Human actions are more than mere body movements. In contrast to other dynamic events in the natural world, human actions involve mental processes that enable willful bodily movements. We reported two experiments to demonstrate that human observers spontaneously assign the role of cause to relative limb movements, and the role of effect to body motion (i.e., the position changes of the body center of mass) when observing actions of others. Experiment 1 showed that this causal action constraint impacts people’s impression on the naturalness of observed actions. Experiment 2a/b revealed that the causal constraint guides the integration of different motion cues within a relational schema. We developed an ideal observer model to rule out the possibility that these effects resulted from the learning of statistical regularity in action stimuli. These findings demonstrate that causal relations concerning bodily movements play an important role in perceiving and understanding actions.