The ‘culturomics’ and other big data approaches have been widely used to trace the development of human cognition and social change. In this cross-lingual study, we examine historical changes in the frequency of discourse connectives (DCs) in Chinese, English, French, German and Spanish over the last two centuries. Our analyses reveal a robust and long-term decline in the frequency of most DCs in English, French, German and Spanish between 1800 and 2000. These diachronic trends closely parallel changes in other stylistic indicators, pointing to coherent shifts in language use and register evolution. Although our findings align with previous studies and recent observations of changing patterns in linguistic rationality, they should not be interpreted as evidence of declining human capacities for logic or rational reasoning. Instead, the observed patterns reflect a broad and sustained process of ‘colloquialization’ in written language, driven by socio-cultural transformations and evolving communicative norms. This study advances our understanding of historical language change and its underlying mechanisms, offering insights into the coevolution of language, cognition and society.