Repeating something doesn't make it true, does it?
Repetition is effective almost across the board when people are paying little attention, but when they are concentrating and the argument is weak, the effect disappears. This is why TV advertising can be persuasive – we hear a weak argument over and over while we are paying little attention. When the argument is strong, though, it doesn't matter whether or not the audience is concentrating hard, repetition will increase persuasion.
This is what psychologists call the illusion of truth effect and it arises at least partly because familiarity breeds liking. As we are exposed to a message again and again, it becomes more familiar. Because of the way our minds work, what is familiar is also true. Familiar things require less effort to process and that feeling of ease unconsciously signals truth (this is called cognitive fluency).
As every politician knows, there's not much difference between actual truth and the illusion of truth. Since illusions are often easier to produce, why bother with the truth?
The exact opposite is also true. If something is hard to think about then people tend to believe it less. Naturally this is very bad news for people trying to persuade others of complicated ideas in what is a very complicated world. …