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This research explores the effect of source certainty—that is, the level of certainty expressed by a message source—on persuasion. The authors propose an incongruity hypothesis, suggesting that source certainty effects depend on perceived source expertise. In three experiments, consumers receive persuasive messages from sources of varying expertise and certainty. Across studies, low expertise sources violate expectancies, stimulate involvement, and promote persuasion when they express certainty, whereas high expertise sources violate expectancies, stimulate involvement, and promote persuasion when they express uncertainty. Thus, nonexpert (expert) sources can gain interest and influence by expressing certainty (uncertainty). Suppose that a person is reading the entertainment section in a city newspaper to find a good restaurant for celebrating a special occasion, and a food critic has listed several strong arguments in favor of a particular bistro. It seems reasonable to surmise that the more certainty, or confidence, the critic expresses about this recommendation, the more likely the reader will be to take the advice and book the reservation. However, is it possible that by voicing certainty, the critic will undermine the persuasiveness of the recommendation? Are there conditions under which the critic could gain influence by expressing uncertainty? This research explores the possibility that the effect of expressed certainty on persuasion varies according to the source’s perceived expertise. We hypothesize that whereas nonexpert sources can become more persuasive by expressing certainty about their recommendations, expert sources can become more persuasive by expressing uncertainty. In three exper-
Karmarkar et al. (Tue,) studied this question.