ABSTRACT
The IQ-achievement discrepancy based definition of dyslexia involves the identification of children who are reading below their “potential”, where “potential” is determined by an IQ score. The discrepancy between the actual and the expected reading level is the central core of the definition. The use of the discrepancy based definition implicitly relies on three main premises, namely: 1) intelligence tests are able to measure potential intellectual capacity, 2) dyslexia is caused by some form of highly circumscribed cognitive deficit which does not affect performance on IQ tests (i.e., IQ is independent of dyslexia), and 3) dyslexics are qualitatively different from poor readers who have low IQ scores (i.e., no discrepancy between IQ score and reading). All three assumptions have yet to be convincingly proven. This chapter critically examines the evidence, rationale and assumptions behind the IQ discrepancy based definition of dyslexia. After reviewing 21 studies that have explicitly compared poor readers and dyslexics, it was found that there were more similarities than differences between these groups on reading tasks such as word recognition, decoding, comprehension, orthographic and phonological awareness. On these tasks, dyslexics and poor readers do not differ significantly from each other but both groups show performance that is significantly below that of normally achieving readers. Most of the differences found between the dyslexics and poor readers were limited to IQ correlated tasks such as mathematics, vocabulary, and syntax, thus casting doubt on the validity of using the reading-IQ score discrepancy to define dyslexia.
