Focus, repacking, and the judgment of grouped hypotheses

Previous research has found that judged probabilities of two complementary singleton hypotheses sum to one. However, there may be important differences between judgment of singleton and disjunctive hypotheses. We suggest that because of a general preference for singletons as the focus of judgment, disjunctions are more likely to be repacked into singletons when focal than when alternative. This prediction of differential repacking implies that a given disjunction will be perceived as less likely when focal. Thus, we predict that when disjunctions are under consideration, the sum of judged probabilities for two complementary hypotheses will be less than one. We observe this pattern in judgment of both probability and relative frequency, and for both externally-generated and self-generated hypotheses. Copyright © 1999 John Wiley & Sons, Ltd.