Abstract: Accessibility and animacy hierarchies are two prominent typological generalizations that have been extensively supported by synchronic research. However, the potential relationship between these synchronic generalizations and diachronic changes has been largely unexplored. This study systematically examines how the diachronic change of English relative clauses is constrained by these two generalizations, using the American State of the Union corpus (1790-2023). Our findings reveal that 1) the influence of accessibility remains stable, with subject relative clauses consistently showing a higher frequency than object relative clauses. 2) When the voice is considered, subject relative clauses in the passive voice show a decreasing trend, while corresponding object relative clauses increase. 3) Taking both accessibility and animacy into account, relative clauses exhibit a stable, polarized distribution when their effects align. However, when these effects compete, the effect of accessibility is stronger than that of animacy of antecedents, though it is constrained and attenuated by the animacy of both antecedents and clausal nouns.