Genetic Evidence for Bidirectional Effects of Early Lexical and Grammatical Development.

Two cohorts of same-sex twin pairs were assessed on grammar and vocabulary. Findings indicated that vocabulary and grammar correlated strongly at 2 and 3 years in both cohorts, with a consistently high genetic correlation between vocabulary and grammar at both ages. Findings suggest that the same genetic influences operate for vocabulary and grammar, a finding incompatible with traditional autonomy hypothesis. (Author/KB)