There have been no reports of children with developmental dyslexia who have not also had a developmental dysgraphia. However, there have been reports of children with developmental dysgraphia who were thought to have good reading skills (Frith, 1980). Spelling difficulties are therefore more common than reading difficulties, but despite this they have received substantially less investigation than reading difficulties. This is as true for the developmental disorders of spelling as it is for acquired disorders of spelling amongst neurological patients. Even Hinshelwood, who in his 1917 book on congenital word-blindness, wrote in such detail about his interest in children with reading difficulties, made remarkably little comment about their spelling skills and abilities. He did note that his children were able to correctly form written letters and could copy pieces of writing placed in front of them. However, as a rule they failed when they were asked to write to dictation. His explanation for this also related to the deficit he believed the children had in the visual memory centre for words (Hinshelwood, 1917):
Although this visual memory centre is not destroyed, it is functionally in abeyance, as it has not yet been furnished with the visual memory of words, and hence stimulation of the graphic centre is impossible, (p.61)
view that spellings are retained in memory by being analysed as sound symbols. The knowledge of printed word spellings also acts within the language to constrain phonological drift and maintains greater consistency in pronunciation from one generation to the next than for cultures without written language. Moreover the rigidity of spelling is evident in the observation that a culture will more rapidly accommodate a shift in the pronunciation of a written spelling than an alteration of the structure of the spelling itself.