In adults and children, oxytocin is a neuropeptide positively associated with social engagement. Endogenous peripheral oxytocin can be unobtrusively measured through saliva. However, little is understood about the early development of the endogenous oxytocin system in infancy. Here, we studied whether infants' salivary oxytocin can be reliably measured, is developmentally stable with age, and is linked to social behavior. We longitudinally collected human infants' saliva at 4, 8, and 14 months of age before and after they watched a video of an unfamiliar woman smiling while we tracked their emotional state. We found within-age reliability in infants' salivary oxytocin levels between the first and second samples we collected at each age, and developmental stability in individual differences of infants' oxytocin levels across these ages. At the group level, we detected no changes in infants' salivary oxytocin levels between 4 and 8 months, but found an increase from 8 to 14 months, a novel finding. As we hypothesized, infants' salivary oxytocin levels were positively associated with their positive emotions to a smiling stranger at 4 months of age; however, unexpectedly, this association disappeared at 8 months of age, and then reversed at 14 months of age. Together, these findings suggest that infant salivary oxytocin may be a stable and reliable measure in infancy, but that it may undergo developmental changes in what it reflects. Further studies are needed to more fully explore whether infants' salivary oxytocin levels are associated with other aspects of infants' social and emotional development.
