Smiling in time: the interactive development of infant smiling with mothers, fathers and strangers.

A developmental examination of the temporal dynamics of infant smiling during early social interaction is lacking. We ask how infants and adults influence each other's expressions of positive affect by examining the timing of transitions among dyadic states defined by the presence and absence of each partner's smiles. Fifty-eight infants interacted with their mothers, fathers and a stranger at 4 and 8 months. A survival analysis of the time course of infant (and adult partner) smiles and non-smiles indicated that infants initiated smiles and reciprocated smiles at lower rates than their adult partners, yet ended smiles at higher rates. Nevertheless, adult smiles were associated with higher infant smile rates, while infant smiles prolonged adult smiles, suggesting a dyadic path to mutual positivity. Eight-month-olds initiated smiles at higher rates than 4-month-olds when neither partner was smiling, suggesting increasing infant smiling initiative. Partner-specific patterns revealed selective infant responsiveness. Although strangers initiated and sustained smiles at higher rates than mothers, infants were more probably to reciprocate mother than stranger smiles. In the first year of life, infants showed a tendency to down-regulate positive affect expressions across age and context, as well as partner-specific smile reciprocation proclivities and a developmental increase in smile initiation. This article is part of the theme issue 'Mechanisms, development, phylogeny and functions of emotional expressions'.