Language-Based Deficits

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We have argued that reading disabilities are best characterized as developmental language disorders. From a theoretical perspective, such a claim is well founded. Reading is first and foremost a language activity. Reading relies heavily on one's knowledge of the phonological, semantic, syntactic, and pragmatic aspects of language. As such, deficiencies in one or more of these aspects of language could significantly disrupt one's ability to read. Not only is a language-based account of reading disabilities theoretically sound, considerable evidence has accumulated over the last twenty-five years to support this view.

Longitudinal Study of Language-Impaired Children

The relationship between language deficits and reading disabilities has been examined from several different perspectives. One approach has been the longitudinal study of children with early spoken language impairments (Aram, Ekelman, & Nation, 1984; Bishop & Adams, 1990; Catts, 1993; Catts, Fey, Tomblin, & Zhang, 2002; Silva, McGree, & Williams, 1987; Stothard et al., 1996; Tallal, Curtiss, & Kaplan, 1989). In this work, children displaying significant impairments in language (generally in semantic-syntactic aspects) have been identified in preschool or kindergarten and tested for reading and academic achievement in the later grades. Evidence that children with language impairments (LI) are more likely than typically developing children to have subsequent reading disabilities indicates that language deficits precede and play a causal role in reading disabilities.

The results of longitudinal studies have consistently shown that children with LI often have reading disabilities. In general, research indicates that 50 percent or more of children with LI in preschool or kindergarten go on to have reading disabilities in primary or secondary grades. In the most comprehensive study to date, the first author and colleagues (Catts, Fey, Tomblin, & Zhang, 2002) investigated the reading outcomes of 208 kindergarten children with LI. These children were a subsample of children who participated in an epidemiological study of developmental language impairments in children (Tomblin, Records, Buckwalter, Zhang, Smith, & O'Brien, 1997).

Results indicated that the group of children with LI read well below expected levels in second and fourth grades. Approximately 50 percent of the children with LI performed one or more SDs below the mean on a composite measure of reading comprehension. Although the remaining children with LI did not meet this criterion, many were, nevertheless, poor readers. When the criterion for a reading disability was changed to below the twenty-fifth percentile, nearly 70 percent of children with LI were classified as poor readers. Furthermore, analyses showed that children with low nonverbal abilities in addition to language problems performed significantly less well in reading than those with normal nonverbal IQs. Finally, those children who continued to have language deficits in second and fourth grade were at a much higher risk for reading disabilities than those whose language abilities had improved by the early school grades.

Language Problems in Poor Readers

The fact that many children with LI exhibit reading disabilities does not necessarily mean that most children with RD have a history of language impairments. To better investigate such a claim, studies have directly examined the language abilities of children with RD. In one body of research, investigators have selected school-age children identified as reading disabled (or in some cases, learning disabled) and studied their performance on traditional measures of language development. This work has shown that children with RD often have problems in receptive and/or expressive vocabulary (Fry, Johnson, & Muehl, 1970; Wiig & Semel, 1975) or in the use and/or comprehension of morphology and syntax (Doehring, Trites, Patel, & Fiedorowitcz, 1981; Fletcher, 1981; Stanovich & Siegel, 1994; Vogel, 1974). Deficits have also been reported in the production and/or comprehension of text-level language (Roth & Spekman, 1986; Stothard & Hulme, 1992; Yuill & Oakhill, 1991).

Although this research clearly shows that children with RD have language deficits, it does not necessarily indicate that these deficits are causally related to reading disabilities. A major problem for the interpretation of this work is that in most cases language abilities were examined in children who had reading problems for several years. This makes it difficult to determine if the observed language deficits were the cause or the consequence of a reading problem. Recall that earlier in the chapter we argued that Matthew effects can lead to language deficits in children with RD. Thus, at least some of the language problems observed in children with RD will be the consequence and not the initial cause of their reading difficulties.

Not all studies of language problems in children with RD have examined reading and language abilities concurrently. Some studies have investigated language deficits in children with RD prior to their learning to read. Scarborough (1990,1991), for example, investigated the early language development of children who later developed reading disabilities. In this study, the language abilities of children with a family history of dyslexia (N = 34) and children without a family history (N = 44) were assessed at age 2½years, and at six- or twelve-month intervals through age 5. Language assessments included measurements of receptive and expressive vocabulary, sentence comprehension, and grammatical production (not all measurements were administered at each age). In second grade, children's reading abilities were assessed. Of the 34 children with a family history of dyslexia, 22 were themselves diagnosed as dyslexic in second grade. The early language abilities of these dyslexic children through 4 years of age were found to be significantly poorer than those of the children without a family history of dyslexia. By age 5, however, only expressive vocabulary differentiated the two groups. Several other studies employing the same design have reported early language deficits in children at risk for reading disabilities (Lyytinen, Poikkeus, Laakso, Eklund, & Lyytinen, 2001; Snowling, Gallagher, & Frith, 2003).

In another study, the first author and colleagues (Catts, Fey, Zhang, & Tomblin, 1999) investigated the language abilities of a large group of poor readers. We identified 183 second-grade children who performed at least one SD below normal on a composite measure of reading comprehension. We did not exclude children on the basis of low IQ (except for those with mental retardation) as others have done in the past. The latter practice may bias results concerning language deficits in poor readers because IQ tests often measure verbal abilities. We compared the poor readers' performance on a battery of kindergarten language tests to that of a normal control group. We also used weighted scores based on epidemiological data (Tomblin et al., 1997) to better ensure that our results were representative of poor readers from the population at large. Our findings indicated that the poor readers performed significantly less well than the good readers on tests of oral language. In addition, a large percentage of poor readers performed at least one SD below the mean on tests of vocabulary (39%), grammar (56%), and narration (44%).

Our results further indicated that the poor reader's early language deficits extended beyond vocabulary, grammar, and narration. Poor readers were also found to have difficulties in phonological awareness and phonological retrieval in the kindergarten assessment. Specifically, 56 percent of the poor readers performed at least one SD below that of the normative sample on a measure of phonological awareness (syllable/phoneme deletion) and 45 percent performed below that level on a test of phonological retrieval (rapid naming). These deficits, however, rarely occurred in isolation from problems in vocabulary, grammar, and narration. Our findings concerning deficits in phonological awareness and retrieval are consistent with a large body of research that has documented the prevalence of phonological processing deficits in children with RD. Phonological processing deficits refer to difficulties in linguistic operations that make use of information involving the sounds of speech (e.g., verbal short-term memory, phonological awareness) (see Catts, 1989b; Rack, Hulme, Snowling, & Wightman, 1994; Wagner & Torgesen, 1987). Phonological processing deficits are the primary language problems associated with dyslexia and a prominent characteristic of other reading disabilities. In the sections that follow, the research findings concerning the relationship between phonological processing deficits and reading disabilities will be reviewed.