Vocabulary development in 12-18 month-olds: Interactions between maternal input and infants' communicative skills

Individual differences in infants' vocabulary growth and developmental trajectory emerge as influenced by both child-related and environmental factors. This dissertation explores the dynamics between maternal language input, infants' nonverbal communication skills, and early vocabulary development in Turkish-learning infants. The focus is on two primary factors: (1) the influence of maternal input, including both verbal and gestural, and (2) infants' nonverbal communicative skills, particularly their ability to produce and comprehend pointing gestures. Another key focus is the extent to which mother-reported language measures reflect infants' language skills and whether mothers' estimations are associated with their verbal input directed to their infants. Chapter 1 presents a review of the relevant literature, summarizing the key empirical findings that inform the current dissertation. Chapter 2 examines whether infants' whole-hand and index-finger pointing, as well as mothers' pointing, at 14 months predict infants' concurrent and prospective vocabulary skills, measured with both direct (lexical processing efficiency - LPE; looking-while-listening - LWL) and indirect (vocabulary size; Turkish CDI) methods at 14 and 18 months. Results revealed that infants' index-finger pointing at 14 months predicted later lexical processing efficiency, as measured by reaction time and accuracy scores of LWL at 18 months. Neither infants' whole-hand pointing nor mothers' pointing predicted infants' concurrent or prospective vocabulary skills assessed via direct and indirect measures. These results extend the previous findings on the unique role of index-finger pointing in early vocabulary development with a more direct measure of vocabulary. Chapter 3 examined the association between maternal pointing and infants' later receptive vocabulary with a focus on how infants' point-following skills may influence this association. Specifically, the study examined whether mothers' pointing at 12 months predicted infants' receptive vocabulary at 14 months and whether this effect was moderated by infants' ability to follow points, particularly to targets outside their visual field. The study found that maternal pointing frequency was associated with infants' later receptive vocabulary at 14 months, but only for infants who could follow points to objects outside their immediate visual field. This suggests that infants' point-following skills enhance their ability to benefit from maternal pointing, potentially by sharing attention and better connecting the referred objects with verbal input. This chapter underlines how child-related factors, such as infants' early socio-communicative skills (point-following), interact with environmental input (maternal pointing) to support language development. Chapter 4 investigated how mothers with different levels of education provide estimations of their infants' word comprehension, how these estimations relate to the verbal input quantity and quality that they provide for their infants, and how their verbal input relates to infants' receptive vocabulary skills. Results demonstrated that while mothers make overestimations and underestimations of the words their infants comprehend regardless of educational background, the overestimations were positively associated with the number of words and lexical diversity in their input for their infants. Moreover, neither the quality nor the quantity of maternal input predicts infants' receptive vocabulary. This chapter suggests that mothers may adjust their verbal input to their infants' language competence, but based on their own overestimations. Overall, this dissertation demonstrates that infants' communicative skills, particularly their point-production and point-following abilities, play a significant role in early vocabulary development. Maternal verbal and gestural input would facilitate vocabulary development better, if infants have the social-communicative skills to process the input and maternal input meets infants' language development needs.