Hohnen, Bettina Louise; (1997) Individual differences in literacy, language and related cognitive abilities: A twin analysis. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

A general population sample of one hundred and twenty-six pairs of twins were seen in a cross-sectional design at the ages of 5 1/2 and 7 years. The general aim of the study was to add to our understanding of literacy development by examining the etiology of individual differences in literacy, language and related skills. The study fills a gap in the literature regarding the causes of individual differences in these skills at a time when children are beginning to become literate. Strong genetic influences were found on all aspects of literacy at this age. These were in line with previous twin studies on older children. Phonological processing skills were also found to be highly heritable. Separate aspects of language functioning were influenced by genetic factors to different degrees, expressive language being more heritable than receptive language. A multivariate approach to the genetic analysis was employed in order to identifying shared etiology. The findings add to our understanding of early literacy development by showing a shared genetic etiology for phonological and orthographic coding. This was interpreted as supporting Ehri's (1992) integrative theory of reading development. Support was found for the view that different aspects of phonological processing all tap a core construct which is heritable. In addition both serial naming and nonword repetition were influenced by genetic factors independent of phonological awareness suggesting that performance on these tasks reflects more than just phonological processing skills. A shared genetic etiology was found for general language ability, phonological awareness and literacy. This was interpreted as showing that there is a single underlying dimension of individual difference with a genetic basis that is specific to verbal skills. Phonological awareness and literacy were independently linked at both ages via environmental mechanisms. Finally, nonword repetition was found to have a specific link with language skills but not with literacy.

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