The Emergence of Positive Affect: The Development of Interactive Smiling with Mothers, Fathers, and Strangers

Social smiling emerges through real-time coordination between infants and adults, yet surprisingly little is known about the temporal dynamics underlying interpersonal coordination. We asked how infant and adults shape each other's experience of positive affect by examining their smile onsets and offsets during face-to-face interaction. Fifty-eight infants interacted with their mother, father, and a stranger at 4 and 8 months, centering the influence of interactive context in a longitudinal framework. Infants typically initiated smiles at lower rates, ended smiles at higher rates, and reciprocated partner smiles at lower rates than their partners, suggesting that infants are less likely to create states of mutual dyadic positivity than adults. Eight-month-olds initiated smiles at higher rates than 4-month-olds when neither partner was smiling, suggesting increasing infant smiling initiative with age. Adults terminated their own smiles at higher rates with 8- than 4-month-old infants, suggesting that developmental changes drive interactive reorganization. Partner-specific patterns revealed infant agency through selective responsiveness. Infants were more likely to reciprocate mother than stranger smiles, despite strangers initiating and sustaining their smiles at higher rates than mothers. The complex, context-dependent temporal dynamics of infant smiling with diverse partners distinguish the ontogeny of positive engagement with parents from engagement with strangers.