This paper investigates the uses of modal auxiliary verbs in a corpus of PhD theses written by native speakers of English. The texts in the corpus come from two departments in the same British university: Agricultural Botany and Agricultural Economics. Variation in the frequency of use of the modal verbs between theses in either department, and also between theses within the same department is examined. Furthermore, variation in use in different rhetorical sections is considered. In all cases, marked variation is observed. An account of the uses of the modal auxiliary verbs in two of the theses is given.
Treatment of the modal verbs in published academic writing textbooks suggest that insufficient attention is given to the range of functions that modal auxiliaries perform in academic writing at the doctoral level, with excessive attention being paid to the role of modals in expressing tentativeness. The analysis also points to the diversity of rhetorical practices possible within the PhD thesis genre.