Sleeper Effect

First studied by Carl Hovland and colleagues during WWII, when the U.S. Army used propaganda films and wanted to understand their long-term persuasive impact. They found that messages paired with a "discounting cue" (a reason to be skeptical) could actually gain credibility over time, as the source faded from memory faster than the message itself. The classic example: "you swallow eight spiders a year in your sleep" — dubious origin, but strangely persistent.