“Sleeper effect” describes a phenomenon in which messages from sources with originally low credibility cause opinion change over time. The credibility of a source as perceived by receivers of its message constitutes a central issue in the theory of persuasion, in particular with regard to its impact on attitude change. A highly credible communicator (e.g., by virtue of trust, expertise, or reliability) commands an increased probability that the receivers of a message will accept and absorb the persuasive intent of the communication. However, the effect of credibility varies during the course of persuasive effects over time. In general, the impact of a persuasive message peaks immediately after exposition and declines over time. The sleeper effect describes a contrary phenomenon for messages from low-credibility sources. Here, the immediate effect is overruled by the long-term effect: The sleeper effect is thus defined as the absolute increase in attitude change over time for receivers of a low credibility message (Hovland et al. 1949).
Increase in agreement for low-credibility communication in the long term might be due to the diminishing of initial skepticism over time. Generally, arguments and other content supporting a communicator’s conclusion are subject to being forgotten over time. For a credible communicator, receivers of a message will show a fairly high agreement immediately after exposure, but this will gradually decline over time. For a non-credible communicator, on the other hand, skepticism or antipathy will lead the receivers of the message to show little initial agreement with the communicator’s position. Over time, however, the effects of low-credibility communication take a different course: “If then the discounted source is forgotten more quickly than the content (or ‘dissociated’ from the content) agreement with the recommended opinion should increase with time” (Hovland et al. 1953, 254).
Hovland & Weiss (1951) tested the sleeper effect in an experiment by using identical communications for four different topics. These stimuli were then presented to half each of the test persons by sources considered trustworthy and untrustworthy respectively. Opinion questionnaires were presented to the subjects before the communication, immediately after the communication, and four weeks afterward. The results show that initially the communications presented by the untrustworthy source were “discounted” by the audience and had less effect on opinion than those presented by the trustworthy sources. With the passing of time, the initial differences attributable to the source disappeared. After a period of four weeks, the amount of opinion change retained from the two sources was approximately equal: “Thus there was a forgetting effect when the presentation was by a trustworthy communicator and a sleeper effect when the communication was presented by a negative communicator” (Hovland et al. 1953, 255). Hovland and Weiss’s results indicate that there is a decreased tendency over time to reject the communication by an untrustworthy communicator. “This may or may not require that the individual must be less likely with the passage of time to associate spontaneously the content with the source” (Hovland & Weiss 1953, 648). Some evidence for this was provided by Kelman and Hovland (1953), who reinstated the source at the time of the delayed opinion test by repeating the introduction of the communicator before passing out the questionnaire. As soon as the source was reactivated in the recipient’s memory by the playback, the original effects were reproduced: the highly credible source yielded a greater impact on attitude change than the less credible source.
Even today, the sleeper effect remains controversial. From a methodological point of view, the experimental design has been criticized (Capon & Hulbert 1973; Gillig & Greenwald 1974), since the original studies included no control groups, which – given that measurement took place at three points in time – would have been necessary to control for a measurement contamination. Still, Praktkanis et al. (1988) found supporting evidence for the sleeper effect given the following presuppositions: subjects have to (1) be aware of the central arguments of a message, (2) receive the disrating stimulus (i.e., the low trustworthiness of a source of opposing arguments) after the reception of the message, and (3) evaluate the credibility of the communicator after having received the disrating stimulus. Such a procedure facilitates the message and the disrating stimulus being central to the effects of the communication. Here, the sleeper effect occurs only if the effect of the disrating stimulus fades sooner than the effect of the message. This interpretation of a different course of effects depending on source credibility opposes the dissociation hypothesis elaborated in the original and in subsequent studies. A further explanation is provided on the basis of the effect of forgetting (forgetting model). Here, immediately after reception, the message of a trustworthy source is assumed to be more readily remembered than that of an untrustworthy source. Over time, as both kinds of messages are forgotten, the difference in the persuasive effects of messages presented by high- and low-credibility sources diminishes. Differences in the effects of high- and low credibility sources can be observed only temporarily.
Given the findings of the studies discussed, the sleeper effect has to be summarized as a relative effect. Over time, the advantage of high-credibility sources diminishes. The sleeper effect suggests only a temporary impact of credibility. Highly credible message sources do indeed create additional attitude change, but this is leveled out over time as either (1) dissociation or (2) forgetting occur. In the long term, a convergence of the effects of the low- and the high-credibility sources is found.
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- Kelman, H., & Hovland, C. I. (1953). “Reinstatement” of the communicator in delayed measurement of opinion change. Journal of Abnormal and Social Psychology, 48, 327–335.
- Praktkanis, A., Greenwald, A., Leippe, M., & Baumgardner, M. (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, 203–218.
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