Rapid facial reactions (RFRs) to observed emotional expressions are proposed to be involved in a wide array of socioemotional skills, from empathy to social communication. Two of the most persuasive theoretical accounts propose RFRs to rely either on motor resonance mechanisms or on more complex mechanisms involving affective processes. Previous studies demonstrated that presentation of facial and bodily expressions can generate rapid changes in adult and school-age children's muscle activity. However, to date there is little to no evidence to suggest the existence of emotional RFRs from infancy to preschool age. To investigate whether RFRs are driven by motor mimicry or could also be a result of emotional appraisal processes, we recorded facial electromyographic (EMG) activation from the zygomaticus major and frontalis medialis muscles to presentation of static facial and bodily expressions of emotions (i.e., happiness, anger, fear, and neutral) in 3-year-old children. Results showed no specific EMG activation in response to bodily emotion expressions. However, observing others' happy faces led to increased activation of the zygomaticus major and decreased activation of the frontalis medialis, whereas observing others' angry faces elicited the opposite pattern of activation. This study suggests that RFRs are the result of complex mechanisms in which both affective processes and motor resonance may play an important role.

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During the first year of life face discrimination abilities narrow toward adult human faces of the most frequently encountered ethnic group/s. Earlier studies showed that perceptual learning under laboratory-training protocols can modulate this narrowing process. Here we investigated whether natural experience acquired in everyday settings with an older sibling's face can shape the trajectory of perceptual narrowing towards adult faces. Using an infant-controlled habituation procedure we measured discrimination of adult (Experiment 1) and child faces (Experiment 2) in 3- and 9- month-old infants with and without a child sibling. Discrimination of adult faces was observed for infants at both ages, although accompanied by posthabituation preferences in opposite directions, whereas at both ages the discrimination of child faces critically depended on sibling experience. These results provide the first evidence that natural experience acquired with siblings affects the tuning properties of infant face representation.

Human societies are organised around cooperative interactions, the origins and development of which have become a timely topic. In this study, we investigated the development of indirect reciprocity in 18-24-month-old toddlers, and infants aged 6 months, on a two-phase sharing task with non-familiar individuals. In the first phase, we observed whether infants and toddlers differentiated and manifested a preference toward an individual altruistically sharing or acting selfishly. In the second phase, infants and toddlers interacted with the same prosocial and antisocial individuals seen in the first phase, and we observed whether they were willing to share with one of the two. Indirect reciprocity was assessed as the match between the preferences for the prosocial individual in phase one, and the firstperson sharing in the second phase. Evidence showed that toddlers, but not infants, indirectly reciprocated the prosocial individual, suggesting that understanding of such a complex behaviour as indirect reciprocity may require prolonged experience in order to emerge.