Genetic and environmental influences on early speech, language and literacy development

Abstract

The genetic and environmental etiology of speech and broader language skills was examined in terms of their concurrent relationships in young children; their longitudinal association with reading; and the role they play in defining the 'heritable phenotype' for specific language impairment (SLI). The work was based on a large sample of 4 1/2-year-old twins, who were assessed at home on a broad range of speech and language measures as part of the Twins Early Development Study. We found that genetic factors strongly influence variation in young children's speech in typical development as well as in SLI, and that these genetic factors also account for much of the relationship between early speech and later reading. In contrast, shared environmental factors play a more dominant role for broader language skills, and in relating these skills to later reading; isolated impairments in language as opposed to speech appear to have largely environmental origins.

Learning outcomes: (1) Readers will be able to discuss some of the ways in which behavioural genetic methods can make a useful contribution to the field of communication disorders. (2) Readers will be able to compare the genetic and environmental contributions to general language versus speech skills in young children. (3) Readers will be able to describe the likely relationship between early speech and language and later reading development, in terms of shared genetic and environmental resources. (4) Readers will be able to discuss how different ascertainment methods for clinical samples may lead to very different understandings of the nature of a disorder such as specific language impairment.

Figure 1

Path diagram for multivariate common pathways model, with two correlated latent factors (General Language and Speech). Square boxes represent the measured variables, while circles represent latent variables. These are the two phenotypic latent factors (above the boxes), with their genetic (A), shared environmental (C), and nonshared environmental (E) variance components. The A, C, E variance components that are unique to the measured variables are shown at the bottom of the figure, below the boxes.

Figure 2

The bivariate relationship between the general language factor at 4½ years, and each of the three reading variables: teacher-rated reading, TOWRE and PIAT. The overall height of the bars represents the total phenotypic relationship between the 4½ year language factor, and each of the reading variables. The different shadings on the bars represent the genetic and environmental contributions to the total bivariate relationship.

Figure 3

The bivariate relationship between the speech factor at 4½ years, and each of the three reading variables: teacher-rated reading, TOWRE and PIAT. The overall height of the bars represents the total phenotypic relationship between the speech factor, and each of the reading variables. The different shadings on the bars represent the genetic and environmental contributions to the total bivariate relationship. The two ‘X’s on the bars highlight non-significant contributions.

Figure 4

The heritability of ‘psychometric SLI’ and ‘clinical SLI’.