Knowing before doing: Review and mega-analysis of action understanding in prereaching infants.

The relationship between experience and knowledge is one of the oldest and deepest questions in psychology. In developmental science, research on this question has focused on prereaching infants who cannot yet retrieve objects by reaching for and grasping them. Over the past 2 decades, behavioral research in this population has produced two seemingly contradictory findings: After first-person experience with reaching via "sticky mittens" training, (a) infants come to expect that people reach efficiently, toward goal objects, but (b) under some conditions, they can express these expectations without training.