Does familiarity arise from direct access to memory representations (a mnemonic account) or from inferences and diagnostic cues (an attributional account)? These theoretically distinct explanations can be difficult to distinguish in practice, as is shown by the positivity effect, the increase in feelings of familiarity that accompanies positive emotion. Experiment 1 manipulated mnemonic and attributional sources of positivity via word valence and physical expressions of emotion, respectively. Both sources influenced the tendency to call items old, but receiver-operating characteristic analysis revealed a change in accuracy only with the mnemonic source. To further contrast the mnemonic and attributional accounts, Experiment 2 varied the ratio of positive to neutral words. A higher proportion of positive words exaggerated the pattern of increased old judgments and decreased accuracy for positive words, relative to neutral ones, consistent with the mnemonic account but inconsistent with the attributional account.