Fig. 3: Knowledge decision-making task and behaviour in study 2 (n = 473).

a In addition to the perception task, participants also completed a task which tested knowledge of national populations. On each trial, participants judged which of two countries had the higher human population and provided a confidence rating (scale of 1–6, where 1 represented “not confident (guessing)” and 6 represented “certain”). Eight evidence discriminability bins were created by grouping pairs of countries with similar population log ratios. The log ratio bins amounted to the following, ranging from least to most discriminable: bin 1 (log10 ratio = 0–0.225), bin 2 = (0.225–0.45), bin 3 (0.45–0.675), bin 4 = (0.675–0.9), bin 5 (0.9–1.125), bin 6 = (1.125–1.35), bin 7 (1.35–1.575), bin 8 = (1.575–1.8). b In both tasks, group-averaged d’ increased as a function of evidence strength. c The systematic type-1 leftward biases (here indexed by the mean type-1 c’) decreased as a function of evidence level for both tasks but were systematically stronger for the perceptual task. d Group-averaged overall mean confidence ratings increased as a function of evidence strength. All error bars reflect 95% confidence intervals for the mean.