When political candidates say things that aren't true they're exploiting the marketplace of ideas at its weakest point. That's because although the marketplace handles the clash of opinion reasonably well, it has never done a good job filtering out falsity.

Falsehoods are to the world of thought what cockroaches are to the animal kingdom. They withstand being refuted and discredited the way roaches survive radiation, drought and flood. Even when they're recanted people keep on believing them, especially if they dovetail with firmly held beliefs. Even among skeptics, the stain on someone who's wrongly vilified is rarely wiped away.

Lately, the news media have gotten aggressive on falsehoods, and the practice of debunking fallacious statements put out by candidates is surging in popularity.

That's partly the legacy of the 2004 campaign, particularly the widespread conviction among journalists that they failed the public by parroting the so-called Swift Boat Veterans' attack on Sen. John Kerry's war record without having the guts to brand it as fallacious.

And it's partly the boiling-over of a long-simmering suspicion that the venerable journalistic practices of literal accuracy and careful balance often make a mockery of truth, that a false accusation isn't nullified by letting the slandered person deny it, that ''30 seconds for Hitler, 30 seconds for the Jews'' is a formula not for fair reporting but for demagoguery and deceit.

Some organizations have created fact-checking services, among them PolitiFact (St. Petersburg Times and Congressional Quarterly), The Fact Checker (Washington Post) and www.factcheck.org (the University of Pennsylvania's Annenberg Public Policy Center). Others routinely test the accuracy of campaign claims.

That's the good news. The bad news is that exposing falsehoods doesn't seem to silence the people who propagate them. We're still hearing that Sarah Palin said ''no thanks'' to the ''bridge to nowhere,'' and that John McCain wants to halve Social Security benefits.

The worse news is that this truth-squadding may not stop people from believing falsehoods. Debunking sometimes has the opposite effect, with more people believing lies after they're discredited.

Those are among recent findings of university research on how people handle controversial information, and they raise prickly questions about how ardently the media should be testing and refuting -- all the while showcasing -- false allegations.

Brendan Nyhan of Duke and Jason Reifler of Georgia State not only conclude that ''corrections frequently fail to reduce misperceptions,'' but found a ''backfire'' effect in which ``corrections actually increase misperceptions among the group in question.''

Conservative test subjects who believed Saddam Hussein destroyed his weapons of mass destruction (WMD) right before the United States invaded were even more convinced after seeing authoritative information to the contrary.

A study by John Bullock of Yale, as reported in The Washington Post, found that among the ideologically predisposed, debunking falsehoods does little. The proportion of Democrats who regarded then-Supreme Court nominee John Roberts unfavorably soared, to 80 from 56 percent, after they watched an abortion-rights ad criticizing him, and slipped only to 72 percent when they learned the ad was so fallacious its sponsors withdrew it.

So? Plainly, truth-squadding is risky. Some people don't hear the lies until you discredit them, others will believe them even more fervently. Lies work -- not because they aren't exposed, but because on balance, exposure doesn't matter. To the faithful, the moral of the WMD story isn't high-level U.S. deceit, it's steadfast U.S. resolve despite the momentary success of a sworn enemy's trickery.

Still, I suppose that even if refutation isn't an antidote, it's still a partial remedy, and for voters who care about the probity of candidates it's information they should have It may actually spare us the most egregious lies.

To me, the problem with truth-squadding is something else: It deludes journalists into believing they're actually operating independently, when in fact it binds them even more tightly to the campaign claptrap that moves the overnight ratings, instead of the issues that the public needs addressed.

Even a truthful campaign, if that's imaginable, can still be a disgrace, if the candidates confine themselves to matters that don't matter because they've determined that addressing the ones that do does neither of them any good. By heavying up on fact-checking, the media must take care not to give drama and urgency to inconsequential lies while letting candidates skate around truths they'd rather not confront at all.

Edward Wasserman is Knight professor of journalism ethics at Washington and Lee University.

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