Predictors of individual differences and language delay in children learning English

Abstract

This thesis examines individual differences in productive vocabulary development in 2-year-old children. Many factors are implicated in such individual differences in productive vocabulary development when children are 2 years old, but less is known about whether or not these factors also have a role in late talking. It is possible that the factors contributing to individual differences in vocabulary development can also be used to predict late talking. However, it is not necessarily a given that these factors will be sensitive enough to correctly identify late talking children. In addition, little is known about the predictors of language delay over time. Many children who experience an early delay in vocabulary development catch up by the time they start school, while a substantial proportion do not. This thesis investigates these issues in detail. Chapter 1 provides an overview of the literature on individual differences in vocabulary development, on the predictors of late talking, and of later developmental delays, as indexed by parental or professional concern about development. Chapter 2, the first empirical chapter, examines the predictors of individual differences in productive vocabulary development at 24 months in a large sample of typically developing English-learning children. This chapter also establishes if those factors, implicated in individual differences, can also be used to distinguish between children who are and are not slow to learn to talk. The purpose of this was to establish if the same factors that predict individual differences can be used to categorise the speech of children with lower ability. Chapter 3 examines if a subset of these factors predicts individual differences in productive vocabulary at 25 months in a different sample of children who were identified, on the basis of vocabulary scores at 15-18 months as children likely to be very delayed in productive language (so called late-talking children), as well as typically developing children. The purpose was to determine whether those factors that successfully predict individual differences in the typical range can also be used to distinguish between typically developing and late talking children. The final empirical chapter, Chapter 4, investigates if the language measures used in chapters 2 and 3, as well as risk factors and earlier parental concern for language development can predict concern for language development late in life, at 4-6 years of age; the age at which language disorders are starting to be diagnosed. Overall, language proficiency and cognitive measures were the most robust predictors of individual difference in productive vocabulary development at 2 years and can be used to identify children who are slow to talk and late talking children. However, language proficiency, in addition to risk factors, did not successfully identify children for whom there was concern for their language development when they were 4-6 years old. The findings of these studies and their implications for future research are discussed in the final chapter, chapter 5.