Cognitive Psychology 39, 116–157 (1999) Article ID cogp.1999.0717, available online at http://www.idealibrary.com on 2.5-Month-Old Infants’ Reasoning about When Objects Should and Should Not Be Occluded Andre ´a Aguiar and Rene ´e Baillargeon University of Illinois The present research examined 2.5-month-old infants’ reasoning about occlusion events. Three experiments investigated infants’ ability to predict whether an object should remain continuously hidden or become temporarily visible when passing behind an occluder with an opening in its midsection. In Experiment 1, the infants were habituated to a short toy mouse that moved back and forth behind a screen. Next, the infants saw two test events that were identical to the habituation event except that a portion of the screen’s midsection was removed to create a large win- dow. In one event (high-window event), the window extended from the screen’s upper edge; the mouse was shorter than the bottom of the window and thus did not become visible when passing behind the screen. In the other event (low-window event), the window extended from the screen’s lower edge; although the mouse was shorter than the top of the window and hence should have become fully visible when passing behind the screen, it never appeared in the window. The infants tended to look equally at the high- and low-window events, suggesting that they were not surprised when the mouse failed to appear in the low window. However, positive results were obtained in Experiment 2 when the low-window event was modified: a portion of the screen above the window was removed so that the left and right sections of the screen were no longer connected (two-screens event). The infants looked reliably longer at the two-screens than at the high-window event. Together, the results of Experiments 1 and 2 suggested that, at 2.5 months of age, infants possess only very limited expectations about when objects should and should not be occluded. Specifically, infants expect objects (1) to become visible when passing between occluders and (2) to remain hidden when passing behind occluders, irre- This research was supported by Grant BEX-2688 from CAPES-Brasilia/Brasil to the first author and by a grant from the National Institute of Child Health and Human Development (HD-21104) to the second author. We thank Noam Chomsky, Dov Cohen, Cindy Fisher, Ro- chel Gelman, Valerie Makin, Jean Mandler, and Brian Ross for helpful comments and Rebecca Bloch, Laura Brueckner, Beth Cullum, Susan Hespos, Lisa Kaufman, Marsha Keeler, Laura Kotovsky, Melsie Minna, Helen Raschke, Teresa Wilcox, and the undergraduate assistants at the Infant Cognition Laboratory at the University of Illinois for their help with the data collec- tion. We also thank the parents who kindly agreed to have their infants participate in the research. Correspondence and reprint requests concerning this article should be sent to Andre ´a Aguiar, Department of Psychology, University of Waterloo, 200 University Ave. W., Waterloo, On- tario, Canada N2L 361. Fax: 519-746-8631. E-mail: aaguiar@watarts.uwaterloo.ca. 116 0010-0285/99 $30.00 Copyright  1999 by Academic Press All rights of reproduction in any form reserved.