Task-related motor response inflates confidence

Studies on confidence in decision-making tasks have repeatedly shown correlations between confidence and the characteristics of motor responses. Here, we show the results of two experiments in which we manipulated the type of motor response that precedes confidence rating. Participants decided which box, left or right, contained more dots and then reported their confidence in this decision. In Experiment 1, prior to confidence rating, participants were required to follow a motor cue. Cued-response type was manipulated in two dimensions: task-compatibility (the relation between response set and task-relevant decision alternatives), and stimulus-congruence (spatial correspondence between response key and the location of the stimulus that should be chosen). In Experiment 2, a decision-related response set was randomly varied in each trial, being either vertical (task incompatible) or horizontal (task-compatible, spatially congruent and incongruent). The main results showed that choice confidence increased following task-compatible responses, i.e. responses related to the alternatives of the choice in which confidence was reported. Moreover, confidence was higher in these conditions, independently of response accuracy and spatial congruence with the correct stimuli. We interpret these results as suggesting that action appropriate in the context of a given task is an indicator of successful completion of the decision-related process. Such an action, even a spurious one, inflates decisional confidence.