Sensitivity to Evidential Dependencies in Judgments Under Uncertainty

Published on May 18, 2022

Imagine you’re trying to solve a mystery, and you have two people giving you information. The first person is completely on their own and tells you what they saw. The second person, however, takes into account what the first person said before giving their own version of events. According to Bayesian models of judgment, the first person’s testimony should carry more weight because it is independent. But are we really that rational when it comes to evaluating evidence? Three experiments tested our sensitivity to this idea. Using a task involving balls and urns, participants had to decide which urn was the most likely source of evidence presented by multiple informants. The results showed that although participants did update their beliefs with additional evidence, they were generally insensitive to whether the testimonies were dependent or independent. However, those who believed that independent evidence was more valuable did give it more weight in the task. These findings suggest that most people do not conform to the Bayesian predictions for weighing independent and dependent evidence under uncertainty.

Abstract
According to Bayesian models of judgment, testimony from independent informants has more evidential value than dependent testimony. Three experiments investigated learners’ sensitivity to this distinction. Each experiment used a social version of the balls-and-urns task, in which participants judged which of two urns was the most likely source of evidence presented by multiple informants. Informants either provided independent testimony based solely on their own observations or dependent-sequential testimony that considered the testimonies of previous informants. Although participants updated their beliefs with additional evidence, this updating was generally insensitive to evidential dependency (Experiments 1 and 2). A notable exception was when individuals were separated according to their beliefs about the relative value of independent and sequential evidence. Those who viewed independent evidence as having greater value subsequently gave more weight to independent testimony in the balls-and-urns task (Experiment 3), in line with the predictions of a Bayesian model. Our findings suggest that only a minority of individuals conform to Bayesian predictions in the relative weighting of independent and dependent evidence in judgments under uncertainty.

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