When expertise backfires: contrast and assimilation effects in persuasion.

It was proposed that source cues bias message processing in a direction opposite to cue valence if message content violates cue-based expectancies (contrast hypothesis), but consistent with cue valence if message content is ambiguous (bias hypothesis). In line with these hypotheses, students (N = 123) reported less favourable thoughts and attitudes after reading weak arguments presented by a high (vs. low) expertise source (Expts 1 and 2), and reported more favourable thoughts after reading strong arguments presented by a low (vs. high) expertise source (Expt 2). Conversely, students' thoughts and attitudes were more (less) favourable when a high (low) expertise source presented ambiguous arguments (Expt 2). Results are discussed in relation to dual- vs. single-process accounts of persuasion and models of assimilation and contrast in social judgment.