The reading skills of 285 pairs of 13-year-old twins drawn from the general population were studied. The twins were independently tested using standardized measures of intelligence, reading and spelling ability. The genetic contribution to reading ability was assessed by examination of correlations in monozygotic (MZ) and same-sex dizygotic (DZ) twins, and by analysing differences between MZ and DZ twins in concordance of reading disability rates. The results suggested that, at this age, genetic factors play only a moderate role in general reading backwardness or specific reading retardation. However, when spelling ability was investigated, a heritability of 0.53 was obtained, increasing to 0.75 when intelligence was controlled. Strong genetic influences on spelling were also found when concordance rates for spelling disability were compared for MZ and DZ pairs.

-The influences of social and family characteristics on individual differences in reading and spelling ability and IQwere investigated for 550 twin children aged 13 yrs. Measures of family social circumstances, parental background, the emotional atmosphere at home and the family's reading behaviour were used as predictor variables in multiple regression analyses with reading, spelling and IQ as dependent variables. There were consistent relationships between many of these measures and the dependent variables. The findings are compared to a previous biometrical genetic analysis of the same data set. It is concluded that the results are within the limits predicted by the genetic analysis. Most of the effect of these environmental influences on children are general, i.e. related to IQ, and not specifically related to reading. After controlling for the effects of IQ on reading, only family size and some aspects of parent-child relationships were signifcant predictors of reading ability.

This study investigates whether 13‐year‐old retarded (SRR) and backward (RB) readers show differences in their use of two reading strategies: phonological and whole word. Thirty‐eight SRR and 63 RB children were compared on their ability to read non‐words (phonological strategy), irregular words (whole word strategy) and the relative facility with which they read regular compared to irregular words (regularity effect). In order to control for IQ and reading level two different two‐group designs were employed: a ‘mental‐age control’ design where only IQ was controlled and a ‘reading‐age match’ design in which reading level and IQ were both taken into account. The findings suggest that in terms of single word reading, retarded and backward readers are not distinct groups, at least at 13 years of age. The overall level of reading proficiency explains most of the difference in reading between the two groups and they do not necessarily use different reading strategies. The implications of these findings for the controversy over subtypes of reading disability and the continuum of reading skill are discussed.