Proposal for a Taxonomy of Dyslexia

The word 'taxonomy' means 'way of classifying' or 'classificatory principle'. Although the word has been used primarily in biology and medicine, there is no reason why it should not be used more widely. How to classify anomalies of development is very much a live issue at the present time.

Classifications can be made for many different purposes. For a start, it seems to me helpful to draw a distinction between strong and weak taxonomies. I illustrate this distinction by a series of examples.

Let us suppose, in the first place, that the organiser of a conference needs to distinguish those who intend to go on an excursion and those who do not. There is, of course, nothing wrong with such a classification. On the other hand no one would be tempted to say it was a powerful or strong one; rather it is a classification made for a strictly limited purpose without any implication that it is of any lasting value. In particular there is no suggestion that biological markers - neurological or biochemical differences, for instance - will be discovered which differentiate excursion-goers from non-excursion-goers; nor would one expect biologists to classify them as 'two distinct species' or claim that the distinction heralded a significant scientific breakthrough. In practice the word 'taxonomy' is not used of cases of this kind, but, if it were, one would have to speak of an extremely weak taxonomy.

It would also be a manifestation of a weak taxonomy if someone were to classify a whale as a fish on the grounds that it lived in the sea, or a duck-billed platypus as a bird because it possessed a beak. In both cases it is known that there are a large number of anatomical features which point, beyond doubt, to the classification 'mammal'.

Medicine constitutes an interesting example because some of its taxonomies are very much stronger than others. For example, the terms 'fever' and 'nervous breakdown' still survive in common use since neither is wholly uninformative; if, however, they are contrasted with terms such as 'tuberculosis' and 'phenylketonuria' the differences are plain. It is characteristic of strong medical taxonomies that they imply a theory of causation, accurate prognosis and distinctive methods of treatment (compare the arguments in Davis and Cashdan, 1963, which were discussed in Chapter 11). Some taxonomies, one might say, are 'nosologic' and some 'nosographic' in the sense given to these words in Chapter 10. A strong or nosologic taxonomy is more than a description; it implies a full understanding of what is happening.

The word 'syndrome' is interesting in this connection. Butterworth's Medical Dictionary (Critchley, 1978, p. 1647) defines a syndrome as 'a distinct group of symptoms or signs which, associated together, form a characteristic clinical picture or entity'. Similarly Churchill's Medical Dictionary (Konigsberg, 1989, p. 1838)

refers to 'signs, symptoms, or other manifestations' and adds that the word is 'used especially when the cause of the condition is unknown'. One can therefore say that use of the term 'syndrome' implies partial knowledge; it indicates that we are in possession of a taxonomy of moderate strength even though much more remains to be discovered.

If one makes classifications of any kind, this involves being both a 'lumper' and a 'splitter'. A lumper is one who wishes to group (or 'lump') certain phenomena together, whereas a 'splitter' is one who treats the phenomena under investigation as separate (or 'splits' them). The issue of substance in disputes over developmental anomalies of all kinds is where to 'lump' and where to 'split'.

There is no special merit either in lumping or in splitting as such. Hypothesis building may require us to lump together phenomena which seemed prima facie unconnected, and this can sometimes constitute a significant advance; on the other hand, as knowledge increases and researchers become more sensitive to differences, there may be increased pressures towards splitting. Whether one should be a lumper or a splitter will therefore depend on the circumstances of a particular case.

Once this point is recognised, the way becomes open for some of the apparent conflicts in disputed areas to be resolved: both parties may be right, since lumping may be appropriate for one purpose and splitting for another. Moreover, since boundaries often need to be changed as science advances, there is no need to fan the flames of controversy by insisting that a particular boundary is the only correct one to draw. This, however, does not justify a laissez-faire attitude towards the selection of a taxonomy. As has already been pointed out, some taxonomies are stronger than others, and, if one's objective is scientific research, it is inefficient to use a weaker taxonomy when a stronger one is available.

An analogy supplied by my colleague Professor Rod Nicolson may be of help in this connection. There are all kinds of places where one might apply pressure to a rock, but, if one applies pressure exactly along a fault line, any resultant division of the rock will provide meaningful 'chunks' for further study. In contrast, if one applies pressure in other places, one will be confronted merely with a collection of disconnected fragments, and it will be difficult to make sense of them.

