Abstract
Accounts of language development vary in whether they view lexical and grammatical development as being mediated by a single or by separate mechanisms. In a single mechanism account, only one system is required for learning words and extracting grammatical regularity based on similarities among stored items. A strong non-linear relationship between early lexical and grammatical development has been demonstrated in English and, more recently, in Italian supporting a single mechanism view (Caselli, Casadio & Bates 1999, Marchman & Bates 1994). The present study showed a comparable non-linear relationship between vocabulary size and the emergence of verb inflection and sentence complexity in two-year-old speakers of English and Icelandic, a highly inflected language. The study included 96 children within a narrow age range, but varying extensively in language proficiency, demonstrating continuity in lexical and grammatical development among children with typical language development as well as very precocious children and children with expressive language delay. Cross-linguistic differences were noted as well, suggesting that the Icelandic-speaking children required a larger critical mass of vocabulary items before grammatical regularity was detected. This is probably a result of the more complex inflectional system of the Icelandic language compared with English.
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