Behaviour Change

PROPAGANDA FOR CHANGE is a project created by the students of Behaviour Change (ps359) and Professor Thomas Hills @thomhills at the Psychology Department of the University of Warwick. This work was supported by funding from Warwick's Institute for Advanced Teaching and Learning.

Wednesday, February 3, 2016

“..Because the dentist said so”

This commercial is an example of employing a credible source to persuade consumers to buy this particular brand of toothpaste over any other toothpaste brand available. This method of persuasion dates back to around 333 BCE, where Aristotle defined “ethos” (good character) as a facet for effective communication.  A credible source can be identified as an expert and trustworthy source (Hovland and Weiss (1951), a physically attractive source (Chaiken, 1979), similar to the target (Brock, 1965), an authority (Bickman, 1974) and having high social status (Lefkowitz, Blake & Mouton, 1995). This advertisement cleverly endorses all these attributes by using an expert in the field, a dentist (or someone who appears to be one) to announce the benefits of this toothpaste. A dentist also fulfills the role of authority for who else can tell you which toothpaste is the best besides a dentist? Further, a dentist is also regarded to be highly educated, experienced and knowledgeable thus maintaining a high social status. The commercial also recruits two attractive women who represent different ethnic groups so consumers can identify themselves with these women.

Hovland and Weiss (1951) demonstrated this effect by examining the effectiveness of using a “trustworthy” communicator and an “untrustworthy” communicator. Two groups were exposed to conditions of “high credibility source” and “low credibility source” in four different topics; prescription of anti histamine drugs without a doctor’s prescription, building an atomic-powdered submarine, blaming steel industry for steel shortage and decrease in movie theatres as a result of TV.  For example, for the topic of prescribing anti histamine drugs without a doctor’s prescription, the high credibility source was the New England Journal of Biology and Medicine and the low credibility source was a monthly mass circulation pictorial magazine called Pravda. Opinion questionnaires were used to assess communicator effectiveness. 

When subjects were asked about the fairness and justifiability of the information presented, their responses showed a significant difference between the high credibility sources and low credibility sources. Information received from a high credibility source was considered more fair and justifiable compared to information received from a low credibility source. Results further showed that subjects’ changes in opinion were significantly related to the trustworthiness of the source. 

So why do we rely on source credibility when making decisions? Hovland et al. (1953) states that it is because people have a desire to hold a correct attitude, thus relying on a source who is an expert and is trustworthy is an effective method in meeting this need. 


Aristotle (circa 350 B.C.E./1954). Rhetoric. New York: Modern Library

Bickman, L. (1974). The social power of uniform. Journal of Applied Social Psychology, 4, 47-61.

Brock, T. (1965). Communicator-recipient similarity and decision change. Journal Of Personality And Social Psychology, 1(6), 650-654.

Chaiken, S. (1979). Communicator physical attractiveness and persuasion. Journal Of Personality And Social Psychology, 37(8), 1387-1397.

Hovland, C., & Weiss, W. (1951). The Influence of Source Credibility on Communication Effectiveness. Public Opinion Quarterly, 15(4), 635.

Lefkowitz, M., Blake, R., & Mouton, J. (1955). Status factors in pedestrian violation of traffic signals. The Journal Of Abnormal And Social Psychology, 51(3), 704-706. 

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