A New Foundation for Support Theory

A new foundation is presented for the theory of subjective judgments of probability generally known as “Support Theory.” It is based on a new complementation operation that, unlike those of classical probability theory (settheoretic complementation) and classical logic (negation), does not satisfy the principle of The Law Of The Excluded Middle. Interrelationships between the new complementation operation and the Kahneman and Tversky judgmental heuristics of availability and representativeness are described, and alternative explanations are provided for experimental studies involving subjective judgments of probability. Subjective evaluations of degrees of belief are essential in human decision making. Numerous experimental studies have been conducted eliciting numerical judgments of probability, and many interesting phenomena have been uncovered. A cognitive theory was proposed by Amos Tversky and colleagues to explain some of the more prominent regularities revealed in these studies. This theory, known today as Support Theory, has its foundational basis in the articles of Tversky and Koehler (1994) and Rottenstreich and Tversky (1997), and incorporates Kahneman’s and Tversky’s seminal research on judgmental heuristics (e.g., Tversky and Kahneman, 1974). Support Theory has an empirical base of results showing that different descriptions of the same event often produce different subjective probability estimates. It explains these results in terms of subjective evaluations of supporting evidence. It ∗I would like to thank R. Duncan Luce for helpful comments on earlier drafts of this article.