Today's Washington Post has an article by Shankar Vedantam about the difficulty of debunking misinformation:

[A] series of new experiments show that misinformation can exercise a ghostly influence on people's minds after it has been debunked -- even among people who recognize it as misinformation. In some cases, correcting misinformation serves to increase the power of bad information.

Vedantam wrote a similar article for the Post almost exactly a year ago:

The conventional response to myths and urban legends is to counter bad information with accurate information. But the new psychological studies show that denials and clarifications, for all their intuitive appeal, can paradoxically contribute to the resiliency of popular myths.

[...]

The experiments do not show that denials are completely useless; if that were true, everyone would believe the myths. But the mind's bias does affect many people, especially those who want to believe the myth for their own reasons, or those who are only peripherally interested and are less likely to invest the time and effort needed to firmly grasp the facts.

The research also highlights the disturbing reality that once an idea has been implanted in people's minds, it can be difficult to dislodge. Denials inherently require repeating the bad information, which may be one reason they can paradoxically reinforce it.

Indeed, repetition seems to be a key culprit. Things that are repeated often become more accessible in memory, and one of the brain's subconscious rules of thumb is that easily recalled things are true. [emphasis added]

The bolded portions of Vedantam's September 4, 2007 article should hold obvious lessons for journalists.

First: it should never, ever be considered acceptable to quote a candidate or official making a false claim without noting its falsity. Reporters do this all the time, justifying it by saying they're just presenting both sides, or that they aren't making the false claim, they're just reporting it, or saying they corrected three other false claims in the article. That is not sufficient: if a journalist includes a false or misleading claim in their news report -- in any form -- without indicating that is false, they are actively helping to spread misinformation.

Second: the way in which news reports debunk misinformation matters a great deal. If Candidate A lies about Candidate B, for example, the fact that Candidate A is lying should be the lede - otherwise the news report just drills the false claim into readers' and viewers' minds, allowing the misinformation to take hold before it is corrected. As I wrote in my column on Friday, the news media too often privileges lies rather than punishing them.

Here's one example from last week: the Washington Post repeated the allegation that Barack Obama had made a sexist comment in five different paragraphs before it finally got around to indicating that the allegation was false (and even then, the Post did not say clearly that it was false.)

Here's another from last week: CBS devoted 5 minutes to “lipstick,” other McCain attacks before reporting that “lipstick” attack was bogus

Much more here.