Abilities underlying decoding differences in children with intellectual disability

Researchers in recent years have made much progress towards understanding why some children struggle to learn to read. However, little of this research has involved children with intellectual disability associated with an IQ < 70 (ID, also called mental retardation). In the present analysis, the authors examined cognitive similarities and differences between stronger and weaker decoders, all of whom have ID. The 65 children with ID in the present analysis were initially referred by their teachers for a study that involved training basic phonological reading skills. The present analysis compares 21 children who were excluded from the training study because their decoding skills were already too high with 44 children whose decoding skills were low enough for the training study. The groups were compared on general intelligence, language ability, phonemic awareness and phonological memory. Initial analyses showed that the stronger decoders were significantly better than weaker decoders in language ability, phonemic awareness and rehearsal in phonological memory, but not in intelligence. They were also significantly older than weaker decoders. When age was covaried out, the groups differed significantly only in rehearsal in phonological memory, although the difference for phonemic awareness was marginally significant when the poorest performers were excluded. When intelligence is substantially limited, the ability to rehearse or refresh phonological codes in working memory plays a major role in determining children's success in learning to read. This ability appears to be more important than intelligence, language ability and phonemic awareness. It is possible that the reason the phonemic awareness measure was not as good at distinguishing the groups as the phonological rehearsal measure was because the former did not involve assembling phonological output. It is suggested that it is the combination of poor phonological representation and poor phonological output assembly that makes decoding difficult for some children with ID.