By watching each other's lower oscillating leg, 2 seated Ss kept a common tempo and a particular phase relation of either 0° (symmetric mode) or 180° (alternate mode). This study investigated the differential stability of the 2 phase modes. In Experiment 1, in which Ss were instructed to remain in the initial phase mode, the alternate phase mode was found to be less stable as the frequency of oscillation increased. In addition, analysis of the nonsteady state cycles revealed evidence of a switching to the symmetric phase mode for the initial alternate phase mode trials. In Experiments 2 and 3, Ss were instructed to remain at a noninitial phase angle if it was found to be more comfortable. The transition observed between the 2 phase modes satisfies the criteria of a physical bifurcation-hysteresis, critical fluctuations, and divergence-and is consonant with previous findings on transitions in limb coordination within a person.The coordination of movements between people is an omnipresent aspect of daily life. Such coordinations consist of the very natural and commonplace coordinations exhibited by people walking and talking together and the very practiced and refined coordinations exhibited by people playing sports or music, or dancing. The degree of coordination between basketball players moving downcourt or the degree of coordination between two ballet dancers is quite obvious. The coordination of movements between speaker and listener (Kendon, 1970), however, or between mother and infant (Bernieri, Reznick, & Rosenthal, 1988) is more subtle and is apparent only through study. In all such cases a coordinative relationship is formed through an interaction of two individuals in order to produce some goal (e.g., score a basket, have a conversation). The unique challenge for an account of between-persons coordination-the durations, spacings, and phasings of movements and their components-resides in the fact that the two individuals share neither a common cognitive nor neural mechanism.Because of the cooperative nature of the relationship, the two individuals can be thought of as a single organism (Asch, 1952;Newtson, Hairfield, Bloomingdale, & Cutino, 1987 , 1987); that is, the spatial and temporal aspects of the two individuals' movements are related congruently-they are entrained-and the movements repeat (either periodically or stochastically)-they are rhythmic. Entrainment and sustained periodic behavior are properties of nonlinear dissipative systems, and it is from this dynamical perspective that we will attempt to understand between-persons coordination.The experiments reported in this article are directed at the questions of whether certain entrainment phenomena found in within-person coordination also hold for between-persons coordination, and whether the same very general dynamical principles govern both. There are two kinds of entrainment that two physically coupled oscillators can enter into. When the two oscillators are related stably in their timing, they are said to be frequency entrained. When they ...
