The double-deficit hypothesis: A comprehensive analysis of the evidence

The double-deficit hypothesis of developmental dyslexia proposes that deficits in phonological processing and naming speed represent independent sources of dysfunction in dyslexia. The present article is a review of the evidence for the double-deficit hypothesis, includ-ing a discussion of recent findings related to the hypothesis. Studies in this area have been characterized by variability in methodology— how dyslexia is defined and identified, and how dyslexia subtypes are classified. Such variability sets limitations on the extent to which conclusions may be drawn with respect to the double-deficit hypothesis. Furthermore, the literature is complicated by the persistent find-ing that measures of phonological processing and naming speed are significantly correlated, resulting in a statistical artifact that makes it difficult to disentangle the influence of naming speed from that of phonological processing. Longitudinal and intervention studies of the double-deficit hypothesis are needed to accumulate evidence that investigates a naming speed deficit that is independent of a phono-logical deficit for readers with dyslexia. The existing evidence does not support a persistent core deficit in naming speed for readers with dyslexia. The double-deficit hypothesis ofdevelopmental dyslexia impli-cates naming speed as a second, independent core deficit of dyslexia (Wolf & Bowers, 1999). The develop-ment of this hypothesis stemmed fro