Whether it’s the giggling of your child or the enthusiastic hollers of a talk show’s studio audience, we hear laughter every day.

Nothing could be more common. But just because it’s common doesn’t make laughter any less strange.

For instance, the next time you’re at the movies enjoying some comedy blockbuster, listen hard to the laughter around you. Why are all these strangers, in unison, exploding into such weird, gasping, grunting noises? Their laughs may suddenly stop seeming familiar, and more like the inhuman chatter of birds or the screeches of monkeys at the zoo.

Once you start looking at laughter as behavior, it can lead to some odd questions. Why do we do it? Do animals laugh? And why do we expect that any decent James Bond villain will cackle diabolically when revealing his plan for world domination? What’s so funny?

To answer these and other mysteries of laughter, WebMD delved into the surprisingly contentious world of laughter research.

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Why Do We laugh?

The answer may seem obvious: We laugh when we perceive something funny. But the obvious answer is not correct, at least most of the time.

“Most laughter is not in response to jokes or humor,” says Robert R. Provine, a professor of psychology and neuroscience at the University of Maryland Baltimore County. Provine should know. He has conducted a number of studies of laughter and authored the book "Laughter: a Scientific Investigation." One of his central arguments is that humor and laughter are not inseparable.

Provine did a survey of laughter in the wild -- he and some graduate students listened in on average conversations in public places and made notes. And in a survey of 1,200 “laugh episodes,” he found that only 10-20 percent of laughs were generated by anything resembling a joke.

The other 80-90 percent of comments that received a laugh were dull non-witticisms like, “I’ll see you guys later” and “It was nice meeting you, too.” So why the laughs?

Provine argues it has to do with the evolutionary development of laughter. In humans, laughter predates speech by perhaps millions of years. Before our human ancestors could talk with each other, laughter was a simpler method of communication, he tells WebMD.

It’s also instinctual. “Infants laugh almost from birth,” says Steve Wilson, MA, CSP, a psychologist and laugh therapist. “In fact, people who are born blind and deaf still laugh. So we know it’s not a learned behavior. Humans are hardwired for laughter.”

But perhaps because laughter is so ancient, it’s much less precise than language.

“Laughter isn’t under our conscious control,” says Provine. “We don’t choose to laugh in the same way that we choose to speak.” If you’ve ever had an inopportune laughing fit -- in a lecture, during a high school play, or at a funeral, for instance -- you know that laughter can’t always be tamed.

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Laughing Is Contagious

The cynical answer is that sitcoms are so witless and unfunny that we need to be told where the jokes are. But this misses the point. Why does hearing other people laugh make us more likely to laugh ourselves?

Everyone’s experienced this on a small scale. Seeing someone in hysterics -- even if you don’t know who the person is or why she’s laughing -- can set you laughing too. Why?

The answer lies in the evolutionary function of laughter. Laughter is social; it’s not a solo activity, says Provine.

“We laugh 30 times as much when we’re with other people than we do when we’re alone,” says Provine.

You might assume that the ‘purpose’ of a laugh is to express yourself -- to let people know that you think something is funny. But according to a 2005 article published in the Quarterly Review of Biology, the primary function of laughter may not be self-expression.

Instead, the purpose of a laugh could be to trigger positive feelings in other people. When you laugh, the people around you might start laughing in response. Soon, the whole group is cheerful and relaxed. Laughter can ease tension and foster a sense of group unity. This could have been particularly important for small groups of early humans.

In some cases, laughter can in fact become literally contagious. History is dotted with accounts of laughter epidemics. In 1962, in the African country that is now Tanzania, three school girls began to laugh uncontrollably. Within a few months, about 2/3 of the school’s students had the symptoms, and the school closed. The contagion spread, and eventually affected about a thousand people in Tanzania and neighboring Uganda. There were no long-lasting effects, but it shows how responsive people can be to seeing another person laugh.

So sitcoms -- or anything else -- seem funnier to us when we hear other people laughing at them. We’ve evolved to be that way.

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Clearly, there are many different types of laughter. The explosion of laughter after being tickled is obviously different from the tight-lipped chuckle you force out of yourself when your boss tells a bad joke.

To account for the differences, some researchers divide laughter into two groups. The first includes spontaneous laughter. The other group includes laughter that is less spontaneous: it includes fake laughter, nervous laughter, and other social laughter that is unconnected to humor.

Some argue that this nonspontaneous laughter might also include a diabolical cackle or the cruel, jeering laughter that we once heard on the playground.

“Laughter does have a dark side,” says Provine. “When gangs or groups of militants attack someone, they are often reported to laugh while doing it.” It’s the sinister aspect of laughter’s power to form group cohesion. Sometimes, those bonds can be used to exclude or persecute others.

According to some researchers, these two types of laughter -- spontaneous and nonspontaneous -- actually have different origins in the brain. The spontaneous laughter originates in part from the brainstem, an ancient part of the brain. So it might be a more original form of laughter. The other type of laughter comes from parts of the brain that developed more recently, in evolutionary terms.

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Do Animals Laugh?

While humans might fancy themselves as the only animal capable of laughter, evidence suggests otherwise. In fact, apes seem to laugh after a fashion. They make a distinctive open-mouthed ‘play face’ and pant rapidly.

