The control of standing body posture is affected by mechanical perturbations, such as motion of the support surface. Postural activity also is responsive to subtle social factors: When 2 people interact there is spontaneous interpersonal coordination of their movements. We asked whether interpersonal postural coordination based on visual contact would be robust in the presence of mechanical perturbations that characterize sea travel. During an ocean voyage, pairs of participants stood facing together or facing apart. Interpersonal coordination of body sway was stronger when participants faced each other than when they faced apart. Furthermore, overall body movement was reduced when individuals faced together, suggesting that the sight of another person improved individuals' ability to compensate for ship motion. These findings provide the first evidence that the "soft" constraint of interpersonal visual contact can influence interpersonal postural coordination as people simultaneously adjust postural sway in response to powerful mechanical (i.e., "hard") constraints.
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