Toward a Rational Taxonomy of the Developmental Dyslexias

In societies where the educational system aims to produce 100 per cent literacy, a substantial minority of children experience great difficulty in learning to read and many never achieve a fluent command of visual language skills. How is this population to be further characterized? It has sometimes been argued that reading retardation is always subsiduary to some more general disability (low intelligence, lack of motivation, sensory-motor impairment, or a cross-modal learning deficit, for example). Such a view is now almost impossible to uphold (Benton and Pearl, 1978). At the other extreme, the World Federation of Neurology has postulated a unitary syndrome of ‘specific developmental dyslexia’ that was defined as “a disorder manifested by difficulty in learning to read despite conventional instruction, adequate intelligence and socio-cultural opportunity. It is dependent upon fundamental cognitive disabilities which are frequently of constitutional origin” (Benton and Pearl, 1978). This definition has been justifiably savaged by Benton (1975) and by Rutter (1978), although these scholars do not, of course, deny that one can find a group (or, better, groups) of dyslexic children for whom reading failure is, in some sense, ‘unexpected’.