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Adults possess a great deal of knowledge about the physical world, and developmental researchers have long been interested in uncovering the roots of this knowledge in infancy. Two main questions have guided this research: What expectations do infants possess, at different ages, about physical events, and how do they attain these expectations?

Piaget (1952, 1954) was the first researcher to systematically investigate the development of infants’ physical knowledge. He examined infants’ actions in various tasks and concluded that young infants understand very little about the physical events they observe. For example, Piaget noted that infants younger than 8 months do not search for objects they have watched being hidden, and concluded that they do not yet realize that objects continue to exist when hidden. One difficulty with Piaget’s (1952, 1954) experimental approach is that action tasks do not test only infants’ physical knowledge (eg, Boudreau & Bushnell, 2000; Berthier et al., 2001; Keen & Berthier, 2004; Hespos & Baillargeon, 2006, 2008). In order to search for a hidden object, for example, infants must not only represent the existence and location of the object but also plan and execute appropriate actions to retrieve it. Because young infants’ information-processing resources are sharply limited, they may fail at a search task not because they do not yet understand that objects continue to exist when hidden but because the combined demands of the task overwhelm their processing resources. Because of the problems inherent in interpreting negative results in action tasks, researchers have developed alternative experimental …