Etiology of Reading Difficulties as a Function of Gender and Severity

• Published in: Reading & writing Vol. 20; no. 1-2; pp. 13 - 25
• Main Authors: Hawke, Jesse L, Wadsworth, Sally J, Olson, Richard K, DeFries, John C
• Format: Journal Article
• Language: English
• Published: Dordrecht Springer 01.02.2007; Springer Nature B.V
• Subjects: Children; Environmental Influences; Etiology; Females; Gender; Genetics; Goodness of Fit; Hypothesis Testing; Independent sample; Independent Samples; Males; Reading; Reading Difficulties; Severity (of Disability); Twins
• ISSN: 0922-4777; 1573-0905
• DOI: 10.1007/s11145-006-9016-z

To test the hypothesis that the etiology of reading difficulties may differ for males and females in more severely impaired samples, reading performance data from monozygotic (MZ), same-sex dizygotic (DZ[subscript ss]), and opposite-sex dizygotic (DZ[subscript os]) twin pairs were analyzed using a model-fitting implementation of the DeFries-Fulker (DF) model (Purcell & Sham, 2003, "Behavior genetics", 33, 271-278). Five non-independent samples were selected using cut-offs of -1 (N = 737 pairs), -1.5 (N = 654), -2 (N = 468), -2.5 (N = 335), and -3 (N = 198) standard deviations (s) below the mean composite reading score of control twins. Male/female gender ratios for children with reading difficulties were significantly higher than 1.0 for all five samples and increased as a function of severity (viz., 1.15, 1.17, 1.40, 1.61, and 1.88, respectively). When the DF model was fit to the data, estimates of heritability (h[subscript g [squared]] ) and shared environmental influences (c[subscript g [squared]] ) were not significantly different for males and females in any of the groups. Consequently, the most parsimonious model that provided a good fit to the data at all five levels of severity equated the heritabilities and shared environmental influences for males and females, and fixed the DZ[subscript os] coefficient of genetic relatedness at 0.5. Thus, these results provide no evidence for a differential etiology of reading difficulties as a function of gender in more severely impaired samples, and suggest that the same genetic and environmental influences contribute to reading difficulties in males and females, irrespective of severity.