Reasoning about collisions involving inert objects in 7.5-month-old infants

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 no-contact 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.