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What are sleeper effects in propaganda research?

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In the context of propaganda research, sleeper effects refer to long-delayed reactions to a persuasive message. These effects occur when propaganda penetrates a person's psychological resistances and insinuates itself into their mind, but the resulting change in attitude or behavior does not become visible until a significant amount of time has passed—often long after the initial exposure or the conclusion of a formal research experiment.1

Because these reactions are not immediate, they pose a significant challenge to the scientific method of measuring propaganda effectiveness, particularly in controlled laboratory settings where the observation period is limited. To identify these delayed responses, researchers may employ panel interviews, which involve repeated, long-term engagement with a representative group of individuals to track shifts in their belief systems and opinions over time.1