Psychologists and psychiatrists at the University of Surrey, Guildford, Surrey, and the Hospital for Sick Children, Great Ormond Street, London, UK studied the reading skills of 285 pairs of 13 year-old twins using standardized measures of intelligence, reading and spelling ability and correlations in monozygotic and same-sex dizygotic twins. Genetic factors played only a moderate role in general reading backwardness and specific reading retardationwhereas strong genetic influences for spelling disability were found. 
COMMENT: Of a total of 96 twin pairs reported in the literature, 36 (88%) monozygotic twins and only 16 (29%) dizygotic twins were concordant for dyslexia (Dyslexia: as the Neurologist and Educator read it. Charles C Thomas, Springfield, Illinois, USA, 1986). Between 25 and 50% of children with reading disability demonstrate transmission within families. Hallgren (1950) concluded that his data best fitted an autosomal dominant genetic mechanism and others have proposed alternative genetic models: autosomal dominant with reduced penetrance in females, and sex-linked recessive. These studies are at variance with the present authors’ conclusions that emphasise the complexity of genetic influences on literacy skills and the importance of changes that occur with development in our understanding of the causation of reading difficulties.