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 difference s between judgment of singleton and disjunctive hypotheses. We suggest that because of a general preference for singletons as the focus of judgment, d isjunctions 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 pre dict that when disjunctions are under consideration, the sum of judged prob abilities for two complementary hypotheses will be less than one. We observ e this pattern in judgment of both probability and relative frequency, and fbr both externally-generated and self-generated hypotheses. (C) 1999 John Wiley & Sons, Ltd.