Literacy enhances lexical variation, not quantity, in adult oral production

Abstract Adult language use varies substantially across speakers, with literacy experience emerging as a crucial but understudied factor in creating this variation. While written language exposes speakers to broader, more diverse vocabulary than speech alone, most psycholinguistic research focuses on highly literate populations, leaving gaps in our understanding of how literacy shapes oral production. This study therefore addresses a critical question: Does literacy acquisition affect lexical diversity in spontaneous oral narrative production in Turkish? We compared lexical diversity results between illiterate and literate adult Turkish speakers during a structured storytelling task. Using Root Type-Token Ratio analyses across six parts of speech, we found that literate speakers consistently demonstrated significantly higher lexical diversity than illiterate speakers (d = 1.18–2.08 for most categories). Crucially, this occurred without increased word production, indicating that literacy enhances vocabulary variation rather than quantity. The largest effects emerged for elaborative categories: conjunctions, adverbs, and adjectives. These findings reveal that literacy fundamentally affects lexical organization and deployment in oral productions.