An Inverse Belief–Bias Effect

Abstract

Two studies examined the hypothesis that accepting false premises as true in order to make the modus ponens (MP) inference requires inhibition of contradictory knowledge. Study 1 presented both MP and affirmation of the consequent (AC) inferences using either false, but plausible premises or completely unbelievable premises, with standard logical constructions using either an evaluation or a production paradigm. The rate of acceptance of the MP inferences was significantly greater with unbelievable premises than with plausible premises, in both evaluation and production, while no such effect was observed with the AC inferences. Study 2 used a computer-generated presentation allowing for measures of response times. Participants who tended to accept the MP inference with unbelievable premises took longer to do so with plausible premises than with unbelievable premises. Participants who tended to reject the MP inference showed an opposite pattern. In both studies, the observed effects were not shown for the AC inferences. The overall pattern of results was consistent with the hypothesis that inhibition is a key component of logical reasoning with false premises.