Amazingly, just the opposite is true for propaganda. If it strikes a chord with someone, this influence will only increase over time. Why? Psychologist Carl Hovland, who led the study for the war department, named this phenomenon the sleeper effect. To date, the best explanation is that, in our memories, the source of the argument fades faster than the argument. In other words, your brain quickly forgets where the information came from (e.g. from the department of propaganda). Meanwhile, the message itself (i.e., war is necessary and noble) fades only slowly or even endures. Therefore, any knowledge that stems from an untrustworthy source gains credibility over time. The discrediting force melts away faster than the message does.
Excerpt from: The Art of Thinking Clearly by Rolf Dobelli