Barbara Spellman

How two causes are different from one: The use of (un)conditional information in Simpson's paradox

CO-AUTHORS Christy M. Price and Jessica M. Logan
Memory and Cognition

UVA Law Faculty Affiliations

Barbara A. Spellman


In a causally complex world, two (or more) factors may simultaneously be potential causes of an effect. To evaluate the causal efficacy of a factor, the alternative factors must be controlled for (orconditionalized on). Subjects judged the causal strength of two potential causes of an effect that covaried with each other, thereby setting up a Simpson's paradox—a situation in which causal judgments should vary widely depending on whether or not they are conditionalized on the alternative potential cause. In Experiments 1 (table format) and 2 (trial-by-trial format), the subjects did conditionalize their judgments for one causal factor on a known alternative cause. The subjects also demonstrated that they knew what information was needed to properly make causal judgments when two potential causes are available. In Experiment 3 (trial-by-trial), those subjects who were not told about the causal mechanism by which the alternative cause operated were less likely to conditionalize on it. However, the more a subject recognized the covariation between the alternative cause and the effect, the more the subject conditionalized on it. Such behavior may arise from the interaction between bottom-up and top-down processing.


Jessica M. Logan, Christy M. Price & Barbara A. Spellman, How two causes are different from one: The use of (un)conditional information in Simpson’s paradox, 29 Memory and Cognition 193-208 (2001).

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