The effect of quality of evidence on attitude change and source credibility

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Abstract
Previous research on the probative power of evidence in persuasive communication has focused on the impact of high‐quality evidence. Results have indicated that inclusion of high‐quality evidence enhances the communicator's credibility and increases the impact of an initially low‐credibility communicator. This study examined the effect of defective evidence, evidence from questionable sources, and evidence not relevant to the issue discussed. Results indicated that inclusion of defective evidence retarded positive attitude change, particularly for a communicator with moderate initial credibility, and that inclusion of defective evidence led to significantly less positive perceptions of the communicator.