The quality of meriting belief or confidence. In social psychology, the persuasiveness of a person or message source, generally associated with prestige. Research has shown that it consists of two major components, namely perceived trustworthiness and perceived competence or expertness, and a multiplicative model is required to represent the way they combine: if either approaches zero, then so does credibility, irrespective of the value of the other. The first serious discussion of it was by the Greek philosopher Aristotle (384–322bc) in his Rhetoric, the key passage being: ‘Persuasion is achieved by the speaker's personal character when the speech is also spoken as to make us think him credible. We believe good men more fully and more readily than others; this is true generally whatever the question is, and absolutely true where exact certainty is impossible and opinions are divided’ (Book I, Chapter 2, Bekker edition, p. 1356a). See also prestige suggestion, sleeper effect.