Reading and Writing: An Interdisciplinary Journal 13: 133–145, 2000.
© 2000 Kluwer Academic Publishers. Printed in the Netherlands.
Reading disability in boys and girls: No evidence for a differential
SALLY J. WADSWORTH, VALERIE S. KNOPIK & J.C. D
University of Colorado, Boulder
Abstract. In order to test the hypothesis that genetic inﬂuences are more important as a cause
of reading disability in girls than in boys, composite reading performance data from iden-
tical and fraternal twin pairs were analyzed using both concordance and multiple regression
methods. The sample included 206 identical (99 male, 107 female), 159 same-sex fraternal
(90 male, 69 female), and 117 opposite-sex fraternal twin pairs, in which at least one member
of each pair had reading difﬁculties. Although the difference between the concordance rates
for reading disabilities in female identical and same-sex fraternal twin pairs was somewhat
greater (65% for identical twins vs. 32% for fraternal twins) than the difference in concordance
rates for boys (68% vs. 39%), loglinear analysis of these categorical data revealed that the
interaction between sex, zygosity, and concordance was not signiﬁcant (p ≥ 0.70). Moreover,
the heritability of reading disability (h
) estimated from regression analysis of the reading
performance data in males was nearly identical to that in females (h
= 0.58 and 0.59, respect-
ively; p ≥ 0.90). Thus, results of this study provide little or no evidence for the hypothesis of
greater genetic inﬂuence on reading difﬁculties in girls than in boys.
Keywords: Gender, Genetic, Heritability, Reading Disability, Twins
A plethora of previous studies have compared reading difﬁculties in boys
and girls, noting both mean differences in reading-related measures and
differences in prevalence rates (e.g., DeFries 1989; DeFries, Wadsworth &
Gillis 1990; Finucci & Childs 1981; Geschwind 1981; Knopik, DeFries &
Alarcón 1996; Shaywitz, Shaywitz, Fletcher & Escobar 1990; Tallal, Ross &
Curtis 1989; Stevenson 1992; Vogel 1990). In studies of clinical and referred
samples of children with reading disabilities, sex ratios have ranged from
2:1 to 15:1 males to females (Finucci & Childs 1981; Vogel 1990), whereas
sex ratios in research-identiﬁed populations are closer to 1:1 (Shaywitz et al.
1990; Stevenson 1992; Wadsworth, DeFries, Stevenson, Gilger & Pennington
1992). Although the magnitude of these sex ratios apparently differs depend-
ing upon how the sample was ascertained and/or how reading disability was
diagnosed, an excess of males has typically been noted (Stevenson 1992).