The prevalence of reading difficulties is typically higher in males than females in both referred and research-identified samples, and the ratio of males to females is greater in more affected samples. To explore possible gender differences in reading performance, we analyzed data from 1,133 twin pairs in which at least one member of each pair had a school-history of reading problems and from 684 twin pairs from a comparison sample with no reading difficulties. Although the difference between the average scores of males and females in these two samples was very small, the variance of reading performance was significantly greater for males in both groups. We suggest that a greater variance of reading performance measures in males may account at least in part for their higher prevalence of reading difficulties as well as for the higher gender ratios that are observed in more severely impaired samples. KeywordsDyslexia; Gender Ratio; Prevalence; Reading Disability; VarianceThe ratio of males to females in samples of children with reading difficulties varies widely depending upon the method of ascertainment. In studies where subjects are ascertained employing clinical or referral methods, gender ratios range from 2:1 to 15:1 males to females (e.g., Finucci & Childs, 1981;Vogel, 1990); however, in research-identified samples, gender ratios are closer to 1:1 (e.g., Harlaar, Spinath, Dale, & Plomin, 2005;Hawke, Wadsworth, Olson, & DeFries, 2007;Shaywitz, Shaywitz, Fletcher, & Escobar, 1990;Stevenson, 1992). Nevertheless, in both referred and research-identified samples, greater numbers of males with reading problems have typically been reported. For example, in a recent review of sex differences in reading disability, Rutter et al. (2004) reported the gender ratios in four independent epidemiological studies in which the samples had been ascertained using research criteria. In all four of the studies, significantly more males than females with reading disabilities Correspondence to: Jesse L. Hawke. NIH Public Access Author ManuscriptDyslexia. Author manuscript; available in PMC 2010 August 1. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript were reported. Moreover, gender ratios for reading difficulties are greater in more severely affected samples (Hawke et al., 2007;Olson, 2002).Several biological and environmental hypotheses have been proposed to account for this gender difference in prevalence rates including X-linked recessive inheritance (Symmes & Rapoport, 1972), differences in brain functioning due to differential exposure or sensitivity to androgens Nass, 1993;Tallal & Fitch, 1993), immunological factors, sexual imprinting, perinatal complications, and differential resilience to neural insult (Liederman, Kantrowitz, & Flannery, 2005). It has also been suggested that females may be less susceptible to environmental factors such as teaching methods and socioeconomic status , and that genetic influences may be more important as a cause of reading difficulties in females than in m...

To test the hypothesis that the genetic etiology for reading disability may differ in males and females, data from identical and fraternal twin pairs were analysed using both concordance and multiple regression methods. The sample included 264 identical (129 male, 135 female) and 214 same-sex fraternal (121 male, 93 female) twin pairs in which at least one member of each pair had reading difficulties. The difference between the identical and fraternal twin pair concordance rates was slightly larger for females than for males, suggesting a possible sex difference in etiology; however, a loglinear analysis of the three-way interaction of sex, zygosity, and concordance was not significant (p50.17). The estimate of group heritability ðh 2 g Þ, a standardized measure of the extent to which reading difficulties are due to genetic influences, was somewhat greater for females than males (0.65 vs 0.54), but this difference was also not significant (p50.35). Gender differences in h 2 g were larger for younger children (less than 11.5 years of age) than for older children. However, the three-way interaction of sex, zygosity, and age was not significant when age was treated either categorically (p50.86) or continuously (p50.71). Thus, results of this study provide little or no evidence for a differential genetic etiology of reading difficulties in males and females.

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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 ss ), and opposite-sex dizygotic (DZ 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 g 2 ) and shared environmental influences (c g 2 ) 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 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.