Now here's a disturbing thought: people will tend to believe more in an assertion simply because they here it repeated. This was originally discussed in a paper by Hasher, Goldstein, and Toppino (1977). A more recent discussion can be found in this 1997 paper. This helps explain why repeated canards (like the "Obama's a muslim" meme) eventually results in vast swaths of the population reporting the same opinion in national polls. As Napoleon is said to have said (and I've heard this repeated many a time), the only rhetorical device needed for serious topics is repetition.