Edit: I hacked this down quite a bit to try to make it more readable and pertinent to the question, which was helped by some comments below illustrating what the question actually was.
As pointed out below, the question is more accurately, "All else being equal (subject knowledge, intelligence, etc.), will the more eloquent individual be more persuasive?"
I'll go along with the Wiki definition found HERE:
Eloquence (from Latin eloquentia) is fluent, forcible, elegant or persuasive speaking. It is primarily the power of expressing strong emotions in striking and appropriate language, thereby producing conviction or persuasion.
- HERE is a preview for book, "Public Communication Campaigns." See the section "Conventional analysis of source variables." Among the factors increasing speaker persuasiveness are credibility (seeming honest and perceived expertise). Then follows:
Perceived source expertise, in turn, derives from characteristics such as the sources's general education level, familiarity with the subject matter, and speaking in an authoritative tone.
Unfortunately, the section entitled "Message variables that increase persuasive impact" is not available via Google, but a summary chart of factors that increase persuasiveness features this:
Message (appeal, inclusion/omission, organization, style, repetitiveness, etc.)
- THIS paper on credibility with respect to Artificial Intelligence states the following, building on the work of Aristotle:
Thus, persuasion is not solely concerned with developing sound arguments and proofs; it also involves putting the audience into a receptive frame of mind and convincing them that the speaker is a credible person.
- HERE is an article by Dr. Doric Little, a professor in speech skills, discussing the role of rhetoric in persuasion (also referring to Aristotle). Note this:
Despite the very reasonable judgement that a person with the most proof or facts should be the most persuasive, Aristotle and numerous speech researchers in this century have concluded that the credible person, the person who is perceived as trustworthy, is most persuasive.
She also cites research by Samuel Becker at the University of Iowa (LINK) as having shown the following:
... Becker found that... Content/analysis and delivery equaled about 45% each of the perceived speaking effectiveness while language constituted 10%.
I believe that THIS is the Becker study being referenced. I would bundle "facts, figures, data" with content/analysis and "eloquence, rhetoric, mastery of language, etc." with the delivery and language categories. If you agree, then Becker's study supports the idea that eloquence is at least a component of the factors that sway 65% of one's perceived credibility.
- Click on the preview for THIS book, "The Art of Public Speaking," and look at the Table of Contents. Note chapters devoted to "Analyzing the Audience," "Using Language," and "Delivery." Note that the chapters are not all variants of "Improve knowledge of the subject matter" -- in other words, eloquence matters.
The fact that anything related to eloquence is mentioned in addition to knowledge of the subject matter alone shows that eloquence is a component, and thus given two speakers:
- a subject expert, and
- a subject expert who is also eloquent
The eloquent speaker will have a higher persuasive impact.
You also asked why this might be. I'm not exactly sure. One factor might be that "practice makes perfect." The more one studies, discusses, and teaches, the more familiar one becomes with the subject matter, corresponding vocabulary, terminology, and minute details of the subject. I have found that in my area of work and in my personal areas of more intense study/reading, I find myself able to access somewhat esoteric vocabulary words and phrases that might have once caused met to previously stumble while looking for the exact wording to express a concept.
Thus, "sounding like an expert" may cause the unfamiliar to think one "is an expert." While this can be used to mislead, it can also assist in preventing others from being misled. Take the "expert sounding" speakers on creationism or homeopathy. Say you're extremely confident that such areas are false, but are not a geological expert or haven't been through med school. I consider it advantageous to be eloquent when discussing such matters as to not be outdone by my opponents and lose others into murky waters.