01 NRDC Dyslexia 1-88 update - Texthelp

Developmental dyslexia in adults: a research review 35
three times as large as a sample identified in the same population by a cut-off of 2 SD below
the mean on the same measure. Because human traits are distributed in multidimensional
space (Cloninger et al., 1997), the two groups will also differ in kind as well as in size (Ellis,
1985). In neither case will extreme scores indicate a reason for their extremity (Frith, 2001).
Groups defined on quantitative measures such as IQ-discrepancy are by their nature
heterogeneous and different patterns of heterogeneity result from different definitions.
Diagnostic criteria inferred from one group may thus be too broad, or too narrow, for the
other group.
Some problems in making IQ-related identifications have been analysed in a study of children
with ‘specific reading retardation’ (Bishop & Butterworth, 1980), a construct which despite its
similarity to ‘dyslexia’ carries no implication as to cause and does not imply a single cause
(Yule & Rutter, 1976). The argument runs like this (Bishop & Butterworth, 1980):
■
■
■
If learners with ‘specific reading retardation’ are defined as those with a nonverbal IQ of 90 or
more who are reading 12 months or more below age level, then, since reading is strongly
related to verbal IQ, poor readers would generally have low verbal IQs. However, since verbal
IQ and nonverbal IQ are positively correlated, most specifically reading-retarded learners
would have low nonverbal IQs, yet the definition would exclude those learners.
If, on the other hand, specifically reading-retarded learners are defined as those with a fullscale
IQ of 90 or more who are reading 12 months below age level, then the poor readers by
this definition (who have a low verbal IQ as poor readers) would have a relatively high
nonverbal IQ and so there would be a larger proportion of learners with verbal-nonverbal
discrepancy among the specifically reading-retarded group.
Alternatively, if specifically reading-retarded learners are defined as those with a verbal IQ of
90 or more who are reading 12 months or more below age level, then there would be no
verbal-nonverbal discrepancy relationship with specific reading retardation. What is more, if
verbal IQ were the basis for selection, it would increase the proportion of males classified as
specifically reading-retarded.
Learners might be most appropriately described as specifically ‘reading-retarded’ only if their
reading ability is disproportionately poor in relation to their verbal (rather than nonverbal or
full-scale) IQ, but such people are rare and their rarity would frustrate researchers (Bishop &
Butterworth, 1980). Nevertheless, to define specific reading retardation in terms of poor
reading relative to nonverbal IQ would include many learners whose reading is quite
consistent with their low verbal ability and could not, on this account, be regarded as
‘unexpected’ (Bishop & Butterworth, 1980).
This is not merely a matter of fine-tuning. In a study in which several different methods of
identification were compared, the criteria for dyslexia were progressively relaxed until every
poor reader was included, at which point the lowest and highest estimates of prevalence were
found to differ by a factor of seven (Snowling et al., 2000).
In brief, people may be ‘dyslexic’ according to one method of identification but not according
to a different method. In as much as ‘dyslexia’ is a construct, the characteristics of ‘dyslexics’
are necessarily artefacts resulting from the identification procedures; they may not
necessarily reflect an innate cognitive dysfunction in the people identified by those
procedures. Most research findings reflect statistical tendencies, not systematic rules; the
given effect is sometimes present in only a minority of subjects and caution is needed in
drawing explanatory models from such studies (Habib, 2000).