1.5.1 Definition by exclusion
A person is ‘dyslexic’ if no alternative explanation can be offered for their reading and writing difficulties.
In the UK, interest in children who showed a specific lack of ability in literacy grew as all children became entitled to a basic education. For the first time there was an expectation that all adults should be literate. Initially, it was proposed that specific difficulties in learning to read and write were visual in nature, and the term congenital word blindness was used by James Hinshelwood (a Scottish eye surgeon) to refer to:
…a congenital defect occurring in children with otherwise normal and undamaged brains characterized by a difficulty in learning to read so great that it is manifestly due to a pathological condition, and where the attempts to teach the child by ordinary methods have completely failed.
(Hinshelwood, 1917, p. 40)
Notice that Hinshelwood identifies the dyslexic population by reference to two norms: the children have normally functioning brains, and that normal methods of teaching result in failure. What this quote reflects is how little was understood about the causes of reading difficulties. This lack of knowledge resulted in dyslexia being defined and diagnosed by exclusion. One example of this is the definition from Critchley:
A disorder manifested by difficulty in learning to read despite conventional instruction, adequate intelligence, and sociocultural opportunity. It is dependent upon fundamental cognitive disabilities which are frequently of constitutional origin.
(Critchley, 1970, p. 11)
That is, if the children's disability could not be attributed to other potential causes (i.e. the children are apparently ‘normal’ in key respects), then they were given the label of ‘dyslexic’ – diagnosis by exclusion.
Diagnosis by exclusion is problematic as it is based on assumptions about what factors might affect one's ability to learn to read. The most problematic exclusionary criterion is that of ‘adequate intelligence’. While IQ tends to correlate with reading ability, a low IQ is not a barrier to learning to read. The idea that someone with a low IQ is ‘expected’ to fail to read is no longer socially acceptable. Dyslexia is not a set of difficulties that can only be experienced by the intelligent. Similarly, the criterion of sociocultural opportunity implies that people from households with few books, or children who are read to less by their parents, either because of financial, cultural or linguistic reasons can also be ‘reasonably expected’ to fail to learn to read.
You may find that you yourself satisfy one of these criteria. Did you experience difficulties learning to read and write? How reasonable do you believe these criteria to be? Think also about what they say about our expectations of children from diverse social and cultural backgrounds, in terms of standards that are being set concerning the conditions necessary for ‘normal’ development.
More fundamentally, definition by exclusion is unsatisfactory on logical grounds. If dyslexia represents a particular ‘abnormal’ pattern of development with a biological basis, then there is no reason why it could not coincide with other factors that may also be disadvantageous to the development of literacy skills. Dyslexia can in fact be found across the whole range of intelligence, although it may be more easily recognised in a child who otherwise appears to be highly talented, and it occurs at fairly similar rates in all countries and cultures where universal literacy is demanded. Social, economic, cultural and personal factors can certainly influence the rate and extent of someone's development in literacy skills, as we shall discuss later. However, these factors may also be quite independent of the predisposition to dyslexia. The fact that they have often been used to ‘explain away’ reading failure amongst children whose background does not conform to desired standards may account for why dyslexia has been mocked as a ‘middle-class disease’.