There is a further reason why some taxonomies can be better than others. If a taxonomy is proposed from a position of strength - from a position of knowledge - a particular decision to lump or split merits nothing but respect. If two researchers are both aware of all the reasons for lumping and of all the reasons for splitting, even if one person decides to be a lumper and the other a splitter, there is nothing significant left for them to disagree about - at most there might be a disagreement about what similarities and differences are the important ones to emphasise. In contrast, if proposals for classification show a lack of such awareness, they can justifiably be faulted. I suspect that those who opposed the concept of dyslexia in the 1970s (see Chapters 11 and 12) were unaware of some of the reasons for lumping.

My thesis is that dyslexia is a syndrome - one which can supply a taxonomy of considerable strength if research findings in a number of areas are seen as interrelated and are therefore lumped together. The syndrome comprises a pattern of difficulties which is easily recognised once one knows what to look for. Its power lies in the fact that it encourages researchers to lump together a number of seemingly disparate phenomena. In particular there is anatomical evidence that dyslexics show some distinctive differences in brain organisation beyond the limits of normal variation; there is genetic evidence based on the fact that the same pattern of difficulties often runs in families, and there is the evidence for phonological difficulties, that is difficulties in the ordering, identification and remembering of speech sounds. The concept of dyslexia, in the sense given to it in this book, straddles these three research areas - anatomical research, genetic research and the behavioural research which points to the existence of a phonological deficit. Any advance in one of these areas is likely to be of help to the other two.

There remains the question of whether within the group of dyslexics there is any justification for further splitting. In Chapter 14 I reported that I could find no evidence for saying that 'visual dyslexics' were a different group from 'auditory dyslexics', and it is suggested in Miles and Miles (1994) that subtyping based on acquired dyslexia (deep dyslexics, surface dyslexics, phonological dyslexics) did not make much sense when applied to developmental dyslexia. Future research may, of course, suggest some different subtypes, but my personal view is that this is unlikely.

The demarcation of a taxonomy can be achieved either by coining a new word or by making stipulations about the more precise use of a current one. As the word 'dyslexia' lies ready to hand I propose to use it. There is now, I believe, enough consensus that dyslexia should be regarded as a syndrome, and not simply as a synonym for 'poor reading', to justify the stipulation to use the word in this way. A specially coined atheoretical term, such as 'the alpha syndrome', would have done as well, except for the fact that any theoretical link with the word 'dyslexia' would have been lost.

At present the links between these different research areas are suggestive rather than firmly proven. We do not know, for instance, whether the typical dyslexic who has been diagnosed on the basis of the Bangor Dyslexia Test (Miles, 1997) would usually or always be found to show distinctive patterns of activation if their brains were scanned. It is the sign of a strong taxonomy that it generates problems of this kind for research.

In contrast to all this my contention is that 'poor reading' provides only a very weak taxonomy; it lumps together a whole variety of phenomena which may have arisen from quite different causes and which may lead to quite different outcomes. As far as the concept of 'poor reading' is concerned there is no reason to be anything other than a splitter. This does not, of course, mean that the concept of 'poor reading' is never of any value to anyone. It may be that a Local Education Authority wishes to know the reading standards in the area for which it is responsible. Such enquiries may, of course, be of value, but they are not, and do not claim to be, the stuff of which scientific advances are made.

There is, however, a difficulty with my proposal for lumping which needs to be addressed. It is that much of the research into dyslexia has used 'poor reading' as a criterion for subject selection. It therefore seems inconsistent that in this book I have sometimes drawn on research findings based on poor reading as the criterion for dyslexia and then asserted that that criterion is unacceptable.

What I suspect has happened is that the great majority of the subjects investigated in these researches have been genuinely dyslexic. Those who are poor readers from lack of opportunity are a decreasing number, since, at least in Britain, the opportunities for learning to read are extremely widespread. The amount of 'noise' in the selection of subjects has, I suspect, been minimal. My quarrel is at the theoretical level: some researchers may not have used the word 'dyslexic' at all, and some of those who used it may have supposed that they were studying poor readers when in fact most of their subjects were manifesting the syndrome of dyslexia.

As we saw in Chapter 15, Ramaa et al. (1993) were able to draw a distinction between those poor readers who were and were not dyslexic, and one must suppose that lack of opportunity is more common in parts of India than it is in Britain. Attempts by myself and my colleagues to distinguish those with literacy problems who were and were not dyslexic will be reported in Chapters 19 and 20.

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Adult Dyslexia

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