Culture-specific pathways to the emerging sense of agency in early vocal development

Learning that one's vocalisations reliably elicit responses from others (i.e., instrumental vocal learning) is thought to support an emerging sense of agency over vocal behaviour and to represent a key milestone in early communicative development. Yet little is known about how this socially situated developmental process unfolds across cultural contexts characterised by different parenting strategies. The present longitudinal study investigated the sensitivity of infant vocal behaviour to perturbations in social responsiveness using the still-face paradigm at 3 and 4.5 months of age in two cultural contexts: a proximal-care, Indigenous-heritage Kichwa community in the Northern Andes, rural Ecuador, and a distal-care, Western middle-class sample from Münster, urban Germany. The vocal extinction burst, defined as an increase in the relative frequency of non-distress vocalisations during the still-face episode compared to baseline, was used as an index of infants' emerging sense of agency over vocalisations. Contrary to predictions, 4.5-month-old infants did not yet exhibit a robust vocal extinction burst in either culture. However, Münster infants showed a trend in this direction, displayed substantial inter-individual variability in the still-face episode, and vocalised significantly more overall than Kichwa infants. Furthermore, vocalisation duration significantly increased in the still-face episode relative to baseline in the Münster sample only. Examination of developmental pathways from 3 to 4.5 months of age indicated limited evidence of individual-order stability, with small-to-moderate stability apparent only in the Münster sample. These findings suggest that the emergence of a sense of agency in vocal development is a gradual and culturally shaped process.