A warning to lecturers: do not rest on your laurels. Psychologists have found that an expert with a good reputation will provoke a more hostile response than someone perceived as a non-expert if he or she presents weak arguments, writes Steve Farrar.

While people are often persuaded more by experts than non-experts, researchers noted that the message had to be credible or a backlash was likely to follow.

A team of German psychologists - Gerd Bohner at Universitat Bielefeld, Markus Ruder at Universitat Erfurt, and Hans-Peter Erb at Martin-Luther-Universitat Halle-Wittenberg - gave students at the University of Mannheim a series of presentations and then tested their responses.

Groups of volunteers listened to a man described as an 18-year-old school student who was a member of a Dutch youth group, and a second man who was described as an award-winning professor and director of a renowned institute for ecology and infrastructure who was working for the Dutch government.

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Both alternately gave strong, ambiguous or weak arguments in favour of the construction of a tunnel beneath Rotterdam harbour.

The psychologists found that ambiguous arguments were more likely to be well received by the experimental subjects if they were presented by the professor.

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Predictably, the opposite reaction greeted the ambiguous argument given by the school student, a person with less expertise.

When the message was strong, a higher perceived level of expertise resulted in a better reception.

But when the arguments were weak, the psychologists detected a strong backlash effect: the volunteers had a far more negative reaction to the professor and his message than to the student.

The psychologists believed this was because the reputation of the expert heightened the students' expectation.

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When these were dashed by the presentation of a poor argument, the readers reacted in a hostile fashion.

The findings are published in the British Journal of Social Psychology.