Assessing the consequences of actions and events often requires comparing a mental stimulation of the world in which the action is present to one in which the action is absent. We propose that people perceive more impact when asked to assess whether an action would increase the likelihood or degree of a potential outcome (mental addition) than when asked whether it would reduce the probability or extent of a potential consequence (mental subtraction). This judgmental asymmetry occurs because people (a) give more weight to features of the particular mental stimulation (the action or its absence) serving as the subject of comparison and (b) give more weight to factors that produce as opposed to inhibit the relevant outcome. In 4 studies, Ss assessed the impact of personal actions (e.g., studying for an exam). Ss perceived more impact when the assessment was placed in an additive frame (e.g., "how many more questions will you get right if you study?") as opposed to a subtractive one (e.g., "how many fewer will you get right if you do not study?"). This effect was not influenced by the hedonic value of the event or by whether the S had actually experienced it. Discussion centers on the relevance of these results for the undoing of scenarios and causal attribution.