie, PhD, a Portland State University communications professor who recently published a review of the topic in Metaphor and Symbol (Vol. 20, No. 1, pages 275-294). However, if a joke is going to garner more than a chuckle, it needs some emotional fire behind it, he says. Some jokes do this by tapping into some uncomfortable or unspoken truth, Martin notes.

Take this one-liner (please): By the time Mary had her 14th child, she'd finally run out of names to call her husband.

According to Ritchie, the joke includes the requisite frame shift--from naming a child to throwing invectives at a husband. But what makes it really funny, he says, is the way it overturns the cultural value that children are unqualified bundles of joy.

"For a long time, a genre of humor has been humor that…points to truths that everyone knows but nobody admits," says Ritchie.

However, for such a joke to be very funny, the listener has to agree with the revealed truth. Otherwise, they may find it baffling or offensive, says Timothy Moore, PhD, a humor researcher and chair of the psychology department at York University in Toronto.

In an early study of cartoon humor published in Sex Roles (Vol. 16, No. 9, pages 521-531), Moore found that men and women who endorsed beliefs such as, "Women should be less worried about their rights and more about becoming good wives and mothers," rated cartoons that depicted women negatively as more funny than people who scored low on sexism. (Though both groups found the cartoons at least somewhat funny.)

"It was true for both men and women," says Moore. "The more liberated you are about women's status, the less likely you are to find sexist humor funny."

A related study, published in a 2002 issue of the Psychology of Women Quarterly (Vol. 26, No. 4, pages 341-350) found a slightly different pattern of results. Men who scored high on benevolent sexism--those who agreed with statements such as "Women should be cherished and protected by men"--found "dumb blonde" jokes much more amusing than women who scored high on the same scale.

"However sweet these impulses might be, the underlying premise is that women are weaker, in need of protection, unable to compete in a man's world," says study author Dara Greenwood, PhD, a University of Michigan communications and psychology professor.

Those who do not agree with such stereotypes may mostly appreciate the joke on the level of a logic puzzle, says Greenwood. But if the punch line resonates with someone's deeply held beliefs, it can spur a belly laugh, she notes.

Jokes can certainly reinforce prejudices, Vaid says, but comedy also has the power to uncover hidden beliefs and then make fun of them. For instance, few people are willing to admit that they try to tree-pose better than everyone else in yoga class, but once that truth has been revealed in a playful way, competitive exercisers can have a laugh at their own expense.

"Improv comedy humor works by exploring the comedic potential in everyday situations," she says. "They subvert what we normally think of a situation by taking a silly perspective that exposes how rigid we are."