Major theories propose that spontaneous responding to others' actions involves mirroring, or direct matching. Responding to facial expressions is assumed to follow this matching principle: People smile to smiles and frown to frowns. We demonstrate here that social power fundamentally changes spontaneous facial mimicry of emotional expressions, thereby challenging the direct-matching principle. Participants induced into a high-power (HP), low-power (LP), or neutral state watched dynamic happy and angry expressions from HP and LP targets while we measured facial electromyography (fEMG) over the zygomaticus major ("smiling muscle") and corrugator supercilii ("frowning muscle"). For smiling, LP participants smiled to all targets, regardless of their expression. In contrast, HP participants exhibited standard smile mimicry toward LP targets but did not mimic the smiles of HP targets. Instead, HP participants smiled more when those HP targets expressed anger. For frowning, all participants showed a more intense mimicry pattern to HP targets. These results demonstrate that spontaneous facial responding-detected by sensitive, physiological measures of muscle activation-dynamically adapts to contextual cues of social hierarchy.