From court ruling to legislative agenda through party competition: Diffusion of Gender Identity Reforms in multiparty local congresses.

Gender identity reforms—amendment of birth certificates for trans people—have spread unevenly across Mexican states. What explains the variation in the incorporation and approval of gender identity reforms in local congresses? This study builds a dataset of 918 state-legislative-year observations, covering the composition of all 32 Mexican state congresses from 2000 to 2026, tracking the introduction and approval of gender identity reforms. Using discrete-time event history analysis, the results show that the intervention of the Mexican Supreme Court increased diffusion coverage; however, its effect was stronger on agenda-setting than on legislative approval. Beyond court ruling, competition between left- and right-wing parties is the strongest catalyst of diffusion. This study contributes to diffusion studies by showing how vertical and horizontal catalysts interact; distinguishing agenda-setting and policy adoption as different stages of policy diffusion; and demonstrating the importance of party competition in policy diffusion across multiparty local congresses.