Movements in social interactions are often unintentionally synchronized and dance-like. Recent research in social neuroscience has proposed that a special evolved "mirror" subsystem may be the neural substrate that facilitates these social interactions as well as their more mental concomitants such as rapport, empathy, and dominance relations. One wedge into understanding how the interpersonal coordination patterns emerge from the neural substrate comes from principles of dynamical entrainment. In order to establish the dynamical interpersonal synergy that is the basis for both intentional and spontaneous coordination, people must be attuned to the relevant information in the interpersonal situation. The proposed research explores the perceptual basis of interpersonal entrainment. Studies record the movements of people when they interact with each other in a common task. Movement records are analyzed using spectral and nonlinear time series methods to determine how the coordination patterns are affected by specific properties of the interaction. One such property is the perceptual pickup rhythm (e.g., as indexed by eye movements), which may play a role in interpersonal coordination patterns. Other experiments investigate how interpersonal coordination is affected by the biological/social nature of environmental stimuli, the relationship between mimicry phenomena and more tacit interpersonal coordination, and how behavioral interpersonal coordination is affected by psychological coordination (e.g., rapport) of the individuals involved. This research will provide a better understanding of the tacit dimension of movement coordination in human social interactions and will constrain our understanding of the role that the nervous system plays in creating social behavior. Understanding the bases of interactional synchrony is also important for understanding psychological dysfunction, in that such synchrony breaks down in pathologies such as schizophrenia, autism and even marital dissatisfaction.