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Notes that viewers can determine the gender of a walker from sagittally projected, dynamic displays of point-lights attached to prominent joints. Three interrelated approaches are explored in search of a biomechanical invariant that viewers might use. The 1st, an index of torso structure, accounts for the data but seems inappropriate because it is not directly revealed in the dynamic stimuli. The 2nd, a dynamic index of visible torsion in the trunk of a walker, also fits the data but seems to have a logical problem and a difficulty in accounting for performance in certain conditions of several previous studies. The 3rd has the strengths of the 1st 2 indices, and it can account for some other data as well. It is the center of moment and is a" deeper," more general description of the invariant. This center is a point around which all movement occurs. It can be thought of as one specification of the gestalt law of common fate and may …