The psychology of dynamic probability judgment or

Authors: Jean Baratgin Guy Politzer

Publish Date: 2007/01/19

Volume: 6, Issue: 1, Pages: 53-66

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Abstract

The Bayesian model is used in psychology as the reference for the study of dynamic probability judgment The main limit induced by this model is that it confines the study of revision of degrees of belief to the sole situations of revision in which the universe is static revising situations However it may happen that individuals have to revise their degrees of belief when the message they learn specifies a change of direction in the universe which is considered as changing with time updating situations We analyze the main results of the experimental literature with regard to elementary qualitative properties of these two situations of revision First the order effect phenomenon is confronted with the commutative property Second an apparent new phenomenon is presented the redundancy effect that is confronted with the idempotence property Finally results obtained in this kind of experimental situations are reinterpreted in the light of pragmatic analysisLet the prior probabilities be Pfreed A Pfreed B and Pfreed C they are equal to 1/3 “all three equally likely” The likelihoods are P“executed B”freed A = 1/2 because if A is freed the warder can indicate that either B or C will be executed and we are supposing that the warder has no preference between B and C P“executed B”freed B = 0 because if B is freed the warder cannot indicate B executed and P“executed B”freed C = 1 because if C is freed the warder can only indicate B executedThree men A B and C were in jail A knew that one of them was to be set free and the other two were to be executed But he didn’t know who was the one to be spared To the jailer who did know A said “since two out of the three will be executed it is certain that either B or C will be at least You will give me no information about my own chances if you give me the name of one man B or C who is going to be executed” Accepting this argument after some thinking the jailer said “B will be executed” Thereupon A felt happier because now either he or C would go free so his chance had increased from 1/3 to 1/2 This prisoner’s happiness may or may not be reasonable What do you thinkThe version we used in the experiment was similar to Shimojo and Ichikawas 1989’ version The main difference is that we asked participants to give two judgements of probability A first judgment took place before the warder released his message This step was meant to know which prior degrees of belief participants used and to see if they were equal to 1/3 as assumed by the experimenter A second judgement was asked after the release of the warder’s message in order to analyse participants’ dynamic process of revision

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