Calibration-based reasoning about collision events in 11-month-old infants

Previous research indicates that, when a moving object collides with a stationary object, infants expect the stationary object to be displaced. The present experiment examined whether infants believe that the size of the moving object affects how far the stationary object is displaced. In the experiment, 11-month-old infants sat in front of a horizontal track; to the left of the track was an inclined ramp. A wheeled toy bug rested on the track at the bottom of the ramp. The infants in the midpoint condition were first familiarized with an event in which a medium-sized cylinder rolled down the ramp and hit the bug, causing it to roll to the middle of the track. Next, the infants saw one of two test events. In both events, novel cylinders were introduced, and the bug now rolled to the end of the track. The two test cylinders were identical to the familiarization cylinder in material but not in size: one was larger (large-cylinder event) and one was smaller (small-cylinder event) than the familiarization cylinder. The infants in the endpoint condition saw the same familiarization and test events as the infants in the midpoint condition except that the bug rolled to the end rather than to the middle of the track in the familiarization event. The infants in the midpoint condition looked reliably longer at the small-than at the large-cylinder event, whereas the infants in the endpoint condition tended to look equally at the two events. These results indicated that the infants (a) believed that the size of the cylinder affected the length of the bug's displacement and (b) used the familiarization event to calibrate their predictions about the test events. After watching the bug roll to the middle of the track when hit by the medium cylinder, the infants were surprised to see the bug roll to the end of the track with the small but not the large cylinder. After watching the bug roll to the end of the track when hit by the medium cylinder, however, the infants were not surprised to see the bug do the same with either the small or the large cylinder. Parallel results were obtained with adult subjects. The present findings have implications for research on the nature and development of infants' physical reasoning as well as for assessments of causal reasoning in infancy.