Humans' (Homo sapiens), capuchin monkeys' (Sapajus [Cebus] apella), and rhesus macaques' (Macaca mulatta) size judgments shift when stimuli change in frequency.

When making decisions, humans often strive to uphold objective, absolute standards, such as about what is small versus large, blue versus purple, or unfair versus fair, suggesting that our judgments should not be swayed by extraneous factors such as the sequence or frequency of events to be judged. Yet in previous research, when some items (e.g., threatening faces) became less frequent, humans responded by expanding their concept (of "threatening") to include more ambiguous stimuli. We assessed the origins of this perceptual frequency bias by testing 25 capuchins, seven rhesus monkeys, and 102 humans on a computer task in which they had to classify one circle at a time (pulled from a continuum of 50 circle sizes) as either small or large. Small and large circles initially appeared with equal probability but over time small circles either became less frequent, more frequent, or did not change in frequency. All three species showed changes in judgment, but contrary to predictions, they contracted, rather than expanded, their size judgments of the less frequent category. In other words, when small circles became rare, participants were more likely to judge ambiguous circles sizes as large (and vice versa). This may have been due to the immediate explicit feedback, as has recently been found in humans, and we consider possible mechanisms driving our participants' responses. These results suggest that humans' difficulties in maintaining absolute standards are shared with other animals. (PsycInfo Database Record (c) 2024 APA, all rights reserved).