Environmental influences on the longitudinal covariance of expressive vocabulary: measuring the home literacy environment in a genetically sensitive design.

BACKGROUND Despite the well-replicated relationship between the home literacy environment and expressive vocabulary, few studies have examined the extent to which the home literacy environment is associated with the development of early vocabulary ability in the context of genetic influences. This study examined the influence of the home literacy environment on the longitudinal covariance of expressive vocabulary within a genetically sensitive design. METHODS Participants were drawn from the Western Reserve Reading Project, a longitudinal twin project of 314 twin pairs based in Ohio. Twins were assessed via three annual home visits during early elementary school; expressive vocabulary was measured via the Boston Naming Test (BNT), and the Home Literacy Environment (HLE) was assessed using mothers' report. RESULTS The heritability of the BNT was moderate and significant at each measurement occasion, h(2) = .29-.49, as were the estimates of the shared environment, c(2) = .27-.39. HLE accounted for between 6-10% of the total variance in each year of vocabulary assessment. Furthermore, 7-9% of the total variance of the stability over time in BNT was accounted for by covariance in the home literacy environment. CONCLUSIONS These results indicate that aspects of the home literacy environment, as reported by mothers, account for some of the shared environmental variance associated with expressive vocabulary in school aged children.