["Developmental Science, Volume 29, Issue 4, July 2026. ", "\nABSTRACT\n\nThis study aimed to test the causal role of sensorimotor experience in the development of facial mimicry. We systematically manipulated 4‐month‐old infants’ experience with their own facial actions, and measured the effect on their sensorimotor cortex activation and facial mimicry when they observed others’ facial actions. Infants in the mirror condition received two weeks of daily sensorimotor experience with their own facial actions via a toy mirror, while infants in the control condition played with the same toy without the mirror for the same amount of time. Before and after this experience, we measured infants’ facial mimicry using electromyography (EMG) and their sensorimotor cortex activation using electroencephalography (EEG) while they observed videos of other infants’ facial actions. As predicted, infants in the mirror condition showed a greater increase in sensorimotor cortex activation during the observation of other infants’ facial actions than infants in the control condition. However, this greater neural activation did not translate into a greater increase in facial mimicry in the mirror group. These findings suggest that although the neural processing of others’ facial actions was enhanced as a result of the sensorimotor training, longer training periods may be necessary for this to lead to greater facial mimicry.\n\n\nSummary\n\nWe investigated the role of sensorimotor experience in infant facial mimicry by assigning infants to either a mirror or control condition\nInfants in the mirror condition received two weeks of daily mirror exposure\nMirror training enhanced sensorimotor cortex activity during the observation of facial actions\nNo corresponding increase in facial mimicry was observed\n\n\n"]