The External Validity of Age- Versus IQ-Discrepancy Definitions of Reading Disability

Abstract

Recent research has raised the question of whether age- and IQ-discrepancy forms of reading disability (RD) are distinguishable in terms of either their underlying linguistic deficit or their response to treatment, thus threatening the external validity of the traditional distinction between specific reading retardation and reading backwardness. The present study pursued the external validity of this distinction in three domains: (a) genetic etiology, (b) sex ratio and clinical correlates, and (c) neuropsychological profiles. Each of these domains was explored in the RD (n = 640) and control (n = 436) twins participating in the Colorado Reading Project (514 males, 562 females, with an overall mean age of 12.42 years). Little evidence for external validity was found in terms of the clinical correlates of attention deficit-hyperactivity disorder (ADHD), immune disorders, or handedness. Most importantly, there was no evidence of differential genetic etiology of the two phenotypes in this sample, in that deficits in both phenotypes were similarly heritable (h2g = .40 and .46 for age and IQ phenotypes, respectively) and the genetic correlation between them was high (rG = .88 to .96). However, the genetic and neuropsychological profile analyses did suggest that age- and IQ-discrepancy definitions of RD may relate differentially to component reading processes, such as phonological awareness and orthographic coding.

Are Specific Language Impairment and Dyslexia Distinct Disorders?

“…Separate analyses were undertaken using either estimates of Full Scale IQ or nonverbal IQ as the index for intelligence. Whereas Full Scale IQ is most often used in defining dyslexia (Pennington, Gilger, Olson, & DeFries, 1992; S. E. Shaywitz, Shaywitz, Fletcher, & Escobar, 1990), a few researchers have used nonverbal IQ in studies of the reading outcomes of children with SLI (e.g., Bishop & Adams, 1990). The latter approach, although less common, reduces the role of verbal intelligence in identifying dyslexia and therefore might be expected to lead to more children with a history of SLI being identified as having dyslexia than if Full Scale IQ is used.…”

Subgrouping Poor Readers on the Basis of Individual Differences in Reading-Related Abilities

“…These include a lack of support for a strong and unidirectional relationship between IQ and reading achievement (Siegel, 1989;Stanovich, 1991). Critics also have noted the lack of evidence of qualitative differences between IQ-achievement discrepancy subgroups on reading-related factors (Ellis, McDougall, & Monk, 1996;Fletcher et al, 1994;Flowers, Meyer, Lovato, Wood, & Felton, 2001;Pennington, Gilger, Olson, & DeFries, 1992;Stanovich & Siegel, 1994). Another major issue of particular relevance to the present investigation has been the inability of the IQachievement discrepancy approach to provide guidance for intervention (Aaron, 1991;Forness, Sinclair, & Guthrie, 1983).…”

The Misconception of the Regression-Based Discrepancy Operationalization in the Definition and Research of Learning Disabilities

“…By so doing, these studies implicitly extended the intra-individual underachievement notion of learning disability with the interindividual notion of poor achievement. For example, in some studies (e.g., Share, 1996;Shaywitz et al, 1992) following Rutter and Yule's (1975) selection procedure, a group of discrepant readers, irrespective of reading level, was compared with a group of nondiscrepant poor readers, whereas in other studies (e.g., Pennington et al, 1992;Stanovich & Siegel, 1994), discrepant poor readers were compared with nondiscrepant poor readers. As a matter of fact, the introduction of a double criterion partly Note.…”