Semantic congruity affects numerical judgments similarly in monkeys and humans

Monkeys (Macaca mulatta) were trained to order visual arrays based on their number of elements and to conditionally choose the array with the larger or smaller number of elements dependent on a color cue. When the screen background was red, monkeys were reinforced for choosing the smaller numerical value first. When the screen background was blue, monkeys were reinforced for choosing the larger numerical value first. Monkeys showed a semantic congruity effect analogous to that reported for human comparison judgments. Specifically, decision time was systematically influenced by the semantic congruity between the cue ("choose smaller" or "choose larger") and the magnitude of the choice stimuli (small or large numbers of dots). This finding demonstrates a semantic congruity effect in a nonlinguistic animal and provides strong evidence for an evolutionarily primitive magnitude-comparison algorithm common to humans and monkeys.