Asymmetric contagion: the radical right and the erosion of parliamentary civility

Does party system fragmentation erode parliamentary civility? Analyzing 130,000 Spanish legislative speeches (1996–2023) using a transformer-based model (BETO), this article identifies an “asymmetric contagion” effect. While the 2015 entry of “new politics” mainstream challengers did not alter the chamber’s tone, the 2019 arrival of the radical right coincided with a structural break, characterized by high intrinsic negativity and a subsequent spillover among established parties. These results demonstrate that rhetorical polarization is not an automatic byproduct of fragmentation, but is specifically associated with the strategic entry of radical right actors who alter the communicative equilibrium of representative assemblies.