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Parental responsiveness to infant behaviors is a strong predictor of infants' language and cognitive outcomes. The mechanisms underlying this effect, however, are relatively unknown. We examined the effects of parent speech on infants' visual attention, manual actions, hand‐eye coordination, and dyadic joint attention during parent‐infant free play. We report on two studies that used head‐mounted eye trackers in increasingly naturalistic laboratory environments. In Study 1, 12‐to‐24‐month‐old infants and their parents played on the floor of a seminaturalistic environment with 24 toys. In Study 2, a different sample of dyads played in a home‐like laboratory with 10 toys and no restrictions on their movement. In both studies, we present evidence that responsive parent speech extends the duration of infants' multimodal attention. This social “boost” of parent speech impacts multiple behaviors that have been linked to …