The Dynamics of Looking and Smiling Differ for Young Infants at Elevated Likelihood for ASD.

Social smiling is the earliest gained social communication skill, emerging around 2 months of age. From 2 to 6-months, infants primarily smile in response to caregivers. After 6 months, infants coordinate social smiles with other social cues to initiate interactions with the caregiver. Social smiling is reduced in older infants with autism spectrum disorder (ASD) but has rarely been studied before 6 months of life. The current study therefore aimed to understand the component parts of infant social smiles, namely look to caregiver and smile, during face-to-face interactions in 3 and 4-month-old infants at elevated (EL) and low likelihood (LL) for ASD. We found that EL and LL infants looked to their caregiver and smiled for similar amounts of time and at similar rates, suggesting that social smiling manifests similarly in both groups. A nuanced difference between groups emerged when considering temporal dynamics of looking and smiling. Specifically, 3-month-old EL infants demonstrated extended looking to the caregiver after smile offset. These findings suggest that social smiling is largely typical in EL infants in early infancy, with subtle differences in temporal coupling. Future research is needed to understand the full magnitude of these differences and their implications for social development.