Data from: A metacognitive illusion in monkeys

Like humans, monkeys can make accurate judgements about their own memory
by reporting their confidence during cognitive tasks. Some have suggested
that animals use associative learning to make accurate confidence
judgements, while others have suggested animals directly access and
estimate the strength of their memories. Here we test a third,
non-exclusive possibility: perhaps monkeys, like humans, base
metacognitive inferences on heuristic cues. Humans are known to use cues
like perceptual fluency (e.g. how easy something is to see) when making
metacognitive judgements. We tested monkeys using a match-to-sample task
in which the perceptual fluency of the stimuli was manipulated. The
monkeys made confidence wagers on their accuracy before or after each
trial. We found that monkeys' wagers were affected by perceptual
fluency even when their accuracy was not. This is novel evidence that
animals are susceptible to metacognitive illusions similar to those
experienced by humans.