COTO DE CAZA, Calif. — This is a story about the decade’s most influential, most prolific, most dynamic cultural product.

It is a story about the new way people tell stories in the 21st century: using grainy video, dramatic music, lightning-quick edits and a seemingly endless supply of people willing to play themselves on camera.

Ten years ago, a handful of shows, including “Survivor,” “Big Brother” and “Trading Spaces,” introduced a new kind of entertainment to American audiences — shows built around and starring real people and every bit as engaging as popular dramas and comedies.

Since that big bang, the universe of reality has grown exponentially. By the end of this year, nearly 600 reality series will have aired on dozens of American networks — competitions, celebrities at home, law enforcement, science, history, docu-soaps, even spoofs of other reality shows.

Political figures are acknowledging the power of reality TV. President Barack Obama recently made a cameo appearance on the Discovery Channel show “MythBusters.” Sarah Palin has drawn stellar ratings for TLC with her new reality series.

In 2001, reality shows of all kinds accounted for 20 percent of TV’s prime-time schedule; today they account for about 40 percent. And they were a key factor in Americans’ migration from network television to cable, where 90 percent of reality TV is seen.

“Nothing else has been as transformational as reality TV — the variety show, the sitcoms, there’s been nothing like it,” says Robert Thompson, professor of television and popular culture at Syracuse University.

The sheer volume of reality entertainment has fueled a demand for untrained talent that simply didn’t exist 10 years ago.

That’s your neighbor on an episode of MTV’s “True Life.” That’s a local bar-band singer winning “American Idol.” That’s your landlord getting fired by Donald Trump on “The Apprentice.”

Reality TV has given thousands of people a small piece of celebrity and gotten millions of others dreaming of it. In a 2007 poll, 51 percent of 18-to-25-year-olds said that a primary goal was to “get famous.”

With its excesses and its sometimes odd obsessions, reality TV makes an easy target for people mourning the country’s loss of civility, meaning and purpose.

“This is not reality,” says documentary filmmaker Ken Burns, creator of a series of epic takes on American history and culture. “Nobody eats bugs in front of millions of people. Nobody proposes or checks people out in front of millions of people. The notion that this is reality is beyond the pale. And what it does is become a vehicle for the same shallow consumerist mentality that is driving our country into the dirt.”

But those who are seduced by reality television say criticism like that misses the point.

“The best parts of these shows are the parts that are contrived,” says Thompson. “I feel I know a little bit more about the human condition by watching people who behave without the usual sorts of rules. Freud would have liked this sort of stuff.”

American reality TV began with the voyeuristic antics of “Candid Camera” in the 1940s and ’50s but got a serious boost when documentary filmmakers began shooting real life for television some years later.

In recent years, filmmakers increasingly conceded that their work was subject to what they call the Heisenberg Principle. The idea, adapted from quantum physics, is that the very act of documenting reality — just flipping on a camera — disturbs and, thus, alters it.

That effect is central to the debate over a police shooting in Detroit earlier this year. Seven-year-old Aiyana Stanley-Jones was killed in a raid that was being filmed by a camera crew for the A&E reality show “The First 48.” Critics say the presence of cameras may have spurred police to use more force than they would have otherwise.

But even in more ordinary dramas that make up most reality TV, opinions differ as to whether participants are performing for camera crews and their directors.

“This is participatory scripting,” says media scholar Patricia Aufderheide of American University in Washington, D.C. “The amateur is working very hard to figure out how to dramatize their life for you.”

But Douglas Ross, whose company produces “The Real Housewives of Orange County,” says that while episodes are heavily edited and slickly produced, they are taken from unforced situations.

Even though every reality show begins with an artificial premise — a competition or a dozen strangers enduring life together in one house — the format almost always lets the real-life performers write their own endings. They do this themselves, without coaching, and anything else, show producers say, would feel phony.

Many of reality TV’s “spontaneous” moments are so familiar by now they’re cliches: the shocking eliminations, the angry confrontations, the last-minute twists.

In response, reality producers “have tended to up the ante in terms of the pressures they put on contestants in order to elicit extreme reactions from them,” says television scholar James Bennett, author of “Television Personalities: Stardom and the Small Screen.”

Bennett cites the example of a recent cycle of “America’s Next Top Model,” when the producers chose contestants of varying racial and class backgrounds to live together during the show’s taping.

“This led to a constant state of conflict in the model house, which overtook the modeling as the main source of drama,” Bennett says.

However, even on reality shows where producers do relatively little gaming, the reality format has a built-in unpredictability that sets it apart from scripted TV shows.

Vicki Gunvalson talks with a reporter on Oct. 13 in her house in Coto de Caza, California. Gunvalson is the only original cast member of the “The Real Housewives of Orange County” show on Bravo, which has been picked up for a sixth season.

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