Charges of media bias have been flying like a bloody banner on the campaign trail. Newt Gingrich excoriated the “elite media” in a richly applauded moment during one of the Republican debates. Rick Santorum chewed out a New York Times reporter. Mitt Romney said this month that he faces “an uphill battle” against the press in the general election.

Meanwhile, just about every new poll of public sentiment shows that confidence in the news media has hit a new low. Seventy-seven percent of those surveyed by the Pew Research Center in the fall said the media “tend to favor one side” compared with 53 percent who said so in 1985.

But have the media really become more biased? Or is this a case of perception trumping reality?

In fact, there’s little to suggest that over the past few decades news reporting has become more favorable to one party. That’s not to say researchers haven’t found bias in reporting. They have, but they don’t agree that one side is consistently favored or that this favoritism has been growing like a pernicious weed.

On the conservative side, the strongest case might have been made by Tim Groseclose, a political science and economics professor at the University of California at Los Angeles. Groseclose used a three-pronged test to quantify the “slant quotient” of news stories reported by dozens of media sources. He compared these ratings with a statistical analysis of the voting records of various national politicians. In his 2011 book, “Left Turn: How Liberal Bias Distorts the American Mind,” Groseclose concluded that most media organizations aligned with the views of liberal politicians. (Groseclose determined that The Washington Post’s “slant quotient” was less liberal than news coverage in the New York Times and Wall Street Journal.)

Even with conservative-leaning sources such as the Drudge Report and the Washington Times factored in, “the aggregate slant is leftward,” said Groseclose, who describes himself as a conservative.

But that’s not the end of the story. A “meta-analysis” of bias studies — that is, a study of studies — shows something different: When all is said and done, left-leaning reporting is balanced by reporting more favorable to conservatives. “The net effect is zero,” said David D’Alessio, a communications sciences professor at the University of Connecticut at Stamford.

D’Alessio drew his conclusion from reviewing 99 studies of campaign news coverage undertaken over six decades for his newly published work, “Media Bias in Presidential Election Coverage 1948-2008: Evaluation via Formal Measurement.” The research, he says, shows that news reporting tends to point toward the middle, “because that’s where the people are, and that’s where the [advertising] money is. . . . There’s nuance there, but when you add it all and subtract it down, you end up with nothing.”

So why the rise in the public’s perception of media bias? A few possibilities:

l T he media landscape has changed.

There’s more media and more overtly partisan media outlets, too. The Internet has given rise to champions of the left — Huffington Post, Daily Kos, etc. — as well as more conservative organizations such as Drudge and Free Republic. This means your chance of running into “news” that seems biased has increased exponentially, elevating the impression that “bias” is pervasive throughout all parts of the media.

“There’s a kind of self-fulfilling perception to it,” said Robert Lichter, a pioneering media-bias researcher who heads the Center for Media and Public Affairs at George Mason University. “Once people see something they don’t like, they notice things that reinforce the belief that there’s bias” in the media as a whole.

l There are more watchdog groups focused on rooting out media bias.

Long ago, a few watchdog groups, such as the conservative AIM (Accuracy in Media) and its more liberal counterpart FAIR (Fairness and Accuracy in Reporting), kept an eye on reporters’ work. Nowadays, not just politicians criticize the media for their alleged bias; an entire cottage industry exists to highlight the media’s alleged failings. This includes ideological outfits such as Media Matters for America and the Media Research Center; the satirical “Daily Show” and “Colbert Report”; and blogs by the hundreds.

All that scrutiny of the press may suggests an inescapable conclusion: There’s something wrong with the news media. All the time.

Journalists have gotten that message, too. “Reporters have heard the criticism from the right so often that they lean over backwards to be fair to them,” said Eric Alterman, a journalist, college professor and the author of the best-selling “What Liberal Media? The Truth About Bias and the News.”

l In the public’s mind, “the news media” encompasses the kitchen sink.

Few people make a distinction between news reporting — which attempts to play it straight — and opinion-mongering, which is designed to provoke and persuade. Tellingly, when asked what they think of when they hear the phrase “news organization,” the majority of respondents (63 percent) in Pew’s news-bias survey cited “cable news,” and specifically Fox News and CNN. But while cable news networks do some straightforward reporting, their most popular programs, by far, are those in which opinionated hosts ask opinionated guests to sling opinions about the day’s news.

“A big part of the conversation on cable is [people] telling you how the rest of the media is getting the story wrong,” said Mark Jurkowitz, a former press critic and newspaper ombudsman who is now associate director of the Project for Excellence in Journalism, a Washington-based research group affiliated with Pew. That, he noted, is likely to sow more doubt about the media’s integrity or accuracy.

Of course, reporters have helped blur the very lines they want the public to respect, Lichter said, by writing up news stories and then appearing on TV or going on social media to tell people what to think about their stories.

“The modern way [for journalists] is to be edgy and opinionated and to call attention to yourself,” Lichter said.

l We know more and can second-guess.

Thanks to technology, people have more access to more sources of news than before. Which means they can check several accounts of the same event. This can create its own kind of suspicion; savvy readers often ask reporters why they ignored or played down facts that another reporter emphasized.

l People believe their preferred news sources are objective and fair, while the other guy’s are biased.

Pew’s research suggests that people think the other guy’s media are spreading lies while one’s own are, relatively, a paragon of truth.

A clear majority (66 percent) say news organizations in general are “often inaccurate.” But the figure drops precipitously (to 30 percent) when people are asked the same question about the news organization “you use most.” Jurkowitz said this is the analogue of how people feel about Congress — most give low marks to lawmakers in general, but they vote to reelect their incumbent representative more than 90 percent of the time.

“If you watch the Channel 2 newscast night after night, you trust the people on the air,” he said. “The mere fact that you’re a habituated user makes you think better of them.”

Despite the low esteem the public seems to hold for “the news media,” the good news may be that it’s all relative. Pew found last year that people said they trusted information from the news media more than any other source, including state governments, the Obama administration, federal government agencies, corporations and Congress.

The lowest degree of trust? By far, people named “candidates running for office.”