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 p
erformance data from identical 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 g
reater than or equal to 0.70). Moreover, the heritability of reading disabi
lity (h(g)(2)) estimated from regression analysis of the reading performanc
e data in males was nearly identical to that in females (h(g)(2) = 0.58 and
0.59, respectively; p greater than or equal to 0.90). Thus, results of thi
s study provide little or no evidence for the hypothesis of greater genetic
influence on reading difficulties in girls than in boys.