We hypothesized that people are ‘vigilant’ for differences between stimuli. In particular, we compared the reactions elicited by unexpected differences versus unexpected similarities. Participants imagined themselves in several situations where they learned something unexpected. This unexpected information showed either that two things previously thought to be similar were actually different or that two things previously thought to be different were actually similar. Also, the new information was either beneficial, neutral, or detrimental to the perceiver. Participants indicated for each situation how surprised they would be and how they would feel. Unexpected differences were rated as more surprising than unexpected similarities for positive and negative events, though not for neutral events. Participants also reported that beneficial differences would produce more positive affect than beneficial similarities, whereas detrimental differences would produce more negative affect than detrimental similarities. These findings support the asymmetrical impact hypothesis that differences have more psychological impact than similarities. Copyright © 2002 John Wiley & Sons, Ltd.