Comparative analyses of twin data.
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Biomedical subjects
D W Fulker
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Evidence for a genetic aetiology in reading disability of twins.
Reading disability (dyslexia) is a major social, educational, and mental health problem. Although estimates of prevalence vary, up to 10-15% of school-age children have severe reading deficits in spite of average intelligence and adequate educational opportunity. That reading disability may have a constitutional basis has long been recognized, and results of twin and family studies suggest that one or more of its forms may be heritable; however, definitive evidence for a genetic aetiology has not been reported. Establishing a heritable basis for reading disability could suggest possible causes, give improved risk estimates, facilitate early diagnosis, and provide validity tests for ostensible subtypes. In this report, we apply a recently developed multiple regression analysis to data collected from a sample of 64 pairs of identical twins and 55 pairs of fraternal twins, in which at least one member of the pairs is reading disabled, and present evidence for a significant genetic aetiology.
Dyslexia↗
Genetic influence on general mental ability increases between infancy and middle childhood.
Adoption studies can provide direct evidence for the independent effects of family environment and heredity that are always confounded in intact nuclear families. When children are separated from their biological mothers shortly after birth and placed nonselectively in adoptive homes, adoptive-parent/adopted-child resemblance can be ascribed to cultural transmission, whereas biological-parent/adopted-child similarities are due to heritable factors. Furthermore, a longitudinal adoption study facilitates examination of changes in these two main sources of variation during development. The Colorado Adoption Project is the first large-scale longitudinal adoption study of behavioural development and was initiated in 1975. Data were collected from biological parents of 245 adopted children, the adoptive parents and parents of 245 matched nonadopted children. The children have subsequently been tested at 1, 2, 3 and 4 years of age, and at the end of their first year in primary school (average age, 7.4 years). The number of subjects tested is now adequate for analysis of data over 7 years. The results provide conclusive evidence for increasing heritable variation of general mental ability, ranging from 9% at 1 year of age to 36% at 7 years.
Adoption↗