Sleeper Effect In Psychology: Why Persuasion Takes Time

Sleeper effect in psychology is that attitudes can change months after a persuasive message is delivered.

sleeper effect

Sleeper effect in psychology is that attitudes can change months after a persuasive message is delivered.

The sleeper effect in psychology is the finding that persuasive messages increase in power over time.

In other words, the sleeper effect means that when you try to change someone’s mind, it can take a while to work.

The sleeper effect was first documented in U.S. soldiers during the Second World War.

History of the sleeper effect

In the 1940s during WWII, the US Department of War wanted to know if their propaganda films were really working.

So they carried out a series of experimental studies into how they affected soldier’s attitudes.

The complacent assumption was that the films should easily influence the average GI.

Producers and psychologists alike expected to see a huge shift in attitudes towards the war after they were viewed.

What they found was nothing of the sort and the results came as a bitter blow to propagandists everywhere.

While the films were informative and did strengthen some existing attitudes, experiments showed they were extremely unlikely to make soldiers more optimistic about the war in general (Hovland et al., 1949).

In retrospect this should have come as little surprise since the soldiers knew these were propaganda films designed to change their attitudes, so their defences were up.

What they did discover, though, was that some of the films did have an effect on soldiers after months had passed.

While attitudes didn’t change immediately, subtle shifts were picked up nine weeks later.

US soldiers who watched one film about The Battle of Britain showed little extra sympathy towards the British five days later, but, after nine weeks, they had softened.

Yale University’s Carl Hovland and colleagues called this the ‘sleeper effect’.


Since then the sleeper effect has had a rockier history than the average soap opera character.

The sleeper effect has gone up and down in the esteem of psychologists over the years as some experiments have confirmed the effect and others have found nothing.

Quite naturally many people wondered whether the sleeper effect really exists, especially as it goes against common sense.

Changes in attitudes should really be strongest just after a message is delivered.

Over time the persuasive effect should weaken as people’s attitudes return to how they were before—and this is what many other studies have shown.

Nevertheless, when researchers have weighed up all these studies, it seems likely the sleeper effect really does exist (Kumkale & Albarracin, 2004).

It’s just it only appears under two circumstances:

  1. Big initial impact: the sleeper effect only emerges if the persuasive message has a major initial impact. If it isn’t powerful enough, it won’t hunker down in our minds, biding its time before it boomerangs back.
  2. Message discounting: it should be obvious that the source of the message can’t be trusted so that we discredit it; like when the soldiers were watching the propaganda film.

What seems to be going on is this: people are convinced by the arguments until they see that the source of the message can’t be trusted.

But, people don’t tend to process the discounting cue very thoroughly.

So, over time, people forget they discounted the information and the content of the persuasive message, which was processed thoroughly, does its devilish work.

Avoid the sleeper effect

The sleeper effect isn’t the great news for advertisers (and advertising agencies) you might imagine.

The sleeper effect is strongest when the message is discounted afterwards.

When we watch adverts, we usually know beforehand that they can’t be trusted, so our minds are already on alert for the distinctive smell of half-truths.

There are all kinds of everyday situations where the sleeper effect occurs.

For example, when the travel supplement recommends a great resort, then we read at the bottom that the trip’s cost was covered by the resort.

Or, there’s an article telling us about the health benefits of milk and then we read at the bottom that the author is the head of the Milk Marketing Board.

Any time we receive a persuasive message before we find out who the source is, the sleeper effect can come into play.

Naturally, then, canny information consumers will want to know the source of a message before they read it.


Author: Dr Jeremy Dean

Psychologist, Jeremy Dean, PhD is the founder and author of PsyBlog. He holds a doctorate in psychology from University College London and two other advanced degrees in psychology. He has been writing about scientific research on PsyBlog since 2004.

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