Although vocabulary has long been an important criterion for assessing second language (L2) writing proficiency, recent research on academic discourse has positioned word study as a leading method to improve learner writing. As a result, teachers must make decisions on which words warrant instructional time and how to help learners deploy these words effectively in production. While there has been a growing trend in research investigating word lists, comparatively fewer studies have examined how words are actually used to achieve writing quality.

Thus, the present paper reports the findings of a quantitative study that examined the extent to which vocabulary size and lexical diversity contributed to writing scores on 172 native and advanced non-native English speakers’ academic essays. Results revealed that lexical diversity had a significantly greater impact on writing score than vocabulary size in both native and non-native speaker essays. Nevertheless, vocabulary size did initially facilitate writing scores at the lower score levels; however, it was lexical diversity that promoted an essay into the higher score range. Additional findings demonstrated that vocabulary size had only a moderate relationship to lexical diversity.

Outcomes from this study suggest that variation of mid-range vocabulary may play a more important role in writing proficiency than the use of infrequent terms that signal a larger productive lexicon. Furthermore, the results indicate that it is not enough to simply teach vocabulary words in the L2 composition classroom, but to also guide learners in how to employ these words in a varied manner within their writing.