Early language experience and later vocabulary among Australian infants from diverse socioeconomic backgrounds

Abstract

Relationships between socioeconomic status (SES) and children's vocabulary are evident from toddlerhood. The existing literature makes clear that quantitative and qualitative differences in early linguistic experience appear to play a mediating role. However, to date, most research has focused on children during the second year of life and beyond, so the research investigating those differences during the first year of life is sparse. However, we know that the foundations of first language acquisition begin early in infancy, well before infants speak their first words. The aim of this thesis is to add to the literature by providing a deeper empirical understanding of the relationship between family SES and early vocabulary development for Australian infants, beginning in the first year of life. A longitudinal study, carried out from 2015 to 2016, focused on quantitative aspects of infants' linguistic experience as they occurred during whole--"day recording sessions at home, to assess SES--'related differences and their potential association with later expressive vocabulary skills. Fifty families participated, from working and middle-class Australian English-speaking backgrounds, representing two levels of maternal education (higher ≥ bachelor degree, lower ≤ bachelor degree). At infant age 6 to 9 months and 12 to 15 months, day-long digital audio recordings were obtained in families' homes on two days. Recordings were processed automatically using the Language Environment Analysis system (LENATM) to provide 12-hour estimates of three aspects of linguistic experience (i.e. adult word count, conversational turn count, and child vocalisation count). At infant age 12 to 15 months and 19 months, parents reported on children's expressive vocabulary using the Australian English Communicative Development Inventory (OZI). A secondary aspect of the study measured mothers' verbal and nonverbal IQ, and a selection of other maternal characteristics were assessed via questionnaire data. Based on previous research in SES, we predicted that higher SES (measured as maternal education) would be associated with 1) greater amounts of linguistic experience in infancy and, later 2) children's larger expressive vocabularies. As predicted, despite considerable individual variation, higher maternal education was associated with greater amounts of linguistic input at 6 to 9 and 12 to 15 months. Maternal education was directly related with infants' vocabulary, but only at 12 to 15 months. At 19 months, the relationship was moderated by sex differences. A key finding of the study is that the quantity of verbal interactions occurring between infants and their caregivers from 6-9 months, but not infants' exposure to overheard speech, was a significant predictor of infants' vocabulary up to a year later. Mothers' IQ and education were related and, like maternal education, IQ appeared to contribute to infant vocabulary via infants' earlier language experiences. The current research represents the first in-depth look at input and vocabulary for lower SES Australian families. Its findings highlight some early environmental factors that warrant further investigation in relation to early vocabulary development.

Date of Award	2017
Original language	English