Space or physics? Children use physical reasoning to solve the trap problem from 2.5 years of age.

By 3 years of age, children can solve tasks involving physical principles such as locating a ball that rolled down a ramp behind an occluder by the position of a partially visible solid wall (Berthier, DeBlois, Poirer, Novak, & Clifton, 2000; Hood, Carey, & Prasada, 2000). However, the extent to which children use physical information (the properties of the wall) remains unclear because spatial information would suffice (the location of the wall in relation to the ball). We confronted 2- to 6-year-old children with a ball resting on a shelf inside a clear plastic-fronted box. To retrieve the ball, children had to roll it away from a trap or barrier using their fingers. Crucially, a single object acted as a barrier or supporting surface in different conditions, thus requiring a flexible response. Preschoolers solved the task and the critical transfers from 2.5 years of age (Study 1). Interestingly, 2.5-year-olds required to use a tool to displace the ball performed significantly worse than those who could use their fingers (Study 2). In contrast, 2.5- to 4.5-year-olds failed a covered trap box that provided only 2-dimensional predictive cues without any visible physical information, and even 6.5-year-olds performed significantly worse on the covered task compared to the uncovered one (Studies 3 and 4). Our results suggest that children from around 2.5 years of age integrate spatial and physical information when solving problems like the trap box task, rather than simply exploit spatial relationships between features.