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Abstract:
Abstract: The overall goal of this research was to examine
the nature of the relationship between two disorders, reading
disability (RD) and phonological disorder (PD). Although there is
robust evidence that these disorders overlap at symptomatic and
cognitive levels, the reason for this overlap has remained unclear.
The present study examined a sample of RD and control twin
children, some of whom had earlier PD, in order to further
elucidate the nature of the cognitive and etiological relation
between these two disorders. Four competing hypotheses (the
severity, pleiotropy, cognitive phenocopy, and RD subtype
hypotheses) were tested. Each posits different combinations of
etiological and cognitive influences to account for the phenotypic
overlap between RD and PD. Group performance on cognitive tasks
(non-word repetition, non-word reading, and phoneme awareness
measures) was examined in a series of 2x2 [(RD: yes or no) x (PD:
yes or no)] ANOVAs. Univariate and bivariate heritability estimates
of RD and PD were calculated using proband-wise concordance and
cross-concordance analyses. Finally, the relations between the PD
groups and several subtypes of RD were explored in order to
determine which of the subtypes is most related to PD. Each
disorder was heritable considered singly and there was also
evidence that they shared genetic influences. Further, PD alone was
found to be etiologically unrelated to RD. These and the other
results best supported the severity hypothesis.
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