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, January 28, 2015

Used by professionals...and you too!

TREsemme is a successful brand of haircare products and they cleverly advertise their products by stating ‘used by professionals’ on most of their bottles. By using the word ‘professional’ they imply that experts use their brand and thus it must be the correct product for the desired outcome; which subsequently attracts consumers towards considering buying it. It is clear that people desire to hold a correct attitude towards a certain issue and therefore they rely on an expert and trustworthy source, which is rewarding in terms of meeting the goal (in this case, having the best hair). This is a clear example of using a credible expert to influence people’s decisions.

Hovland & Weiss (1951) showed that expert and trustworthy sources were more effective in securing persuasion to various issues compared to communicators lacking in expertise and trust. Four different topics (future of movie theaters, atomic submarines, anti-histamine drugs and steel shortage), with two alternative versions were prepared; one presenting an ‘affirmative’ position and one ‘negative’ position on each issue. For each version one ‘trustworthy’ (e.g. New England Journal of Biology and Medicine, Robert J. Oppenheimer) and one ‘untrustworthy’ (e.g. Pravda, a pictorial magazine) source was used, meaning that each topic had four versions. Opinion questionnaires on each topic were administered to all subjects before the communication, immediately after the communication and a month after the communication. These obtained data on the amount of factual information acquired and the extent to which the opinion of the subject was changed in the direction of the position advocated by the communicator.

The results showed no significant difference in the amount of factual information acquired when the material was attributed to a high credible source compared to a low credible source. However, a significant difference was obtained in the extent to which opinion on an issue was changed when the material was attributed to different sources. Subjects changed their opinion in the direction advocated by the communicator when the material was attributed to a ‘high credibility’ source than when attributed to a ‘low credibility’ source. Table 4 presents these findings and it is clear on average the ‘trustworthy’ sources caused higher change in opinion than ‘untrustworthy’ sources.

By associating their product with ‘professional’ use, TREsemme are more than likely to attract customers by implying to them that their product is trustworthy and therefore, should be chosen over the thousands of other hair products available.  


Hovland, C.I., & Weiss, W. (1951). The influence of source credibility on communication effectiveness. Public Opinion Quarterly, 15, 635-650.

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