Another excellent Washington Post article on bias by Shankar Vedantam: 

The federal Centers for Disease Control and Prevention recently issued a flier to combat myths about the flu vaccine … When … social psychologist Norbert Schwarz had volunteers read the CDC flier, however, he found that … three days later, they remembered 40 percent of the myths as factual. … Most troubling was that people … now felt that the source of their false beliefs was the respected CDC.  … The research, which has been confirmed in a number of peer-reviewed laboratory experiments, have broad implications for public policy. …

The psychological studies highlight … the potential paradox in trying to fight bad information with good information. … 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. …

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. … People are not good at keeping track of which information came from credible sources and which came from less trustworthy ones, or even remembering that some information came from the same untrustworthy source over and over again. …

Ruth Mayo … found that for a substantial chunk of people, the "negation tag" of a denial falls off with time. … explaining why people who are accused of something but are later proved innocent find their reputations remain tarnished. … 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 … Myth-busters, in other words, have the odds against them.

I guess this goes some way to answering Ute’s question, and "what evidence in silence."