I've got an article in this week's New Yorker that tries to answer two questions.

1) How did the individual mandate go from being a policy Republicans supported and Democrats resisted in 1993 to a policy Democrats support and Republicans loathe in 2012?

2) How did we go from having “a less than one-per-cent chance that the courts will invalidate the individual mandate" -- that was the estimation of Orin Kerr, a George Washington University law professor who had clerked for Justice Anthony Kennedy -- to, as he puts it now, "a 50-50 chance"?

Charles Dharapak / AP

And it's not just the individual mandate. You can see a similar dynamic at work in the DREAM Act, which Republicans introduced into the Senate in 2001 and opposed by 2010. You can see it in a cap-and-trade plan to reduce carbon emissions, which Republican Senator John McCain introduced into the Senate in 2003 and included in his presidential campaign in 2008, and which no Senate Republican, to my knowledge, supports today. You can see it in Republican attitudes towards deficit-financed stimulus plans, which President George W. Bush pushed and signed in 2008.

The point here isn't to cry "hypocrisy" and let slip the dogs of Nexis. It's to ask a broader question about Congress. In a system that typically requires compromise for anything to get done, how can compromise be reached if parties change positions this quickly, and this remorselessly?

Coming to any conclusion about that requires disentangling the mechanisms by  which these shifts are happening. One popular perspective -- see this post by Brad DeLong -- is that this is rank cynicism. The Republican Party hasn't changed any of its positions, or if it did, it didn't hold those positions in the first place. Rather, the Republican Party is coldly taking whatever position happens to be most strategically convenient, and if that requires pretending to jettison their previous beliefs, then so be it.

My view -- based both on my experience as a reporter in Washington and on an increasingly voluminous psychological literature about how partisans form opinions -- is that human beings are very good at convincing themselves of whatever their self-interest would have them believe, and that Washington has become an increasingly sophisticated machine for encouraging and accelerating this sort of "motivated reasoning." That is to say, strategic incentives do drive shifts in opinion, but the new opinions are sincerely held.

This distinction is, I think, crucial to make, because there's a real difference between bargaining with a political party that has fixed policies it would like to achieve but shifting strategic incentives and a party that has shifting policy and strategic incentives. In the former case, the fixed policy preferences give you something to hold onto, something you can use to fight against the shifting strategic incentives. In the latter case, there's nothing to hold onto, and in a political system where elections are zero-sum and so compromise is rarely in the minority party's strategic interest, it's very difficult to see how many major compromises can be reached. This model, I think, does a much better job explaining the previous few years.

Anyway, I hope you'll read the full piece, which has a lot more the background of the mandate, the reflections of key players in Washington, and the experiments that have convinced leading psychologists that human beings rationalize much more often than they reason. As always, I welcome your feedback.