Variation of sentence length across time and genre
This study investigates whether mean sentence length in written English has consistently decreased over time (1810–2000), using the Corpus of Historical American English (COHA), and examines its relationship with grammatical change—particularly the decline of the non-finite purposive marker *in order to*—and genre evolution. Methodologically, it employs large-scale corpus querying, diachronic mixed-effects modeling, multivariate correlation analysis, and genre-normalized frequency computation. Results confirm a statistically significant reduction in mean sentence length; crucially, they reveal a robust cross-temporal association between this reduction and the declining use of *in order to*, with systematic genre-specific variation observed in news and fiction. Moving beyond descriptive sentence-length analyses, the study integrates syntactic change, register evolution, and macro-level sentence-length trends within a unified empirical framework. It thus offers both methodological innovation—through genre-aware diachronic modeling—and theoretical advancement for historical syntax.