A comparative analysis of cognitive profiles: Monozygotic and dizygotic reading-disabled twins

Abstract

AEKTRACT: Previous twin studies have provided evidence for genetic intluences on reading disability (RD) or "normal" reading ability. The existence of subtypes of RDeach with its characteristic cognitive profile -has been suggested. Whether the etiology of the particular subtypes is genetic is uncertain. The present study examined within-pair comparisons of cognitive profiles (with respect to composites of general reading, symbol processing speed and sequential memory) in RD and control twins. Twin profile analysis were applied to these cognitive composites on 120 pairs of RD and control twins. Results showed a greater monozygotic (MZ) than dizygotic (DZ) resemblance in profiles indicating that individual differences in the patterning of cognitive abilities related to reading are genetically influenced.Past twin studies on reading have focused on the genetic etiology of either reading disability (RD) or "normal" reading ability. Studies on RD have utilized samples of twins in which the index case has been diagnosed as having a reading disorder whereas studies on normal reading ability have utilized samples of "normal" twins. Evidence of significant genetic influence on both normal reading ability and reading disability has been provided from these studies (Harris 1986). For example, twin studies on "normal" reading ability have consistently 'reported higher intraclass correlations for monozygotic (MZ) twins than for dizygotic (DZ) twins (e.g. Husen 1960, Vandenberg 1962, Matheny and Dolan 1974. Likewise, studies on RD twins (e.g. Bakwin 1973, DeFries Fulker and LaBuda 1987, Zerbin-Rudin 1967 have supported a genetic etiology of individuals with deviant reading performance. Past studies have not utilized a design that includes both affected and control twins. The assessment of the same battery of psychological tests on both groups is useful as a first step to test whether the etiology of the disability differs from that of individual variations within the normal range of reading ability.While older studies were based largely on rather subjective measures Reading and Writing: An Interdisciplinary Journal 1 (1989) 6 l-72.0 Kluwer Academic Publishers.

A Twin Research Perspective on Reading and Spelling Disabilities

“…Furthermore, the reading disabled MZ twins' resemblance was significantly greater than that of the DZ twins. Differences on a general measure of reading ability between the reading disabled twins and control twins were also found to be genetically influenced (Ho & Decker, 1989). It was concluded that genetic factors may underlie specific cognitive deficits that may be implicated in reading disability, but the etiology of the reading disorder itself remains unresolved.…”