Confidence judgments by humans and rhesus monkeys

Researchers have begun to evaluate animals' metacognitive capacities. Continuing this evaluation, the present authors asked whether monkeys could use the analog of a confidence-rating scale to judge retrospectively their accuracy in a psychophysical discrimination. Monkeys and human participants classified stimuli as dense or sparse without feedback. Then, they made a secondary confidence judgment by choosing responses that risked different levels of timeout but could earn different levels of reward. Human participants were instructed to use these responses to express levels of confidence. They used 2- and 3-level confidence-rating scales appropriately. Monkeys used the 2-leveI scale appropriately-especially one who performed almost identically to humans. Neither monkey used the 3-level scale as the humans did. These studies place humans and animals for the first time in the identical risk-taking situation that collects declarative confidence judgments from humans. They demonstrate the first retrospective reports of uncertainty by nonhuman primates while also suggesting their limits in this area.