Monkeys overestimate connected arrays in a relative quantity task: A reverse connectedness illusion.

Humans and many other species show consistent patterns of responding when making relative quantity ("more or less") judgments of stimuli. This includes the well-established ratio effect that determines the degree of discriminability among sets of items according to Weber's Law. However, humans and other species also are susceptible to some errors in accurately representing quantity, and these illusions reflect important aspects of the relation of perception to quantity representation. One newly described illusion in humans is the connectedness illusion, in which arrays with items that are connected to each other tend to be underestimated relative to arrays without such connection. In this pre-registered report, we assessed whether this illusion occurred in other species, testing rhesus macaque monkeys and capuchin monkeys. Contrary to our pre-registered predictions, monkeys showed an opposite bias to humans, preferring to select arrays with connected items as being more numerous. Thus, monkeys do not show this illusion to the same extent as humans.