Moderating political extremism: single round vs runoff elections under plurality rule

We compare single ballot vs dual ballot elections under plurality rule, assuming sincere voting and allowing for partly endogenous party formation. Under the dual ballot, the number of parties is larger but the influence of extremist voters on equilibrium policy is smaller, because their bargaining power is reduced compared to a single ballot election. The predictions on the number of parties and on policy volatility are consistent with data on municipal elections in Italy, where cities with more (less) than 15,000 inhabitants have dual (single) ballots respectively.