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.