Common wisdom would suggest that the more certain a person is on a subject, the more persuasive and credible we perceive them to be. However a study looking looking at how certainty affects persuasiveness and perceived credibility found that the opposite is true:
Experts are more persuasive when they seem tentative about their conclusions [â€¦] but the opposite is true of novices, who grow more persuasive with increasing certainty.
This result held across the three experiments described in the paper (pdf, doi), but it’s worth noting that this only applies in situations where there is no objective truth — such as in consumer situations (the experiments used restaurant reviews, and I imagine product reviews would give similar results):
Earlier research [â€¦] had made the case that expressing certainty generally increases people’s persuasive power, because it boosts their perceived credibility. [However] those studies concerned topics such as witnesses testifying in court or stock market advisers giving stock recommendations where there is an objective truth or correct answer. In those instances [â€¦] people might rely on a person’s certainty as an indicator of his or her credibility. “In more subjective domains like consumer contexts, though, [â€¦] expressing certainty appears to have a more dynamic effect, giving a message more or less impact depending on who is expressing it.”