Date of Award

Spring 1-1-2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Psychology & Neuroscience

First Advisor

Michael C. Stallings

Second Advisor

Sally J. Wadsworth

Third Advisor

Erik G. Willcutt

Abstract

Specific reading disability (RD) or dyslexia is often defined as an unexpected problem with learning to read despite having normal intelligence, no sensory impairments, and the opportunity to learn from reasonable instruction (Lyon et al., 2003). Research has shown that deficits in reading are both stable and heritable suggesting that genetic influences may be largely continuous throughout development.

This dissertation employs data from twins and their nontwin siblings in the Colorado Learning Disabilities Research Center (DeFries et al., 1997) and the Longitudinal Twin Study of Reading Disability (Wadsworth et al., 2007) to investigate factors which contribute to the heritability and stability of reading difficulties. Early twin studies compared “concordance” rates in pairs of identical and fraternal twins as a test for genetic etiology. However, DeFries and Fulker (1985, 1988) proposed fitting a multiple regression model to data from selected twin pairs to more rigorously assess genetic and environmental influences on extreme scores.

First, data from twin and nontwin siblings were fitted to DF multiple regression models to investigate the heritable nature of reading deficits in addition to examining evidence for a “special twin environment”. Second, twin data was employed to examine the etiology of genetic and environmental influences on the stability of reading deficits. Third, we examined heritability and stability utilizing data from a larger sample of twin pairs and their nontwin siblings. Our fourth study examined the differential etiology of genetic and environmental influences for reading disability as a function of gender.

Results from the first study indicated that reading deficits are substantially heritable; in addition, there were significant results for a special twin environment. Findings from the second study indicated that reading deficits were not only heritable, but also highly stable. Our third study suggested that reading disabilities are heritable and stable for both twins and their nontwin siblings. There was no finding for a special twin environment influencing the stability of reading deficits. The fourth study examined the etiology of the heritability and stability of reading deficits as a function of gender. Results were highly heritable and stable, however, for this sample there were no significant gender differences. Implications for these findings are discussed.

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