Implicit and explicit categorization: A tale of four species

► Humans dimensionalize perceptual stimuli and learn category rules preferentially. ► Macaques and capuchins-therefore, primates broadly-show the same learning advantage. ► Pigeons show none of this learning advantage for dimensionally based categorization. ► Pigeons may reveal the character of the ancestral vertebrate categorization system. ► Nonhuman primates share some aspects of humans' capacity for explicit cognition.