Parliamentary Efficiency under Majority and Supermajority Rules: The Role of Independent Legislators
Parliaments dominated by two political blocs often face legislative inefficiencies as polarization increases. A central institutional question concerns how majority and supermajority rules interact with parliamentary composition to balance governability and minority protection. This article examines how the inclusion of independent legislators, introduced through sortition, affects collective decision-making under different majority and supermajority quorum requirements. Using an agent-based model that combines analytical threshold derivations with numerical simulations, we identify four critical thresholds that partition the parameter space into distinct coalition regimes. These regimes range from majority-party dominance to minority veto and, at high levels of diversity, to structural fragmentation in which coordination becomes increasingly difficult. Parliamentary efficiency emerges from non-linear interactions among quorum thresholds, party-size distribution, and the proportion of independents. Under simple majority, the system exhibits an interior efficiency maximum associated with a transition from unilateral control to pivot-based coalition formation. Under strong supermajorities, however, the same increase in diversity may induce minority veto dynamics or coordination breakdown, significantly reducing legislative performance. These results show that institutional performance is not an intrinsic property of a particular decision rule but an emergent outcome of the interaction between approval thresholds, parliamentary composition, and the coalition structures they generate.