From group diffusion to ratio bias

Judgment and Decision Making. 2009;4(6):436-446

 

Journal Homepage

Journal Title: Judgment and Decision Making

ISSN: 1930-2975 (Online)

Publisher: Society for Judgment and Decision Making

Society/Institution: Society for Judgment and Decision Making and European Association for Decision Making

LCC Subject Category: Philosophy. Psychology. Religion: Psychology | Social Sciences: Economic theory. Demography: Economics as a science

Country of publisher: United States

Language of fulltext: English

Full-text formats available: PDF, HTML

 

AUTHORS

Paul C. Price
Teri V. Matthews

EDITORIAL INFORMATION

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Editorial Board

Instructions for authors

Time From Submission to Publication: 12 weeks

 

Abstract | Full Text

The group-diffusion effect is the tendency for people to judge themselves to be less likely to experience a negative outcome as the total number of people exposed to the threat increases --- even when the probability of the outcome is explicitly presented (Yamaguchi, 1998). In Experiment 1 we replicated this effect for two health threat scenarios using a variant of Yamaguchi's original experimental paradigm. In Experiment 2, we showed that people also judge themselves to be less likely to be selected in a lottery as the number of people playing the lottery increases. In Experiment 3, we showed that explicitly presenting the number of people expected to be selected eliminates the group-diffusion effect, and in Experiment 4 we showed that presenting the number expected to be affected by a health threat without presenting the total number exposed to the threat produces a reverse effect. We propose, therefore, that the group-diffusion effect is related to the ratio bias. Both effects occur when people make risk or likelihood judgments based on information presented as a ratio. The difference is that the group-diffusion effect occurs when the denominator of the relevant ratio is more salient than the numerator, while the ratio bias occurs when the numerator is more salient than the denominator.