When a small number of lights are placed on the limbs and joints of a moving human (or animal), the motions of the lights (biological motion) are sufficient to enable adult observers to perceive immediately the activity of the human. This perception of biological motion has been hypothesized to be an intrinsic capacity of the visual system. The results of this experiment, which demonstrate that infants 4 to 6 months of age exhibit a preference for biological motion patterns, support that hypothesis.