Sleeper effect
A message from a doubtful source gains persuasive power over time.
What it means
A delayed increase in the persuasive impact of a message originally accompanied by a discounting cue, such as a low-credibility source. At first the weak source suppresses persuasion, but over time people forget the source faster than the message itself, so the discounting cue's dampening effect fades and the argument's influence rises. The effect is fragile and appears reliably only under specific conditions — notably when the message is encoded before the discounting cue is learned. It matters because it shows that the credibility of a source and the content of its message can decay independently in memory.
Examples
Weeks after dismissing a tabloid claim, people recall the claim but not its dubious origin, and find it more believable.
Months after an attack ad, voters remember the accusation but not that it came from the rival's own campaign, so the smear lands better than it did on the night.
A colleague passes on a health tip they heard from a stranger at a party, prefaced with 'probably nonsense.' A year later the caveat is gone and only the tip remains.
First described in Hovland & Weiss (1951); Pratkanis et al. (1988).