Recent studies have demonstrated subadditivity of human probability judgment: the judged probabilities for an event partition sum to more than 1. We report conditions under which people's probability judgments are superadditive instead: the component judgments for a partition sum to less than 1. Weinterpret both directions of deviation from additivity in a common framework, in which probability judgments are often mediated by judgments of evidence. The two kinds of nonadditivity result from di#erences in recruitment of supporting evidence together with reduced processing of nonfocal propositions. 2 Introduction Suppose that an event E has been partitioned into two or more mutually exclusive subevents, and that probability assessments are made for E and for each of these subevents. The assessments are said to be additive if the probability assigned to E is approximately equal to the sum of the probabilities of the subevents. They are subadditive if the probability assigned to E...
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Macchi et al. (1999) studied this question.