Polarization, a scholar has quipped, is a problem that no one can explain or solve.
We know that a vast ideological gulf now separates most Democratic from most Republican politicians. We also know that highly polarized politicians poorly represent many of their constituents, who remain comparatively moderate. But we’re not sure why polarization has arisen or what we can do about it.
As a pathology of American politics that has developed over decades, polarizat...