  Our understanding of goal-directed behavior is characterized by a principle of rationality; that is, that all things being equal, agents take the easiest, most direct, and most efficient means available to achieve their goal. In a series of studies, Csibra, Gergely and their colleagues provide evidence that infants use this principle (Csibra et al. 1999 and 2003). In Csibra et al. 2003, 12-month-old babies were habituated to a ball rolling along a path, apparently jumping while its path is hidden by a screen, and then continuing rolling along its path once it has emerged from behind the screen. In the test trials, the screen was removed and babies were shown one of two displays: one with an obstacle on the path, one with no obstacle. Longer looking times at the display with no obstacle indicate that jumping for no apparent reason is unexpected for the infant. In contrast, when there is an obstacle on the path, jumping over it is a direct and efficient means to achieving one’s goal and is therefore not a violation of expectation.