Abstract

Phonological coding, measured by the oral reading of nonwords, and orthographic coding, measured by the discrimination of words from homophonic nonwords (e.g., rane, rain), were compared for pairs of older children with reading disabilities (RD) and younger nondisabled readers matched on word recognition. Phonological coding was substantially lower for most children with RD, indicating a unique developmental deficit in phonological coding rather than an equal developmental lag across all component reading skills. Data from identical and fraternal twins indicated that the phonological coding deficit of the children with RD was highly heritable and accounted for most of the heritable variance in their word recognition deficits. The deficits of the twins with RD in segmental language skills (rhyming and phoneme segmentation) were related to the heritable variance in their phonological coding deficits. Orthographic coding was not significantly heritable, and it accounted for much of the environmental variance in word recognition deficits. Implications of the results for the remediation of reading disability are discussed.