SUMMARYThe widespread conviction that perceiving another person must rest on ambiguous and fakeable information is challenged. Arguing from biomechanical necessities inherent in maintaining balance and coping with reactive impulses, we show that the detailed kinematic pattern is specific to an acting person's anatomical makeup and to the working of his or her motor control system. In this way information is potentially available about gender, identity, expectations, intentions, and what the person is in fact doing. We invoke the lawfulness of human movement, as elucidated by recent advances in motor control theory, to demonstrate the virtual impossibility of performing truly deceptive movements and to argue in general terms for the specification power inherent in human kinematics.The outcome of the analysis is subsumed under a principle of kinematic specification of dynamics (KSD), which states that movements specify the causal factors of events. Generally, a linked multiple degrees-of-freedom system does not exhibit substitutability; a change in one of its "input" factors cannot substitute for, or cancel, the multivariable effects of a change in another factor.Six explorative experiments are reported. Displaying humans in action with Johansson's (1973) patch-light technique, we show that (a) the influence of an invisible thrown object on the kinematics of the thrower enable observers to perceive the length of throw; (b) the lead-in movements of a person lifting a box allow perception of what weight the lifter expects; (c) a person lifting a box cannot deceive observers about the weight of the box, only convey the deceptive intention; (d) gender of adults and children in complex activity is recognizable to about 75% of presentations; (e) gender recognition rises to about 85% correct when the observed persons are.not self-conscious about gender; (f) real gender and expressed (acted) gender are simultaneously, but independently, perceived; and (g) observer instructions to judge only "gender" yields results that erroneously indicate a deception effect.We conclude that the experiments have demonstrated the considerable effectiveness of kinematic information in enabling perception of persons and action. Judgments of good precision were often obtained. Some perceived properties were relatively subtle states of the seen person. True conditions were perceived despite deceptive endeavors. The KSD principle therefore appears an appropriate conceptual guide, and the patch-light technique a useful empirical method, for the study of social knowing.The concluding discussion argues that person perception has a dual nature in that true person properties and communicative or deceptive expressions are co-specified in the kinematic pattern. Hence, they constitute alternative foci for perception, and attention can switch freely between them in normal social interaction. Extensive parallels exist between this view of person perception and Gibson's (1979) treatment of pictures and picture perception.Furthermore, we argue that the stu...

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