Comparability effects in probability judgments.

Psychological theories of subjective probability judgments assume that accumulated evidence (support) mediates the relation between the description of a to-be-judged event (hypothesis) and the judgment. To scale a probability from the support for a hypothesis, these psychological theories make a strong independence assumption. This assumption is stated in the form of the product rule, in which the support garnered for a particular hypothesis is independent of the support for the alternative hypothesis. In the study reported here, I asked participants to judge the likelihood of a bicyclist winning a simulated race. Results showed that the independence assumption was systematically violated. Observed judgments suggest that when a probability judgment is made, the comparability or similarity of the hypotheses on one dimension increases the weight that judges allocate to differences on the other dimensions. These results speak against the simple scalability-processing assumption of support theory, and they illustrate the need for a theory of judgment processes that describes how the similarity between hypotheses shapes judgment.