experiments investigated the development of children's ability to recognize perceptual relational commonalities such as symmetry or monotonicity. In Experiment 1, 6-and 8-year-olds were able to recognize higherorder relational similarity across different dimensions (e.g., size/saturation) and across different polarities (e.g., increase/decrease), whereas 4-year-olds could recognize higher-order relational matches only when they were supported by lower-order commonalities (e.g., size/size but not size I saturation matches). Further experiments tested how the processes of comparison and categorization affected 4-year-olds' ability to recognize relational similarity. The results of the experiments supported the hypothesis that comparison and categorization processes lead to changes in children's representations of relational structure, enabling them to recognize more abstract commonalities. A computational model lent further support to the claims.

experiments investigated the development of children's ability to recognize perceptual relational commonalities such as symmetry or monotonicity. In Experiment 1, 6-and 8-year-olds were able to recognize higherorder relational similarity across different dimensions (e.g., size/saturation) and across different polarities (e.g., increase/decrease), whereas 4-year-olds could recognize higher-order relational matches only when they were supported by lower-order commonalities (e.g., size/size but not size I saturation matches). Further experiments tested how the processes of comparison and categorization affected 4-year-olds' ability to recognize relational similarity. The results of the experiments supported the hypothesis that comparison and categorization processes lead to changes in children's representations of relational structure, enabling them to recognize more abstract commonalities. A computational model lent further support to the claims.1 The relational shift is not a shift away from object similarity; even adults find it easier to process overall similarity (that includes object matches as well as relational matches) than to process pure analogy, and there is evidence that object matches are processed before relational matches (Goldstone & Medin, 1994; Markman & Gentner, 1996).2 These levels of similarity are more precisely defined using a propositional representation. Attributes are defined as predicates taking one argument (e.g., BIG [a]), and relations as predicates taking two or more arguments (e.g., BIGGER [a,b];Gentner, 1983Gentner, , 1989Palmer, 1975). The difference between first-order and higher-order relations is that first-order relations take objects or functions as arguments, and higher-order relations take propositions as arguments. That is, a higher-order relation is a relation between relations. Using this representation, object similarity involves a match at the level of attributes, lower-order relational similarity involves a match at the level of first-order relations, and higher-order relational smilarity involves a match at the level of higher-order relations.

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The present research asked whether 7.5-month-old infants realize that an object cannot displace another object without contacting it. The infants in Experiment 1 were assigned to a contact or a no-contact condition. The infants in the nocontact condition saw static familiarization displays in which a tall, thin barrier stood across the bottom of a ramp; a cylinder rested against the left side of the barrier and a wheeled toy bug against its right side. The infants in the contact condition saw similar displays except that a large portion of the barrier's lower half was removed so that the cylinder rested directly against the bug. Next, a small screen was placed in front of the bottom of the ramp; only the upper portion of the barrier was visible above the screen. The infants in the two conditions watched the same test event. The cylinder was released and rolled to the bottom of the ramp, partly disappearing behind the screen's left edge; next, the bug rolled down the track, as though launched by the cylinder. The infants in the no-contact condition looked reliably longer at the test event than did those in the contact condition. This result suggested that the infants (a) viewed the bug as an inert object that could move only when acted upon; (b) believed that the cylinder could not act on the bug without contacting it; (c) realized that the cylinder could contact the bug when the half-barrier but not the barrier was present; (d) remembered after the screen was raised whether contact was possible between the cylinder and bug; and (e) were surprised in the no-contact condition when the bug was launched down the track. A second experiment confirmed the results of Experiment 1. Previous research comparing infants' responses to no-contact and contact events has typically made use of self-moving rather than inert objects. These experiments have consistently found that infants do not look reliably longer at no-contact than at contact events. In the General Discussion, we examine the contrast between these prior results and the present results and speculate on how infants' expectations about inert and self-moving objects may be best characterized.