We used structural equation modeling to investigate sources of individual differences in oral reading fluency in a transparent orthography, Russian. Phonological processing, orthographic processing, and rapid automatized naming were used as independent variables, each derived from a combination of two scores: phonological awareness and pseudoword repetition, spelling and orthographic choice, and rapid serial naming of letters and digits, respectively. The contribution of these to oral text-reading fluency was evaluated as a direct relationship and via two mediators, decoding accuracy and unitized reading, measured with a single-word oral reading test. The participants were "good" and "poor" readers, i.e., those with reading skills above the 90 th and below the 10 th percentiles (n = 1,344, grades 2-6, St. Petersburg, Russia). In both groups, orthographic processing skills significantly contributed to fluency and unitized reading, but not to decoding accuracy. Phonological processing skills did not contribute directly to reading fluency in either group, while contributing to decoding accuracy and, to a lesser extent, to unitized reading. With respect to the roles of decoding accuracy and unitized reading, the results for good and poor readers diverged: in good readers, unitized reading, but not decoding accuracy, was significantly related to reading fluency. For poor readers, decoding accuracy (measured as pseudoword decoding) was related to reading fluency, but unitized reading was not. These results underscore the importance of orthographic skills for reading fluency even in an orthography with consistent phonology-to-orthography correspondences. They also point to a qualitative difference in the reading strategies of good and poor readers.

This article describes the results of an epidemiological study of developmental language disorder (DLD) in an isolated rural Russian population. We report an atypically high prevalence of DLD across all age groups when contrasted with a comparison population. The results are corroborated by a set of comparisons of school-aged children from the target population with their age peers and mean length of utterance matches from the comparison population. We also investigate the relationship between nonverbal cognition, verbal working memory, and expressive language performance in the population, and find statistically significant but small effect sizes. Finally, we describe the complex and heterogeneous structure of the phenotype in the population along with patterns of its vertical transmission on the basis of the exemplar pedigrees, and discuss the implications of our findings for genetic and clinical studies of DLD.

BACKGROUND AND OBJECTIVE: Developmental language disorder (DLD) is a highly prevalent neurodevelopmental disorder associated with negative outcomes in different domains; the etiology of DLD is unknown. To investigate the genetic underpinnings of DLD, we performed genome-wide association and whole exome sequencing studies in a geographically isolated population with a substantially elevated prevalence of the disorder (ie, the AZ sample).