SUPPORT THEORY - A NONEXTENSIONAL REPRESENTATION OF SUBJECTIVE-PROBABILITY

This article presents a new theory of subjective probability according to which different descriptions of the same event can give rise to di fferent judgments. The experimental evidence confirms the major predic tions of the theory. First, judged probability increases by unpacking the focal hypothesis and decreases by unpacking the alternative hypoth esis. Second, judged probabilities are complementary in the binary cas e and subadditive in the general case, contrary to both classical and revisionist models of belief. Third, subadditivity is more pronounced for probability judgments than for frequency judgments and is enhanced by compatible evidence. The theory provides a unified treatment of a wide range of empirical findings. It is extended to ordinal judgments and to the assessment of upper and lower probabilities.