The Impact of Evidence Reliability on Sensitivity and Bias in Decision Confidence

Human observers effortlessly and accurately judge their probability of being correct in their decisions, suggesting that metacognitive evaluation is an integral part of decision making. It remains a challenge for most models of confidence, however, to explain how metacognitive judgments are formed and which internal signals influence them. While the decision-making literature has suggested that confidence is based on privileged access to the evidence that gives rise to the decision itself, other lines of research on confidence have commonly taken the view of a multicue model of confidence. The present study aims at manipulating one such cue: the perceived reliability of evidence supporting an initial decision. Participants made a categorical judgment of the average color of an array of eight colored shapes, for which we critically manipulated both the distance of the mean color from the category boundary (evidence strength) and the variability of colors across the eight shapes (evidence reliability). Our results indicate that evidence reliability has a stronger impact on confidence than evidence strength. Specifically, we found that evidence reliability affects metacognitive readout, the mapping from subjectively experienced certainty to expressed confidence, allowing participants to adequately adjust their confidence ratings to match changes in objective task performance across conditions. Public Significance Statement People constantly face various types of decisions, which are commonly accompanied by an inherent feeling of (in)correctness: Just as realizing the tennis ball we played will most likely hit the net, we can feel more or less confident regarding our recent car purchase. People's confidence judgments have been found to be surprisingly accurate. However, little is known about the underlying mechanisms that give rise to them. In this study, we suggest that variability in the information we receive from the outside world is of particular importance for how confident we feel in our decisions-more so than the extent to which evidence favors one over another choice option. Specifically, we find that this variability affects how people translate their internal feelings of certainty into the confidence judgments they express.