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Why is American Politics Uni-dimensional?

Paul Krugman briefly mentions a somewhat intriguing puzzle. Most people, if you ask them about it, would say that political beliefs are “multidimensional.” We often think of a simple 2-dimensional models like the Nolan Chart in which people should be sorted along both a left-right axis about economics, and then along a second axis about social/cultural issues like gay rights. But as Krugman observes, Congress doesn’t work this way:

That’s what I would have thought a few years ago. But then I became familiar with the Poole-Rosenthal work on Congressional voting. They use a clever algorithm to jointly map bills and members of Congress in a hypothetical issues space. The number of dimensions in that space is arbitrary — but they found that historically just two dimensions accounted for the great bulk of voting. One dimension corresponded to left-right on economic issues; the other was basically race/segregation.

And since the 1960s, with the great Southern realignment, the race dimension has collapsed. So Congressional politics is left versus right — end of story. Oh, and polarization along that dimension has increased hugely: the center did not hold, and there really isn’t any middle ground.

To offer some qualitative examples, Susan Collins and Olympia Snowe are pro-choice Republican Senators. But they’re also the two senators who seem like they might possibly vote for a national health care bill. Rather than representing some kind of ideal type of upscale northeasterner who’s socially liberal but economically conservative, they’re less conservative across-the-board than their colleagues from the South and the Mountains. Conversely, when you stroll down to Arkansas’ Democratic Senators, you don’t see cultural conservatives with populist economics, they’re just more conservative across-the-board than their coastal colleagues.

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Something to consider along these lines is the map of states with anti-union “right to work” laws that substantially prevent union organizing:

This is about as purely an economic issue as you can think of, but it’s the liberal coastal elites who have union-friendly laws. My guess is that this winds up creating a higher degree of unidimensionality among practical politicians than exists in the population. The presence of a meaningful labor movement on the ground has huge implications for how politicians will vote on economic issues. And unions tend to be strong in the same places where people have relatively left-wing views on cultural issues.