The compatibility heuristic in non-categorical hypothetical reasoning: Inferences between conditionals and disjunctions

• People use a compatibility heuristic to make hypothetical inferences from one set of alternatives to another.
• A compatibility effect occurs for inferences between conditionals and inclusive disjunctions.
• The compatibility effect is amplified for ‘A only if B’ conditionals, and for implicit negation.
• The compatibility effect is eliminated for inferences between bi-conditionals and exclusive disjunctions.
• The heuristic generalizes to inferences between universals, ‘All A are B’ and disjunctions or conditionals.

A new theory explains how people make hypothetical inferences from a premise consistent with several alternatives to a conclusion consistent with several alternatives. The key proposal is that people rely on a heuristic that identifies compatible possibilities. It is tested in 7 experiments that examine inferences between conditionals and disjunctions. Participants accepted inferences between conditionals and inclusive disjunctions when a compatible possibility was immediately available, in their binary judgments that a conclusion followed or not (Experiment 1a) and ternary judgments that included it was not possible to know (Experiment 1b). The compatibility effect was amplified when compatible possibilities were more readily available, e.g., for ‘A only if B’ conditionals (Experiment 2). It was eliminated when compatible possibilities were not available, e.g., for ‘if and only if A B’ bi-conditionals and exclusive disjunctions (Experiment 3). The compatibility heuristic occurs even for inferences based on implicit negation e.g., ‘A or B, therefore if C D’ (Experiment 4), and between universals ‘All A’s are B’s’ and disjunctions (Experiment 5a) and universals and conditionals (Experiment 5b). The implications of the results for alternative theories of the cognitive processes underlying hypothetical deductions are discussed.