Even worse was that people now attributed these false beliefs to the CDC itself! In an effort to dispel misconceptions about the vaccine, the CDC had inadvertently lent its prestige to them. In many cases, truth and elucidation can actually strengthen misconceptions and make them more psychologically available.
Related studies by Kimberlee Weaver of Virginia Polytechnic University and others (link below) have shown that being repeatedly exposed to information from a single source is often tantamount to hearing it from many sources. People simply forget where they heard something after a while, and the repetition makes it more psychologically available and hence credible.
In one simple experiment, groups of students were given an e-mail to read, some groups receiving a version that seemed to have a software bug in it since the same crucial paragraph was reprinted two more times at the bottom of the e-mail. Those students with the repeated information were more persuaded by the e-mail and overestimated the information's general appeal than did students who had read the non-repetitive e-mail.
Trying to dispel myths can backfire. It's not just confirmation bias that helped sell people on the Iraq War. When war opponents denied the Bush administration's repeated assertions that Saddam Hussein was behind the 9/11 attacks, they were probably strengthening, at least in the minds of some, the belief that he was indeed involved. Likewise, the effect on many in the Arab world of denials that the hijackers were really Westerners, that Jews were warned to stay home, et cetera, also may have been to strengthen that flapdoodle.
The difficulty in processing denials is probably part of the reason for their frequent ineffectiveness. Complexity and logical connectives get lost in transmission. (Quick, what does the following sentence mean? "It's not the case that Waldo denied that evidence was lacking that he did in fact fail to do X.") In fact, the new research suggests that people quite often mentally transmute a denial into an assertion. They hear "X is not this bad thing Y," and soon enough what remains with them is the link between X and Y, which eventually becomes "X is Y."
These foibles, it should be repeated, affect our beliefs about many disparate phenomena, not just political issues. Health scares are a rich source for them. Sticking with the vaccine theme, I note that the denial by medical researchers and scientists of a connection between mercury in childhood vaccines and the increased incidence of autism (defined now to include Asperger's syndrome and other "autism spectrum disorders") has not quelled the controversy over the vaccines or weakened the belief that mercury was the culprit. The number of cases cited for all such disorders also serves to anchor our estimate of the prevalence of autism proper.
Being aware of these biases will presumably help us minimize their effects. One specific lesson from this research seems clear. Denials of assertions should in general not repeat the assertions. It's better to say "X is this good thing Z" rather than "X is not this bad thing Y."