a The learning problems differed in rule complexity (see Supplementary Table 1). The low complexity problem was unidimensional (e.g., antennae size), medium complexity required a conjunction of two features (e.g., leg size and mandible shape), and high complexity required all three features. b Learning trials consisted of presentation of a stimulus for 3.5 s, followed by a fixation cross for 0.5–4.5 s, and then a feedback display for 2 s that included the stimulus, accuracy of the response, and the correct category. Learning trials were separated by a delay of 2–6 s of fixation. c The probability of a correct response increased across stimulus repetitions. The rate of learning differed according to the complexity of the problems. Bands represent 95% confidence intervals of the logistic regression model and error bars represent 95% prediction intervals for the midpoint of accuracy across participants.