© 2021 The AuthorsDuring feeding, parents have been described to move their mouth as if they were eating themselves. Such matching of behaviours between parents and their infants during face-to-face interactions represents an example of behavioural synchrony. To date, however, the function of these synchronous eating-like mouth movements by the caregiver remains unexplored. To address this question, two competing hypotheses were tested: 1) the instructional hypothesis proposing that parents make eating-like mouth movements, such as opening and closing their mouth, to demonstrate to their infants what they need to do; 2) the mimicry hypothesis suggesting that parents imitate their infant's mouth actions to enhance affiliation. To test these hypotheses, we examined the temporal dependencies between parents’ and infants’ mouth movements. We reasoned that parents’ mouth movements would occur before their infants’ if they serve an instructional purpose, but that they would happen after, if parents mimic their infants. Additionally, we expected that parents’ matching mouth movements would be more likely when their infants looked at them in both cases. To examine these hypotheses, fifteen caregivers were observed as they were feeding their six-month-old infants. Time-window sequential analysis was conducted to quantify how likely parents were to display mouth opening and closing before or after their infants did. The results revealed that parents’ mouth movements were more likely to follow infants' movements and are thus in line with the mimicry hypothesis. Interestingly, these mouth movements of parents were independent of infant's gaze.