It has been known for over a century that there are children who appear to have a special difficulty with literacy that would not be expected on the basis of their other abilities (such as non-verbal reasoning). This is usually considered to involve developmental dyslexia, which is distinct from acquired dyslexia.
The former is present from the early stages of reading; the latter is not present in childhood but is usually the result of a brain injury in later life. Here we use the term dyslexia to refer to developmental dyslexia.
As we have seen, phonological abilities appear to play an important part in the development of literacy abilities. Keith Stanovich’s ‘phonological core hypothesis’ put forward in 1986 supposes that difficulties with phonological processes are the main cause of dyslexia.
One way of explaining this link is the suggestion that children with dyslexia have imprecise or, as sometimes termed, ‘fuzzy’ representations of phonemes. As a result, when they are learning to read it is more difficult for them to detect consistent patterns between the letters and sounds.
There are many studies that have findings consistent with the phonological core hypothesis. Lynnette Bradley and Peter Bryant found that children with dyslexia were worse at phonological awareness tasks than children with a similar reading age, but who were much younger.
Other evidence comes from a study conducted by Hollis Scarborough in 1990, and who followed up 32 children from two to seven years who had at least one parent with dyslexia. 65 per cent of the children were later identified as having dyslexia; these children were found to have at two-and-a-half years the same vocabulary size, but more speech mispronunciations and simpler sentences, than other children in the sample. At five years they performed less well on assessments of phonological awareness.
However, there are also findings which suggest that the nature of literacy difficulties may differ from language to language.
German children have a comparatively easy task in learning to relate letters to sounds because this relation is very consistent (unlike in English). Hans Wimmer and his colleagues reported in 1988 that German children with literacy difficulties tended to be slower at reading than their peers, but they did not have appreciable difficulties in decoding (i.e. converting letters into sounds).
Thus it may be that phonological skills are not as important in languages where there are regularities between letters and sounds.
Explanations of dyslexia have also concerned the visual system. Many clinical reports exist of children with reading difficulties who have problems with: letters moving on a page; letters changing place in words; or letters like ‘c’ and ‘l’ being fused and read as ‘d’.
There have been some researchers, such as Miles who have argued that dyslexia involves a range of difficulties, including: unstable vision when reading print, difficulties with the sequencing of sounds, attentional difficulties and poor motor coordination.
Although we now know much more about the reasons why some children have reading difficulties there is still uncertainty about whether the same difficulties affect all children. Even so, there is an increasing amount of evidence that interventions which target phonological abilities can be a significant help for young children.