Party discipline, representation, and the condorcet jury theorem

Abstract How does district magnitude affect the quality of political representation? The Condorcet Jury Theorem (CJT) predicts that majority decisions improve with group size when members vote independently and are more likely than not to be correct. We test whether this aggregation mechanism applies to political decisions by examining whether larger legislative delegations better reflect constituent preferences. Nominal district magnitude often overstates the number of independent decision-makers because of party discipline. We therefore introduce effective district magnitude as a measure that more accurately captures the number of independently voting politicians within a constituency. Using 263 Swiss referenda (1992–2024) matched to parliamentary decisions, we find that the probability that a majority of a district’s delegation aligns with the preferences of its constituents increases with (effective) district magnitude, consistent with the CJT. The CJT aggregation mechanism closely predicts delegation-constituency congruence only for effective, not nominal, district magnitude. Our results provide empirical evidence that the CJT applies to representative democracies. They also show how party discipline limits the advantages of larger districts.