High- and low-task-importance Ss read a strong or weak unambiguous message or an ambiguous message that was attributed to a high- or low-credibility source. Under low task importance, heuristic processing of the credibility cue was the sole determinant of Ss' attitudes, regardless of argument ambiguity or strength. When task importance was high and message content was unambiguous, systematic processing alone determined attitudes when this content contradicted the validity of the credibility heuristic; when message content did not contradict this heuristic, systematic and heuristic processing determined attitudes independently. Finally, when task importance was high and message content was ambiguous, heuristic and systematic processing again both influenced attitudes. Yet, source credibility affected persuasion partly through its impact on the valence of systematic processing, confirming that heuristic processing can bias systematic processing when evidence is ambiguous. Implications for persuasion and other social judgment phenomena are discussed.

Ss received consensus information that was either congruent or incongruent with the valence of persuasive message content. In Experiment 1 Ss believed that their processing task was either important or unimportant whereas in Experiment 2 all Ss believed that their task was unimportant. In accord with the heuristic-systematic model's sufficiency principle, high-task-importance Ss exhibited a great deal of systematic processing regardless of congruency, whereas low-importance Ss processed systematically only when they received incongruent messages; in the congruent conditions heuristic processing dominated. Attitude data generally reflected these processing differences and confirmed the additivity and attenuation assumptions of the model. The utility of the sufficiency principle for understanding motivation for elaborative processing and the relevance of the findings to understanding the processing and judgmental effects of expectancy disconfirmation are discussed.

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Studies examining message framing effects on persuasion have produced mixed results. Some studies show positively framed messages, which specify attributes or benefits gained by using a product, to be more persuasive than negatively framed messages, which specify attributes or benefits lost by not using a product. Reverse outcomes have been obtained in other studies. The authors explore a theoretical explanation for such findings by investigating whether differences in the degree to which people engage in detailed message processing account for the mixed results. The findings support the view that positively framed messages may be more persuasive when there is little emphasis on detailed processing, but negatively framed messages may be more persuasive when detailed processing is emphasized.

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