Infants’ intentionally communicative vocalisations elicit responses from caregivers and are the best predictors of the transition to language:a longitudinal investigation of infants’ vocalisations, gestures, and word production.

What aspects of infants’ prelinguistic communication are most valuable for learning to speak, and why? We test whether early vocalisations and gestures drive the transition to word use because, in addition to indicating motoric readiness, they 1) are early instances of intentional communication and 2) elicit verbal responses from caregivers. In study 1, 11-month-olds (N = 134) were observed to coordinate vocalisations and gestures with gaze to their caregiver’s face at above chance rates, indicating that they are plausibly intentionally communicative. Study 2 tested whether those infant communicative acts that were gaze-coordinated best predicted later expressive vocabulary. We report a novel procedure for predicting vocabulary via multi-model inference over a comprehensive set of infant behaviours produced at 11- and 12-months (n = 58). This makes it possible to establish the relative predictive value of different behaviours that are hierarchically organised by level of granularity. Gaze-coordinated vocalisations were the most valuable predictors of expressive vocabulary size up to 24 months. Study 3 established that caregivers were more likely to respond to gaze-coordinated behaviours. Moreover, the dyadic combination of infant gaze-coordinated vocalisation and caregiver response was by far the best predictor of later vocabulary size. We conclude that practice with prelinguistic intentional communication facilitates the leap to symbol use. Learning is optimised when caregivers respond to intentional vocalisations with appropriate language.