Lexical and grammatical development: a behavioural genetic perspective

The relation of lexical and grammatical knowledge is at the core of many co ntroversies in linguistics and psycholinguistics. Recent empirical findings that the two are highly correlated in early language development have furt her energized the theoretical debate. Behavioural genetics provides an illu minating new tool to explore this question, by addressing the question of w hether the empirical correlation simply reflects the fact that environments which facilitate one aspect of language growth also facilitate the other, or whether the same underlying acquisition mechanisms, influenced by the sa me genes, are responsible for the correlation. We explored this issue in a study of 2898 pairs of two-year-old twins born in England and Wales. Langua ge development was assessed by their parents using an adapted version of th e MacArthur Communicative Development Inventory which assesses vocabulary a nd grammar. Moderate heritabilities were found for both. As in previous stu dies, measures of vocabulary and sentence complexity were substantially cor related (r = 0 66). Behaviour-genetic modelling of the relation of vocabula ry and grammar produced an estimated value of 0.61 for the genetic correlat ion, a measure of the overlap of the genetic effects that contribute to the two aspects of language development. In contrast, a measure of nonverbal c ognitive development, the PARCA, was only weakly correlated at both the phe notypic level and at the level of genetic correlations with the language me asures. Thus, although the distinction between verbal and nonverbal skills has a genetic basis underlying the phenotypic dissociation, there is little evidence either genetically or phenotypically for a dissociation between v ocabulary and grammar within language.