Which aspects of processing text mediate genetic effects?

Genetic influences on reading are investigated in a sample of 285, 13 year old twins. Using a multiple regression procedure, the heritability of disability (h _g ² ) for Reading Recognition was found to be non-significant. However the h _g ² for spelling disability was found to be 0.58 (P<0.05), after controlling for individual differences in IQ. The twins in this study were an unselected sample from the general population. Therefore it was possible to estimate h _g ² for differing degrees of severity of disability. These analyses showed that for spelling but not for Reading Recognition or Reading Composite, there were substantial genetic contributions to all levels of disability. For indices of Orthographic Coding there were no significant values of h _g ² . In contrast measures of Phonological Coding and Homophone Recognition have consistently high values of h _g ² . More detailed analyses suggested that there were possibly two independent aspects of phonological ability, each influenced by genetic factors.