G. Reid Lyon
Jack M. Fletcher
Marcia C. Barnes
Since learning disabilities (LDs) were federally designated as “handicapping conditions” in 1969, children identified with LDs now represent approximately one-half of all children receiving special education services in the United States (U.S. Department of Education, 1999). In the years since publication of the first edition of this chapter (Lyon, 1996b), progress has been made on understanding and treating LDs, especially in the area of reading. Here significant advances have been made in classification and definition issues (Fletcher et al., in press; Lyon et al., 2001), assessment practices (Fuchs & Fuchs, 1998; Speece & Case, 2001; Torgesen & Wagner, 1998; 2002), neurobiological correlates involving the brain (Eden & Zeffiro, 1998; Joseph, Noble, & Eden, 2001; Shaywitz et al., 2000), genetics (Grigorenko, 2001; Olson, Forsberg, Gayan, & DeFries, 1999; Wood & Grigorenko, 2001), and interventions (Felton & Brown, 1993; Foorman, Francis, Fletcher, Schatschneider, & Mehta, 1998; Fuchs et al., 2001; Lovett, Lacarenza, Borden, Frijters, Steinbach, DePalma, 2000; Lovett, Steinbach, & Frijters, 2000; Mathes, Howard, Allen, & Fuchs, 1998; Vellutino et al., 1996; Torgesen et al., 1999, 2001; Vaughn, LinanThompson, & Hickman-Davis, 2002). The advances in interventions are especially promising, as the research shows that reading disabilities are preventable in many children, and that intensive interventions can be effective with older children who have severe reading difficulties. Moreover, in the reading area, research is converging on a comprehensive model of the most common LD—dyslexia—that accounts for biological and environmental factors as well as for the effects of intervention, and is grounded in reading development theory (Grigorenko, 2001; Lyon et al., 2001; Rayner, Foorman, Perfetti, Pesetsky, & Seidenberg, 2002). Indeed, the same theory that explains how children develop reading skills explains why some fail, unifying research on LDs in reading and the normative development of reading ability. Given these advances for dyslexia, similar advances for other LDs cannot be far behind.
Within the context of this significant progress, the next section of this chapter reviews briefly the historical events that have molded the general field of LDs into its present form, with a focus on the origins of current policy-based definitions of LDs. Subsequent sections address in detail the core features of specific types of LDs. The reader should note from the outset that LDs do not constitute a homogeneous disorder. In fact, LDs by definition refer to deficits in one or more of several domains, including reading disabilities, mathematics disabilities, and disabilities of written expression. Since each type of LD is characterized by distinct definitional and diagnostic issues, as well as issues associated with heterogeneity, each is covered separately in this chapter. Thus, for each LD domain, a review of critical background information, constructs, and research and policy trends is provided. More specifically, a review of each major domain of LD is organized to address (1) a review of current