Now, a paper on polarization and inequality released in August by political scientists from Princeton, Georgetown, and the University of Oregon (and highlighted this week in a Washington Post article) provides some empirical evidence that Democratic Party’s leftward drift is more pronounced than the GOP’s rightward drift, at least at the state level. The study’s overall argument is that income inequality has increased political polarization at the state level since the 1990s. But the authors find that that this happens more by moving state Democratic parties to the left than by moving state Republican parties to the right. As the Democratic Party lost power at the state level over the past 15 years, it also effectively shed its moderate wing. Centrist Democrats have increasingly lost seats to Republicans, “resulting in a more liberal Democratic party” overall. The authors find that the ideological median of Republican legislators has shifted much less.

One study does not a thesis prove, but the paper is certainly interesting, and it coheres with the trends we’ve been seeing. So while Democrats from President Obama on down often give the impression that their party is moderate and in line with public opinion while Republicans have undergone a sudden jolt to the right, it may not be that simple. Our discussions about polarization need to reflect the fact that it is a bipartisan affair.