Genetic influence for adult slow and/or inaccurate reading ability was studied from selfreported answers, using a dichotomous question on having difficulties in reading the Danish subtitles on foreign TV programs. The data from 33,424 twins were population based and were used for biometric analysis in order to estimate the heritability of reading difficulties. The rate of reading difficulties were 6–9 percent, higher for males than females. Tetrachoric correlations were estimated under univariate saturated models, specified with appropriate constraints. Hierarchical x2 tests showed that the fit of these models was not significantly worse than that of the more general models without the constraints. The probandwise concordance rates and tetrachoric correlations were substantially and significantly higher for monozygotic compared to dizygotic pairs, suggesting genetic influence. Univariate genetic analysis showed that additive genetic (A) and unique (unshared) environmental (E) factors best explained the observed concordance patterns for males. For females, and possibly also for males, a small proportion of non-additive genetic factors (D) were included. But the AEmodel had the same goodness-of-fit as the ADE-model, so therefore we selected the simplest model, the AE-model. The heritability is 0.63 with the 95 % confidence interval [0.55; 0.68], adjusted for the effects of age and gender. Both age and gender have significant influence. With the limitations related to adult self-reports this study demonstrates substantial genetic, almost only additive, influence on reading difficulties. The environmental factors affecting reading difficulties were unique and unshared (E). Unshared environmental factors are those factors that are specific to an individual, thus contributing to differences between family members. Unshared environmental factors are environmental factors related to perinatal factors, illness, traumas, foster home, individual peers etc. With a simple question about difficulty in reading subtitles we got valid information about the heritability of reading disorders, well in accordance with other studies involving very time consuming assessments.
Specialusis Ugdymas / Special Education, 2012, Vol 1, Issue 26, p. 19-28