Recent research has shown that information presented by untrustworthy endorsers is likely to be thoughtfully elaborated, whereas information presented by trustworthy endorsers is likely to be unthinkingly accepted (Priester & Petty, 1995). Study 1 manipulated argument quality and assessed cognitive responses to demonstrate that this influence of trustworthiness on persuasion holds for familiar endorsers likely to be used in actual advertisements. Study 2 demonstrated that trustworthiness can be influenced by individuals endorsing too many products, with similar persuasion consequences (i.e., untrustworthy endorsers prompt greater scrutiny of product-related attributes than trustworthy endorsers). Study 2 also found that the attitudes that resulted from an untrustworthy endorser came to mind faster, even when those attitudes were equivalently positive-thus demonstrating that elaboration can influence attitude accessibility. This research provides evidence that attitude extremity may not be a sufficient indicator of advertising effectiveness, in that equally extreme attitudes can vary as to the bases by which they were formed, their underlying strength, and their effectiveness. Further, this research provides theoretical and strategic insight into the use of trustworthy and untrustworthy endorsers.The goal of advertising is to present information to potential customers. This information, it is hoped, will result in customers adopting more favorable attitudes toward the advertised product or service. These attitudes, in turn, should result in a greater probability of the customer purchasing the advertised product or using the service than if the customer had not been exposed to the advertisement. 1 A common strategy in advertising is to have an endorser present information. But what attributes are desirable for an endorser to possess? This question is particularly important given research suggesting that the process by which an attitude is formed or changed has crucial implications for the effectiveness of that attitude to guide future behavior, such as purchasing a product (see Petty, Priester, & Wegener 1994;Petty & Wegener, 1997;Priester & Fleming, 1997; see also). The goal of this article is to examine how endorser trustworthiness influences message elaboration and advertising effectiveness. A trustworthy endorser is one whom people perceive to be honest and sincere, whereas an untrustworthy endorser is one about whom people feel skepticism and suspicion (i.e., people question whether one can be trusted to communicate the truth). 2 Contemporary theories in attitudes and persuasion have introduced the notion that persuasion can be the result of qualitatively different processes. That is, the Elaboration Likelihood Model of persuasion (ELM; Petty & Cacioppo, JOURNAL OF CONSUMER PSYCHOLOGY, 13(4), 408-421 Copyright © 2003, Lawrence Erlbaum Associates, Inc.Requests for reprints should be sent to Joseph R. Priester, University of Michigan Business School, 701 Tappan Street, Ann Arbor, MI 48109-1234. E-mail: priest...