How we change what others think, feel, believe and do
The impact of a persuasive message will generally tend to decrease over time. However, under the right circumstances the sleeper effect predicts that a message from a low-credibility source can actually increase in persuasiveness.
Low credibility may be caused by a discounting cue, such as when a prediction of improving economic conditions is given by a government spokesperson (who is presumed to be biased). However, when the message eventually gets separated from its source (by dissociation), the message may gain more credibility.
Evidence for the sleeper effect is limited and inconsistent. One of the findings is that if the impact of a persuasive message does not increase with time, if it is given with a low-credibility source with a discounting cue, then the impact decline is at least slowed.
I was going to the races and a work friend (who knows little about horses) wrote down the name of three horses of which he had heard. When I pulled out the piece of paper I had forgotten who wrote it, but noticed that one of the horses had won. I consequently bet on all of the other horses. I did not win.
Make the message more dramatic than the deliverer. Once the message catches on, the source may be safely (and desirably) forgotten.
When making a decision based on specific evidence, deliberately recall the source and hence credibility of the data.