Causal Pathways for Specific Language Impairment: Lessons From Studies of Twins

Abstract

Purpose This review article summarizes a program of longitudinal investigation of twins' language acquisition with a focus on causal pathways for specific language impairment (SLI) and nonspecific language impairment in children at 4 and 6 years with known history at 2 years. Method The context of the overview is established by legacy scientific papers in genetics, language, and SLI. Five recent studies of twins are summarized, from 2 to 16 years of age, with a longitudinal perspective of heritability over multiple speech, language, and cognitive phenotypes. Results Replicated moderate-to-high heritability is reported across ages, phenotypes, full population estimates, and estimates for clinical groups. Key outcomes are documentation of a twinning effect of risk for late language acquisition in twins that persists through 6 years of age, greater for monozygotic than dizygotic twins (although zygosity effects disappear at 6 years); heritability is greater for grammar and morphosyntax than other linguistic dimensions, from age 2 years through age 16 years, replicated within twin samples at subsequent age levels and across twin samples at age 16 years. Conclusion There is consistent support for legacy models of genetic influences on language acquisition, updated with a more precise growth signaling disruption model supported by twin data, as well as singleton data of children with SLI and nonspecific language impairment. Presentation Video https://doi.org/10.23641/asha.13063727.

Figure 1.

Schematized within twin pair expected pattern of correlations for an inherited trait.

Figure 2.

Comparisons of correlations and levels of performance. Schematized correlations between Twin 1 and Twin 2 of a twin pair for vocabulary standardized scores in a sample of twins (left side) versus possible differences in levels of performance within twin pairs contrasting high performing pairs similar to typically developing singletons (blue line) versus lower performing pairs showing twinning effects at lower than expected age levels (red line). Note that although the accuracy level differs, in this example the within-group correlations are similar, ~ r = .80, as might be the case for MZ pairs. Thus, within MZ twin pairs there could be high similarity between the two children and low accuracy of performance.