Variability in Reading Ability

Abstract

Why individuals succeed or fail in learning to read is one of the most investigated domains in cognitive psychology. Reading compels interest for several reasons. First, reading is widespread but not innate. Unlike spoken language, reading must be explicitly taught. But, like spoken language, reading ability can vary tremendously among individuals, and also within and among families. Thus there is a genetic component to what is ostensibly a trait that is not genetically coded. Secondly, reading ability is not one single process, but is rather the outcome of multiple interacting and supporting processes. It can be decomposed and measured in a variety of ways. Finally, reading ability is relatively distinct from general intelligence. Most psychometric tests correlate substantially with intelligence, but dyslexia, impaired word decoding in the presence of normal intelligence, and hyperlexia, skilled word decoding in the absence of concomitant language comprehension, provide evidence that word decoding ability reflects a latent trait distinct from general intelligence. These characteristics of reading raise several questions: Notably, if reading is not "innate," what is being transmitted genetically? If reading is built on multiple processes, are there many or few processes that influence individual variability in reading ability? Finally, how can we explain individuals with hyperlexia, who appear to decode words fluently in the absence of appropriate spoken language? This dissertation is built around three papers that aim to answer these questions. In the first paper, we examine the genetic etiology of reading comprehension and lexical decision reaction time. Specifically, we show that although both reading comprehension and lexical decision are heritable and contributed to by several genes, the two traits appear to share virtually no genes between them. In the second paper, we examine the relative contributions of a same different response time task and phonemic awareness tasks to reading comprehension and word decoding measures. We found that the response time measures contributed unique variance above that of phonological awareness. Importantly, these response time measures, when decomposed to reflect various cognitive processes, predicted more unique variance and provided a stronger theoretical footing to interpret the results. Finally, in the last paper we examined the performance of children with autism and hyperlexia on an auditory artificial lexicon task. We found evidence that these individuals incorporate and activate lexical items differently than do typically developing individuals. We suggest these differences account for how these individuals learn to decode words so easily as compared with their typically developing peers.