Decades of research have investigated the complex role of source credibility in attitude persuasion. Current theories of persuasion predict that when messages are thoughtfully scrutinized, argument strength will tend to have a greater effect on attitudes than source credibility. Source credibility can affect highly elaborated attitudes, however, when individuals evaluate material that elicits low attitude extremity. A recently proposed model called the guru effect predicts that source credibility can also cause attitudinal change by biasing the interpretation of pragmatically ambiguous material. The present studies integrate models of explanatory pragmatics and persuasion in order to empirically assess these hypotheses. Experiment 1 found that text difficulty and attitude neutrality reflect independent persuasion variables. Experiment 2 found that higher source credibility causes more favorable attitudes toward messages eliciting low attitude extremity. Text difficulty was not found to have a significant effect on attitudes. These results confirm the predictions of prior social cognition research but no do not support the guru effect model. The implications of these studies for pragmatics and persuasion research as well as the value of interdisciplinary research between these fields are discussed.