Coherence and the Conjunction Fallacy

The conjunction fallacy occurs when, after reading about an introductory event a, participants rate a combination of two events (b _˄ c) as more likely than a single event b. It has been proposed that the fallacy is more likely to occur when the coherence of a, b, c-the extent to which the three events fit together to form a united and complete story-is greater than that of a, b. In the current research, participants were presented with a three-event sequence made to appear coherent or non-coherent. The events were presented in one of three modalities (verbal, physical, and musical) with two studies in each modality. Compared to chance, participants were more likely to commit the fallacy in the coherent condition and less likely to commit the fallacy in the non-coherent condition. Coherence can thus serve as a conduit by which different circumstances can induce the conjunction fallacy.