Sleeper Effect

The sleeper effect is a phenomenon in the psychology of persuasion in which a message's persuasive impact increases over time rather than fading, provided the message is delivered alongside a discounting cue such as a source of low credibility or a negative association; a message delivered by a credible source, by contrast, may produce an immediate positive response that weakens as time passes. The effect was first observed among United States Army soldiers exposed to wartime propaganda films, with researchers hypothesizing that the soldiers gradually forgot the discounting cue, the fact that the message was propaganda, while the content of the message itself remained. Although it has been studied extensively, the sleeper effect has proven notoriously difficult to reproduce under controlled conditions, leading some researchers to doubt whether it is a genuine phenomenon at all.