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Abstract The current study used functional near‐infrared spectroscopy (fNIRS) to investigate whether and how individual differences in positive social engagement among 5‐month‐old (N= 109;N= 35 final sample) infants relate to variability in functional connectivity in the human brain's Default‐Mode Network (DMN). Neuroimaging results showed that on average infants displayed greater functional connectivity in the right than in the left hemisphere of the DMN, adding to prior work indicating faster connectivity development in the right hemisphere. Results did not show any positive associations between our preregistered measures of positive social engagement and functional connectivity in the DMN. However, an additional analysis revealed that higher levels of infants’ smiling and laughter during daily social interactions with their caregivers positively predicted DMN functional connectivity in the left hemisphere. This suggests that individual differences in connectivity in a long‐range brain network implicated in a host of social and cognitive functions are associated with some aspects of infants’ positive social‐interactive behaviors. 

Abstract Dorso-medial prefrontal cortex (dmPFC) plays a vital role in social cognition and behavior among humans. Enhanced responses in dmPFC when viewing social scenes predict increased levels of sociability in adults. The current longitudinal study examined the association between dmPFC response and sociability in early development. Brain responses were measured in response to social smiles and frowns using functional near-infrared spectroscopy (fNIRS) at 11 months. Individual differences in sociability were measured using the Early Childhood Behavior Questionnaire (ECBQ) at 18 months. Our longitudinal results show that greater engagement of the dmPFC when processing social smiles, but not frowns, at 11 months predicts higher levels of sociability at 18 months. This demonstrates that early variability in dmPFC response during positive social interactions is linked to individual differences in overtly displayed social behavior. This supports the view that dmPFC plays an important role in social cognition and behavior from early in human ontogeny.