Two fluency heuristics (and how to tell them apart)

People experience an illusion of familiarity when the fluency of their performance is enhanced without their knowledge. This illusion helps to reveal the source of appropriate feelings of familiarity occurring in the presence of repeated stimuli. That feeling might occur for one of two reasons: because of the increase in the absolute magnitude of fluency, or because the fluency becomes surprisingly great, relative to an expectation for that event. We demonstrate that people use each of these bases of decision, in different circumstances. We also offer a procedure that can be broadly applied to discriminate them.