Numerosity illusions occur when the spatial arrangement of stimuli influences the quantity perception of a set.

We tested the Solitaire illusion among preschool children and task-naïve capuchin monkeys.

Monkeys perceived the numerosity illusion, although there were large individual differences.

Younger children performed similarly to the monkeys, whereas older children more consistently perceived the illusion like adults.

Human-unique perceptual experiences may play a key role in the emergence of the Solitaire illusion in humans.