Source credibility is traditionally defined as the ability or willingness of a message source to provide reliable and truthful information (Kelman and Hovland 1953). Traditionally, source credibility has been conceived as consisting of two dimensions: source expertise and source trustworthiness (Mills and Jellison 1967; Rhine and Severance 1970). Source expertise refers to the extent of which a source is perceived of being knowledgeable on the message topic whereas source trustworthiness refers to the text of which the source is believed to make unbiased conclusions about a message topic. Thus, although expertise may affect a message source of making valid claims, trustworthiness is likely to influence a message source willingness to communicate valid claims (Hovland et al., 1953; McCracken, 1989). While most research does not manipulate source expertise and trustworthiness, source credibility research continues to underline the general effects of source credibility (O’Keefe 2002). It is likely, however, that assumed source e
xpertise and trustworthiness could have different individual and situational factors (Moore, Hausknecht, and Thamodaran 1986; DeBono and Klein 1993; Pornpitakepan 2004). With regard to expertise, consumers have been found to not believe advertising messages when they do not perceive that they know what they are talking about i.e. that they are experts in the related area of the message that they are passing (Karmarkar and Tormala, 2010). To explain this, in the sales context, an expert salesperson generated far greater sales than the non-expert salesperson did (Woodside & Davenport, 1974). These propositions were confirmed by Speck, Schumann and Thompson [68] whose study found that celebrities with expertise to the endorsed product would develop significantly higher brand recall than a non-celebrity; however, the difference was not statistically important. More precisely, a positive attitude towards the endorsed product may be driven by more reliable information about the product provided by the expertise of the celebrity [69].