https://astralcodexten.substack.com/p/which-party-has-gotten-more-extreme

Matt Yglesias has written a couple of posts (1, 2) on the subject of this meme (originally by Colin Wright, recently signal-boosted by Elon Musk):

He concludes that, contra the image where the Right stays in the same place and the Left moves, both Republicans and Democrats have "changed a lot" since 2008. He wisely avoids speculating on whether one party has moved further or faster than the other.

I'm less wise, so I've been trying to look into this question. My conclusion is: man, people really have strong emotions on this.

I think a lot of the disagreement happens because this is more than one question. You can operationalize it a couple different ways:

• Which party's policy positions have changed more in their preferred direction (ie gotten further left for the Democrats, or further right for the Republicans) since 2008 - or 1990, or 1950, or some other year when people feel like things weren't so partisan?

• Which party has diverged further from ordinary Americans?

• Which party has become more ideologically pure faster than the others (ie its members all agree and don't tolerate dissent)?

• Which party has become crazier in terms of worldview and messaging, in a way orthogonal to specific policy proposals? That is, suppose one party wants 20% lower taxes, and plans to convene a meeting of economists to make sure this is a good idea. The other party wants 10% higher taxes, and says a conspiracy of Jews and lizardmen is holding them back, and asks its members to riot and bring down the government until they get the tax policy they want. The first party has a more extreme policy position (20% is more than 10%), but the second party seems crazier.