A modified Dimensional Change Card Sort (DCCS) task was used to test cognitive flexibility in adult cotton-top tamarins and children aged 19 months to 60 months. Subjects had to infer a rule from the experience of selecting between two cards to earn a reward, and the pairs of stimuli defined the rule (e.g., pick blue ones, not red ones, or pick trucks, not boats). Two different tests measured subjects' ability to shift to a reversal of the rule (intradimensional shift) and to shift to a new rule defined by a dimension previously irrelevant (interdimensional shift). Both adult tamarins and children aged 49-60 months were able to learn the initial rule and switch to a reversal and to a rule based on a different dimension. In contrast, the two younger groups of children, aged 19-36 months and aged 37-48 months, could switch when a reversal was imposed but took significantly longer to learn a new rule on a former irrelevant dimension. Experiment 2 presented a wider set of novel stimuli which shared some features with the original set to further explore the basis of rule learning. The result was that tamarins and 52- to 60-month-old children both chose novel stimuli that fit the rule and had no a priori associative strength, suggesting a rule application not solely based on associative strength. Importantly, novel items introduced some risk for choice, and children showed themselves to be risk-averse, whereas tamarins were risk-prone within a novel context. (PsycInfo Database Record (c) 2022 APA, all rights reserved).