Determinants of bill co-sponsorship in open-list proportional representation systems: the case of Chile, 2018–2022

Bill co-sponsorship allows legislators to signal policy priorities, build alliances, and cultivate constituencies. Yet, it is unclear whether legislators choose collaborators primarily because they share partisan ties, represent similar territorial interests, belong to the same coalition, or share personal traits such as gender, especially in countries where electoral reforms alter opportunities for legislative collaboration. We examine co-sponsorship in Chile’s Chamber of Deputies during the 2018–2022 term, after a reform that increased district magnitude (DM), reduced the number of districts, fragmented the party system, and introduced gender quotas. Building on the pre-reform study by Alemán and Calvo, we explore if the determinants of legislative collaboration changed under the new rules. Analyzing 1,890 legislator-initiated bills and 13,203 legislator pairs, we use ERGM models to estimate tie formation and C-ERGM models to estimate tie intensity. The results show that party affiliation remains the central organising principle of legislative co-sponsorship after reform.