If we were told that sometimes we think we are more of a political speech or a propaganda announcement after a few months of watching it than at the very moment we are receiving it, we would surely say that this is simply impossible.

However, in both social and experimental psychology, the existence of the sleeper effect has been raised , a strange phenomenon that occurs when, after a few weeks, our attitudes towards a persuasive message change significantly.

This phenomenon is extremely rare and it has even been suggested that it is not something that really happens, but several explanations have been given and attempts have been made to address it experimentally. Let’s see what this is all about.

Sleeper effect: what is it?

The effect of the sleeper is a curious phenomenon, proposed from social and experimental psychology, which holds that, sometimes, a message that was intended to be persuasive from the beginning, instead of being assimilated immediately, will receive greater force when a certain time has passed .

Normally, when something is said or shown that carries a message, be it political, opinion, ethical or of any other nature, the person usually manifests a series of immediate attitudes with respect to the content of the message itself. Depending on how credible he or she finds what the message is saying, the person will basically make one of two decisions: accept what he or she is being told or simply not accept it.

Regardless of whether you believe the information you just received to be true, it is normal that, after a certain time, you will forget the content of the message. In other words, if a person is exposed to a message of any kind, they will usually be more impressed by it immediately after receiving it than a few weeks later.

However, and according to how the effect of the sleeper is defined, what sometimes happens is that the message, which originally was not taken as credible, is taken into consideration after a few weeks . Not only does the person continue to remember what was said to him/her long ago, but he/she also manifests a whole series of attitudes that are favorable or in accordance with what was said to him/her at the beginning.

This phenomenon, as described here, may seem counterintuitive. If you doubt the content of a message from the beginning, especially because you doubt the veracity of the source of information, it is normal that, with the passage of time, you will either forget or be even more critical of what was said in it.

Historical background

The origins of the definition of this particular phenomenon can be found in the time of the Second World War. The United States had a clear interest in keeping morale high among the ranks, as well as convincing its soldiers of the need to help their allied countries, including Britain. To do this, the U.S. War Department used propaganda entertainment, especially movies, to spread a message of optimism and sympathy for the Allies.

However, despite the large investment that the United States was making in the development of these films did not seem to obtain the desired effects . That is why, through a series of experiments, he set out to see how the message was getting through to the troops. Through these experiments, it became clear that the message they intended to spread was not as well received as they thought.

It was found that those short films that were informative and that sought to strengthen certain existing attitudes related to war seemed to have a very moderate impact in the short term. However, after a few weeks, it was apparent that among the troops there was a marked increase in this optimism and support for both their nation and the allied countries.

Theories behind this phenomenon of persuasion

As we have already mentioned, the sleeper effect is a rather counter-intuitive phenomenon. Normally, when faced with a message that we doubt, the content of the message is seen in an even more critical way as time goes by , not that it ends up being seen as something true after a few weeks.

Several aspects have been proposed that attempt to explain why and how the sleeper effect occurs, although to date there is still controversy about this and it seems that, experimentally, it is difficult to replicate.

1. Forget that it is doubtful

The first to describe this phenomenon were Hovland, Lumsdaine and Sheffield in 1949. These researchers, taking the case of the American soldiers, hypothesized that some time after the message is received, it is forgotten that it has dubious aspects and the content of the message itself remains.

That is, with the passage of time , the attitudes that were manifested at the beginning are forgotten, which makes the content of the message itself gain more prominence , generating new attitudes.

This, however, is not so simple. It is quite simplistic that people, after a while, are going to change their attitudes simply by forgetting where a particular message came from or they are going to believe what was said in it all of a sudden.

The other proposal of the same research group is that in reality the origin of the message is not forgotten, what happens is that it is dissociated from the message . That is, it is known that it had a dubious origin, but it is not known which one.

In the face of this fact, the person gives it greater importance, and even gives it another opportunity to ‘see’ it more objectively, which can affect their attitudes if the original persuasive objective of the message is satisfied.

2. Different processing of content and origin

Years after the Hovland group proposed what we have seen in the previous point, the group of Pratkanis, Greenwald, Leipe and Baumgardner offered an alternative hypothesis to the previous explanation in 1988.

This research group proposed that the effect was due to the fact that people code the content of the message differently in comparison to the source from which it comes .

In other words, we know how to differentiate objectively what the message entails compared to who the source is.

As content and origin are processed differently, the origin is forgotten or lost over time, while the content or message itself remains .

By looking at the content separately from its origin, it is more likely that it can be taken as truthful.

How does it happen?

Whichever mechanism can give a more objective explanation to this strange phenomenon, in order for the message to be remembered over time it must meet the following two conditions:

1. Strong initial impact

The sleeper effect can only occur if the message that was originally broadcast has a marked and remarkable persuasive impact .

Although the person will not believe it, the fact that this message is strong makes it remain in his long-term memory.

2. Message discarded a posteriori

When a message is issued by a source of information that is not considered reliable, there is a tendency to discredit that message right from the start.

However, if the source of information is found to be unreliable, but after the message has been delivered, then the message will be better remembered, at the risk of being more suggestible in the long run .

For example, we are watching a political meeting on television and, at the end of the candidate’s speech, a presenter comes out highlighting, with evidence, all the electoral promises broken by the same candidate when he won a previous election.

Despite the fact that we have been given proof that this politician cannot be trusted, having seen the evidence after having heard the speech does not stop us from remembering what he said as he set out what he would do if he won this election.

After a few months, it is more likely that we will remember the content of the speech than the tests that were given after it ended.

Criticisms of this phenomenon

The main controversy to which this phenomenon has been exposed is the way in which it occurs. It is very difficult to contemplate the possibility that a message that has just been broadcast and whose audience has not believed it or is very much in doubt, will eventually be taken into account and even significantly change the attitudes of those who originally received it.

It has been practically impossible to replicate this phenomenon in laboratory conditions . The theories put forward by both Hovland’s and Pratkanis’ groups are not at all clear about what they mean by persuasive message and unreliable source. Experimental psychology strongly doubts that this phenomenon is plausible in real life beyond its hypothetical approach.

Bibliographic references:

  • Capon, N. & Hulbert, J., “The Sleeper Effect – An Awakening”, Public Opinion Quarterly, Vol.37, No.3, (Autumn 1973), pp. 333-358.
  • Cook, T. D., Gruder, C. L., Hennigan, K. M., & Flay, B. R., “History of the Sleeper Effect: Some Logical Pitfalls in Accepting the Null Hypothesis,” Psychological Bulletin, Vol.86, No.4, (July 1979), pp. 662-679.
  • Hovland, C.I., Lumsdale, A.A. & Sheffield, F.D, Experiments on Mass Communication: Studies in Social Psychology in World War II: Volume III, Princeton University Press, (Princeton), 1949.
  • Hovland, C.I., Weiss, W., “The Influence of Source Credibility on Communication Effectiveness”, Public Opinion Quarterly, Vol.15, No.4, (Winter 1951), pp. 635-650.
  • Pratkanis, A. R., Greenwald, A. G., Leippe, M. R., & Baumgardner, M. H. (1988). In search of reliable persuasion effects: III. The sleeper effect is dead: Long live the sleeper effect. Journal of Personality and Social Psychology, 54(2), 203-218. https://doi.org/10.1037/0022-3514.54.2.203