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The Sleeper Effect

    People Remember the Message, But Forget the Messenger
    Working as a senior psychologist in the War Department during World War II, Carl Hovland and colleagues studied attitude change and persuasion (Experiments on Mass Communication by Hovland, Lumsdaine and Sheffield, 1949).

    The military wanted to know if its propaganda films, such as Frank Capra's Why We Fight. were making the impact they expected on soldiers' opinions and attitudes, and were disappointed to find they were not.

    It was eventually learned that some of the films did make a impact, but only after months had passed. The soliders knew the films were army propaganda when they saw them, so they discounted their objectiveness, their credibility. But as time passed, while they remembered the messages, they forgot the untrustworthy source. Thus, the films made a stronger impact on attitudes and opinions with the passage of time. Hovland and colleagues called this the sleeper effect.

    After the war, Hovland and Yale colleague Walter Weiss did a much more controlled study (e.g. multiple sources/ communicators) to understand the influence of the prestige factor (low-prestige vs high-presitge sources) on the acquisition and effect of information and its retention over a period of time (The Influence of Source Credibility on Communication Effectiveness).

    The study created essays on four different topics (e.g, Can an atomic-powered submarine be built at the present time?, Should anti-histamine drugs be sold without a prescription?) and two different versions for each topic: an affirmative and negative position on each issue. And then for each version, one trustworthy and untrustworthy source was used. For the atomic submarine topic, for example, the trustworthy source was Nobel Prize-winning nuclear physicist Robert J. Oppenheimer, and the untrustworthy source was the Soviet newspaper Pravda.

    The study's Yale grad student test subjects were each give a booklet with four different essays, two with negative and two with affirmative positions, and two with trustworthy sources and two with untrustworthy subjects. In other words, everything was carefully mixed.

    The subjects were tested on their knowledge, attitudes and opinions on the four subjects prior to reading the essays, immediately after reading them, and then a month later.

    The results of the study were that source credibility initially made a big difference in how the information was processed by the subjects:
    Subjects changed their opinion in the direction advocated by the communicator in a significantly greater number of cases when the material was attributed to a "high credibility" source than when attributed to a "low credibility" source.

    But then after a month, the difference in opinion and knowledge disappeared. The subjects could also no longer remember the source.
    There was a decrease after a time interval in the extent to which subjects agreed with the position advocated by the communication when the material was presented by trustworthy sources, but an increase when it was presented by untrustworthy sources.