01 NRDC Dyslexia 1-88 update - Texthelp

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Research Report
their turn, a variety of causal assumptions. But before we consider the causes of dyslexia, we
need to remind ourselves that exclusionary definitions of dyslexia are primarily research
definitions (Vellutino, 1979). Although exclusionary definitions relying upon a discrepancy
between measured IQ and attainment may be crucially important in developing the theory of
dyslexia, their sole justification is to identify potential research participants (Nicolson, 1996;
Torgesen, 1989). Otherwise, the observed differences between dyslexics and non-dyslexics
may simply reflect differences in intellectual ability and so tell us nothing about dyslexia itself
(McCrory, 2003).
Inclusionary definitions may be misleading in a different way. A ‘pattern of difficulties’
approach (Miles, 1983) is sometimes assumed to imply an absolute distinction between
impaired and normal abilities, by analogy with the ‘holes in the mind’ found in stroke or brain
injury patients. However, the presence of fully-realised abilities in other domains cannot be
taken for granted in developmental disorders. If there is a genetic abnormality, then it is
possible that the whole of a person’s development will be atypical (Karmiloff-Smith et al.,
2003), so that their ‘pattern’ of difficulty is one of muted contrasts.
For the purpose of intervention, several studies have questioned the validity of the IQdiscrepancy
criterion (Aaron, 1997; Fletcher et al., 1992; Gustafson & Samuelsson, 1999;
Hatcher, 2000; Klicpera & Klicpera, 2001; Siegel, 1992). Contrary to what people may
sometimes assume, IQ is not a measure of intellectual potential (Stanovich, 1999); neither is
it a measure of ability to benefit from intervention (Vellutino et al., 2000). Meta-analysis has
shown that ‘any classification of poor readers based on IQ-discrepancy is an artefactual
distinction based on arbitrary subdivisions of the normal distribution’ (Stuebing et al., 2002).
On measures of decoding ability, there is no great difference between IQ-discrepant and nondiscrepant
poor readers (Vellutino, 2001).
Given a conjectural association between dyslexia and ‘right-hemisphere’ abilities in a
‘pathology of superiority’ (Geschwind, 1984), it is unlikely that the last word has been written
on this topic. On anecdotal evidence (Davis, 1994; West, 1997), the belief that ‘difficulty in
learning to read is not a wholly tragic life sentence but is often accompanied by great talents’
(Stein & Talcott, 1999) may seem attractive. However, systematic investigation has found little
if any support for it (Adelman & Adelman, 1987; Everatt, 1997; Thomas, 2000; von Karolyi et
al., 2003; Winner et al., 2001).
Adult literacy tutors need to be wary of any reading disability construct with its origins in a
‘folk psychology’ based on conjecture, correlation and inferences from anecdotal evidence.
‘Folk’ taxonomies differ greatly from scientific, theory-based taxonomies (i.e. principled
classification schemes) in the overall patterns they see in nature (Ziman, 2000); folk
taxonomies may be ecologically valid, but reasoning about them as if they were theory-based
often leads people into error (Atran, 1998). (To take an analogy, the frog, the eel and the
elephant seal are each viable on land or in water. Ecologically they have something in
common, but no taxonomic theory can make their association meaningful.)
It will become evident in the course of this review that current methods of identification of
who is (or can be described as) ‘dyslexic’ map loosely, if at all, to current theories. This is
entirely proper. Although the research programmes in cognitive psychology, physiology and
genetics are exciting, they may need to continue for many years before their findings inform
methods of diagnosis or prevention. The scientists themselves advise that much of their work
is, in the nature of scientific enquiry, conjectural. While the research must be supported, too
much should not be expected of it too soon.