Belief bias is response bias: Evidence from a two-step signal detection model.

When asked to determine whether a syllogistic argument is deductively valid, people are influenced by their prior beliefs about the believability of the conclusion. Recently, two competing explanations for this belief bias effect have been proposed, each based on signal detection theory (SDT). Under a explanation, people set more lenient decision criteria for believable than for unbelievable arguments. Under the alternative explanation, believability affects the reasoning stage of processing an argument, with believable and unbelievable arguments differing in subjective strength for both valid and invalid items. Two experiments tested these accounts by asking participants to make validity judgments for categorical syllogisms and to rate their confidence. Conclusion-believability was manipulated both within group (Experiment 1) and between groups (Experiment 2). A novel two-step version of the signal detection model was fit to receiver operating characteristic (ROC) curves for believable and unbelievable arguments. Model fits confirmed that in both experiments there was a shift in decision criterion but not argument discriminability as a function of argument believability. Crucially, when believability is manipulated between groups, this shift is expected under the response bias account but not under the argument strength account. Therefore, the results support the view that belief bias primarily reflects changes in response bias: people require less evidence to endorse a syllogism as valid when it has a believable conclusion. This has important implications for theories of deductive reasoning. (PsycINFO Database Record (c) 2019 APA, all rights reserved).