Abstract
Abstract
Syntactic complexity is a multidimensional feature of second-language (L2) writing, but most developmental research treats the essay as the unit of analysis. Much less is known about development within rhetorically consequential micro-units such as thesis statements. This quantitative, cohort-longitudinal corpus study examines how thesis statements change across an undergraduate EFL programme. Seventy learner essays were included in the analysis at three time points (beginning of Year 1, n = 26; end of Year 1, n = 23; end of Year 4, n = 21) and were compared descriptively with 20 argumentative essays from the Louvain Corpus of Native English Essays (LOCNESS). Thesis statements were coded for occurrence, placement, rhetorical pattern, structural completeness, and sentence type. Clearly identifiable thesis statements increased from 2/26 essays at entry to 13/23 at the end of Year 1 and 16/21 by Year 4. Beginning placement increased from 4/26 to 14/21 essays, while end placement disappeared by Year 4. Among thesis statements that could be evaluated for structural completeness, 15/20 final-year instances met the study's criteria for a well-formed thesis statement, compared with 11/19 in the benchmark corpus. Complex sentences were the dominant sentence type in both the final-year learner data (13/20) and the benchmark data (14/19). These patterns indicate substantial development in the explicit rhetorical realization and sentence-level structuring of thesis statements across the programme. Because the study uses short rhetorical units and descriptive group-level counts rather than inferential longitudinal modelling, the findings are interpreted as evidence of cohort-level developmental change rather than causal effects of instruction or statistical convergence with a benchmark norm. The study argues that thesis statements can provide a focused unit for tracing the interaction of genre knowledge and sentence-level complexity in L2 argumentative writing.