Reasoning about the height and location of a hidden object in 4.5- and 6.5-month-old infants.

The present experiments examined 6.5- and 4.5-month-old infants' ability to represent and to reason about the height and location of a hidden object. In Experiments 1 and 2, the infants were habituated to a screen that rotated back and forth through a 180 degree arc, in the manner of a drawbridge. Following habituation, a box was placed behind the screen, and the infants saw two test events. In one (possible event), the screen rotated until it reached the occluded box; in the other (impossible event), the screen rotated through either the top 80% or the top 50% of the space occupied by the box. The results indicated that (a) the 6.5-month-old infants were surprised when the screen rotated through the top 80%, but not the top 50%, of the box and (b) the 4.5-month-old infants failed to be surprised even when the screen rotated through the top 80% of the box (4.5-month-old infants do show surprise, however, when the screen rotates through the entire (100%) box (Baillargeon, 1987a]. Experiments 3 and 4 tested whether infants would be better at detecting that the screen rotated farther than it should if provided with a second, identical box to the side of the box behind the screen. This second box stood out of the screen's path and so remained visible throughout the test trials. The results indicated that with the second box present (a) the 6.5-month-old infants showed surprise when the screen rotated through the top 50% of the occluded box and (b) the 4.5-month-old infants were surprised when the screen rotated through either the top 80% or the top 50% of the box. The results of Experiment 5 revealed that the improvement in performance brought about by the second box disappeared when this box was no longer in the same fronto-parallel plane as the box behind the screen. Different models are considered to describe the impressive quantitative and qualitative physical reasoning abilities revealed by these findings.