The present study investigated the language development of 50 children with intellectual disabilities (ID) and 42 typically developing children from age 4 to 5 years, and was designed to shed more light on the respective roles of phonological working memory (WM) and nonverbal intelligence in vocabulary and syntax development. Results showed that nonverbal intelligence predicted phonological WM, vocabulary and syntax of children with ID at age 4 and 5, and that it only predicted these skills at age 4 in typically developing children. Furthermore, syntax at age 5 was predicted by vocabulary at age 4 in children with ID, which points to children with ID requiring a larger critical mass of vocabulary for syntactic development to be initiated.
The language development of children with intellectual disabilities (ID) often shows delays, and the cognitive disabilities of these children are thought to be the main cause of these delays (e.g., cognition hypothesis; Cromer, 1991). This view, however, seems to be too limited (Rondal, 2001). Although in many studies the language level has been found to be in pace with the mental age of the children with ID (Rondal, 2001), other studies found great variation in linguistic areas that develop faster or slower than could be expected based on the mental level of the children. For example, children with Down syndrome show specific weaknesses in the areas of speech production, syntax, and the intelligibility of their speech (Roberts, Price, & Malkin, 2007). On the other hand, some children with ID have vocabulary levels far above their mental age (Facon, Facon-Bollengier, & Grubar, 2002), and with appropriate support, children with ID can acquire high levels of communication and literacy skills (Kaiser et al., 2001, Koppenhaver and Erickson, 2003 and Van der Schuit et al., 2010).
These varying language levels of children with ID raise the question of what the influence of different levels of intelligence is on the language development of children. Not all studies have found developmental level to be predictive of later language skills (Calandrella and Wilcox, 2000 and McCathren et al., 1999), while others did not include intelligence measures as predictors of language development. Often, studies only included children with the same etiology (e.g., Down syndrome), which gives valuable information about strengths and weaknessess of the specific syndrome, but does not shed further light on the influence of intelligence on language development of all children with low intelligence. The present study, therefore, included children with a range of etiology to shed more light on the influence of intelligence levels on the language development of children with ID and their typically developing peers.
This study examined the influence of cognitive factors on the language development of children with ID and typically developing children. The aim was to describe how phonological WM affects the development of vocabulary and syntax and to find out what the role is of nonverbal capacity on the language development from age 4 to 5. As expected, children with ID were far behind their typically developing peers on nonverbal intelligence, phonological WM, vocabulary, and syntax, a finding that is in line with previous research (e.g., Kaiser et al., 2001 and Roberts et al., 2007). Even when nonverbal intelligence was taken into account, children with ID still showed significant delays on phonological WM, and syntax. No significant differences, however, were found for vocabulary at both ages 4 and 5. This means that although children with ID have lower vocabulary levels, controlling for intellectual level results in non-significant differences on vocabulary between the two groups. This finding suggests that the vocabulary levels of the children with ID are in pace with their mental level, but that both phonological WM and syntax level are below their mental level.
When studying the role of phonological WM in longitudinal language development, two different models were found for the two groups. For the typically developing children no effect from phonological WM on vocabulary or syntax development from age 4 to 5 was found. Also, no structural relationships were found between vocabulary and syntax development from age 4 to 5. This means that for these children, the development of phonological WM, vocabulary and syntax is quite autonomous. Previous research has also shown this emerging modularity, in which different language domains become more autonomous during development (Jones Moyle et al., 2007). This means that, while during early language development tight relationships exist between the different language domains, from a certain point in development these domains dissociate. For example, research has shown both lexical and syntactic bootstrapping in typical language development (Dionne et al., 2003), however, after the age of 3 years and 6 months no cross-lagged relationships have been found between lexical and grammatical development (Jones Moyle et al., 2007).
