How Two Causes Are Different from One:

The Use of (Un)Conditional Information in Simpson’s Paradox


Barbara A. Spellman

University of Virginia


Christy M. Price

University of Texas


Jessica M. Logan

Washington University



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 (or "conditionalized 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), subjects did conditionalize their judgments for one causal factor on a known alternative cause. Subjects also demonstrated that they knew what information is needed to properly make causal judgments when two potential causes are available. In Experiment 3 (trial-by-trial), 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 and the legal system.