Abstract

The conjunction fallacy is the well-documented reasoning error on which people rate a conjunction A∧B as more probable than one of its conjuncts, A. Many explanations appeal to the fact that B has a high probability in the given scenarios, but Katya Tentori and collaborators have challenged such approaches. They report experiments suggesting that degree of confirmation—rather than probability—is the central determinant of the conjunction fallacy. In this paper, we have two goals. First, we address a confound in Tentori et al.’s experiments: they failed to control for the fact that in their stimuli where B is confirmed, it is also conversationally relevant in the sense that it fits with the topic or question under discussion. Conversely, when B has a high probability but is not confirmed, it is conversationally irrelevant. Consequently, it is possible that conversational relevance, rather than confirmation, is responsible for the differences they found between confirmed and probable hypotheses. Second, inspired by recent theoretical work, we aim to give the first empirical investigation of the hypothesis that this type of conversational relevance on its own—independently of degree of confirmation—can be an important factor in the conjunction fallacy. We report on two experiments that vary Tentori et al.’s design by making B relevant without changing its degree of probability or confirmation. We found that doing so increases the rate of the conjunction fallacy, suggesting that relevance plays an important role in the conjunction fallacy.