Eight-and-a-half-month-old infants' reasoning about containment events.

The present research examined whether 8.5-month-old infants take into account the width and compressibility of an object when determining whether it can be inserted into a container. The infants in Experiment 1 saw 2 test events. At the start of each event, a tall container rested on the apparatus floor. Next, the container was hidden by a screen, and a large ball attached to the lower end of a rod was introduced into the apparatus and lowered behind the screen into the container. Finally, the screen was removed to reveal the ball's rod protruding above the container's rim. The only difference between the 2 test events had to do with the width of the containers: in 1 event (large-container event), the container was slightly wider than the ball; in the other event (small-container event), the container was only half as wide as the ball, so that it should have been impossible for the ball to be lowered into it. Infants in a control condition saw identical test events except that a small ball was used that could fit into either the large or the small container. The infants in the experimental condition looked reliably longer at the small- than at the large-container event, whereas those in the control condition tended to look equally at the 2 events. These results suggested that, although the infants never saw the ball and the container simultaneously, they realized that the large ball could fit into the large but not the small container, whereas the small ball could fit into both containers. In Experiment 2, the large ball used in Experiment 1 was replaced with an equally large but compressible ball. The results were negative, suggesting that the infants understood that the large compressible ball could be inserted into either the small or the large container. Finally, Experiment 3 confirmed the results of the experimental condition in Experiment 1, with a slightly different procedure. Together, the present results indicate that, by 8.5 months of age, infants are already capable of sophisticated reasoning about containment events.