once an idea has been implanted in people's minds, it can be difficult to dislodge. Denials ... paradoxically reinforce BAD information ...

Persistence of Myths Could Alter Public Policy Approach | By Shankar Vedantam | Washington Post Staff Writer | Tuesday, September 4, 2007; Page A03

The federal Centers for Disease Control and Prevention recently issued a flier to combat myths about the flu vaccine. It recited various commonly held views and labeled them either "true" or "false." Among those identified as false were statements such as "The side effects are worse than the flu" and "Only older people need flu vaccine."

When University of Michigan social psychologist Norbert Schwarz had volunteers read the CDC flier, however, he found that within 30 minutes, older people misremembered 28 percent of the false statements as true. Three days later, they remembered 40 percent of the myths as factual.

Younger people did better at first, but three days later they made as many errors as older people did after 30 minutes. Most troubling was that people of all ages now felt that the source of their false beliefs was the respected CDC.
..
... 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 research is painting a broad new understanding of how the mind works. Contrary to the conventional notion that people absorb information in a deliberate manner, the studies show that the brain uses subconscious "rules of thumb" that can bias it into thinking that false information is true. Clever manipulators can take advantage of this tendency.
...
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.
...
... In politics and elsewhere, this means that whoever makes the first assertion about something has a large advantage over everyone who denies it later.

Furthermore, a new experiment by Kimberlee Weaver at Virginia Polytechnic Institute and others shows that hearing the same thing over and over again from one source can have the same effect as hearing that thing from many different people -- the brain gets tricked into thinking it has heard a piece of information from multiple, independent sources, even when it has not. Weaver's study was published this year in the Journal of Personality and Social Psychology.
...
Mayo found that rather than deny a false claim, it is better to make a completely new assertion that makes no reference to the original myth.
...
So is silence the best way to deal with myths? Unfortunately, the answer to that question also seems to be no.

Another recent study found that when accusations or assertions are met with silence, they are more likely to feel true, said Peter Kim, an organizational psychologist at the University of Southern California. He published his study in the Journal of Applied Psychology.

Myth-busters, in other words, have the odds against them.