If a candidate says something, and it isn't true, people have a tendency to believe it, and that's especially true if the charge is ambiguous, like so-and-so raised taxes, or the bum cheats on his wife.
And denying that it's true, reinforces the bias to believe that it's true, even if the denial comes from the candidate who leveled the charge in the first place.
"You get free negative advertising when someone gets up there and says it's not true," Dale said. "If they mention it enough times, people are going to remember it as something that was true."
That's not to say that ambiguity and repetition always lead to confirmation, he added. In time, people can overcome their bias to believe the affirmative, especially if there is evidence it isn't true. But the bias to believe will remain in effect, and "that's scary," Dale said.
Lee Dye is a former science writer for the Los Angeles Times. He now lives in Juneau, Alaska.