Dyslexia and the double deficit hypothesis

The double deficit hypothesis (Bowers and Wolf 1993) maintains that children with both phonological and naming-speed deficits will be poorer readers than children with just one or neither of these deficits. In the present study, we drew on this hypothesis to help understand why some children have a serious reading impairment. In addition, by adding an orthographic factor, we extended it to a triple deficit hypothesis. Participants were 90 children aged 6 to 10 years. Dyslexic children, whose reading was low for age and for expected level, garden-variety poor readers, reading-level matched younger children, and low verbal IQ good readers, were compared. The dyslexic group was significantly lower then the garden-variety poor readers and the low verbal IQ good readers on most measures, and lower than the younger group On phonological measures. Findings support the double deficit hypothesis of Bowers and Wolf, and also the triple deficit hypothesis. Most of the poorest readers, nearly all of whom qualified as dyslexic, had a double or triple deficit in phonological, naming-speed, and orthographic skills. Conclusions were that dyslexia results from an overload of deficits in skills related to reading, for which the child cannot easily compensate.