Political polarization and primary elections

I provide a causal test of the impact of primary elections on political polarization. I exploit the staggered introduction of primary elections for US senators and representatives in Indiana and New York State. Using a difference-in-differences design, I show that primaries deliver less-polarized politicians, reducing the ideological gap between parties that existed before the reform by one-fifth. I interpret the results in light of a conceptual framework in which primaries reduce the cost of participating in the candidate selection processes, thereby also giving moderate voters incentives to participate. The findings suggest that primary elections are an institution able to reduce the welfare costs associated with polarization.