Child-directed speech and infant vocal development in rural, highland Bolivia and in immigrant families in urban United States

Decades of research have established links between speech input and children's vocabulary growth. However, it is unclear if input also facilitates phonological development. Phonology has a strong biological component-implicating fine motor development-so language socialization factors like speech input may matter less. Further complicating matters, children in many cultures are exposed to vastly different amounts of speech input, and yet these children still reach many major language development milestones on age-appropriate timelines. How does speech input relate to infant phonological development in the first years of life? We estimated infants' (1) speech input and (2) vocal maturity using daylong audio recordings taken in an Indigenous Quechua- and Spanish-speaking community in Bolivia (n = 10, M age = 12 months, 5 females, 5 males) and an immigrant Spanish- and English-speaking community in the United States (n = 10, M age = 9 months, 4 females, 6 males; all Hispanic or Latino). Although we found no differences in the overall amount of speech input to adults and children between communities, infants in the United States were 2.5× more likely to hear speech directed to them than the infants in Bolivia. When child-directed speech (CDS) was instead characterized to include all speech directed to any child within the infants' vicinity, there were no differences between communities. When employing these different definitions of CDS, we found positive relationships between quantity of speech input and different metrics of the Bolivian infants' vocal maturity. These results paint a nuanced picture showing that directed speech input, even when less common, is related to early phonological development, and that by expanding the definition of speech input to accommodate diverse cultural settings we can understand how infants' language development is resilient to differences in speech input.