(Non) complementary updating of belief in two hypotheses

Past research has led to the conclusion that two competing hypotheses are r epresented dependently, and confidence in them is updated in a complementar y manner. It is argued here that confidence in two hypotheses can be repres ented either dependently or independently. Changes in confidence in the for mer case are always complementary, but changes in the latter case are compl ementary only under certain conditions. In three simulated medical diagnosi s experiments, subjects learned about two illnesses in a manner expected to lead to either dependent or independent confidence. They were then present ed with two symptoms sequentially (for each of several patients), updating confidence after each. Experiment 1 demonstrated that changes in confidence in the two illnesses were largely complementary for subjects with dependen t, but not independent, confidence. Experiment 2 showed that encouraging co nsideration of the alternative led to more complementary changes for subjec ts with independent confidence. Experiment 3 succeeded in producing complem entary changes from these subjects. Thus, complementarity does not imply de pendent confidence, nor does independent confidence imply non-complementari ty.