There has been evidence to suggest that an overheard communication is more effective than a usual communication. That’s why advertisers sometimes use the “overheard communicator” trick, in which the source tells a buddy about a new product that really works. As if eavesdropping on a personal conversation, viewers assume that what one friend says to another can be trusted.
If a communication is overheard, the communicator can have no ulterior motive. If it is not overheard, but presented directly, then the communicator may be suspected of giving desirable information for his own ends.
A study by Walster and Festinger (1962) supported this notion. In their first experiment, participants (who are students in the Standford University) were told to try out a common technique for analysing group behaviour, “blind listening”, during a tour of the social psychology laboratory’s observation room. The students were also told that the observation room is rarely in use for experiments and graduate students usually use it as a lounge. One-half of the groups were given instructions to convince them that the graduate students in the lounge would know they were listening while the other half were led to believe that the graduate students would be unaware of their presence. In both conditions, subjects were told to try out a little blind listening on the graduate students who were talking. The “conversation” was actually a taped 6-minute persuasive communication discussing the common “misconception” that smoking causes lung cancer. A series of (non-existent) studies were described which showed there was no relationship between smoking and cancer when all confounding factors were eliminated. In fact, the data were said to suggest that smoking might even be beneficial since it released tension. A week after the “tour”, a mimeographed questionnaire purporting to be from the National Institutes of Health was given to the participants during a regular class by a second experimenter. The “medical survey” included four questions concerning students’ opinions about the causal relationship between smoking and cancer. The results showed that when subjects touring the observation room “overheard” a persuasive communication, they significantly changed their attitudes more than when they were told that speakers knew they were listening.
Walster, E., & Festinger, L. (1962). The effectiveness of "overheard" persuasive communications. Journal of Abnormal and Social Psychology, 65, 395-402.