Experts are authorities in their chosen field and hence persuasive because of their grasp of facts and the certainty it breeds.
In fact, the most oft-repeated advice given to budding speakers is to imitate an expert by speaking with confidence and with authority. This confidence or the manner of speaking with authoritative is considered as the most persuasive method of convincing people.
Simply put, most of us who have to interact with others, whether to convince a sales prospect or an audience put on an authoritative tone and swagger to our act.
However, Zakary L. Tormala's and Stanford Graduate School of Business doctoral candidate Uma R. Karmarkar’s research flies in the face of logic.
Their persuasion research particularly when it comes to consumers reveals "is that although non-experts can become more persuasive by expressing high certainty about their opinions, experts can become more persuasive when they express some degree of uncertainty. Across several studies, we found that expert sources gained interest and influence by expressing minor doubts about their own opinion."
Tormala said incongruity between the source’s expertise and level of certainty makes his or her message more intriguing. "Whether it’s a person without established expertise in a given domain expressing very high certainty, or a person with clearly established expertise in a domain expressing low certainty," Tormala said, "the inconsistency is surprising. It draws people in. And as long as the arguments in a message are reasonably strong, being drawn in leads to more persuasion."
Therefore, in order to be more persuasive one need not be overtly confident and all knowing. By injecting a measure of human frailty to your supporting arguments, you can indeed persuade more convincingly.