The elusive illusion: Do children (Homo sapiens) and capuchin monkeys (Cebus apella) see the Solitaire illusion? Audrey E. Parrish a,b,⇑ , Christian Agrillo c , Bonnie M. Perdue d , Michael J. Beran a,b a Department of Psychology, Georgia State University, Atlanta, GA 30302, USA b Language Research Center, Georgia State University, Atlanta, GA 30302, USA c Department of General Psychology, University of Padova, 35131 Padova, Italy d Department of Psychology, Agnes Scott College, Decatur, GA 30030, USA article info Article history: Received 17 July 2015 Revised 17 September 2015 Available online 26 October 2015 Keywords: Solitaire illusion Human children Capuchin monkeys Visual illusion Gestalt laws Perception abstract One approach to gaining a better understanding of how we perceive the world is to assess the errors that human and nonhuman animals make in perceptual processing. Developmental and comparative perspectives can contribute to identifying the mechanisms that underlie systematic perceptual errors often referred to as perceptual illusions. In the visual domain, some illusions appear to remain constant across the lifespan, whereas others change with age. From a comparative perspective, many of the illusions observed in humans appear to be shared with nonhuman primates. Numerosity illusions are a subset of visual illusions and occur when the spatial arrangement of stimuli within a set influences the perception of quantity. Previous research has found one such illusion that readily occurs in human adults, the Solitaire illusion. This illusion appears to be less robust in two monkey species, rhesus macaques and capuchin monkeys. We attempted to clarify the ontogeny of this illusion from a developmental and comparative perspective by testing human children and task- naïve capuchin monkeys in a computerized quantity judgment task. The overall performance of the monkeys suggested that they perceived the numerosity illusion, although there were large differences among individuals. Younger children performed similarly to the monkeys, whereas older children more consistently perceived the illusion. These findings suggest that human-unique http://dx.doi.org/10.1016/j.jecp.2015.09.021 0022-0965/Ó 2015 Elsevier Inc. All rights reserved. ⇑ Corresponding author at: Language Research Center, Georgia State University, Atlanta, GA 30302, USA. E-mail address: audrey.parrish1@gmail.com (A.E. Parrish). Journal of Experimental Child Psychology 142 (2016) 83–95 Contents lists available at ScienceDirect Journal of Experimental Child Psychology journal homepage: www.elsevier.com/locate/jecp