Should all stationary objects move when hit? Developments in infants' causal and statistical expectations about collision events.

Four experiments examined 8- and 9-month-old infants' expectations about collision events. The infants saw test events in which a small cylinder rolled down a ramp and hit one of several different boxes. These boxes varied in width and height and always remained stationary when hit. The results revealed two separate developments. The first involved infants' knowledge of the variables relevant to collision events. At 8 months, the infants expected all of the boxes to move when hit, regardless of their sizes; at 9 months, the infants began to take into account the size of the boxes to predict whether they should move when hit. The second development concerned infants' ability to generate explanations for outcomes that violated their collision knowledge. At both ages, upon observing that a box with a salient vertical dimension did not move when hit, the infants apparently concluded that the box must be one of those objects we term pillars-vertical objects that are attached at one or both ends to adjacent surfaces. At 8 months, the infants considered any vertical box as a potential pillar; at 9 months, the infants considered only boxes that were both vertical and narrow as potential pillars. The development of infants' knowledge about collision events is thus one that is complex and protracted and weaves together many separate developments.