When a politician apologizes for a nasty rumor he or she spread about an opponent, and asks people not to believe it, you can bet on one thing: More people will believe it's true. That's one of the reasons why negative campaigns so often work.
Cognitive scientists have even found evidence that people are predisposed to believe a statement is true, even if it obviously isn't. Humans' urge to affirm, rather than deny, is manifested even in body motions, according to the new study out of the University of Memphis and Cornell University.
"What our research suggests is your whole body seems to show this tendency," to want to believe, psychologist Rick Dale of the University of Memphis, said in an interview. Dale is co-author of a paper describing the research in the January issue of Psychological Science.
Dale and Michael J. Spivey of Cornell completed a project that was created by a computer scientist, Chris McKinstry, who took his own life while the research was underway.
The researchers found that participants in their study really wanted to confirm a question, especially if it was ambiguous.
"Specifically, we found that evaluating a proposition as false, exhibits more difficulty, compared with evaluating a proposition as true," they said in their report.
But that was a bonus prize. What the researchers set out to show was that, contrary to widespread beliefs, the brain shares its biases with the body, even before the cognitive processes are completed. That was revealed in the arm motions of 141 college-age participants, who were asked to decide if 11 questions were true or false.
The participants — 97 females and 44 males — were seated at a computer, and asked to move the cursor from the bottom center of the screen to one of two boxes at the top left and top right corner of the screen, if they believed a statement was true or false.
The statements, read randomly into headphones, had different levels of probability, ranging from "Should you brush your teeth everyday," an obvious yes, to "Is murder sometimes justifiable," maybe yes and maybe no, to "Is a thousand more than a billion," an obvious no.
The computer recorded the movement of the cursor, including the curvature of its course, and the time it took to check yes or no.
It took longer for the participants to decide that a statement was false, than that it was true, and in many cases, the cursor traveled first toward the yes, and then curved over to no, indicating two things: The body was in motion before the cognitive processing was completed, and the participants really wanted to believe most of the statements were true, even though they decided quickly that some of them were not.
"Thus, reasoning about the truth value of a proposition exhibits a significant prior bias toward 'truth,' and this bias must be overcome before a 'false' response can come to fruition," the researchers added in their report.
That was especially true when the statements were a bit ambiguous.
"It's very hard for you to take something and evaluate it as being not true," Dale told ABCNews.com. "It's harder for you to do that than say it's true."
He suspects the tendency, to believe something is true, plays a significant role in political campaigns.
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.