Convergent Evidence From Longitudinal Studies On Dyslexia

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Convergent evidence from longitudinal studies on dyslexia supports this causal hypothesis. Scarborough (1990) identified PA and LK as candidate causal factors by reporting weakness in these two factors in pre-readers who later became dyslexic. However, dyslexic children in the study were less intelligent than normal readers, which cast doubt on the findings. Results from Scarborough’s study are further supported by carefully conducted studies. For example, Snowling, Gallagher, and Frith (2003) followed FR children and control children from the age of 3 years, 9 months to 8 years, and found that language and letter knowledge assessed at Phase 1 predicted phonological awareness and grapheme-phoneme skill at Phase 2 (6 years), which then predicted word-level reading skills at Phase 3 (8 years).

Additionally, it is important to notice that Snowling et al. (2003) found that so-called unaffected children from high-risk family performed significantly better than impaired children in the tests of phonological skills, while they performed significantly poor in digit span and rhyme oddity, and performed marginally poor than the control group in the rest of tasks at 6 years. FR children for dyslexia who had milder phonological deficits might “escape” literacy problems, which emphasizes that causes do not operate in an all-or-none condition, and weakness in PA or LK alone might not sufficient to cause dyslexia. In other words, dyslexia appears to be multi-componential. Phonological

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