Probabilistic theories of reasoning need pragmatics too: Modulating relevance in uncertain conditionals

According to probabilistic theories of reasoning in psychology, people’s degree of belief in an indicative conditional ‘if A, then B’ is given by the conditional probability, P(B | A). The role of language pragmatics is relatively unexplored in the new probabilistic paradigm. We investigated how consequent relevance affects participants’ degrees of belief in conditionals about a randomly chosen card. The set of events referred to by the consequent was either a strict superset or a strict subset of the set of events referred to by the antecedent. We manipulated whether the superset was expressed using a disjunction or a hypernym. We also manipulated the source of the dependency, whether in long-term memory or in the stimulus. For subset-consequent conditionals, patterns of responses were mostly conditional probability followed by conjunction. For superset-consequent conditionals, conditional probability responses were most common for hypernym dependencies and least common for disjunction dependencies, which were replaced with responses indicating inferred consequent irrelevance. Conditional probability responses were also more common for knowledge-based than stimulus-based dependencies. We suggest extensions to probabilistic theories of reasoning to account for the results.