“The ‘ha, ha’ noise of human laughter,” Provine tells WebMD, “ultimately has its origins in the ritualized panting laughter of our primate ancestors.” Some researchers have found laugh-like behavior in other animals, even in the rat.

But it’s not just coincidence that all stand-up comics have been human. While they may laugh, animals -- with the possible exceptions of some primates -- don’t seem to have a sense of humor.

So if not at jokes, what do animals -- and what did our ancestors -- laugh at? According to Provine, animal "laughter" follows tickling, rough and tumble play, or chasing games. Apes laugh at some of the same things that make infants laugh. While babies aren’t known for subtle wit, they will squeal and laugh when you chase them or tickle them. In all likelihood, early adult humans -- before they started telling jokes -- laughed at the same sort of thing.

Which leads us to an interesting conclusion: Since laughter predates speech, the first human laugh predated the first joke by hundreds of thousands or years, if not millions. It’s a long time to wait for a punch line.

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Dying Laughing

Happily, laughing itself is seldom lethal. But in some people with underlying health conditions, occasionally, jokes can kill. For instance, some unlucky laughers have had heart attacks, strokes, and embolisms when cracking up.

According to Provine, there is some historical evidence that tickling was used as a method of torture and execution in centuries past. In one reported and exceedingly bizarre technique, a victim was tied up and the soles of his feet were covered with salt. A goat was then brought in to lick the salt, causing intense tickling. If kept up for long enough, the stress and exertion of laughing -- and squirming -- could have eventually brought on cardiac arrest or a brain hemorrhage.

Using ‘Laugh Therapy’

We’ve all heard the claim that ‘laughter is the best medicine.’ And according to many media reports, laughter is a panacea that will heal your immune system, dull your pain, improve your memory, lower blood pressure, and perform other wondrous feats.

But does this mean that, soon, insurance companies will start covering your movie tickets to HMO-approved comedies? Is laughter really the best, or for that matter, any kind of medicine?

The research isn’t clear. But nonetheless, the last few decades have witnessed the rise of "laugh therapy" and other approaches that are based on the notion that laughter is healing.

Wilson is a proponent. He calls himself a "joyologist" and teaches people, business groups, and aspiring laugh therapists how to laugh.

Some other laugh therapists might dress as clowns or sell CDs of themselves telling jokes to the laughless. Of course, if being hilarious is so easy for anyone with a certificate in laugh therapy, why do professional comics like Dave Chappelle get $50 million contracts?

This hits on one problem with a treatment based on humor -- it doesn’t account for taste. Some people like Adam Sandler; others would rather put their heads in a vise than see one of his films. Humor is a very subjective thing.

Wilson gets around the troublesome issue of taste by skipping the jokes.

“I don’t use humor,” he says. Instead, he just starts encouraging people to laugh. And because laughter is contagious, they do.

When Wilson leads a group, he aims to produce a spontaneous, unforced mirthful laugh, which he believes may have health benefits. “It can be almost trance-like,” he says. He fuses his approach with some eastern, yoga-like traditions that he claims are “probably about 5,000 years old.” He says that people in his class can laugh for as long as two to three hours.

Laughter for Your Health

However, Provine says he is skeptical about the health benefits of laughter. “I don’t mean to sound like a curmudgeon,” says Provine, “but the evidence that laughter has health benefits is iffy at best.”

He says most studies of laughter have been small and problematically conducted. He also says that the bias of the researchers is too evident; they want to prove that laughter has benefits. After all, we’d all like to believe that good-humored, happy people are rewarded with long lives. Who wants to believe that being a boring, mirthless jerk is the surefire way to live past 100?

Provine also points out that it’s difficult to separate the effects of laughter, specifically, from all of the other things that go with it.

“It’s part of a larger picture,” says Provine. “Laughter is social, so any health benefits might really come from being close with friends and family, and not the laughter itself.”Wilson agrees that there are limits to what we know about the benefits of laughter.

“Laughing more could make you healthier, but we don’t know,” he says. “I certainly wouldn’t want people to start laughing more just to avoid dying -- because sooner or later, they’ll be disappointed.”

But he and Provine agree that whether laughter actually improves your health or not, it undeniably improves your quality of life.

“Obviously, I’m not antilaughter,” says Provine. “I’m just saying that if we enjoy laughing, isn’t that reason enough to laugh? Do you really need a prescription?”

By R. Morgan Griffin, reviewed by Louise Chang, MD

SOURCES: Gervais, M. Quarterly Review of Biology, December 2005; vol 80: pp 395-430. Panksepp, J. Psychological Science, December 2000; vol 9: pp 183-85. Robert R. Provine, professor of psychology and neuroscience, University of Maryland Baltimore County; author, Laughter: A Scientific Investigation. Provine, R. Laughter: A Scientific Investigation, Penguin, 2000. Rosner, F. Cancer Investigation, 2002; vol 20: pp 434-36. Steve Wilson, MA, CSP, psychologist, Columbus, Ohio; founder, World Laughter Tour; board member, American Association for Therapeutic Humor, Columbus, Ohio.