A robber points a gun at a cashier and says: “Only one of these two options is true: If you conceal the combination to the safe, then I kill you; otherwise, if you don´t conceal the combination to the safe, then I kill you.” Hearing this statement, most people conclude that, in either case, “I kill you.” This is an illusory response, in fact; the valid conclusion states “I don´t kill you.” The research reported here studied the roles that different expressions of conditionals (“if-then,” “only if,” and “if and only if”) play in the illusory response. Three experiments show that participants inferred the conclusion “I kill you” from the conditional “if-then” and “I may or may not kill you” from the conditional “only if,” while selecting both options with similar frequency for the biconditional “if and only if.” These results shed light on the main theories of deductive reasoning.
