Genetic and Environmental Influences on Reading Achievement: A Study of First- and Second-Grade Twin Children
Authors: Harris Emily Louann 
Published: July 2016

Previous twin studies provide evidence for genetic contributions to individual differences in reading achievement, but the nature of those genetic effects is uncertain. Reading is a complex behavior composed of many lower-level skills including attention, memory, learning ability, and the integration of auditory and visual information; genetic influences may exert their effects through these lower-level skills. This study evaluated the relationship between auditory-visual integration (AVI) and reading achievement and assessed genetic variance in AVI and reading achievement using the conventional twin model (comparison of identical twins with fraternal twins).The final sample consisted of 109 first- and second-grade volunteer twin pairs. Of the 89 same-sexed pairs, 57 pairs were classified as monozygotic, 31 pairs as dizygotic, and one remained unclassified; zygosity determination included examination of the intrapair similarity of genetic markers, dermatoglyphics, and/or physical appearance. The childrens' test battery included reading achievement tests (Woodcock Reading Mastery Tests) and the AVI task as well as estimates of general intellectual ability (Vocabulary and Block Design tests from the Wechsler Intelligence Scale for Children-Revised), auditory memory (Auditory Sequential Memory subtest of the Illinois Test of Psycholinguistic Abilities), comparison of auditory patterns, and comparison of visual-spatial patterns. Although the presence of heritable variation in reading achievement has been reported previously [22, 23, 30, 34, 50, 57], it does not account for all of the reliable variance; accordingly, two family questionnaires (Attitudes Toward Education and Moos' Family Environment Scale) and one dealing with first-grade method(s) of reading instruction were included to assess possible environmental factors.The most unique aspects of this study were the young age of the children studied and the twin analyses of the auditory-visual integration test battery. Evidence for heritable variation in reading achievement was found in this young sample consistent with the results of studies using older children. The main thrust of this project was to investigate possible “lower-level” sources of this heritable variation, particularly auditory-visual integration. As a first step, it was necessary to establish the presence of a significant relationship between reading achievement and auditory-visual integration, independent of general intellectual ability. The partial correlations between the reading achievement measures and AVI (WISC-R test scores partialled out) ranged from 0.10 to 0.49 and were generally significant. Factor analysis of the children's test battery supported the idea that the integration aspect of the AVI task (auditory-visual and/or spatial-temporal) was the aspect of this skill related to reading achievement rather than auditory memory or visual-spatial ability, which also contribute to success on the AVI task. Twin analyses of the AVI task revealed evidence of the presence of heritable variation. However, the task relates to other skills exhibiting genetic variance, and it could not be determined whether aspects of the AVI task independently related to reading achievement were the sources of the heritable variation observed.Although this study emphasized genetic relationships, environmental contributions to variation in reading achievement were also considered in a preliminary fashion. Family environmental data suggested that exposure to and emphasis on intellectual and cultural activities promote reading achievement.In conclusion, a positive and significant relationship between reading achievement and auditory-visual integration, independent of general intellectual ability, was established for a first- and second-grade sample. One environmental variable, intellectual-cultural orientation of the family, was also related to reading achievement. Evidence indicative of the presence of genetic variance was presented for reading achievement, as previously reported in older twin samples, and for auditory-visual integration.