Idioms in Academic Writing: A Corpus-Based Study of Native English Speakers Across Academic Levels and Disciplines

This study explores the use of idiomatic expressions in academic writing, comparing texts written at undergraduate and master's level in the Cambridge Academic English (CAE) Corpus with an ad hoc corpus of 30 PhD theses (COPLUS EN) written by native English speakers across three European Research Council (ERC) domains: Social Sciences and Humanities (SH), Physical Sciences and Engineering (PE) and Life Sciences (LS).While idioms are often studied in spoken language, this research focuses on their role in formal academic writing and seeks to address three primary objectives: (1) to determine the extent to which native English speakers use idiomatic expressions in academic writing, particularly in PhD theses compared to undergraduate and master's level texts, (2) to assess how proficiency level and linguistic awareness influence the frequency and functions of idiomatic expressions at different stages of academic study, and (3) to identify whether certain idioms are more prominent in specific academic disciplines.A mixed-methods approach combines quantitative analysis of idiom frequency with qualitative analysis of their functions within each ERC domain to examine whether idiomatic usage varies across academic levels and across disciplines.Pedagogical implications include recommendations for English for Academic Purposes (EAP) educators to integrate idiomatic expressions into teaching, focusing on their role in improving students' ability to construct persuasive arguments, establish epistemic positioning and communicate with greater precision in academic contexts.