The present study presents contrastive analyses of task-oriented spoken and written discourse in terms of lexical diversity, lexical density, and word length. In an age-matched within-language comparison (Swedish), written discourse consistently scored higher on these measures. It is suggested that the same type of differences will hold for any language, because of the difference between speech and writing in processing constraints. The absolute scores, however, can vary substantially for reasons of language typology. An extended, cross-linguistic analysis (English, Hebrew, Icelandic, Swedish), focusing on word length, was made to substantiate that claim. Further, cross-age-group comparisons of lexical quanta indicated a dynamic interaction between speech and writing in development. Spoken discourse eventually comes to “learn” from the development of writing.