Suppose that one day you read a post on an online forum saying, “Taking this supplement keeps you from catching colds.” Once you learn that the writer works for the company that sells the supplement, your belief vanishes at once. Yet a few weeks later, as you walk past a pharmacy, who said it has grown hazy, and only the words “I heard it keeps you from catching colds” come to mind clearly.
This phenomenon, in which a message from a hard-to-believe source has little power at first but becomes more persuasive after time passes, is called the “sleeper effect.” Put simply, it means “a message that was asleep waking up as time goes by,” and it is a concept that has long stirred debate in communication studies and persuasion research.
In this article, we will look in turn at how the sleeper effect was first discovered, how it works, the conditions it needs to appear, and finally examples in news, advertising and social media, along with ways to deal with it.
Persuasion That Wakes Up Over Time: The Sleeper Effect
A Discovery That Began with Propaganda Films for Soldiers
The sleeper effect was first observed in the U.S. Army during World War II. Psychologist Carl Hovland and his colleagues showed soldiers propaganda films explaining the meaning of the war, and then studied how the soldiers’ views changed afterward.
The researchers compared the opinions of soldiers who had seen a film with those of soldiers who had not, both 5 days and 9 weeks later. Normally the effect of persuasion fades as time passes, but on some opinions the gap between the two groups actually grew wider after 9 weeks. The researchers named this unexpected result the “sleeper effect” and published their findings in 1949 as Experiments on Mass Communication.
In 1951, Hovland and Walter Weiss followed up with an experiment focused on source credibility. One group was told that the same article had been written by a trustworthy expert or academic journal, and another group was told it came from a hard-to-believe source. Right afterward, the article from the trustworthy source was far more persuasive, but four weeks later the difference between the two groups had almost disappeared. This was because the effect of the trustworthy source shrank, while the effect of the hard-to-believe source actually grew slightly.
How the Message and the Source Drift Apart
To understand the sleeper effect, you need to know about the “discounting cue” attached to a message. A discounting cue is information that makes a message less believable, such as an untrustworthy source, a label marking it as an advertisement, or a warning like “This may not be true.”
(1) The forgetting hypothesis
This was Hovland’s original explanation. As time passes, people forget the discounting cue and remember only the content of the message, so its persuasive power revives. However, later experiments showed that people still remembered the source, so this explanation was revised.
(2) The dissociation hypothesis
This explanation holds that people do not forget the source completely; rather, the link between the source and the message loosens. When they later think about the topic, the content of the message comes to mind easily, but where it came from does not come to mind with it.
(3) The differential decay hypothesis
This explanation was proposed by Anthony Pratkanis and his colleagues in 1988. It treats the message and the discounting cue as two pieces of information pulling in opposite directions, and holds that because the discounting cue weakens faster than the message, only the power of the message remains as time passes.
Conditions for the Sleeper Effect to Appear
The sleeper effect is also famous for being very difficult to reproduce. In the 1970s, when it rarely showed up in experiments, a paper even asked whether it was time to lay the sleeper effect to rest. In response, Thomas Cook and his colleagues argued that the effect appears only when several conditions are all met.
- A persuasive message: The message itself must be sufficiently persuasive from the start.
- A strong discounting cue: The discounting cue must be strong enough to clearly suppress the initial persuasive effect.
- Enough time (time delay): Enough time must pass between the first measurement and the later one.
- A lasting message: The content of the message must still carry influence in memory at the time of the later measurement.
Through a series of experiments, Pratkanis’s team also confirmed that the sleeper effect appears much more readily when the discounting cue comes after the message than when it comes before. A 2004 meta-analysis by G. Tarcan Kumkale and Dolores Albarracín likewise reported that the effect is larger when the discounting cue follows the message and when people take in the message while thinking it over carefully.
The Sleeper Effect in News and Advertising
The sleeper effect matters outside the laboratory too, because the source being forgotten while only the content remains happens constantly in everyday life.
- Political advertising: An attack ad made by a rival candidate is viewed with suspicion at first, but over time who made the ad grows hazy, and only the attack itself remains and can influence judgment.
- Misinformation: Even a story labeled “unverified rumor” runs the risk that, as time passes, the label is forgotten and only the content of the rumor is remembered as fact.
- Advertising and sponsorship (sponsored content): Even with content marked as an advertisement, a good impression of the product may later outlast the fact that it was an ad.
That is why people taking in information need the habit of recalling the source as well when a thought comes to mind, asking, “Where did I hear this?” For those rebutting or correcting information, too, it helps to deliver the correction clearly and repeatedly rather than attaching a warning once and leaving it at that.
The sleeper effect shows that our memory stores content and source separately. Even words we once doubted can, over time, quietly settle in as if they were our own thoughts.
In an age overflowing with information, remembering who we heard something from matters as much as remembering what we heard. Tracing the source of a thought that comes to mind one more time is the first step toward judgment that is not easily swayed.
