Researchers have hypothesized that around the age of 18 months (or around a vocabulary size of 50 words) children experience a marked increase in rate of vocabulary learning ( Bloom, 1973 Nelson, 1973).
First, a large body of work has asked whether and when children experience a vocabulary spurt early in development. The previous work in this area has taken two primary approaches. Not surprisingly, attempts to understand change over time in vocabulary growth have been part of the language acquisition literature for decades. Questions about how and why abilities develop are therefore often of central interest. Why study growth trajectories? Developmental phenomena, by definition, involve change. Thus, this study extends the current body of research by investigating the role of early vocabulary growth in predicting later vocabulary skill. Third, we address the more practical issue of how useful early language data can be in predicting later vocabulary skill. Second, we aim to make use of our detailed longitudinal data to examine whether these early vocabulary growth trajectories can help us predict the language skills children bring with them to school. First, we aim to understand the role that parent, family and child factors play in explaining variation in child vocabulary acquisition across the early childhood period. In this case, looking at rate of change and/or rate of acceleration in vocabulary growth would be useful in predicting vocabulary size at school entry. However, if we knew that one of the two children was increasing her vocabulary at a faster rate than the other (i.e., had a greater slope at 30 months), we might predict that the first child would have a larger vocabulary at school entry than the second.
Using observed vocabulary at 30 months to predict later language skill would lead us to predict identical vocabularies for the two children at school entry. Take, for example, two children who have the same vocabulary size at 30 months. But information gathered at a single point in time can be misleading. We could, for example, focus on background factors and child data gathered at one moment in time during the pre-school period and use this information to predict academic skills in the early elementary years. The obvious way to approach this question is to look to the period prior to school entry. The question we ask is whether differences in the rate at which vocabulary is acquired in the earliest stages predict children's vocabulary skills at school entry. Children's oral language skills when they enter kindergarten predict their later literacy skills and school success ( Dickinson & Tabors, 2001 Duncan, Dowsett, Claessens et al., 2007 Snow, Burns & Griffin, 1998)––those who start behind tend to stay behind ( Stanovich, 1986). It is important to understand not only the causes of variation in vocabulary growth rates, but also the consequences of acquiring vocabulary at different rates. Thus, previous research strongly suggests that variations in early social-interactive environments are associated with variations in vocabulary growth rates, and highlights the importance of understanding the role that parent and family factors play in child language development. This theoretical perspective has led to many studies exploring the relation between parent and family factors (including parental talk to children and family socioeconomic status) and child vocabulary growth, and positive relations have been found between the two (see Hoff, 2006 for a review). It is plausible that children's early environments and social interactions influence the course of language acquisition ( Braine, 1994). Children vary widely in the rate at which their vocabulary grows during early childhood ( Fenson, Dale, Reznick, Bates, Thal & Pethick, 1994).
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