Project Summary Social interactions are the central context for learning the prerequisites that spur social and linguistic advances in development.1,2 While caregiver behaviors are traditionally a central focus, healthy communicative development requires infants to learn that their actions wield communicative power. Although infants are known to be active learners,5-7 we do not yet know if or how self-generated evidence of communicative success shapes their language learning over time. Understanding the mechanisms of how infants process communicative success in ways that may prioritize social learning throughout development could be key to reducing disparities in language outcomes. The goal of the planned research is to understand if early sensitivity to instances of communicative success drives individual differences in infants’ language learning. Central to this goal will be the evaluation of caregiver-infant interactions in families spanning the full spectrum of socioeconomic status (SES). We will quantify variation in infants’ communicative success with their caregivers, examine infants’ sensitivity to vocally-controllable stimuli using functional near-infrared spectroscopy (fNIRS), and test whether infants’ neural processing of controllable events predicts language outcomes. In Aim 1, during two free-play sessions (one with fNIRS, one without), we will measure individual variation in infants’ communicative success with caregivers. In Aim 2, we will use fNIRS to test whether infants show a bias towards controllable stimuli. In Aim 3, we will explore the effects of caregiver responsiveness and infants’ controllability sensitivity on language skill at 24 months. Taken together, using an SES-inclusive sample, this research program will provide insight into behavioral and neural mechanisms that promote effective language acquisition throughout early development. Through these aims, the applicant will gain training in dual-brain neuroscientific techniques, broaden his theoretical training in language and cognitive development, and gain experience working with diverse populations and backgrounds, including families that span a wide range of SES. The sponsor, Dr. Casey Lew-Williams, has extensive experience providing mentorship in all of these training areas. The expertise of three co-sponsors provide important training opportunities as well: Dr. Uri Hasson is an expert in the study of neural coupling in dyadic interactions; Dr. Lombrozo is an expert in children’s curiosity and exploration; and Dr. Goldberg is an internationally recognized expert in language processing. The unique training environment at Princeton University provides the resources necessary to successfully complete the proposed work, as well as additional opportunities for mentorship and professional development. In sum, this project will provide the applicant with training that will support his career goal of becoming a tenure-track professor at a research university.