How pragmatic interpretations arise from conditionals: Profiling the Affirmation of the Consequent argument with reaction time and EEG measures

Conditional reasoning consists in combining a conditional premise with a categorical premise and inferring a conclusion from them. Two well-known conditional arguments are Modus Ponens (MP: If P then Q; P//therefore Q), which is logically valid and Affirmation of the Consequent (AC: If P then Q; Q//therefore P), which is not. The latter is often accepted as true on pragmatic grounds under the generally accepted assumption that the conditional premise is transformed so as to justify a biconditional reading (p if-and-only-if q). We present results from two experiments – one using self-paced measures and the other electroencephalography (EEG) – while comparing participants’ evaluations of MP and AC arguments. Based on prior work, we anticipated finding two types of individuals as a function of their endorsing or rejecting AC arguments. The self-paced task (Experiment 1) shows that Rejecters of AC arguments are linked to especially long response times with respect to Endorsers and, critically, shows that AC argument’s minor premise prompts slowdowns among all participants compared to MP’s. The EEG study (Experiment 2) reveals that the minor premise in AC arguments prompts both Rejecters and Endorsers to produce an N200 wave, which the literature associates with a violation of expectations. These findings suggest that the second premise of AC arguments arrives unexpectedly even among those who are ultimately Endorsers. We describe how these findings impact the conditional reasoning literature.

► Minor premise of AC is unexpected compared to that of MP for all participants.
► Minor premise elicits an N200 for both endorsers and rejecter of AC arguments.
► Longer reading times on conditional arguments for rejectors of AC arguments.