This article tackles factual and counterfactual ‘unless’ expressions such as ‘Virginia will not pass the exam unless she works harder’ and ‘Virginia would not have passed the exam unless she had worked harder’. ‘Unless’ is a negative conditional that is semantically equivalent to ‘if not’. However, some authors have claimed that ‘unless’ is more closely related to ‘only if’ than to ‘if not’. We report two experiments that compare conditional inferences from ‘unless’ to ‘if-not’’ and ‘only if’ factual and counterfactual conditionals. The first experiment compared ‘not-A unless B’ and ‘if not-B then not-A’ and showed a difference between affirmative (i.e. B therefore A, A therefore B) and negative (i.e. not-B therefore not-A, not-A therefore notB) inferences only for factual ‘if not’. The second experiment compared ‘not-A unless B’ and ‘A only if B’ and showed no difference between affirmative and negative inferences for factual ‘unless’ and ‘only if’, whereas the affirmative inferences were higher for counterfactual ‘unless’ and ‘only if’. In both experiments latency results confirmed that inferences from ‘B to A’ were faster than from ‘A to B’ for ‘unless’ and ‘only if’. The implications of the results for the mental representations and processing of counterfactual ‘unless’, ‘if not’ and ‘only if’ are discussed in the context of mental model theory.

Open Access