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. Both directions of deviation from additivity are interpreted in a common framework, in which probability judgments are often mediated by judgments of evidence. The 2 kinds of nonadditivity result from differences in recruitment of supporting evidence together with reduced processing of nonfocal propositions.
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